DOI: 10.1002/chem.201404230

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& Electrochemiluminescence

Molecular Size and Electronic Structure Combined Effects on the Electrogenerated Chemiluminescence of Sulfurated Pyrene-Cored Dendrimers Giovanni Valenti,[a] Andrea Fiorani,[a] Simone Di Motta,[a] Giacomo Bergamini,[a] Marc Gingras,*[b] Paola Ceroni,*[a] Fabrizia Negri,*[a] Francesco Paolucci,[a] and Massimo Marcaccio*[a]

tally and theoretically by molecular mechanics and quantum chemical calculations. It has allowed rationalization of a possible mechanism and the experimental dependence of the transient ECL on the dendrimer generation. The theoretically calculated Marcus electron-transfer rate constant compares very well with that obtained by the finite element simulation of the whole ECL mechanism. This highlights the role played by the thioether dendrons in modulating the redox and photophysical properties, responsible for the occurrence and dynamics of the electron transfer involved in the ECL. Thus, the combination of experimental and computational results allows understanding of the dendrimer size dependence of the ECL transient signal as a result of factors affecting the annihilation electron transfer.

Abstract: The electrochemistry, photophysics, and electrochemically generated chemiluminescence (ECL) of a family of polysulfurated dendrimers with a pyrene core have been thoroughly investigated and complemented by theoretical calculations. The redox and luminescence properties of dendrimers are dependent on the generation number. From low to higher generation it is both easier to reduce and oxidize them and the emission efficiency increases along the family, with respect to the polysulfurated pyrene core. The analysis of such data evidences that the formation of the singlet excited state by cation–anion annihilation is an energy-deficient process and, thus, the ECL has been justified through the triplet–triplet annihilation pathway. The study of the dynamics of the ECL emission was achieved both experimen-

Introduction

electrode surface by the application of rapidly varying potentials, and then they react with one another, within the diffusion layer, to generate the excited state, as predicted by Marcus’ theory,[6] according to the so-called annihilation mechanism. Besides obvious thermodynamic requisites for the downhill generation of the excited state, the two partners of the annihilation reaction need to also display favorable kinetic properties, and in particular high diffusivities and great chemical stability, since the removal of one (or both) species by parasitic reactions would affect drastically the overall efficiency of the annihilation reaction, thus resulting in negligible ECL intensity. Polycyclic aromatic hydrocarbons, such as pyrene and diphenylanthracene, display, in this respect, an optimal combination of spectroscopic and redox properties and great stability of radical cation/radical anion species, and have largely been used in ECL research for organic light-emitting diodes (OLEDs) and biosensing applications.[3] We positioned ourselves in a relatively unexplored molecular area, in which the careful design of our molecular models features multiple phenyl rings, thioether functions, and a peculiar thiosubstituted pyrene core possessing interesting photophysical, optoelectronic, and electrochemical properties. Pyrene thiosubstitution has been extremely rare in the materials science literature.[7] To top it all off, a partial restriction of the degree of freedom of the molecules is also noteworthy in such design,

Electrochemically generated chemiluminescence (ECL) is a redox-induced light emission in which species generated at electrodes undergo high-energy homogeneous electron-transfer reactions to form excited states that emit light.[1] ECL originating from metal complexes,[2] organic molecules,[3] and nanoparticles[4] has largely been investigated in recent years, in particular for analytical applications in which it is known to offer better performance than other analytical techniques.[5] In a typical ECL experiment, the highly energetic radical anion and radical cation of a suitable fluorophore are first generated at the [a] Dr. G. Valenti, Dr. A. Fiorani, Dr. S. Di Motta, Dr. G. Bergamini, Prof. P. Ceroni, Prof. F. Negri, Prof. F. Paolucci, Prof. M. Marcaccio Department of Chemistry “G. Ciamician” University of Bologna Via Selmi 2, 40126 Bologna (Italy) E-mail: [email protected] [email protected] [email protected] [b] Prof. M. Gingras Aix-Marseille Universit, CNRS, CINaM UMR 7325 163 Ave. de Luminy, 13288 Marseille (France) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404230. Chem. Eur. J. 2014, 20, 1 – 13

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Full Paper Herein, we describe the annihilation ECL behavior of the sulfur-containing PYG0–PYG2 family of dendrimers. Only rare annihilation ECL examples with dendrimers have been previously reported. However, they were from sulfur-free dendritic species, without taking advantage of the extraordinary properties brought by sulfur.[11] The present study is also complemented by fluorescence anisotropy measurements, time-resolved luminescence, molecular mechanics, and quantum chemical calculations, thus giving, for the first time, a comprehensive description of the structural and electronic properties that govern the generation of the ECL signal.

Results and Discussion In view of the considerable flexibility of the dendrons, which may influence both the electronic properties of pyrene-cored dendrimers and their dynamical behavior during the annihilation process leading to ECL, a conformational analysis of the pyrene-cored dendrimers was preliminarily carried out. Based on B3LYP/3-21G* geometry optimization, we determined seven different conformers for PYG0 featuring different orientations of the dendrons with respect to the pyrene core. A pictorial representation of the seven structures is shown in Figure 1.

Scheme 1. Formulae of the investigated dendrimers and pyrene model compound.

to attain some beneficial molecular attributes and a limited number of conformers. To such an end, a new class of dendrimers (Scheme 1),[8] which consist of a polysulfurated pyrene core (1,3,6,8-tetra(arylthio)pyrene, PYG0) with thioether dendrons (PYG1 and PYG2), were chosen as suitable molecular model systems. Dendrimers are ideal scaffolds for this purpose, since the same luminescent unit can be placed at the core and the molecular dimension can be tuned by increasing the dendron generation. Indeed, dendrimers are monodisperse and highly branched macromolecules with size typically between 1 and 10 nm.[9] They usually consist of a core upon which radially branched layers are covalently attached. The core can be shielded or protected from the external dendritic environment, depending on the generation number. By using suitable synthetic strategies it is also possible to prepare dendrimers that contain selected functional units in predetermined sites within their structure, with great control over both molecular size and geometry. Polysulfurated pyrene derivatives PYG0–PYG2 have been carefully designed because sulfur substituents are directly bound to an aromatic core. This sulfur contribution is known to stabilize aromatic charged species, thereby giving rise to complexes with thiophilic metal ions and charge-transfer complexes with organic molecules.[10] This family of dendrimers thus shows unprecedented photophysical, redox, and electrochromic properties to warrant novel and important investigations, never tackled before.[8] &

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Figure 1. Graphical representation, through molecular surface, of the seven conformers of PYG0 optimized at the B3LYP/3-21G* level of theory.

