The effect of polyaromatic hydrocarbons on the spectral and photophysical properties of diaryl-pyrrole derivatives: an experimental and theoretical study.

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The effect of polyaromatic hydrocarbons on the spectral and photophysical properties of diaryl-pyrrole derivatives: an experimental and theoretical study† Joa õ Pina, Daniela Pinheiro, Bruno Nascimento, Marta Pin ˜ eiro* and J. Se ´rgio Seixas de Melo* A new class of diaryl-pyrrole derivatives of the polyaromatic hydrocarbons (PAH) benzene, naphthalene, anthracene and pyrene were synthesized in a multicomponent reaction under microwave irradiation and studied in solution at room (293 K) and low (77 K) temperature. The study includes a complete spectroscopic evaluation (singlet–singlet and triplet–triplet absorption, fluorescence and phosphorescence spectra) as well as photophysical evaluation (fluorescence, phosphorescence and triplet lifetimes together with fluorescence and triplet occupation and singlet oxygen sensitization quantum yields). From the above evaluation, a complete set of deactivation rate constants (kF, kIC and kISC) could be obtained. The study

Received 25th April 2014, Accepted 15th July 2014

was further complemented with TDDFT calculations. It is shown that, with the exception of the anthracene

DOI: 10.1039/c4cp01797h

derivative, the diaryl-pyrrole moiety strongly influences the spectral and photophysical properties of the PAH and that with the exception of the benzene derivative, the excited state internal conversion deactiva-

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tion channel of the diaryl-pyrrole derivatives is higher than that of the PAH counterparts.

Introduction Pyrrole is widely recognized as one of the simplest and most important aromatic heterocycles and can be found in a wide variety of natural coloured products such as chlorophyll and hemoglobin, and in a myriad of synthetic compounds.1–4 Pyrrole is present in a large number of bioactive compounds. Antitubercular,5,6 anti-tumour,7 anti-inflammatory,8 HIV fusion inhibitory9 and cholesterol-lowering10 activities are amongst the biological activities demonstrated by several pyrrole derivatives. Furthermore, pyrrole has been used as a building block for the synthesis of new materials for the development of solar cells,11 chiral liquid crystals12 and supramolecular13,14 assemblies. The polyaromatic hydrocarbons (PAH) pyrene, naphthalene and anthracene are catacondensed p-electron systems, which belong to the D2h symmetry group and are totally symmetric in the ground state with A1g symmetry.15,16 The inclusion of these PAH in different types of structures enables them to be used as chemosensors for a variety of applications.17–24 The electronic transitions of these molecules – and also of benzene which is of D6h symmetry – between the ground state and the first (1B3g) Coimbra Chemistry Centre, Department of Chemistry, University of Coimbra, Rua Larga, 3004-535 Coimbra, Portugal. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cp01797h

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and second (1B2u) electronic states are polarized along the short and long axis dipole-moment transitions, respectively.16 Due to its forbidden nature the 1Ag–1B3u transition appears as very weak in the spectrum (low molar extinction coefficients), whereas a very intense absorption is observed from the ground state to the second singlet state, 1Ag–1B2u transition (high molar extinction coefficients).25 According to Platt’s classification for catacondensed p-electron systems the two lowest electronically excited states are defined as 1Lb and 1La states.16 It is the relative position of these two lowest lying p,p* transitions that is responsible for the spectral and photophysical properties of these compounds. N-Phenyl derivatives have been heavily investigated; amongst some of these one can find 1-phenyl and 2-aryl substituted pyrroles.26–28 A particular emphasis has been placed on the charge transfer character involved, where this can be tuned, for example, with the introduction of cyano-phenyl groups in position 2 of the pyrrole unit.29 The current work aims to extend these studies by introducing an additional aromatic unit in position 5 of the pyrrole unit. Indeed, in this work we report the synthesis and photophysical characterization of four 3,5-diaryl-2-methyl-1H-pyrrole derivatives substituted, in position 3, with the aromatic hydrocarbons benzene, naphthalene, anthracene and pyrene. It is shown that N-methyl-pyrrole introduces a charge transfer character which changes the spectral and photophysical properties of the PAH.

