Ultrafast Raman Spectroscopy as a Probe of Local Structure and


Sep 15, 2016 - Biography. Art Bragg received his Ph.D. from UC Berkeley, where he worked with Professor Daniel M. Neumark studying the electronic rela...
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Ultrafast Raman Spectroscopy as a Probe of Local Structure and Dynamics in Photoexcited Conjugated Materials Arthur E Bragg, Wenjian Yu, Jiawang Zhou, and Timothy J. Magnanelli J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01060 • Publication Date (Web): 15 Sep 2016 Downloaded from http://pubs.acs.org on September 16, 2016

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Ultrafast Raman Spectroscopy as a Probe of Local Structure and Dynamics in Photoexcited Conjugated Materials

Arthur E. Bragg,* Wenjian Yu, Jiawang Zhou, Timothy Magnanelli Department of Chemistry, Johns Hopkins University 3400 N. Charles St., Baltimore, MD 21218 *corresponding author: [email protected], 410-516-5616

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Abstract: An important challenge in the study of conjugated organic materials is to relate the properties of transient states underlying macroscopic material responses directly with intra- and intermolecular structure. We discuss recent efforts using the vibrational sensitivity of timeresolved Raman spectroscopy that have interrogated structural properties of transient excited and charge-separated states in conjugated oligomers and polymers in order to relate them to molecular conformation and material microstructure. We focus on our recent work on excitedstate Raman spectroscopy that provides mode-specific signatures of structural relaxation in oligo- and polythiophenes; examination of structural heterogeneities associated with exciton localization in different structural motifs of amorphous polymer; and interrogation of correlations between microstructure and properties of charge-separated states within polymer aggregates. Based on these and related work from other labs, we provide an outlook for further applying this method to elucidate relationships between structure, properties of transient states, and the photoresponses of conjugated materials.

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Conjugated polymers and oligomers have become a common semiconducting component in organic electronics, including photovoltaics1-3 and light-emitting diodes4,5 as well as organic field-effect transistors and other organic semiconductor devices.6,7 Interest in conjugated organic materials arises from their inherent processing advantages and flexibility relative to their solidstate, inorganic counterparts. However, these benefits come with a catch: the properties and responses of organic semiconducting materials are intimately linked with their structure and interactions at the molecular and intermolecular level; for light-responsive applications in particular, local structure determines the nature of transient electronic and charge-separated states and their dynamics that underlie material responses.3,8,9 An important challenge with lightresponsive conjugated organic materials is to relate the properties of transient states directly with local structure, such as molecular conformation and material microstructure or morphology. Transient electronic spectroscopies have been valuable for elucidating mechanisms and timescales for relaxation and kinetics of transient states,10-16 but provide limited explicit information on relationships with local structure or structural dynamics. On the other hand, steady-state vibrational spectroscopies have been used to establish valuable correlations between material structure and macroscopic material responses, but with limited insights about transient states that underlie these responses.17-19 This perspective discusses recent interest and efforts aimed at using the structural sensitivity of time-resolved Raman spectroscopy to interrogate transient excited and charge-separated states in conjugated organic materials in connection with their local molecular or material structure.

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Figure 1: Variation (or “dispersion”) in symmetric C=C stretching frequency of oligofurans (OF, Ref. 31), oligopyrroles (OP, Ref. 32), and oligothiophenes (OT, Ref. 30) with number of monomer units (n). Best-fit trend lines (vs. 1/n) are plotted for comparison, as are the C=C symmetric stretching frequencies for annealed and amorphous films of poly(3-hexylthiophene) (Refs. 17,19). The symmetric stretching mode is illustrated in the inset image of septithiophene (7T). Raman Spectroscopy and the Steady-State Characterization of Conjugated Materials Raman spectroscopy has a long history as a probe of conjugated oligomeric and polymeric materials, used initially for structural fingerprinting of conductive and conjugated polymers first synthesized in the 1980s and 90s.20-23 Raman cross-sections of many vibrational modes of conjugated oligomers and homo-polymers are unusually large due to the fact that these modes induce large changes in polarizability that is associated with significant electron delocalization of excited electronic states; under resonant conditions many of these modes are coincident with the structural displacement between nominally benzoidal ground- and quinoidal excited-state geometries (i.e. large electron-phonon coupling occurs along this structural coordinate).24,25 In particular the Raman spectra of conjugated homo-oligomers and polymers