The lowest-energy computed structure is conformer A (Figure 1) featuring two CH–pi interactions with the p core of pyrene and two CH–pi interactions with the p system of two benzene rings (see Figure S1 in the Supporting Information). The highest-energy structure is G, featuring four CH–pi interactions with the p core of pyrene. The energy difference is, however, very small (less than 2 kcal mol1, see Table S1 in the Supporting Information) and the various conformers are likely to interconvert rapidly at room temperature. Following molecular mechanics (universal force field, UFF) and DFT geometry optimization, the lowest-energy structures of PYG1 are those derived from structure G of PYG0 (see Figure 2 and Table S2 in the Supporting Information). 2

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Full Paper erties with respect to the parent PY, and can therefore be investigated as benchmark systems to establish how, and to what extent, structural and electronic parameters affect and control the dynamics and efficiency of the ECL process. The relevant electrochemical data for all species are summarized in Table 1. In the positive potential region, all species show two reversible one-electron oxidation processes, attribut-

Table 1. Electrochemical and annihilation enthalpy (DHannihil) data of pyrene and the three dendrimer species in CH2Cl2 solutions at 298 K. DHannihil [eV][b]

Compound

I ox

E = [V, vs. SCE) II ox III ox[a]

I red

PY PYG0 PYG1 PYG2

1.30 0.76 1.01 1.00

– 1.11 1.22 1.30

2.16 1.65 1.54 1.54

1

2

Interestingly, in the lowest-energy structures, the pyrene core exposes one side to the environment. Because of the nonspherical distribution of the dendrons, the structures with exposed pyrene core show large dipole moments that are likely to enhance their stability in polar solvents. Similar considerations hold for PYG2 dendrimer (see Figure 3 and Table S2 in the Supporting Information).

ed to the polysulfurated pyrene core (Figure 4).[8] All dendrimers and, in particular, PYG0 are easier to oxidize than parent PY, a likely consequence of the delocalization of the positive charge by the sulfur moieties, as suggested by the shape of the frontier molecular orbitals of, for example, PYG0 shown in Figure 5 (see also Figures S2–S4 in the Supporting Information). The large potential shift (with respect to parent PY) observed for PYG0 (540 mV) is significantly reduced in dendrimers PYG1 and PYG2 (300 mV), in line with the decreasing electron-withdrawing effects of the dendron substituents along the dendrimer generations. The orbital energies depend on the conformation and on the pattern of CH–pi interactions (see Table S3 in the Supporting Information); therefore, averaged HOMO (and LUMO) levels (over the several computed conformations) were compared with the electrochemical data. Although the number of conformers investigated for PYG1 and PYG2 is certainly not complete, the averaged HOMO energies in Table S3 (Supporting Information) indicate an increase of the ionization potential of PYG1 and PYG2 compared to PYG0, which follows closely the experimental trend. At higher potentials, an irreversible multielectronic anodic peak (Figure S5 in the Supporting Information) was attributed to oxidation processes involving the dendrons because the corresponding charge scales linearly with the number of dendron shells (Figure S5), in line with the increasing density of occupied orbital levels calculated for PYG0–PYG2 (Figure S6 in the Supporting Information). The reversibility of the various processes has been checked by a sweep rate study over a range of about three orders of magnitude and up to 40 V s1. Even at the highest scan rate used, the multielectron peaks at high positive potentials remain completely irreversible. Finally, in the negative potential region, all dendrimers undergo a reversible one-electron reduction process at less nega-

Figure 3. Graphical representation, through molecular surface, of the structures of PYG2 optimized at the B3LYP/3-21G* level of theory. Structures G1 and G2 expose one side of the pyrene core to the environment.

Electrochemistry and photophysics For their ideal properties, polyaromatic hydrocarbons such as anthracene and pyrene (PY) may be considered textbook examples of ECL-active species. Dendrimers PYG0–PYG2 display largely conserved or improved redox and photophysical propwww.chemeurj.org

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3.36 2.31 2.45 2.44

[a] Anodic peak potential at 1 V s1; [b] annihilation reaction enthalpy: DHannihil = DGannihil + TDS, in which DGannihil is evaluated from the standard redox potentials (see below) and the estimated entropy contribution term is  0.1 eV.[1]

Figure 2. Graphical representation, through molecular surface, of the structures of PYG1 optimized at the B3LYP/3-21G* level of theory. Structures G1 and G2 expose one side of the pyrene core to the environment.

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– 1.82 1.70 1.57

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Figure 5. The B3LYP/3-21G* computed frontier molecular orbitals of PYG0 conformer G.

dendrimers compared to PY and increasing from PYG0 to PYG2. The photophysical properties of PYG0–PYG2 dendrimers[8] have been summarized in Table 2. All dendrimers exhibit intense and well-defined emissions, with a vibrationally structured band centered at about 460 nm, that is, redshifted relative to PY for the presence of the sulfur substituents (see Table 2, and the top panels of Figure 7). Lifetimes of the fluorescent excited states are much shorter than that of PY,[12] and increase, in dichloromethane, with the dendrimer generation analogously to the emission quantum yield. Interestingly, such trends are not observed in cyclohexane solutions, in which all dendrimers exhibit the same emission quantum yield thus suggesting, in the former case, a deactivation path involving the solvent, which is prevented in PYG2 by a shielding effect of the dendrons. Excited-state calculations carried out on PY and PYG0 allow explanation of the drastic reduction in the fluorescence lifetimes of the three dendrimers compared with PY. It is convenient to recall that the two lowest-energy excited singlet states of most polycyclic aromatic hydrocarbons are labeled La (relatively intense, dominated by the (H!L) excitation) or Lb (usually very weak, dominated by the (H1!L) and (H!L + 1) transitions).[13] Our calculations show that substitution of the pyrene core by dendrons changes the nature of the emitting state. As a result, the emitting state of pyrene-cored dendrimers is La instead of the weak Lb state (see Table S4 in the Supporting Information and the Computational Details section). To further validate this computed inversion of excited states, we simulated the Franck–Condon (FC) vibronic structure of the emission of PY from the La state and compared it with the observed vibronic structure of the emission spectrum of PYG0 (see Figure 6). The agreement is very close and clearly supports the assignment of the emission of dendrimers from the La

Figure 4. Cyclic voltammetric curves of 1 mm a) PYG0, b) PYG1, and c) PYG2 in 0.07 m TBAH/CH2Cl2 solution. Working electrode glassy carbon (GC) disk, 2 mm diameter; reference electrode SCE; scan rate = 0.2 V s1; T = 25 8C. TBAH = tetrabutylammonium hexafluorophosphate.

tive potentials than pyrene; they are easier to reduce than the parent species by 510 mV for PYG0 and by 620 mV for PYG1 and PYG2, a result that is also supported by the averaged LUMO energies in Table S3 in the Supporting Information, thus indicating a remarkable increase of electron affinity for all the

Table 2. Photophysical and ECL data of pyrene and dendrimers PYG0–PYG2 in degassed solution at 298 K. Hydrodynamic volumes have been estimated from the values of steady-state anisotropy (rss) by using Equations (7) and (8). Photoluminescence lmax PY PYG0 PYG1 PYG2

375 457 457 460

Dichloromethane [nm] t [ns] Fem [%] 277 1.4 1.6 2.4

61 33 35 55

lmax [nm]

t [ns]

Cyclohexane Fem [%] rss[a]

370 448 452 457

650 2.5 2.5 2.6

65 60 60 60

< 0.010 0.030 0.060 0.115

q[b] [ns]

Vh[c] [nm3]

Radius [nm]

Electrochemiluminescence Dichloromethane lmax [nm] FECL [%]

– 0.28 0.63 1.62

– 1.18 2.66 6.83

– 0.66 0.86 1.18

375 478 476 480

0.02 0.28 0.10 0.14

[a] Steady-state anisotropy at 298 K, lex = 435 nm; [b] rotational correlation time obtained by Equation (7); [c] hydrodynamic volume estimated by Equation (8).