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Experimental section Detailed synthetic procedures and characterization data for the compounds investigated can be found in ESI.† All of the solvents were of spectroscopic or equivalent grade and unless mentioned all experiments were carried at room temperature (293 K). Absorption and fluorescence spectra were recorded on Shimadzu UV-2100 and Horiba-Jobin-Ivon Fluorog 3-22 spectrometers respectively. Phosphorescence measurements were made in glasses at 77 K using the Horiba-Jobin-Ivon Fluorog 3-22 spectrometer, equipped with a 1934 D phosphorimeter unit. The molar extinction coefficients (e) were obtained from absorption spectral measurements using six solutions of different concentrations and the slope of plots of the absorption vs. concentration. The fluorescence and phosphorescence quantum yields were obtained by comparison with standards of known quantum yield, as described elsewhere.30 The fluorescence quantum yields were measured using quinine sulfate (fF = 0.545) in 0.5 M H2SO4 solution as a standard while the phosphorescence quantum yields were determined using benzophenone (fPh = 0.84) as a standard.31 Fluorescence decays were measured using home-built nanosecond32 and picosecond33 time correlated single photon counting (TCSPC) equipment described elsewhere.30 All solutions were deoxygenated by bubbling with nitrogen for at least 20 minutes. The fluorescence decays and the instrumental response function (IRF) where collected using a time scale of 1024 channels, until 5 103 counts at maximum were reached. Deconvolution of the fluorescence decay curves was performed using the modulating function method as implemented by G. Striker in the SAND program as previously reported.34 The ground state molecular geometry was optimized using the density functional theory (DFT) by means of the Gaussian 03 program,35 at the B3LYP/6-31G** level.36,37 Optimal geometries were determined on isolated entities in a vacuum and no conformation restrictions were imposed. For the resulting optimized geometries time dependent DFT calculations (using the same functional and basis sets as those in the previous calculations) were performed to predict the vertical electronic excitation energies. Molecular orbital contours were plotted using Molekel 5.4. To better understand the specific nature of the intramolecular interactions in the investigated compounds the natural bonding orbital (NBO) analysis was performed using the NBO 3.1 program as implemented in the Gaussian 03 program. The experimental setup used to obtain the triplet–triplet absorption spectra and singlet to triplet intersystem crossing yields has been described elsewhere.32,38 First-order kinetics was observed for the decay of the lowest triplet state. Special care was taken in determining triplet yields, namely to have optically matched dilute solutions (abs E0.2 in a 10 mm square cell) and low laser energy (r2 mJ) to avoid multiphoton and T–T annihilation effects. The triplet molar absorption coefficients (eT) were obtained by the singlet depletion or by the partial

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saturation method. When using the singlet depletion technique the well-known relationship (eqn (1)) was applied39 eT ¼

eS DODT DODS

(1)

where both DODS and DODT are obtained from the triplet– singlet difference transient absorption spectra and eS corresponds to the ground state molar extinction coefficient. For the partial saturation technique the triplet–triplet absorption signal (DODT) was collected at the maximum (in methylcyclohexane solution) as a function of the energy of the laser pulse (E). For all the investigated samples (S) a non-linear region (partial saturation region) was achieved. The experimental data were then fitted with eqn (2), assuming the initial conditions [1S] = [1S]0 and [3S*] = 0:39,40 DODT = a(1 exp[bE])

(2)

a = (eT eS)[1S]0l

(3)

b = 2303eStfT

(4)

with

where l corresponds to the optical path length of the monitoring beam in the sample and [1S]0 is the initial concentration of the sample. The fT values were obtained by comparing the triplet DOD at 525 nm of a benzene solution of benzophenone (lexc = 355 nm) or by comparing the triplet DOD at 415 nm of a naphthalene ethanol solution (lexc = 266 nm) with that of the compounds (optically matched at the laser wavelength) as described elsewhere.32,38 Room-temperature singlet oxygen phosphorescence was detected at 1270 nm using a Hamamatsu R5509-42 photomultiplier, cooled to 193 K in a liquid nitrogen chamber (products for research model PC176TSCE-005), following laser excitation of aerated solutions at 266 nm or 355 nm, with an adapted Applied Photophysics flash kinetic spectrometer, as reported elsewhere.41 Biphenyl in cyclohexane (lexc = 266 nm), fD= 0.73, or 1H-phenalen-1-one (perinaphthenone) in toluene (lexc = 355 nm), fD= 0.93, was used as a standard.42,43

