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Letter pubs.acs.org/JPCL
Evolution of Nonmirror Image Fluorescence Spectra in Conjugated Polymers and Oligomers Joseph K. Gallaher,†,‡,# Kai Chen,†,‡,# Gregory S. Huff,‡,§ Shyamal K. K. Prasad,†,‡ Keith C. Gordon,‡,§ and Justin M. Hodgkiss*,†,‡ †
School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington 6140, New Zealand MacDiarmid Institute for Advanced Materials and Nanotechnology, New Zealand § Department of Chemistry, University of Otago, Dunedin 9016, New Zealand ‡
S Supporting Information *
ABSTRACT: The nonmirror image relationship between absorption and fluorescence spectra of conjugated polymers contrasts with most organic chromophores and is widely considered a signature of interchromopohore energy funneling. We apply broad-band ultrafast fluorescence spectroscopy to resolve the evolution of fluorescence spectra for dilute solutions of conjugated oligothiophenes, where no energy transfer is possible. Fluorescence spectra evolve from a mirror image of absorption, which lacks vibronic structure, toward a spectrally narrower and vibronically structured species on the hundreds of femtosecond to early picosecond time scale. Our analysis of this fluorescence spectral evolution shows that a broad distribution of torsional conformers is driven to rapidly planarize in the excited state, including in solid films, which is supported by Raman spectroscopy and quantum chemical modeling. Our data have important implications for understanding different energy-transfer regimes that are delineated by structural relaxation.
T
soft ground-state potential, while the excited-state potential energy surface is stiffer.26,27 Thus, when a broad thermal distribution of ground-state conformers is photoexcited, they are strongly driven toward a narrow distribution of more planar conformers.28 Time-resolved fluorescence measurements of such oligomers29 and oligomer aggregates7,30 identify conformational dynamics on the tens of picosecond time scale. This time scale is slower than key charge- and energy-transfer processes in organic photovoltaic cells,10 suggesting that their efficiencies may be affected by operating in the nonrelaxed regime. However, other investigations report subpicosecond torsional relaxation time scales,22,23,31 including a recent ultrafast Raman spectroscopy study that directly probes torsional modes coupled to electronic excitations in oligothiophenes.32 This study revealed that the dominant torsional dynamics occur within a few hundred femtoseconds, with a secondary phase extending to a few picoseconds. Ultrafast fluorescence spectroscopy is uniquely suited to probing singlet excited-state relaxation dynamics. However, recovering parameters like the excited-state energy distributions and vibronic coupling from quantitatively fitted fluorescence spectra requires the challenging combination of broad-band spectral resolution and ultrafast time resolution.33 Here, we employ a new transient grating-based broad-band ultrafast fluorescence spectroscopy method complemented by TA
he nonmirror image relationship between broad absorption and sharp, vibronically structured fluorescence spectra in conjugated polymers is widely considered a signature of excitation energy funneling from a broad distribution of absorbing chromophores to a narrow distribution of emitters.1−5 Time-resolved fluorescence measurements added another variable to this picture: conformational relaxation. Intrinsic conformational relaxation dynamics were believed to mix with energy transfer in producing fluorescence decay in tens of picoseconds on the high-energy side of conjugated polymer fluorescence spectra.6−8 Explicitly separating conformational dynamics in such systems is important because structural relaxation may profoundly change the outcome of competing charge- and energy-transfer processes, notably in organic photovoltaic cells. For example, electronic delocalization in nonrelaxed states may favor charge separation,9−11 while dynamic red shifts associated with conformational relaxation may strongly reduce the overlap for resonant energy transfer to other nearby chromophores. Westenhoff et al. found that transient absorption (TA) spectral dynamics for a conjugated polymer solution could only be adequately simulated when including a torsional relaxation component.12 Conjugated oligomers comprised of a single chromophore are ideally suited to isolating these intrinsic conformational dynamics because they cannot support inter- or intramolecular energy transfer in dilute solution. Indeed, theoretical13−17 and experimental18−25 examinations of conjugated oligomers confirm that conformational relaxation can produce the signature nonmirror image spectra. Torsional modes have a © 2016 American Chemical Society