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Full Paper which is lower than the charge-separated state energy. Therefore, triplet state generation is allowed by the anion–cation annihilation. Such a triplet can subsequently annihilate another triplet thus providing the energy to populate the singlet excited state. This mechanism is well known in the literature as a triplet–triplet annihilation (TTA).[14] The mechanism leading to the generation of ECL light is therefore hypothesized as follows [Eqs. (2)–(6)]:

PYGn ! PYGnþ þ e

k ox

ð2Þ

PYGn þ e ! PYGn

k red

ð3Þ

PYGnþ þ PYGn ! PYGnðT1 Þ þ PYGnðS0 Þ Figure 6. Comparison between the observed fluorescence spectrum of PYG0 (top) and the computed FC vibronic structure associated with the emission from the La state of PY (bottom, gray trace).

PYGnðT1 Þ þ PYGnðT1 Þ ! PYGnðS1 Þ þ PYGnðS0 Þ PYGnðS1 Þ ! PYGnðS0 Þ þ hn

Radical cation–radical anion annihilation leads to ECL generation To generate the emitting excited state, the annihilation reaction needs to satisfy energy requirements that are strictly related to both the redox and photophysical properties of the precursor. In fact, the free energy for the generation of the excited state (DGES) can be calculated according to Equation (1):[1] 1240:8 Eex ½nm

ð1Þ

The singlet excited-state energy of dendrimers, Eex, is 2.69– 2.70 eV (3.30 eV for PY), and the corresponding DGannihil, given by the half-wave potential separation of the first oxidation and first reduction processes, ranges from 2.41 (PYG0) to 2.55 (PYG1) to 2.54 eV (PYG2) (3.46 eV for PY). This indicates that, by contrast to parent PY, for which DGES  0.16 eV, for all dendrimers the direct generation of the emitting singlet excited state from the annihilation reaction is thermodynamically disallowed. Quantum chemical time-dependent DFT (TDDFT) calculations predict the presence of a stable triplet level, Eex, at about 1.9 eV (see Table S4 in the Supporting Information), Chem. Eur. J. 2014, 20, 1 – 13

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k TTA

ð5Þ ð6Þ

in which kox and kred are the heterogeneous electron-transfer rate constants with the electrode to form the radical cation and anion, respectively; kann refers to the electron transfer of the annihilation process, which leads to the formation of a triplet excited state (PYGn(T1)) and a ground-state molecule (PYGn(S0)); kTTA is the rate constant for the triplet–triplet annihilation that leads to the formation of the fluorescent excited state (PYGn(S1)); and kfl is the rate constant of the radiative deactivation of the lowest singlet excited state. The photoluminescence (PL; panels a, c, e) and ECL (panels b, d, f) spectra of PYG0–PYG2 are shown in Figure 7 and the relevant data are collected in Table 2. The spectral analysis of ECL signals indicated that the same excited state is involved in PL and ECL light generation, thus substantiating the above hypothesis of the triplet–triplet annihilation route for the electrochemical excitation of the pyrene core. The two sets of spectra reported in Figure 7 differ from one another mainly in the maxima positions and vibronic structures. Although the 20 nm shift of emission maxima to larger wavelengths is typically observed in the ECL spectra and is associated essentially with self-absorption, a selection of specific conformers involved in ECL generation with respect to PL might explain the different vibronic structures of ECL vis--vis PL spectra, as anticipated in the previous discussion on the computed vibronic structure of the emission spectra. Computations (see Figures 3 and 4) have in fact identified some energetically accessible conformers of high-generation dendrimers in which the pyrene core is largely exposed to the environment, and it is plausible that such conformers be preferentially involved in the homogeneous annihilation electron transfer that is preliminary to ECL generation. The suggestion that specific conformers are preferentially involved in ECL generation also comes from the presence of excimeric emissions in the ECL spectra of some dendrimers. Excimeric emission is observed (at 470 nm) in the ECL spectrum of PY.[15] PYG0 also showed, along with a principal emission at 478 nm with a well-defined vibronic structure, a broad emission at 630 nm (see inset in Figure 7 b) attributed to the exci-

state. The vibronic spectrum is governed by the activity of a number of vibrational modes in the 400, 1200–1350, and approximately 1600 cm1 frequency region and is depicted in Figure S7 in the Supporting Information. Calculations on dendrimers indicate that the lowest excited state, of La character, also contains some contribution (conformer-dependent) of charge-transfer excitations from the benzene rings (belonging to dendrons) to the pyrene core. These charge-transfer contributions result in an enhanced vibronic activity of normal modes related to the breathing mode of benzene, and are therefore located at approximately 1000–1100 cm1. These variable vibronic contributions are likely to modulate the final emission spectrum if a selection of specific dendrimer conformations occurs.

DGES ffi DGannihil þ

k fl

ð4Þ

kann

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Figure 7. Normalized photoluminescence spectra (top panels) and ECL spectra (bottom panels) of the species PYG0 (a,b), PYG1 (c,d), and PYG2 (e,f) in CH2Cl2 at 25 8C. Inset in panel (b): ECL of the excimer. Fluorescence emission: lex = 375 nm. ECL: 0.5 mm of dendrimer species (PYG0–PYG2); TBAH 80 mm; working electrode, GC disk 2 mm diameter; double potential step program from first oxidation to first reduction, pulse width 0.1 s, photomultiplier tube (PMT) bias 650 V. See also Figure S8-1 in the Supporting Information.

mer formation. This was supported by TD-CAM-B3LYP/3-21G* geometry optimization of the excimer of PYG0 consistent with its bonding LUMO depicted in Figure 8 (see also the Computational Details in the Experimental Section). Conversely, PYG1 and PYG2 showed bright emission at 476 and 480 nm without any evidence of excimer formation, and such a different behavior can be easily rationalized on the basis of the increasing steric encumbrance of the dendrons, which prevents the p stacking required by excimer formation. Very interestingly, dendrimers showed higher ECL yields than PY (see Table 2), approximately twenty times for PYG0 and five times for PYG1 and PYG2. Transient ECL behavior ECL transients were generated by performing chronoamperometric experiments in which the potential was stepped back and forth between the first reduction and the first oxidation of &

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Figure 8. The CAM-B3LYP computed antibonding HOMOs and bonding LUMOs of the excimer of PYG0.