Results and discussion The structures and acronyms of the investigated compounds are depicted in Scheme 1. These consist of 3,5-diaryl-2-methyl1H-pyrrole derivatives substituted in position 3 with different aromatic chromophores (phenyl, 1a, naphthalene, 1b, anthracene, 1c, and pyrene, 1d). Synthesis Following previous studies on the synthesis of pyrroles with aliphatic substituents,44 pyrroles 1a–d were synthesized in a multicomponent reaction (see Scheme 1). In order to establish the best reaction conditions 3,5-diphenyl-2-methyl-1H-pyrrole 1a was used as a model compound. The results obtained using equimolar amounts of (E)-1,3-diphenylprop-2-en-1-one, benzylamine and a


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Scheme 1

Table 1 Reaction medium, conditions and obtained yields for 1a–1d according to the general synthetic procedure in Scheme 1

Entry

Reaction medium

Reaction conditions

Yielda (%)

1 2 3 4 5 6 7 8 9

SiO2 60 (200–500 mm) SiO2 60 (35–70 mm) SiO2 60 (35–70 mm) SiO2 60 (35–70 mm) SiO2 60 (35–70 mm) SiO2 N (2–20 mm) SiO2 60/H2SO4 (35–70 mm) Al2O3 (50–150 mm) Montmorillonite K-10

MW MW MW MW MW MW MW MW MW

19 28 27 24 —b 23 —b 25 —b

(10 min 100 1C) (10 min 100 1C) (20 min 100 1C) (10 min 150 1C) (5 min 100 W) (10 min 100 1C) (10 min 100 1C) (10 min 100 1C) (10 min 100 1C)

a Yields refer to isolated reaction products obtained using 5 mmol of chalcone and benzylamine and 15 mmol of nitrobenzene. b Only trace amounts of pyrrole 1a were detected by TLC analysis of the crude reaction product.

three-fold molar excess of nitromethane pre-absorbed on the surface of several inorganic solid supports and heated under microwave irradiation are collected in Table 1. Between the silicon dioxide support, aluminium oxide and montmorillonite the best result obtained was an isolated yield of 28% after heating at 100 1C for 10 min using silica 60 as support. A simplification of the procedure was made by eliminating the inorganic solid support, i.e., in solvent-free conditions it led to the formation of only trace amounts of pyrrole under microwave (MW) irradiation for 10 or 20 minutes. The use of a solvent-base approach using glacial acetic acid, ethanol or ethanol doped with a few drops of concentrated sulfuric acid did not give better results. An identical isolated yield of 28% was obtained when open-vessel conditions were employed at the same reaction temperature and time. The chalcone precursors required for the synthesis of 3,5-diaryl-2-methyl-1Hpyrroles were synthesized with very good yields (70–90%) through a classical base-catalysed Claisen–Schmidt condensation reaction45 (for further details, see ESI†). Photophysical studies The present study will be essentially focused on the electronic spectral and photophysical properties resulting from the arylsubstitution on the central pyrrole unit. Furthermore, the effect from the aromatic counterpart in the 3-diaryl-2-methyl-1H-pyrrole


Fig. 1 Room temperature absorption (A, C) and fluorescence emission spectra (B, D) for the investigated compounds (top panels) and their aromatic counterparts (bottom panels) in methylcyclohexane solution.

