Article pubs.acs.org/JPCA

Rigid Medium Effects on Photophysical Properties of MLCT Excited States of Polypyridyl Os(II) Complexes in Polymerized Poly(ethylene glycol)dimethacrylate Monoliths Akitaka Ito,†,‡ Troy E. Knight,† David J. Stewart,† M. Kyle Brennaman,† and Thomas J. Meyer*,† †

Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States Department of Chemistry, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan

‡

S Supporting Information *

ABSTRACT: Higher-energy emissions from the metal-to-ligand chargetransfer (MLCT) excited states of a series of polypyridyl Os(II) complexes were observed at the fluid-to-film transition in PEG-DMA550. The higherenergy excited states, caused by a “rigid medium effect” in the film, led to enhanced emission quantum yields and longer excited-state lifetimes. Detailed analyses of spectra and excited-state dynamics by Franck−Condon emission spectral analysis and application of the energy gap law for nonradiative excited-state decay reveal that the rigid medium effect arises from the inability of part of the local medium dielectric environment to respond to the change in charge distribution in the excited state during its lifetime. Enhanced excited-state lifetimes are consistent with qualitative and quantitative predictions of the energy gap law.

■

transparent films with features conformable to the nanoscale by thermal6,20,21 and/or photochemical polymerization.22,23 We previously reported the spectroscopic and photophysical properties of a series of polypyridyl Ru(II) complexes, including [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine), in a PEGDMA containing nine ethylene glycol spacers (PEG-DMA550, n = 9 in Scheme 1) in both the fluid and film.24 Metal-to-ligand charge-transfer (MLCT) absorption energies and spectral band shapes are similar in both media. By contrast, emission energies and excited-to-ground state 0−0 energy gaps (E0), determined by emission spectral fitting, are blue-shifted, and spectral band widths are decreased at the fluid-to-film transition due to an expected “rigid medium effect”.17,25,26 It arises from the frozen nature of the film and the inhibition in the local medium dielectric to respond to the change in charge distribution induced by the optical excitation. This leads to increased emission energies in the film, enhanced emission quantum yields, and longer excited-state lifetimes, all consistent with qualitative and quantitative predictions of the energy gap law. The rigid medium effect is a general excited-state phenomenon, and we provide further elucidation here based on measurement of the spectroscopic and photophysical properties of a series of polypyridyl Os(II) complexes in PEG-DMA550 fluid and film. We also discuss the role of ligand variations in dictating excited-state properties by application of

INTRODUCTION

Understanding the influences of fluid and rigid/semirigid media (e.g., polymer films,1−6 sol−gels,7−11 glasses,12−17 and so forth18,19) on excited-state properties and photoinduced energy and electron transfer is an important element in exploiting molecular-level excited-state phenomena in device applications at interfaces and in films. An important materials advance in this area has come from use of a class of poly(ethylene glycol)dimethacrylates (PEG-DMAs), Scheme 1. These polymerizable fluids provide a means for preparing optically Scheme 1. Structures of PEG-DMA Monomer and the Polypyridyl Ligands

Special Issue: Current Topics in Photochemistry Received: February 25, 2014 Revised: April 9, 2014 Published: April 10, 2014 © 2014 American Chemical Society

10326

dx.doi.org/10.1021/jp5019873 | J. Phys. Chem. A 2014, 118, 10326−10332

The Journal of Physical Chemistry A

Article

axis. Decay traces were constructed as an average of 500 transients monitored at the emission maximum for each compound. Emission Spectral Fitting. Emission spectra were fit by application of a one-mode Franck−Condon analysis, eq 2.34,35

the energy gap law. Polypyridyl ligand structures are shown in Scheme 1.