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Full Paper fects, such as double-layer charging, interdiffusion of radical cation and radical anion, and rates of the annihilation reaction.[1a]

The dynamics of annihilation–ECL generation To rationalize the increasing delay times as a function of dendrimer generation (Figure 7), an analysis of the rate-determining step (RDS) in ECL signal generation [Eqs. (2)–(6)] is necessary. Reactions (2) and (3) occur under diffusion-limiting conditions, so such processes cannot be the RDS. Furthermore, reaction (6) is also very fast: for all the dendrimers in dichloromethane solutions, kflffi2.3  108 s1 (kfl = Fem/t, in which Fem and t are referred to the lowest singlet excited states, see Table 2). The rate of triplet–triplet annihilation [Eq. (5)] was also estimated near to the diffusion limit by laser flash photolysis experiments. A deaerated dichloromethane solution of PYG0 (1.1  104 m) was excited by a 355 nm laser source and both prompt and delayed fluorescence at 460 nm was observed: the longerlifetime component was strongly dependent on the absorbance at the excitation wavelength and the laser intensity.[16] From the fluorescence intensity decay, a value of kTTA of 1.4  108 m1 s1 was obtained. It has therefore to be concluded that homogeneous electron transfer between radical cation and radical anion [Eq. (4)] represents the RDS of the overall ECL generation process. In such a case, the increase of the ECL signal onset with the dendrimer generations can be reasonably attributed to a combination of the decrease in the homogeneous electron-transfer constant (kET) and the decrease of the diffusion coefficient as a consequence of the molecular size increase. To investigate such an aspect of the complex mechanism, steady-state fluorescence anisotropy was used to obtain information on the molecular size of dendrimers. Experiments were carried out in cyclohexane (Table 2 and Figure S8-2 in the Supporting Information), that is, a low-viscosity solvent in which quenching deactivation processes, previously observed in dichloromethane, are minimized. The steady-state anisotropy (rss) is related to the maximum anisotropy value (r0) and the lifetime t according to Equation (7):[17]

Figure 9. Chronoamperometry (solid gray line), experimental transient ECL (dashed black line), and simulated (dotted black signal) curves of 0.5 mm a) PYG0, b) PYG1, and c) PYG2 in 0.07 m TBAH/CH2Cl2 ; working electrode, GC disk 2 mm diameter. PMT bias 500 V. t1 is the preceding reduction step of the double-step potential cycles. Digital simulations of the ECL transient were carried out with COMSOL multiphysics software (see the Supporting Information for details).

the dendrimers, while the current (solid gray) and ECL light (dashed black) signals were continuously monitored (Figure 9). The current curves display similar features for all three dendrimers. Following the fast double-layer charging (millisecond timescale), a typical Cottrellian behavior ( t = ) is observed, with the diffusion-limited faradaic current generation process.[1a] The ECL signal exhibits analogously a steep increase, which follows the generation of the reacting species at the electrode surface (i.e., after each complete reduction–oxidation cycle). Note that, however, although the current signal was almost simultaneous in all cases to the potential step, the ECL signal onset displayed a delay that was typical for each dendrimer ( 1 ms), increasing apparently with the dendrimer generation: tPYG0 = 7 ms; tPYG1 = 10 ms; tPYG2 = 13 ms. Delayed generation of ECL (with respect to current signal) is often observed[1a] and has been consistently attributed to various ef-

rss ¼

2

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ð7Þ

The rotational relaxation time q can be related to the hydrodynamic volume Vh by the Stokes–Einstein–Debye equation. In particular, for compounds having a van der Waals volume much bigger than the volume of the solvent molecules, “sticking” boundary conditions are applicable, that is, the form of the rotor has no influence since it moves together with solvent molecules, and q can be expressed by Equation (8):

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r0 t 1þq

q ¼ V h h=kB T

ð8Þ

in which h is the viscosity of the solvent, kB the Boltzmann constant, and T the absolute temperature. 7

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Full Paper Upon increasing the dendrimer generation, a strong increase in the rss value is observed relative to a value below the detection limit in the case of PY under the same experimental conditions. The photophysical properties (lowest-energy absorption band, emission maximum and quantum yield, excitedstate lifetimes) are practically identical for all dendrimers, so the observed differences in rss values can be attributed to the increasing dimension of the molecules, which brings about a slower rotation in fluid solution. An estimate of the rotational correlation time and of the hydrodynamic volume can thus be obtained by using Equations (7) (r0 = 0.3)[18] and (8), as reported in Table 2. The corresponding values follow the same trend as the computed values extracted from optimized molecular structures (see the Computational Details section and Table S3 in the Supporting Information). The latter are systematically smaller than the experimentally determined ones probably because of the selected molecular surface employed to evaluate molecular volume and dimension. Finally, to rationalize the transient ECL, a series of digital simulations were carried out using COMSOL Multiphysic software, a program that uses finite element analysis to treat the diffusion and kinetics problems. As already demonstrated by the pioneering work of Bard[19a] and Amatore,[19b] finite element analysis is a powerful tool to simulate the ECL transient. Thus, Butler–Volmer heterogeneous kinetics and Fick’s law were combined with photophysical and electrochemical experimental data (collected in Table S6 in the Supporting Information) to calculate the ECL transients using Equations (2)–(6). Figure 9 shows the comparisons and the good matching between the simulated (black dots trace) and experimental (dashed black trace) ECL transients (simulation details in the Supporting Information).[20] Interestingly, the simulations allowed estimation of the annihilation constant (kann), which is the kinetic parameter that accounts for both diffusion and homogeneous electron-transfer problems. In line with the dendrimer generation there is a decrease in the annihilation constant from 50 to 25 and 20 s1 as consequence of the dendrimer’s molecular size and, thus, as a combination effect between the increase of the diffusion coefficients and the homogeneous electron transfer. To explore the distance dependence on the charge transfer in the ECL process, we considered the rate constant of the electron-transfer process in Equation (4) and computed its magnitude by employing the semiclassical Marcus law [Eq. (9)]:

kET ¼

2p 2 1 ðDGES þ lÞ2 Þ VET pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi expð h  4lkB T 4plkB T

The electronic coupling VET was obtained at the DFT level of theory, as a function of the intermolecular distance r between the pyrene cores assumed to be the molecular sites where charge is localized and from where charge transfer occurs [see Eq. (12) in the Computational Details section]. The reorganization energy l [see Eq. (13) in the Computational Details section] is a function of the distance R between the centers of the two dendrimers and it depends also on the dimension of the dendrimer through the RD and RA parameters, the radii of the spherical donor and acceptor, that were set equal to the radii of the dendrimers (Rdendr) determined from fluorescence anisotropy measurements. Note that the distance R in Equation (13) is different from the distance r between the pyrene cores in Equation (12) and reads R = 2 Rdendr + r. Therefore, the distance/dimension dependence of the rate constant results from a combination of the distance dependence of the electronic coupling, which is expected to be similar for the three generations, and the generation dependence of the outer-sphere reorganization energy which, in contrast, is more remarkably affected by the dendrimer dimension. Using the computed exponential dependence of the electronic coupling and the distance/dimension dependence of l we estimated the rate constants according to Equation (9). The dependence as a function of the intermolecular distance r between pyrene cores is shown in Figure S9 (Supporting Information) for the three generations, for which the computed rates intersect the annihilation constants, determined from the analysis of ECL transients. The kann values include the effects of both diffusion and electron transfer. However, the latter being the slowest process, in the following we make the approximation that kann can be identified with the sole electron-transfer rate constant. In Figure S9 (Supporting Information), the distances (between pyrene cores) at which intersection occurs are very similar in the three cases, and range between 14.04 and 14.50 . This result is in line with the assumption that electron transfer involves the exposed cores of dendrimers and suggests that the major variable parameter is the dendrimer dimension modulating reorganization energies. Accordingly, a plot of the computed rate constants as a function of the new variable q, related to the molecular dimension rather than to the intermolecular distance, and defined as q = (Rhri)/2 (in which hri is the average distance between pyrene cores extracted from Figure S9 in the Supporting Information), shows crossing of the computed kET with the experimentally determined kann for values of q very close to the radii of the dendrimers, as shown in Figure 10, thereby supporting the strong relation between ECL delay, electron-transfer rate, and dendrimer dimension. In other words, the analysis above shows that the assumption of preferential electron transfer between dendrimers, which adopt a conformation in which the pyrene cores are exposed, leads to a dependence of the rate constant versus molecular dimension that is in agreement and supports the observed trend.