unit is also established from a comparison with the spectroscopic and photophysical properties of the former. Singlet state Fig. 1 presents the absorption and fluorescence emission spectra of the investigated diaryl-pyrrole derivatives, 1a–1d, in methylcyclohexane (MCH) solution. Comparison of the room temperature absorption and emission spectra with those of the aromatic counterparts (benzene, naphthalene, anthracene and pyrene) shows a significant decrease in vibrational structure together with a significant red-shift (namely in the fluorescence emission) of the spectra (see Fig. 1). For the investigated diaryl-pyrrole derivatives the molar extinction coefficients (e) obtained from the lowest energy maximum were found to be relatively high (e 4 7000 M1 cm1, see Table 2) thus pointing to an allowed p - p* character of the lowest energy transitions. With the exception of derivative 1a, the e values obtained for compounds 1b–1d are in good agreement with the values found for their PAH counterparts (naphthalene, e B 6000 M1 cm1 @ 275 nm, anthracene, e B 10 000 M1 cm1 @ 355 nm, and pyrene, e B 54 000 M1 cm1 @ 335 nm) (see Table 2).15 Noteworthy is the increase in the e value by two orders of magnitude on going from benzene (e B 200 M1 cm1 @ 255 nm) to derivative 1a (e B 10 990 M1 cm1 @ 275 nm). This behavior was previously described for benzene and PAH derivatives and was attributed to the substituent effect on the aromatic rings affecting the shape and the position of the spectra and the intensity of the transitions (due to the symmetry loss of the compounds).15 The latter effects are mirrored by an increase or decrease in the e value and by a change in the natural fluorescence lifetime. Furthermore, as discussed in the following sections, other photophysical parameters, such as fluorescence quantum yields, are indirectly affected since an increase in e values can enhance the excited state radiative deactivation channel with respect to the radiationless transitions. As elsewhere reported, the N-phenylpyrrole derivative in polar solvents can undergo the formation of an intramolecular


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Table 2 Spectroscopic data (absorption, fluorescence and phosphorescence emission and triplet absorption maxima, extinction – singlet and triplet – coefficients, eSS and eTT) for the investigated compounds in methylcyclohexane solution at 293 K. Data in parentheses for 1a and 1c obtained in ethanol

lPhosp max (nm)

1 !Tn lTmax (nm) eTT (M1 cm1)

10 990, @ 284 nm 347 (357)

493

370

14 800

16 480, @ 291 nm 371

503, 539, 581 360

18 700

Compound lAbs max (nm)

a 1 lAbs cm1) max cal (nm) eSS (M

1a

239, 284 (240, 282)

226, 287

1b

291

284

1c

290, 348, 366, 385 (348, 366, 386) 291, 364, 392

7000, @ 385 nm

1d

277, 325, 352

23 600, @ 352 nm 393, 415

a

291, 330, 372

lFluo max (nm)

422, 441 (475) — 612, 672

430

10 400

430

12 300

Data obtained from TDDFT calculations.

charge transfer state (ICT) in the excited state.26,28 Therefore in order to investigate the occurrence of an ICT state for derivatives 1a and 1c, the absorption and fluorescence emission spectra were further investigated in (the polar solvent) ethanol (see Table 2 and Fig. S1 in ESI†). From the ground-state absorption spectra of these compounds in MCH and ethanol no significant differences were seen in the shape and wavelength maximum of the low-energy absorption band (see Table 2, Fig. 1 and Fig. S1 in ESI†). In contrast to the solvent independent absorption spectra, the fluorescence emission spectra display a clear solvent dependence with a significant bathochromic shift in the emission wavelength maxima with the increase in solvent polarity (ranging from 10 nm for 1a to 53 nm for 1c) (see Table 2). The larger solvatochromic shifts seen in the fluorescence emission spectra compared to those in the absorption spectra clearly suggest the occurrence of intramolecular charge transfer in the excited state. Fluorescence lifetimes obtained with picosecond and nanosecond time resolution were found to fit single exponential decay laws (see Fig. 2 and Table 3).

Theoretical calculations To further investigate the electronic properties of these compounds the ground state optimized geometry structures, the relevant HOMO and LUMO energy levels together with their electron density distribution surface plots and the natural bonding orbital (NBO) analysis were obtained at the DFT/ B3LYP/6-31G* level (Fig. 3). From the geometry optimization it was seen that the phenyl substituents in positions 1 and 5 of the pyrrole moiety are non-planar with respect to the plane formed by the central pyrrole core, displaying dihedral angles of B751 for the substituent in position 1 and B431 for the phenyl group in position 5. The latter dihedral angle was found to be higher than the values previously reported for 2-phenylpyrrole where depending of the computational method used, dihedral angles in the range of 241–311 were obtained.46 However, the most relevant piece of information comes from the substitution in position 3 of the PAH unit. Indeed, depending of the aromatic chromophore attached to the pyrrole unit, dihedral angles—with respect to the plane formed by the pyrrole ring—of B461 (benzene), 601 (naphthalene and anthracene) and 571 (pyrene) were obtained.