■

EXPERIMENTAL SECTION Materials and Sample Preparation. The complexes in this study, [Os(bpy) 3](PF6 )2 , [Os(phen) 3](PF 6) 2, [Os(bpy)2(phen)](PF6)2, [Os(dmb)3](PF6)2, [Os(deeb)3](PF6)2, and [Os(dpb)3](PF6)2 (bpy = 2,2′-bipyridine, phen = 1,10phenanthroline, dmb = 4,4′-dimethyl-2,2′-bipyridine, deeb = 2,2′-bipyridyl-4,4′-dicarboxylic acid diethyl ester, and dpb = 4,4′-diphenyl-2,2′-bipyridine), were prepared by methods previously described27−29 or by similar procedures. Anhydrous acetonitrile was obtained from Burdick and Jackson and used without further purification. Samples for optical measurements in poly(ethylene glycol)dimethacrylate (PEG-DMA550) fluid and film were prepared as described previously.24 PEG-DMA550 liquid monomer (Aldrich) was purified to remove the polymerization inhibitor (MEHQ) by passing the neat monomer through an alumina column. Residual water was removed by evaporation under reduced pressure for over 2 h prior to use. The solutions of the complexes in inhibitor-free liquid PEG-DMA550 were placed in 1 cm path length glass cuvettes and degassed by sequential freeze−pump−thaw cycles. For the polymerized film samples, the solution with 1 wt % of the cross-linking initiator (Vazo-52, DuPont) was heated at 50 °C overnight under vacuum to yield optically transparent PEG-DMA550 films. Steady-State Absorption and Emission Spectra. UV− visible absorption spectra in fluid and films were obtained by using a Varian Cary 50 UV−visible spectrophotometer. Steadystate emission spectra were acquired with a PTI 4SE-NIR QuantaMaster fluorimeter or an Edinburgh Instruments FLS920 spectrometer. Emission intensities at each wavelength were corrected for system spectral response. Spectra were acquired on samples dissolved in thoroughly degassed PEGDMA550 under optically dilute conditions (absorbance ≈ 0.1− 0.3) and sealed under vacuum in 1 cm path length glass cuvettes. Emission quantum yields (Φem) were determined relative to [Ru(bpy)3](PF6)2 (Φem,std = 0.095 in CH3CN)30,31 by using eq 1.32,33 Φem = Φem,std

2 (Iunk /A unk ) ⎛ ηunk ⎞ ⎜⎜ ⎟⎟ (Istd /A std ) ⎝ ηstd ⎠

⎛ E0 − νMℏωM ⎞3⎛ SM νM ⎞ ⎜ ⎟⎜ ⎟ E0 ⎠ ⎝ νM! ⎠ νM = 0 ⎝ ∞

I(ν)̃ =

∑

⎡ ⎛ ν ̃ − E + ν ℏω ⎞2 ⎤ 0 M M ⎥ ⎢ ⎟⎟ × exp − 4 ln 2⎜⎜ ⎢ ν Δ ̃ ⎝ ⎠ ⎥⎦ 0,1/2 ⎣

(2)

In eq 2, I(ν̃) is the emission intensity at the energy ν̃ in wavenumber (cm−1) relative to that of the 0 → 0 transition. E0 is the energy gap between the zeroth vibrational levels in the ground and emitting excited states, ℏωM is the quantum spacing for the averaged medium-frequency vibrational mode, SM is the electron-vibrational coupling constant or the Huang− Rhys factor36 reflecting the extent of change along the normal coordinate for an average medium-frequency mode, and Δν̃0,1/2 is the full width at half-maximum (fwhm) for an individual vibronic line. The photon numbers of the emission spectrum were corrected to a wavenumber scale by using the equation I(ν̃) = I(λ) × λ2.33,37 The parameters E0, ℏωM, SM, and Δν̃0,1/2 were optimized with a least-squares minimization routine by using a generalized reduced gradient (GRG2) algorithm.38 The summation was carried out over 11 vibrational levels (νM: 0 → 10).

■

RESULTS AND DISCUSSION Absorption and Emission Spectra. As a typical example, steady-state absorption and emission spectra for [Os(bpy)3]2+ are shown in Figures 1 and 2, with additional spectra in the

(1)

In eq 1, Iunk and Istd are the areas of emission profiles of the sample and standard, respectively, and Aunk and Astd are the absorbances of the sample and standard at the excitation wavelength (λex = 450 nm), respectively. The ηunk and ηstd are the indices of refraction of the sample and standard solutions (taken as the neat solvents). Time-Resolved Emission. Nanosecond time-resolved emission decays were obtained with a PTI GL-3000 pulsed nitrogen laser (337 nm, 6 Hz, 1 mJ) as the input for a PTI GL301 dye laser (440 nm, 6 Hz, 1 μJ) as an excitation source or by an FLS920 emission spectrometer equipped with a pulsed 444 or 484 nm LED excitation source (Edinburgh Instruments EPL-444 or -485, fwhm ≈ 1.5 ns). Data were acquired at room temperature in thoroughly freeze−pump−thaw degassed PEGDMA550 fluid and film samples having absorbances between 0.1 and 0.3 at the excitation wavelength. Samples were sealed under vacuum in 1 cm path length glass cuvettes. A McPherson 272 monochromator and a Hamamatsu R-928 photomultiplier were used to detect emission at 90° relative to the excitation

Figure 1. Electronic absorption spectra of [Os(bpy)3](PF6)2 in PEGDMA550 fluid (red trace) and film (blue trace) at room temperature.