ð9Þ

in which l is the solvent reorganization energy and the driving force DGES is given by Equation (1) and was estimated to be in the range between 0.25 and 045 eV. From Equation (9), it is clear that, assuming similar values of the DGES across dendrimer generations, at least two factors can contribute to change the rate constant as a function of the generation; the distance dependence of the electronic coupling and the distance/dimension dependence of the outer-sphere reorganization energy. &

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Full Paper Experimental Section Chemicals and electrochemical measurements Dendrimers were synthesized as reported by Gingras et al.[8] Tetrabutylammonium hexafluorophosphate (TBAH; from Fluka), as supporting electrolyte, was used as received. Dry dichloromethane (CH2Cl2) was successively heated at reflux over, and distilled from, CaH2 and activated alumina super I neutral (ICN Biomedicals), and it was stored in a specially designed Schlenk flask over 3  activated molecular sieves, protected from light.[21] Shortly before performing the experiment, the solvent was distilled through a closed system into an electrochemical cell containing the supporting electrolyte and the species under examination. Electrochemical experiments were carried out in an airtight single-compartment cell described elsewhere[22] by using either platinum or glassy carbon (GC) as working and counter electrodes, respectively, and a silver spiral as a quasi-reference electrode. The cell containing the supporting electrolyte and the electroactive compound was dried under vacuum at about 110 8C for at least 60 h before each experiment. All the E = potentials were obtained directly from cyclic voltammetric curves as averages of the cathodic and anodic peak potentials for one-electron peaks and by digital simulation for those processes closely spaced in multielectron voltammetric peaks. The E = values were referred to an aqueous saturated calomel electrode (SCE) and were determined by adding, at the end of each experiment, ferrocene as an internal standard and measuring the values with respect to the ferrocenium/ferrocene couple standard potential. Voltammograms were recorded with a custom-made fast potentiostat[23] controlled by an AMEL model 568 programmable function generator. The potentiostat was interfaced to a Nicolet model 3091 digital oscilloscope, and the data were transferred to a personal computer through the program Antigona.[24] The minimization of the uncompensated resistance effect in the voltammetric measurements was achieved by the positive-feedback circuit of the potentiostat. Digital simulations of the cyclic voltammetric curves were carried out either by Antigona or DigiSim 3.0.

Figure 10. Crossing between experimentally determined kann in glassy carbon (dotted gray horizontal lines) and computed rate constants based on the Marcus equation (dotted black curves). The variable considered to plot the data, q, is related to dendrimer dimensions: see the text. The crossing points indicated in gray occur close to the dimension of dendrimers and support the observation of a dendrimer dimension dependence of the electron-transfer step: PYG0, PYG1, PYG2.

1

2

1

2

Conclusion A family of dendrimers containing a persulfurated pyrene core has been investigated for ECL generation. Indeed, the polysulfurated core is highly luminescent and exhibits a reversible electrochemical behavior to produce the corresponding radical anion and radical cation. The thioether dendrons appended to the core enable tuning of the molecular dimension by changing their generation. This dendrimer generation is thus an ideal benchmark to establish how and to what extent structural and electronic parameters affect and control the dynamics of the ECL properties. The electrochemical and photophysical data show that the cation–anion annihilation is an energy-deficient process for obtaining the electrochemically generated chemiluminescence. Thus, the excited state formation has been explained through the triplet–triplet annihilation pathway. A key experimental finding is the dependence of the ECL transient signal on the dendrimer generation. To rationalize this observation, the study has been complemented by fluorescence anisotropy measurements to estimate the dendrimer dimension in solution, time-resolved luminescence to estimate the rate of triplet–triplet annihilation, molecular mechanics and quantum chemical calculations to explain the photophysical and electrochemical properties of the dendrimers, and, most importantly, the dependence of the homogeneous electron-transfer rate between the electrochemically generated cation and anion on the dendrimer generation. By combining the experimental and computational results, we report for the first time a correlation between the ECL transients and the rate of the homogeneous electron transfer. The dependence that comes out is a function of dendrimer size and it results from a complex combination of distance-dependent factors influencing the final electron transfer.

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Photophysics The experiments were performed at 298 K in dichloromethane or cyclohexane. Deaerated solutions were obtained by three freeze– pump–thaw cycles. Fluorescence steady-state anisotropy measurements were performed by an Edinburgh Instruments FLS920 spectrofluorimeter equipped with a TCC900 card for data acquisition in time-correlated single-photon counting experiments (0.5 ns time resolution) with a D2 lamp and an LDH-P-C-405 pulsed diode laser. The experiments of triplet–triplet annihilation and delayed fluorescence were performed by measuring the fluorescence intensity decay by a Hamamatsu R928 phototube connected to a Tektronix TDS380 (400 MHz) oscilloscope upon excitation with a Continuum Surelite I-10 Nd:YAG laser source (lex = 355 nm). The intensity of the laser source was measured by a laser point power meter model Plus, equipped with a 10 UV-A detector head. The estimated experimental errors were: 2 nm on the band maximum, 5 % on the molar absorption coefficient and luminescence lifetime, and 10 % on the fluorescence quantum yield.