Fig. 2 Room temperature fluorescence decays for 1c and 1d in methylcyclohexane solution obtained with lexc = 373 and 339 nm respectively. For a better judgment of the quality of the fits, autocorrelation functions (A.C.), weighted residuals (W.R.) and chi-square values (w2) are also presented as insets. The dashed lines in the decays are the pulse instrumental response.

The time-dependent density functional theory (TDDFT) was used at the B3LYP/6-31G** level on the previously optimized ground-state molecular geometries to calculate the lowest energy electronic excited state transitions of the compound under study. The predicted values are presented in Table 2 (lAbs max cal) and display, in general, a good agreement with the experimental values giving additional support to the calculated molecular geometries (Fig. 3). For all the investigated diarylpyrrole derivatives the observed lowest energy absorption bands (and more intense bands) are associated to the predicted S0 - S1 transition (see Fig. 3). For the diaryl-pyrrole 1a this transition displays contributions from the HOMO - LUMO


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Table 3 Photophysical properties including quantum yields (fluorescence, fF, phosphorescence, fPh, internal conversion, fIC, triplet formation, fT, and sensitized singlet oxygen formation, fD) for 1a–1d in methylcyclohexane solution

Compound

fF (293 K)

tF (ns) (293 K)

1a Benzenea

0.29 0.06

0.44 34.0

0.50 0.69

0.021 0.15

1b Naphthalenea

0.52 0.19

1.64 96.0

0.15 0.06

0.012 0.033

1c Anthracenea

0.34 0.30

5.27 5.30

0.32 0.08

— 0.0003b

1d Pyrenea

0.40 0.65

3.71 650

0.38 B0

fIC (293 K)

fPh (77 K)

0.0010 0.0021

tPh (ms) (77 K)

fT (293 K)

fD (293 K)

tT (ms) (293 K)

kF (ns1)

kNR (ns1)

2390 4500

0.21 0.25

0.22 —

37 —

0.659 0.002

1.614 0.028

1.136 0.020

0.477 0.007

1240 2300

0.33 0.75

0.32 —

32 175

0.317 0.002

0.293 0.008

0.091 0.001

0.201 0.008

— B40b

0.34 0.62c

0.20 —

97 670

0.065 0.057

0.125 0.132

0.061 0.015

0.065 0.117

0.22 0.37

0.24 —

57 180

0.108 0.001

0.162 0.001

0.102 B0

0.059 0.001

445 550

a Photophysical data obtained in non-polar solvents from ref. 31. dioxane solution at 293 K from ref. 50.

b

kISC (ns1)

Data obtained in EPA glass at 77 K from ref. 31 and 49. c Data obtained in

Fig. 3 DFT//B3LYP/6-31G** optimized ground-state geometry together with the frontier molecular orbital energy levels and the electronic density contours for the investigated compounds.

(major contribution, 60%), HOMO - LUMO + 1 (29%) and HOMO - LUMO + 3 (12%) orbitals. Upon going to 1b this transition involves the orbitals HOMO 1 - LUMO (26%), HOMO - LUMO + 1 (major contribution, 45%), HOMO LUMO + 2 (20%) and HOMO - LUMO + 3 (9%). For 1c and 1d the orbitals HOMO 1 - LUMO (with 39% and 36% contributions respectively) and HOMO - LUMO (major contributions with 61% and 64% contributions) are involved in the S0 - S1 transition. From the molecular orbital contours (Fig. 3) it is possible to observe that the density of the HOMO orbital is, in general, spread over the entire molecule (although displaying higher electron density on-top of the pyrrole core), while the LUMO shows a decrease in the electron density on the pyrrole moiety and a concomitant increase in the peripheral PAH units, thus giving support for the occurrence of ICT in the excited state. A similar effect was previously reported for 2-phenylpyrrole (with the HOMO orbital delocalized over the pyrrole ring while the LUMO was found delocalized over the phenylene unit) and