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are dominated by symmetric C=C stretching, and to lesser degree C-C (intra- and inter-ring) and in-plane CH bending. As oxidized conjugated polymers and oligomers take on a delocalized, quinoidal character, steady-state Raman spectroscopy has been a valuable tool for interrogating charge doping of conjugated materials.21,26,27 In addition to resonant enhancements driven by strong electron-phonon coupling, the intensities and frequencies of Raman-active modes are strongly affected by the length of a conjugated chain.24,25,28-32 Figure 1 illustrates specifically that the symmetric C=C stretching frequencies of various conjugated oligomers shift with length, an effect termed Raman-frequency dispersion.24,29,32

Frequency dispersion can be recognized as a quantum-mechanical effect

associated with electron delocalization for two reasons: firstly, Raman frequency dispersion generally appears as a frequency decrease with oligomer length, whereas pure mechanical coupling between monomers to form a vibrational exciton would give rise to frequency increases with increasing oligomer length;33 secondly, the degree of dispersion with oligomer length is anticorrelated with monomer aromaticity (e.g. thiophene, phenyl, phenylene vinylene < pyrrole, furan << ethene),29 a common metric for electron localization. Effective conjugation coordinate (ECC) theory attributes this behavior to changes in the non-local vibrational coupling of monomer units that is facilitated by increased electron delocalization, giving rise to vibrational potentials that soften with oligomer length.24,25,29 This relationship implies that frequencies of some Raman active modes (notably, C=C symmetric stretches) could serve as a metric of effective delocalization in pi-conjugated polymers. Unfortunately, common semiconducting polymers (such as polythiophenes (PTs) and polyphenylenevinylenes (PPVs)) exhibit high aromaticities in their ground states that limit the extent to which delocalization impacts vibrational frequencies, and therefore the degree to which

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structural disorder impacts these frequencies. However, delocalization of the electron density can be anticipated to reduce the effective monomer aromaticity in an excited oligomer or localized excited state in a conjugated polymer. Hence, Raman spectroscopy can be expected to provide a sensitive metric for probing the relative delocalization of excited-states in disordered polymer materials. Material microstructure or morphology has also been shown to impact the steady-state Raman spectroscopy of conjugated materials. Seminal work by Gao and Grey17 in 2009 and Tsoi et al.19 in 2011 demonstrated correlated changes in the steady-state optical absorption and Raman spectra of the C=C/C-C stretching region between as-cast and annealed films of poly-(3hexylthiophene) (P3HT); both Raman and absorption spectra could be resolved into features associated broadly with either ordered, lamellar-stacked or disordered, amorphous polymer microstructures.34

Differences in the steady-state electronic spectroscopy of these

microstructures result from the formation of excitonic aggregates in stacked, but not amorphous phases.35

In contrast, differences in the Raman spectra of these phases can be traced to the

spectroscopy of oligothiophene films and solutions, which exhibit differences in average frequency and frequency dispersion with oligomer length.30 Using DFT calculations with rangecorrected (CAM-B3LYP) functionals we recovered the correct frequency dispersion trends for flat (planar, torsional dihedral of 0°) vs. minimum-energy (non-planar, dihedral of 21°) geometries of “gas-phase” oligomers.28 Although intermolecular interactions may contribute in some way to differences in Raman frequencies with material microstructure,30 quantum-chemical computations and mechanical modeling indicate that the differences in frequency and frequency dispersion for ordered (planar) and amorphous (non-planar) phases arise primarily from a change in the balance of mechanical and quantum-mechanical coupling with intramolecular geometries