Electrochemiluminescence The ECL measurements were carried out in CH2Cl2 solution with TBAH as supporting electrolyte, under the same strictly aprotic conditions as described above. A one-compartment three-electrode airtight cell, with high-vacuum O-rings and glass stopcocks,

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Full Paper was used for ECL measurements.[25] The working electrode consisted of either a Pt side-oriented disk (2 mm diameter) or a GC disk 2 mm in diameter whereas the counter electrode was a platinum spiral and the reference electrode was a quasi-reference silver wire. In a given solution, two or three records were made to check the temporal stability of the system investigated. The annihilation reaction was obtained by pulsing the working electrode between the first oxidation and the first reduction peak potential of the species, increased by, at least, + 0.1 and 0.1 V, respectively, with a pulse width of 0.1 s. The ECL signal generated by performing the potential step program was measured with a photomultiplier tube (PMT, Hamamatsu R4220p) placed a few millimeters from the cell, and in front of the working electrode, inside a dark box. A voltage in the range 250–750 V was supplied to the PMT. The light/current/voltage curves were recorded by collecting the preamplified PMT output signal (by an ultralow-noise Acton Research mod. 181) with the second input channel of the analog-to-digital converter module of the Autolab instrument.

structures were then optimized with molecular mechanics employing the UFF. The resulting structures were refined with optimization at the B3LYP/3-21G* level. The dimensions of the dendrimers as a function of the generation were also determined from the UFF optimized structures and from the DFT optimized structures by computing molecular volumes and approximating the dendrimers to spheres, thereby extracting the corresponding radii that are collected in Table S3 in the Supporting Information. Molecular orbital shapes and energies discussed in the text are those calculated at the DFT optimized structures. Orbital and molecular pictures were prepared with Molekel 4.3 visual software.[29] All the quantum-chemical and molecular mechanics calculations were carried out with Gaussian 09.[30] Electronic excitation energies and oscillation strengths of the lowest singlet excited electronic states of selected conformers of PYG0 and PY were determined with time-dependent (TD) DFT calculations. The computed absorption spectra of PY and PYG0 are compared with the observed spectra in Figure S10 in the Supporting Information. DFT (ground state) and TDDFT (excited state) optimized geometries, vibrational frequencies, and normal coordinates were employed to evaluate the Bi displacement parameters representing the projection of the geometry change upon excitation on the ground-state normal coordinates. Assuming the harmonic approximation, negligible Duschinsky effect, and identical frequencies in the K and J states, the FC intensity of a band corresponding to the n = [n1,n2,n3,…nN] vibrational quantum in the excited state is given by [Eq. (11)]:

ECL spectra were recorded by inserting the same PMT in a dualexit monochromator (Acton Research mod. Spectra Pro2300i) and collecting the signal as described above. Photocurrent detected at the PMT was accumulated for 1–3 s, depending on the emission intensity, for each monochromator wavelength step (usually 1 nm). Entrance and exit slits were fixed to the maximum value of 3 mm. The ECL yield is defined as the photons emitted per redox event, which is related to the total electrical charge involved in the generation of the reactants. Thus, the ECL efficiency can be rigorously estimated by the annihilation method and obtained by chronoamperometric experiment using the following expression [Eq. (10)]:[1a]

0 ECL

FECL ¼ F

0

0

IK0;Jn /

ð10Þ

ðIQ =I QÞ

in which F0ECL is the ECL efficiency of the standard under the same experimental conditions, I and I0 are the integrated ECL intensity of the species and the standard systems, and Q and Q0 the faradaic charges (in Coulombs) passed for the investigated species and the standard species, respectively. It has been estimated that the experimental error for the ECL band maximum is 4 nm and the ECL efficiency can be confidently given with an error of  20 %. To obtain the ECL yields, measurements of two standard ECL systems (i.e., 9,10-diphenylanthracene (DPA) and [Ru(bpy)3]2 + (bpy = bipyridine), which are among the most efficient ECL systems widely used[26, 27]) in CH2Cl2 solution, under the same experimental conditions as those used for the dendrimers, were performed (Figure S13 in the Supporting Information) and the ECL intensity ratio (Idendrimers/IDPA) was determined. From such an ECL intensity ratio, and using the values of ECL annihilation efficiency of DPA and [Ru(bpy)3]2 + (the values of which, under similar experimental conditions, are reported to be 11 and 5 %, respectively), the ECL yield of the dendrimers can be obtained directly.

1 gi ¼ ðBi Þ2 2 The vibrational coordinates and frequencies displaying the largest g values are collected in Figure S7 in the Supporting Information. In plotting computed vibroelectronic spectra, a Lorentzian line width of 0.1 eV was superimposed onto each computed intensity to facilitate the comparison with experimental spectra. Computed vibrational frequencies were uniformly scaled by a factor of 0.95. One notable difference between dendrimers based on a pyrene core and PY is that the excited lifetime is remarkably shorter (ca. 2.5 versus 650 ns for PY). Based on computed vertical excitation energies we attribute this observation to the inversion of the La and Lb states that takes place on moving from PY to PYG0 and higher-generation dendrimers. It is known that most density functionals yield the wrong excited state order for PY.[31, 32] Indeed, TD B3LYP/6-31G* calculations at the ground state optimized geometry of PY erroneously predict the intense La state at 3.72 eV and the forbidden Lb state at 3.79 eV. Interestingly, however, TD CAMB3LYP/6-31G* calculations at the same reference geometry correctly predict the La state (4.03 eV) above the Lb state (4.00 eV). The slight excitation energy overestimate is typical for the CAM-B3LYP functional. Moving to PYG0, the excited states with dominant La and Lb character are computed at 2.93 (2.70) and 3.58 eV (3.62 eV), respectively, at the TD B3LYP/3-21G* level for conformer A (conformer G). The energy difference between the two states has increased remarkably compared with PY and the state order is confirmed by the corresponding TD CAM-B3LYP/3-21G* calculations

Computational details Atomic structures were optimized with density functional theory (DFT) calculations using the B3LYP hybrid functional[28] with the basis set limited to 3-21G* owing to the large dimension of the chromophores. A number of isomers of the PYG0 dendrimer were considered and generations PYG1 and PYG2 were constructed starting from two selected PYG0 isomers. The PYG1 and PYG2 &

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ð11Þ

in which:

The ECL transient experiments were simulated by using COMSOL Multiphysic software 3.5a; for all the simulation details, see the Supporting Information.

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Y   2 Y ðgj Þnj 0j nj ¼ egj nj ! j j

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Full Paper l decreases with the dimension of the dendrimer, although it increases with the distance between the donor and the acceptor. The previously discussed quantum-chemical calculations have shown that dendrimer configurations exposing the pyrene core to the environment are more stable than those with the pyrene core completely surrounded by dendrons, so it is reasonable to assume that electron transfer will preferentially occur between ion pairs of dendrimers in the pyrene exposed configuration. Accordingly, the distance r used to plot the data in Figure S12 (Supporting Information) is between the centers of the pyrene cores.

placing the state with dominant La character below the Lb state, at 3.37 (3.40) and 3.89 eV (3.92 eV) for conformer A (conformer G). Thus, we conclude that the dramatic decrease observed in the lifetime of the emission from PYG0 and higher-generation dendrimers compared with PY is due to the inversion between the lowest two singlet excited states. To explore the possibility of formation of excimers, we considered a dimer of PY and of dendrimer PYG0 with two molecules initially set at a distance of 3.5 . The lowest excited state of the dimer was optimized with TDDFT calculations employing the CAM-B3LYP and M06HF functionals. The formation of an excimer was computed with both functionals and for both systems. Previous examples of computed excimers of pyrene have already been published.[31] To explore the distance/dimension dependence on the charge transfer in the ECL process, we considered one dimer formed by two PYG0 dendrimers and evaluated the electronic coupling between the initial state formed by the charged (PYG0) + (PYG0) pair and the final (PYG0)*(PYG0) or (PYG0)(PYG0)* states. Assuming that the excited state is reasonably well defined as a (H!L) single excitation and that the charged species are well defined by configurations corresponding to the single occupation of the HOMO or the LUMO, the electronic coupling is proportional to the average between the VHH and VLL couplings„ in which the subscripts indicate the orbitals (HOMO or LUMO) localized on the two molecules forming the dimer. We evaluated these electronic couplings at the B3LYP/3-21G and B3LYP/6-31G** levels of theory, as a function of the distance r between the pyrene cores of the two dendrimers, by adopting the direct approach employed in previous work.[33–35] The DFT computed distance dependence (see Figure S11 in the Supporting Information) was fitted with the exponential function [Eq. (12)]:  a  VET ¼ VJJ ¼ VJJ0 exp  r 2