kIC (ns1)

was attributed to the occurrence of electron transfer between the heterocyclic ring and the phenylene moiety (ICT).46 NBO analysis has been performed on the investigated diarylpyrroles in order to further elucidate the intra-molecular interactions and delocalization of electron density within the molecule. Natural bond orbital analysis provides an efficient method for studying intra- and inter-molecular bonding and interaction among bonds, and also provides a convenient basis for investigating charge transfer or conjugative interaction in molecular systems.47 The results of the second order perturbation theory analysis of Fock Matrix in NBO basis together with electron density occupancy (ED) are presented in Tables S1–S4 in ESI.† This analysis was carried out to evaluate the stabilizing interactions between filled NBOs (donor) of the idealized Lewis structure and empty (acceptor) non-Lewis NBOs, together with an estimation of their energetic importance. These interactions result in a loss of occupancy in the localized donor NBO into an empty non-Lewis orbital. For each donor and acceptor orbital the greater the stabilization energy value, E(2), the more intense is the interaction between electron donors and acceptors and therefore the higher is the extent of conjugation of the whole system. In Tables S1–S4 (ESI†) only the most important interactions, corresponding to the largest E(2) values, are presented. For the investigated aryl-pyrroles, in general, the predominant intramolecular interactions are formed by the orbital overlap between p(CQC) and p*(CQC) bonding orbitals within each individual chromophore (pyrrole, phenyl, naphthalene, etc.), which results from the aromaticity stabilization of these units. This is supported by the high electron density values found within each unit thus showing strong electron delocalization. The most important interaction energy in these molecules is due to charge transfer from the nitrogen lone-electron pair into the antibonding (C1–C2) and (C3–C4) orbitals (in the pyrrole moiety) resulting in stabilization energies of B33 and B35 kcal mol1, respectively. It is also noteworthy that although the interactions between different chromophoric units are masked by the more energetic aromatic interactions, they occur with stabilization energies


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below B10 kcal mol1 (data not shown in Tables S1–S4, ESI†); thus the NBO analysis does not exclude the occurrence of ICT in the excited state previously supported by the solvatochromic study and the HOMO–LUMO molecular orbital contours of these derivatives.

Fig. 4 Phosphorescence emission spectra for the investigated compounds in methylcyclohexane solution at 77 K.

Triplet state With the exception of 1c the phosphorescence emission spectra of the diaryl-pyrrole derivatives were obtained in MCH glasses at 77 K (Fig. 4). The phosphorescence emission spectra of 1a, 1b and 1d samples closely resemble the emission spectra of the aromatic counterparts (benzene, naphthalene and pyrene respectively).28,30,48 With our current phosphorescence setup we were unable to detect any phosphorescence signal for the diaryl-pyrrole 1c, although phosphorescence has been previously reported for anthracene in EPA glasses at 77 K.31,49 The singlet–triplet difference absorption spectra of the diarylpyrrole derivatives were also obtained from laser flash photolysis (Fig. 5). In addition to ground state depletion, at shorter wavelengths, an intense transient triplet absorption in the 250–650 nm region of the spectra is observed. For compounds 1c and 1d the obtained triplet wavelength maxima values are similar to the values previously reported for the aromatic substituents (anthracene, 430 nm, and pyrene, 420 nm),31 although the broader transient absorption spectra suggest delocalization of the triplet state due to conjugation along the diaryl-pyrrole derivatives, in particular in the case of 1b. For compound 1a no transient absorption was observed below 300 nm, the region of triplet–triplet absorption of benzene 1 !Tn (lTmax ¼ 235 nm); however, due to the increased triplet conjugation segment a transient triplet–triplet absorption band at longer 1 !Tn ¼ 370 nm) was observed. wavelengths (lTmax Upon photolysis of aerated MCH solutions of the compounds, singlet oxygen formation quantum yields (fD) were obtained. Observation of the fD values in Table 3 reveals that, within the experimental error, these are in agreement with the fT values, thus giving support for the latter. Photophysical properties: overall discussion

Fig. 5 Room temperature transient triplet–triplet absorption spectra for the investigated compounds in methylcyclohexane solution.