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for each phase. Regardless of origin, this sensitivity of Raman spectroscopy of polymers to microstructure has proven highly valuable for correlating morphologies of P3HT:PCBM nanoaggregates and films with charge transfer efficiencies36 and photovoltaic performance.18 These effects could likewise provide a spectroscopic handle for correlating structural characteristics of transient states to the morphology of their local material environments. It is important to note that steady-state Raman spectroscopy can provide a valuable window into the dynamics of excited conjugated materials. This is because Raman spectra measured in resonance can be interpreted in terms of the evolution of excited-state vibrational wavepackets and their projections onto vibrational levels in the ground electronic state (through the so-called time-dependent theory of spectroscopy37); hence, Raman feature lineshapes and intensities reflect ultrafast vibrational coherences and quenching of these coherences through ultrafast processes in conjugated materials.38 For example, using the time-dependent theory of Raman spectroscopy, Gao and Grey have reported vibrational coherence in amorphous and crystalline P3HT and P3HT:PCBM on sub-100 fs timescales,38 which are generally much shorter than can be accessed through pump-probe type measurements. This connection between the frequency and time domains offers valuable insights on the role of vibrational coherence in energy and charge transfer in conjugated materials; however, it cannot report on properties and dynamics of transient species on longer timescales. Given that steady-state Raman spectroscopy of conjugated polymers and oligomers exhibit sensitivities to changes in charge distribution, electron delocalization, and microstructure or morphology, transient Raman spectroscopy holds promise for interrogating properties of transient excited and charge-separated states and their dependence on local structure. Below we describe its application as a probe of structure and structural dynamics of transient states, briefly

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discussing recent results from our own lab and to related work by others. Our work has focused on canonical conjugated oligomer and polymer materials (oligo- and polythiophenes) in order to develop and demonstrate these spectroscopic sensitivities. In a series of projects described below we have (a) examined the dynamics of conjugated oligomers in order to identify vibrational mode-specific signatures of conformational relaxation in conjugated materials,39,40 (b) used excited-state Raman signatures in combination with electronic spectroscopies (e.g. transient absorption and transient hole-burning) to interrogate structural heterogeneities associated with excited-states localized in different motifs along amorphous polymer chains,41,42 and (c) used Raman measurements to interrogate structural properties of charge-separated states within aggregated conjugated materials.43 Experimentally, our work takes advantage of femtosecond stimulated Raman spectroscopy (FSRS),44-47 a technique that has been developed into its own subfield over the last 15 years. FSRS utilizes a combination of a narrowband Raman-excitation pulse (<10-20 cm-1) and a broadband continuum probe to stimulate Raman transitions over a large frequency range (in all a coherent, (3) light-matter interaction). In the experiments described below, FSRS is used as a probe of transient states created via population dynamics that are initiated by sample photoexcitation (so, in total, time-resolved FSRS is a 3-pulse, (2)+(3) experiment). Not only is FSRS experimentally compatible with transient electronic spectroscopies that are also valuable for characterizing the spectroscopy and dynamics of conjugated materials, it enables higher time resolution than is available from (spontaneous) time-resolved Resonance Raman spectroscopy (TR3).48 A detailed discourse of FSRS is beyond the scope of this perspective; here we only give experimental details as necessary to understand our measurements and refer the reader to numerous extensive reviews of this technique44-47 as well as detailed descriptions of our

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experimental implementation and spectral analysis found in the original sources of the work described below.39-43

Vibrational Spectroscopy and Structural Dynamics of Excited Conjugated Oligomers As there is no reason a priori to expect that vibrational spectroscopy or its sensitivity to molecular conformation will be similar for ground and excited states in conjugated materials, it is valuable to understand the excited-state vibrational spectroscopy of oligomers that are amenable to both detailed experimental and computational studies. (This is similar to previous efforts to use oligomers to understand the optical spectroscopy and dynamics of polymers.49-51) We have investigated specifically the excited-state spectroscopy of quaterthiophenes – e.g. 2,2′:5′,2′′:5′′,2′′′-quaterthiophene

(4T)

and

3,3′′′-dihexyl-2,2′:5′,2′′:5′′,2′′′-quaterthiophene

(DH4T) – in order to establish how a transient Raman probe can illuminate excited-state structural dynamics in thiophene-based materials and, conversely, how Raman spectroscopy of these systems responds to changes in molecular conformation of excited states.39 Quantum-chemical structural calculations predict that the ground- and excited-state minima of these two compounds are non-planar and planar, respectively.39,52 The steady-state fluorescence Stokes shift of these oligomers can be taken as an indicator of a structural change induced by excitation (absorption maxima in chlorobenezene – 395 and 382 nm, respectively; fluorescence – 457 nm at 0-0 transition), and transient redshifting of stimulated emission would be a time-resolved signature of structural relaxation; the latter is observed to occur on timescales of 0.56 and 1.09 ps for 4T and DH4T in chlorobenzene, respectively.39,50 (The slower relaxation rate observed for DH4T would be interpreted as an impact of the bulky hexyl groups on both the initial molecular conformation and the evolution in average inter-ring torsional dihedral.)