Supporting Information (Figures S1−S5). Spectral data for the six complex salts in PEG-DMA550 fluid and film are summarized in Table 1. The higher energy, ligand-localized π → π* transitions in the UV region were not observable due to competitive absorption by the glass cuvettes. Visible absorption spectra are dominated by 1Al → 1MLCT (t2g → π*) transitions at wavelengths from 430 to 520 nm, and less intense, spinforbidden 1Al → 3MLCT (t2g → π*) transitions at wavelengths above 550 nm, which are observable due to spin−orbit coupling at Os with a spin−orbit coupling constant of ∼3000 cm−1.39−42 Absorption band energies are nearly identical in fluid and film 10327

dx.doi.org/10.1021/jp5019873 | J. Phys. Chem. A 2014, 118, 10326−10332

The Journal of Physical Chemistry A

Article

between the fluid and film. SM is related to the change in equilibrium displacement along the normal coordinate for the average mode between the excited and ground states, ΔQM, by eq 3, with MM the reduced mass and ωM the angular frequency. SM =

1 ⎛ MMωM ⎞ 2 ⎜ ⎟(ΔQ ) M 2⎝ ℏ ⎠

(3)

The similarity in SM values between the two media suggests that structural changes between the ground and excited states are similar in the two. On the other hand, 0−0 energies, E0, increased by ∼440 cm−1 in the film and fwhms, Δν̃0,1/2, decreased by ∼210 cm−1. These differences can be explained by the change in the medium reorganization energy (λo) between the two media. The reorganization energy in a fluid is related to Δν̃0,1/2, as shown in eq 4, with kB and T the Boltzmann constant and temperature, respectively.

Figure 2. Corrected emission spectra for [Os(bpy)3](PF6)2 in PEGDMA550 fluid (red trace) and film (blue trace) at room temperature. Excitation wavelength = 450 nm.

PEG-DMA550, suggesting similar local dielectric microenvironments in the two. Emission occurs from 3MLCT → 1Al transitions, which have been studied in both fluid34,43−45 and rigid media.46 Although emission spectral band shapes of [Os(bpy)3]2+* in the fluid and film at room temperature are similar, as shown in Figure 2, a blue shift of ∼30 nm was observed in the film relative to the fluid. Similar behavior was observed for the other complexes. The similarities in emission and absorption band shapes in PEG-DMA550 film and fluid point to similar dielectric properties in the two media. Emission Spectral Fitting. Emission spectral fitting parameters obtained from the single average mode Franck− Condon analysis are summarized in Table 2. The average quantum spacing, ℏωM, includes contributions from a series of ring-stretching vibrational modes in the diimine ligand. The Huang−Rhys factors, SM, showed no significant differences

λo =

(Δν0,1/2 ̃ )2 16kBT ln 2

(4)

In a film, the medium reorganization energy can be separated into frozen, λoo, and nonfrozen, λoi, parts, with λo = λoo + λoi.5,16,17,24,47 Compared to the fluid (fl), in the film, the frozen (fr) part of the reorganization energy becomes part of the energy gap, with E0,fr = E0,fl + λoo, increasing the emission energy. Similarly, the bandwidth at half-height is decreased by the loss of λoo in the film. Microscopically, λoo arises from largeamplitude collective medium dipole reorientations and λoi from individual molecule rotations and local lattice or phonon modes. Photophysical Properties. Emission decay profiles for [Os(bpy)3]2+* in fluid and film PEG-DMA550 are shown in Figure 3 and, for comparison, in CH3CN in the inset. Data for

Table 1. Spectroscopic and Photophysical Properties of the Complexes in PEG-DMA550 Fluid (top entry) and Film (bottom entry) at Room Temperature λabsa/nm

complex [Os(bpy)3](PF6)2

438, 481, 584, 645(sh) 438, 481, 584, 645(sh)

[Os(phen)3](PF6)2

433, 478, 563, 655(sh) 434, 481, 564, 656(sh)

Φem

τem (A), β

krc/105 s−1

knrd/107 s−1

758

0.004

0.96

2.4

728

0.008

1.3

1.6

735

0.015

0.92

0.61

712

0.018

τem = 42 ns (1.00) τem1 = 18 ns (0.14) ⟨τem2⟩ = 63 ns (0.86), β = 0.833 τem = 163 ns (1.00) τem1 = 4 ns (0.05) ⟨τem2⟩ = 240 ns (0.95), β = 0.931 τem = 57 ns (1.00) ⟨τem2⟩ = 103 ns (1.00), β = 0.852 τem = 26 ns (1.00) ⟨τem2⟩ = 47 ns (1.00), β = 0.904 τem = 82 ns (1.00) τem1 = 5 ns (0.04) ⟨τem2⟩ = 135 ns (0.96), β = 0.899 τem = 40 ns (1.00) τem1 = 1 ns (0.03) ⟨τem2⟩ = 78 ns (0.97), β = 0.892 τem = 56 ns

0.75

0.41

1.1 0.98

1.8 0.97

0.77 1.1

3.9 2.1

1.3

1.2

1.1

0.73

1.3

2.5

1.3

1.3

0.90

1.8

[Os(bpy)2(phen)](PF6)2

434, 482, 577, 640(sh) 435, 481, 579, 640(sh)

752 723

0.006 0.010

[Os(dmb)3](PF6)2

463, 490, 597, 658(sh) 463, 490, 597, 658(sh)

775 746

0.002 0.005

[Os(deeb)3](PF6)2

444, 499, 606, 665 445, 498, 605, 666

782

0.011

751

0.015

788

0.005

755

0.010

748

0.005

[Os(dpb)3](PF6)2

459, 514, 612(sh), 680(sh) 459, 512, 610(sh), 681(sh)

[Os(bpy)3](PF6)2e a

λemb/nm

Absorption maxima.