Fig. 6

The overall set of photophysical parameters including quantum yields, lifetimes and rate constants obtained in MCH solution at 293 K is presented in Table 3.

Jablonski-type diagram with the photophysical parameters for the excited-state deactivation of the diaryl-pyrroles 1a–1d and those of the PAH counterparts.



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For the diaryl-pyrrole derivatives, with the exception of 1b, the radiationless deactivation channel (fIC + fT) is the main excited state deactivation pathway. For 1b (the naphthalenesubstituted diarylpyrrole) a similar contribution was found between the radiative (fF) and radiationless (fIC + fT) excited state deactivation processes (see Table 3). The singlet oxygen sensitization values are close to those obtained for the intersystem crossing yield, ranging between 0.20 and 0.32, thus showing that singlet oxygen is efficiently sensitized by these compounds. In general, the low phosphorescence quantum yield (fPh) together with the significant intersystem crossing values found for the investigated compounds indicates that the radiationless channel is the main deactivation pathway of the triplet excited state, a similar behavior to what was found for naphthalene– oligothiophene derivatives.41 A more straightforward comparison between the photophysical parameters for the diaryl-pyrroles 1a–1d and those of the PAH counterparts can now be established (see Fig. 6). In Fig. 6 the singlet energy values for the diaryl-pyrrole derivatives were obtained from the intersection between the normalized spectra of the lowest energy absorption band and the fluorescence emission band (since the 0–0 vibronic band in the absorption spectra could not be distinguished) while for the aromatic counterpart the values were obtained from the lowest energy 0–0 vibronic absorption bands. The latter values were seen to be in good agreement with the data reported in ref. 31. The triplet energy values were taken from the energy of the 0–0 vibronic bands of the phosphorescence spectra and from ref. 31. Indeed, a comparison between the energies of the singlet and triplet excited states of the diaryl-pyrroles 1a–1d and of the PAH counterparts, together with the quantum yields and lifetimes, can provide additional valuable information. The main observation resulting from this comparison is summarized below. (i) All the singlet and triplet energies of the PAH are higher than those of the diaryl-pyrroles 1a–1d, with the exception of the T1 triplet state in 1d and likely 1c (for which we were unable to observe phosphorescence). (ii) The internal conversion deactivation channel is active for all the aryl-pyrrole derivatives whereas with the PAH counterparts this channel is only effective with benzene. (iii) Observation of data in Fig. 6 and Table 3 reveals that with the exception of compound 1c, for all the diaryl pyrrole derivatives, the rate constants for the singlet excited state deactivation (kF, kIC and kISC) are one to two orders of magnitude higher (together with the concomitant decrease in the tF values) relative to their PAH counterparts, thus showing the existence of some degree of coupling between the chromophoric units. (iv) The decrease in the phosphorescence quantum yields and lifetimes (from the PAH to the diaryl pyrrole derivatives) gives further support for the electronic coupling/delocalization between the aromatic substituents and the pyrrole core in the triplet excited state.

Conclusions The electronic spectral and photophysical properties of a set of four diaryl-pyrrole chromophores linked to the aromatic


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benzene, naphthalene, anthracene and pyrene were investigated in solution at 293 K and 77 K and further complemented with theoretical data. A comparison with their analogues has been made showing (i) faster rate constants for the deactivation of the S1 state from the diaryl-pyrrole derivatives, (ii) higher fIC values (with the exception of the benzene derivative) and (iii) close proximity between the properties of anthracene and its 1c derivative. A charge transfer character of the singlet excited state is present in these compounds (absent with the analogues PAH), which was further confirmed by DFT calculations.

Acknowledgements õ para a Cie ˆncia e Tecnologia The authors thank the Fundaça (Portugal) and FEDER-COMPETE for financial support through the Coimbra Chemistry Centre (project PEst-OE/QUI/UI0313/ 2014), Program C2008-DRH05-11-842 (JP) and doctoral grants BN (SFRH/BD/41472/2007) and DP (SFRH/BD/74351/2010).

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