Acknowledgements We thank the University of Bologna, Aix-Marseille Universit, Italian Ministero dell’Istruzione, Universit e Ricerca (MIUR-project PRIN 2012), Fondazione Cassa di Risparmio in Bologna, and the FIRB project “NANOSOLAR”. M.G. thanks the French National Center for Scientific Research (CNRS). Keywords: cyclic voltammetry · dendrimers · electrochemiluminescence · electron transfer · quantum chemical calculations [1] a) Electrogenerated Chemiluminescence (Ed.: A. J. Bard) Marcel Dekker New York, 2004; b) M. M. Richter, Chem. Rev. 2004, 104, 3003; c) E. Rampazzo, S. Bonacchi, D. Genovese, R. Juris, M. Marcaccio, M. Montalti, F. Paolucci, M. Sgarzi, G. Valenti, N. Zaccheroni, L. Prodi, Coord. Chem. Rev. 2012, 256, 1664 – 1681. [2] a) M. Bandini, M. Bianchi, G. Valenti, F. Piccinelli, F. Paolucci, M. Monari, A. Umani-Ronchi, M. Marcaccio, Inorg. Chem. 2010, 49, 1439 – 1448; b) G. Valenti, E. J. O’Reilly, A. McNally, T. E. Keyes, M. Marcaccio, F. Paolucci, R. J. Forster, ChemPlusChem 2013, 78, 55 – 61; c) E. H. Doeven, E. M. Zammit, G. J. Barbante, C. F. Hogan, N. W. Barnett, P. S. Francis, Angew. Chem. Int. Ed. 2012, 51, 4354 – 4357; Angew. Chem. 2012, 124, 4430 – 4433; d) A. Kapturkiewicz, J. Nowacki, P. Z. Borowicz, Z. Phys. Chem. 2006, 220, 525 – 542. [3] a) K. M. Omer, S.-Y. Ku, K.-T. Wong, A. J. Bard, J. Am. Chem. Soc. 2009, 131, 10733 – 10741; b) G. Valenti, C. Bruno, S. Rapino, A. Fiorani, E. A. Jackson, L. T. Scott, F. Paolucci, M. Marcaccio, J. Phys. Chem. C 2010, 114, 19467 – 19472; c) K. M. Omer, S.-Y. Ku, K.-T. Wong, A. J. Bard, Angew. Chem. Int. Ed. 2009, 48, 9300 – 9303; Angew. Chem. 2009, 121, 9464 – 9467. [4] a) G. Valenti, E. Rampazzo, S. Bonacchi, T. Khajvand, R. Juris, M. Montalti, M. Marcaccio, F. Paolucci, L. Prodi, Chem. Commun. 2012, 48, 4187 – 4189; b) Y. Song, C. Zhao, J. Ren, X. Qu, Chem. Commun. 2009, 1975 – 1977; c) F. Deiss, N. C. La Fratta, M. Symer, M. T. Blicharz, N. Sojic, R. D. Walt, J. Am. Chem. Soc. 2009, 131, 6088 – 6089. [5] a) R. J. Forster, P. Bertoncello, T. E. Keyes, Annu. Rev. Anal. Chem. 2009, 2, 359 – 385; b) L. Z. Hu, G. B. Xu, Chem. Soc. Rev. 2010, 39, 3275 – 3304; c) W. Miao, Chem. Rev. 2008, 108, 2506 – 2553; d) V. A. Zamolo, G. Valenti, E. Venturelli, O. Chaloin, M. Marcaccio, S. Boscolo, V. Castagnola, S. Sosa, F. Berti, G. Fontanive, M. Poli, A. Tubaro, A. Bianco, F. Paolucci, M. Prato, ACS Nano 2012, 6, 7989 – 7997. [6] A. R. Marcus, N. Sutin, Biochim. Biophys. Acta Rev. Bioenerg. 1985, 811, 265 – 322. [7] a) G. Heywang, S. Roth, Angew. Chem. Int. Ed. Engl. 1991, 30, 176 – 177; Angew. Chem. 1991, 103, 201 – 203; b) T. Li, R. Giasson, J. Am. Chem. Soc. 1994, 116, 9890 – 9893. [8] M. Gingras, V. Placide, J.-M. Raimundo, G. Bergamini, P. Ceroni, V. Balzani, Chem. Eur. J. 2008, 14, 10357 – 10363. [9] a) Designing Dendrimers (Eds.: S. Campagna, P. Ceroni, F. Puntoriero), Wiley, Hoboken, USA, 2012; b) Dendrimers: Towards Catalytic, Material and Biomedical Uses (Eds.: A.-M. Caminade, C.-O. Turrin, R. Laurent, A. Ouali, B. Delavaux-Nicot), Wiley, Chichester, UK, 2011; c) F. Vçgtle, G. Richardt, N. Werner, Dendrimer Chemistry, Wiley-VCH, Chichester, 2009.

ð12Þ

in which J is H(HOMO) or L(LUMO) and with a/2 = 1.6 1, in line with the recently reported value of 2.1 1 estimated with the INDO Hamiltonian for cofacial pentacene molecules,[36] or with previous ab initio calculations on pentacene dimers[37] and is quite large, as expected for a through-space charge transfer between nonbonded molecules. The electron-transfer rate constant can be described by employing the semiclassical Marcus expression [Eq. (9)]. In contrast with the charge-transfer processes in the solid phase, such as those occurring in organic semiconductors, the reorganization energy for electron transfer in solution is generally dominated by a large outersphere contribution. Such a contribution can be estimated by employing the classical Marcus formula [Eq. (13)]:  lS ¼

 1 1 1 1 1 þ   2RD 2RA R e1 e

ð13Þ

in which RD and RA are the radii of the spherical donor and acceptor, R is the distance between the centers of mass, e is the dielectric constant of the medium, e1 = n2 is the optical dielectric constant, and n is the refractive index. RD and RA were set equal to the radii of the dendrimers determined by fluorescence anisotropy measurements. The computed outer-sphere reorganization energies are depicted in Figure S12 in the Supporting Information for the three generations, as a function of the intramolecular distance. It is seen that Chem. Eur. J. 2014, 20, 1 – 13