437, 480, 583, 646(sh) b3

MLCT → 1Al emission maximum. ckr = Φem/τem. dknr = (1−Φem)/τem. eData in nitrogen-purged CH3CN. 10328

dx.doi.org/10.1021/jp5019873 | J. Phys. Chem. A 2014, 118, 10326−10332

The Journal of Physical Chemistry A

Article

Table 2. Spectral Fitting Parameters for the Complexes in the PEG-DMA550 Fluid (top entry) and Film (bottom entry) complex [Os(bpy)3](PF6)2 [Os(phen)3](PF6)2 [Os(bpy)2(phen)](PF6)2 [Os(dmb)3](PF6)2 [Os(deeb)3](PF6)2 [Os(dpb)3](PF6)2 [Os(bpy)3](PF6)2c a

E0/cm−1

SM

ℏωM/cm−1

Δν̃0,1/2/cm−1

λoa/cm−1

ln[F(calc)]b

13290 13750 13660 14060 13400 13860 13020 13470 12910 13320 12810 13220 13490

0.83 0.85 0.67 0.76 0.78 0.83 0.88 0.87 0.67 0.69 0.72 0.68 0.83

1320 1320 1370 1300 1360 1320 1280 1300 1170 1200 1190 1240 1330

1690 1480 1740 1500 1740 1520 1660 1480 1560 1350 1510 1300 1640

1250 950 1320 980 1310 1010 1200 960 1060 790 990 740 1180

−16.41 −17.23 −17.93 −19.39 −16.53 −17.80 −16.19 −16.79 −20.62 −21.03 −19.32 −20.04 −16.61

Calculated from eq 4. bCalculated from eqs 7. cParameters from data in CH3CN solution.

⎛1⎞ ⟨τem2⟩ = (kem2β)−1Γ⎜ ⎟ ⎝β⎠

Emission lifetimes (τem) and quantum yields (Φem) for the series of complexes in PEG-DMA550 fluid and film are summarized in Table 1. Excited-state decay for [Os(bpy)3]2+* in PEG-DMA550 occurs with ⟨τem2⟩ = 63 ns in the film and τem = 42 ns in the fluid. Radiative rate constants (kr), calculated from τem (or ⟨τem2⟩) and Φem, are nearly the same in the film, 1.3 × 105 s−1, and in the fluid, 0.96 × 105 s−1. Similar behavior was observed for the other Os(II) complexes, consistent with the relatively small effect of the fluid-to-film transition on the energy gap and the expected approximate dependence of kr on E0−3.34,53−55 Nonradiative decay is more sensitive to the fluid-to-film transition. As an example, there is a decrease in the nonradiative decay rate constant (knr) for [Os(bpy)3]2+ in the film (1.6 × 107 s−1) compared to the fluid (2.4 × 107 s−1) of 1.5 with the same ratio observed almost independent of the complex. Similar trends were observed in the earlier study based on Ru(II) complexes24 and explained by invoking the energy gap law.56−58 Energy Gap Law, Accounting for Medium Effects. In the weak vibrational coupling limit, E0 ≫ SMℏωM, with a single average acceptor mode with ℏωM ≫ kBT, ln knr is given by the energy gap law in eq 8 with parameters defined in eqs 7. The Franck−Condon weighted density of states in eq 7a, ln[F(calc)], can be evaluated from the emission spectral fitting parameters in Table 2.34,59

Figure 3. Nanosecond time-resolved emission decay profiles for [Os(bpy)3](PF6)2 in PEG-DMA550 fluid (red trace) and film (blue trace) at room temperature. Excitation wavelength = 444 nm. (Inset) The decay profile of [Os(bpy)3](PF6)2 in nitrogen-purged CH3CN.

the other complexes are shown in Figure S6 (Supporting Information). Emission decay profiles in PEG-DMA550 fluid were exponential, while multiexponential functions were required to fit the data in the films. In a previous study, we reported exponential decay profiles for a series of polypyridyl Ru(II) complexes in PEG-DMA550 films with the exception of [Ru(phen)3]2+*. With the relatively short-lived polypyridyl Os(II) complex excited states, the appearance of a rapid kinetic component (