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However, electronic spectroscopy provides limited, indirect insight on structural evolution in conjugated systems, whereas time-resolved vibrational spectroscopy can serve as a more sensitive probe of ultrafast structural evolution. The time-resolved Raman spectrum of DH4T excited to its lowest excited state is presented in Figure 2(a). In our experiments both oligomers were excited through their lowest-energy transitions and FSRS spectra were measured pre-resonantly via the lowest-energy absorption transition (peaked at 760 nm) for the lowestlying singlet excited state (S1). Three sizable Raman features appear (Fig. 2(a)) and are attributed to thiophene ring deformation (680 cm-1), intraring C=C in-phase stretching (1420 cm-1) and C=C out-of-phase stretching (1520 cm-1) based on comparisons with calculated excited-state vibrations in 4T. Figures 2(a) and 2(b) illustrate that the frequency of the out-of-phase mode exhibits a noticeable time-dependent blue shift (6.2 and 11.5 cm-1 for 4T and DH4T respectively), whereas the peak positions of other modes remain unchanged. The frequency of the out-of-phase mode shifts on timescales of 0.40 and 0.86 ps for 4T and DH4T, respectively, when fitted with a single-exponential function; these are similar to the timescales from shifts in stimulated emission noted above, and indicate that evolution of this frequency is a vibrational probe of the same underlying dynamics.

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Figure 2: Time-resolved Raman spectroscopy as a probe of relaxation dynamics in photoexcited quaterthiophenes. (a) Transient Raman spectra of 3,3′′′-dihexyl-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (DH4T). (b) Time-dependence in the peak frequency of the out-of-phase C=C stretch for DH4T and 2,2′:5′,2′′:5′′,2′′′-quaterthiophene (4T). (c) Out-of-phase and in-phase C=C stretching frequencies computed with bithiophene at various interring dihedral angles, illustrating greater sensitivity of the former to conformational relaxation; 0° corresponds with a planar, “trans-S” structure. (Adapted from Ref. 36 with author permission.) To relate spectral dynamics with structural evolution, we undertook a series of calculations of the frequencies of these two excited-state modes as a function of interring

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torsional dihedral (0 to 40°, where 0° corresponds with a planar, “trans S-S” structure) in a bithiophene model. Results from these calculations, summarized in Figure 2(c), demonstrate that out-of-phase C=C stretching frequency increases as the torsional dihedral decreases (i.e. the molecular geometry approaches planar), whereas the in-phase stretch is predicted to be perturbed only weakly by this change in molecular conformation; in contrast, ground-state frequencies are predicted to red-shift as the structure planarizes.19,28 The reasons for this mode-specific frequency blueshift in terms of changes in delocalization or bond order with evolution in molecular conformation are not fully understood.

The spectral blue-shift also matches

expectations for vibrational cooling within an anharmonic potential,53 yet it is likewise unclear why only this mode might exhibit this behavior. Interestingly, we also observed that the intensities of the in-phase and out-of-phase modes increase and decrease on similar timescales as the frequency shifts observed for each system. Evolution in feature intensities may reflect changes in the Raman cross-sections or energy transfer between these modes with evolution in structure or vibrational excitation, or both; deconvolving these effects will require more extensive theoretical evaluation of excited-state Raman-active vibrations and their cross-sections (including resonant enhancements). Nonetheless, spectroscopy of oligomers demonstrates the potential of time-resolved Raman spectroscopy as a vibrational mode-specific probe of molecular conformation in conjugated materials, and also illustrates that it is not sensible to interpret time-dependence of Raman features of excited states based on correlations between vibrational frequencies and structure in the ground state. In recent work Scopigno and coworkers used FSRS more specifically to interrogate mechanisms associated with ultrafast excited-state electronic relaxation in the conjugated dimer 2-methylphenyl thiophene (2-MPT).54 Previous TAS experiments by Elles uncovered rapid