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Full Paper [10] G. Heywang, F. Jonas, Eur. Pat. Appl., 1989, EP 339419 A2 19891102. [11] J.-S. Yang, H.-H. Huang, J.-H. Ho, J. Phys. Chem. B 2008, 112, 8871 – 8878. [12] In air-equilibrated solution pyrene has Fem = 0.05 and t = 15 ns (see Table 2 for deaerated solution data). [13] J. R. Platt, J. Chem. Phys. 1949, 17, 484 – 496. [14] C. Bohne, E. B. Abuin, J. C. Scaiano, J. Am. Chem. Soc. 1990, 112, 4226 – 4231. [15] a) E. A. Chandross, J. W. Longworth, R. E. Visco, J. Am. Chem. Soc. 1965, 87, 3259 – 3260; b) T. C. Werner, J. Chang, D. M. Hercules, J. Am. Chem. Soc. 1970, 92, 5560 – 5565; c) J. T. Maloy, A. J. Bard, J. Am. Chem. Soc. 1971, 93, 5968 – 5981; d) B. Fleet, G. F. Kirkbright, C. J. Pickford, J. Electroanal. Chem. 1971, 30, 115 – 121; e) T. Kihara, M. Sukigara, K. Honda, J. Electroanal. Chem. 1973, 47, 161 – 166; f) C. P. Keszthelyi, A. J. Bard, Chem. Phys. Lett. 1974, 24, 300 – 304; g) H. Tachikawa, A. J. Bard, Chem. Phys. Lett. 1974, 26, 568 – 573; h) T. Suminaga, S. Hayakawa, Bull. Chem. Soc. Jpn. 1980, 53, 315 – 318. [16] Under our experimental conditions, we observed a square dependence between the delayed fluorescence intensity and the laser excitation intensity, with a saturation effect at high laser power. [17] J. R. Lakowicz, in Principles of Fluorescence Spectroscopy, 3rd edition, Springer, New York, 2006. [18] This value has been estimated by measuring the steady-state anisotropy in a rigid matrix at 77 K. [19] a) M. Shen, J. Rodrıguez-Lopez, J. Huang, Q. Liu, X. Zhu, A. J. Bard, J. Am. Chem. Soc. 2010, 132, 13453 – 13461; b) O. V. Klymenko, I. Svir, C. Amatore, ChemPhysChem 2013, 14, 2237 – 2250. [20] Notice that the difference between the ECL transient simulation and the experimental data at longer times is most likely due to side reactions, such as some quenching processes (e.g., 3*R + R + !R + R + or 3 *R + R !R + R energy-transfer reactions) that are not taken in consideration in the simulation. [21] a) A. A. La Pense, J. Bickley, S. J. Higgins, M. Marcaccio, F. Paolucci, S. Roffia, J. M. Charnock, J. Chem. Soc. Dalton Trans. 2002, 4095 – 4104; b) C. Bruno, M. Marcaccio, D. Paolucci, C. Castellarin-Cudia, A. Goldoni, A. V. Streletskii, T. Drewello, S. Barison, A. Venturini, F. Zerbetto, F. Paolucci, J. Am. Chem. Soc. 2008, 130, 3788 – 3796. [22] a) M. Marcaccio, F. Paolucci, C. Paradisi, S. Roffia, C. Fontanesi, L. J. Yellowlees, S. Serroni, S. Campagna, G. Denti, V. Balzani, J. Am. Chem. Soc. 1999, 121, 10081 – 10091; b) M. Marcaccio, F. Paolucci, C. Paradisi, M. Carano, S. Roffia, C. Fontanesi, L. J. Yellowlees, S. Serroni, S. Campagna, V. Balzani, J. Electroanal. Chem. 2002, 532, 99 – 112. [23] C. Amatore, C. Lefrou, J. Electroanal. Chem. 1992, 324, 33 – 58. [24] Antigona program developed by Dr. Loı¨c Mottier, University of Bologna, Bologna, Italy, 1999.

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[25] L. Della Ciana, S. Zanarini, R. Perciaccante, E. Marzocchi, G. Valenti, J. Phys. Chem. C 2010, 114, 3653 – 3658. [26] a) F. E. Beideman, D. M. Hercules, J. Phys. Chem. 1979, 83, 2203 – 2209; b) K. M. Maness, R. M. Wightman, J. Electroanal. Chem. 1995, 396, 85 – 96; c) M. M. Collinson, R. M. Wightman, P. Pastore, J. Phys. Chem. 1994, 98, 11942 – 11947. [27] W. L. Wallace, A. J. Bard, J. Phys. Chem. 1979, 83, 1350 – 1357. [28] a) C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785 – 789; b) A. D. Becke, J. Chem. Phys. 1993, 98, 5648 – 5652; c) S. H. Vosko, L. Wilk, M. Nusair, Can. J. Phys. 1980, 58, 1200 – 1211; d) P. J. Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, J. Chem. Phys. 1994, 98, 11623 – 11627. [29] Molekel, version 4.3; S. Portmann, H. P. Lthi, Chimia 2000, 54, 766 – 770. http://www.cscs.ch/molekel/. [30] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, N. J. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. [31] R. Huenerbein, S. Grimme, Chem. Phys. 2008, 343, 362 – 371. [32] M. Parac, S. Grimme, Chem. Phys. 2003, 292, 11 – 21. [33] S. Di Motta, E. Di Donato, F. Negri, G. Orlandi, D. Fazzi, C. Castiglioni, J. Am. Chem. Soc. 2009, 131, 6591 – 6598. [34] D. Fazzi, C. Castiglioni, F. Negri, Phys. Chem. Chem. Phys. 2010, 12, 1600 – 1609. [35] A. Troisi, G. Orlandi, Chem. Phys. Lett. 2001, 344, 509 – 518. [36] Y. Olivier, V. Lemaur, J.-L. Brdas, J. Cornil, J. Phys. Chem. A 2006, 110, 6356 – 6364. [37] R. J. Cave, D. V. Baxter, W. A. Goddard III, J. D. Baldeschwieler, J. Chem. Phys. 1987, 87, 926 – 935.

Received: July 3, 2014 Published online on && &&, 0000

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Full Paper

FULL PAPER & Electrochemiluminescence

Generation gap: Pyrene-based dendrimers show stable and intense electrochemiluminescence (ECL) emission thanks to their excellent photophysical and electrochemical properties (see figure; kann = rate of electron transfer in the annihilation process). They are therefore an ideal benchmark to establish how and to what extent structural and electronic parameters affect and control the dynamics of the ECL properties.

Chem. Eur. J. 2014, 20, 1 – 13

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G. Valenti, A. Fiorani, S. Di Motta, G. Bergamini, M. Gingras,* P. Ceroni,* F. Negri,* F. Paolucci, M. Marcaccio* && – && Molecular Size and Electronic Structure Combined Effects on the Electrogenerated Chemiluminescence of Sulfurated Pyrene-Cored Dendrimers

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Molecular size and electronic structure combined effects on the electrogenerated chemiluminescence of sulfurated pyrene-cored dendrimers.

The electrochemistry, photophysics, and electrochemically generated chemiluminescence (ECL) of a family of polysulfurated dendrimers with a pyrene cor...
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