Received: May 31, 2016 Accepted: August 2, 2016 Published: August 2, 2016 3307
DOI: 10.1021/acs.jpclett.6b01185 J. Phys. Chem. Lett. 2016, 7, 3307−3312
Letter
The Journal of Physical Chemistry Letters
chromophores. The spectra are well fit by the Franck−Condon model described by Clark et al.,34 which has previously been applied to polythiophene fluorescence spectra.30,34 By assuming that the vibronic structure arises from an electronic transition coupled to a single vibrational mode (e.g., CC stretch),15,35 the fluorescence spectra are modeled by a progression of Gaussian line shapes displaced by the phonon energy of the stretching mode according to
spectroscopy and DFT calculations to resolve the evolution of fluorescence spectra in a series of conjugated oligothiophenes. We find that fluorescence spectra rapidly evolve from a featureless mirror of absorption to having vibronic structure within a few picoseconds, which is driven by local torsional dynamics. We also observe a slower phase of fluorescence spectral evolution that likely reflects softer backbone arching coordinates, that is, changes in curvature along the polymer backbone. We find that the key fluorescence dynamics are retained in solid films, thus establishing a benchmark to evaluate energy- and charge-transfer regimes in conjugated polymer films and allowing us to quantify the degree of energytransfer enhancement prior to conformational relaxation. The steady-state absorption and fluorescence spectra of dilute solutions of the series of oligothiophenes ranging from 4 to 10 thiophene rings (denoted 4T−10T) with solubilizing side chains, along with the chemical structures, are shown in Figure 1. The absorption maxima progressively red -shift with increasing oligomer length (4T, 3.28 eV; 6T, 2.95 eV; 8T, 2.83 eV; 10T, 2.70 eV), as expected when π-orbitals become more delocalized. All absorption spectra are broad and unstructured, while the fluorescence is narrow with clear vibronic structure. The absence of energy transfer in dilute solution ensures that these spectra are intrinsic to the oligomer
I(ω) ∝ (ℏω)3 n(ω) ×
∑ m=0
Sm Γ(E0−0 − mEph , σ ) m!
(1)
where ℏω is the photon energy, n(ω) is the refractive index at optical frequency (ω) (assumed constant across the energy range), m is the vibronic transition index, and Eph is the phonon energy for a CC stretch, which was fixed at 0.17 eV.30,34,36 The remaining parameters are fit from the spectra; S is the Huang−Rhys factor, E0−0 is the position of the 0−0 transition (in eV), and Γ is a Gaussian line shape function with full width at half-maximum (fwhm), σ. These fitted parameters are listed in Table 1 and also included in Figure 2. From this, we can Table 1. Optimized Fitted Parameters Obtained from the Franck−Condon Model Applied to Steady-State Fluorescence of Oligothiophenes in Dilute Solution and Absorption Maxima Values from Steady-State Absorption oligomer length 4 6 8 10
Huang−Rhys factor 1.17 1.08 0.99 0.95
± ± ± ±
0.01 0.01 0.01 0.01
E0−0 position (eV) 2.73 2.42 2.29 2.25
± ± ± ±
9 7 1 1
× × × ×
10−4 10−4 10−3 10−3
Gaussian fwhm (meV) 160.9 161.2 171.2 169.8
± ± ± ±
1.3 1.0 1.8 1.8
absorption maximum (eV) 3.28 2.95 2.83 2.70
Figure 2. Time-dependent parameters extracted from the Franck− Condon fits of ultrafast fluorescence spectra of oligomers. (A) The Huang−Rhys factor, (B) the position of the 0−0 transition, and (C) the fwhm of the Gaussians.
confirm that the band gap energy, E0, obeys the expected 1/n scaling as a function of length (Supporting Information).37 Additionally, the Huang−Rhys factor decreases as the oligomer length increases, reflected in the comparatively higher intensity in the 0−0 vibronic peak and consistent with previous spectral parametrization of a series of α-oligothiophenes.38 In order to probe length-dependent relaxation dynamics, we performed transient-grating photoluminescence (TGPL) spec-
Figure 1. Normalized ultrafast fluorescence spectra of (A) 4T, (B) 6T, (C) 8T, and (D) 10T showing a spectral narrowing and the emergence of vibronic structure on the early picosecond time scale. Fluorescence spectra at each time point were fitted using the Franck− Condon model, with optimized fits overlaid (solid lines). Also overlaid are the steady-state optical absorption (black line) and fluorescence (gray line) spectra and chemical structures. 3308
DOI: 10.1021/acs.jpclett.6b01185 J. Phys. Chem. Lett. 2016, 7, 3307−3312
Letter
The Journal of Physical Chemistry Letters troscopy.39 The series of fitted broad-band ultrafast fluorescence spectra for each of the oligomers are shown in Figure 1. Immediately after photoexcitation (