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intersystem crossing (ISC) in phenylthiophenes following excitation to the lowest optically accessible singlet state, the rate of which was found to vary strongly with the relative nonplanarity of phenylthiophene.55 FSRS measurements with 2-MPT resonant with either the stimulated emission from the singlet excited state or the transient absorption of singlet and triplet transient absorbance revealed signatures associated with structural and/or vibrational relaxation in the excited singlet state on 1.5 and 10 ps timescales, with ISC occurring much more slowly (135 ps). The authors characterized the singlet and triplet electronic surfaces along the inter-ring dihedral and C-S bond-length coordinates via quantum-chemical computations; using these surfaces they carried out semiclassical trajectories and evaluated the evolution of vibrational frequencies for key modes to compare with experimental results. Computational results revealed two possible pathways into the triplet manifold, one of which involves relaxation to a S1 minimum characterized by a significantly elongated C-S bond.52 Yet the evolution of calculated vibrational frequencies predicted for S1 2MPT as it takes on this minimum-energy structure are incompatible with experimental observations, suggesting that ISC predominantly involves relaxation via a S1-T2 conical intersection preceded by vibrational cooling. These results demonstrate the power of combining the structural sensitivity of FSRS with computational interrogation of excited-state surfaces in order to deduce excited-state relaxation mechanisms. Oligomers are also valuable systems for exploring localization of charge and spin densities in conjugated systems, with Raman providing a probe of associated structural deformations. For example, Tauber and coworkers recently reported a study of the resonance Raman spectra of the lowest triplet (T1) excited states of oligothiophenes56 that are important long-lived states in organic light-emitting diodes (OLEDs),4,5 organic photovoltaics (OPVs)57,58 and singlet-fissible materials.59,60 Comparisons of the experimental and calculated Raman spectra

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of T1 and S0 states of oligothiophenes reflect greater variation in the inter-ring bonding pattern between adjacent thiophene rings in triplet states. This was attributed to a disruption in the regular bonding pattern that is associated with localized quinoidal regions and distributed unpaired electron density in T1. Furthermore, disparities between the T1 spectra of the tetramer (3,3′′′-Didodecyl-2,2′:5′,2′′:5′′,2′′′-quaterthiophene)

and

hexamer

(3′,4′,3′′′′,4′′′′-Dihexyl-

2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′:5′′′′,2′′′′′-sexithiophene) indicate that the quaterthiophene is too short to approach the scale of the T1 electron distribution of a long oligomer or polymer.

Raman Spectroscopy as a Probe of Structure, Dynamics and Disorder of Excited States in Amorphous Polymers Structural disorder of conjugated polymers in amorphous environments (e.g. solutions and films lacking intermolecular order) presents an added challenge for relating the properties and dynamics of transient states to material structure. The origins of structural disorder (or heterogeneity) can be appreciated by considering the scale of an average polymer chain: A typical molecular weight for a polymer may be well in excess of 50,000 Daltons; for a polymer such as poly(3-hexylthiophene), this would correspond with a chain of >300 monomer rings. Small-angle neutron scattering experiments have shown that polymer chains exhibit persistence lengths of a few nanometers (~several monomer units) that are associated with structural deformations (i.e. bending) of the polymer backbone.61 Given this disorder, local structural motifs along the polymer chain that are comparable in length to a conjugated oligomer are thought to facilitate trapping of transient states.62-64

Hence, photoexcitation of amorphous

conjugated polymers will involve structural features and dynamics associated with exciton trapping, and in general exciton dynamics should be correlated with the properties of their local

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structure.13 Experimental techniques with vibrational sensitivity have promise for interrogating local structure and structural dynamics associated with trapping and relaxation of excited and charge-separated states in motifs along the extended macromolecular structure.

Figure 3. Time-resolved absorption spectra of regio-regular poly(3-hexylthiophene). Raman excitation wavelengths used to interrogate structural dynamics and heterogeneity underlying the near-IR singlet excited-state absorption band are shown. (Adapted from Ref. 37 with author permission.)

In our work we have investigated how transient Raman spectroscopy can complement transient electronic spectroscopies for interrogating local structural evolution associated with exciton dynamics as well as spectroscopic manifestations of structural heterogeneity for photoexcited poly-(3-hexylthiophene) (P3HT) and other polythiophenes in solution. Previous studies utilizing transient electronic spectroscopies (e.g. transient absorption,65 stimulated emission,12 and fluorescence11,66) have revealed multi-phasic relaxation of polythiophene excited states occurring on various timescales that are ascribed to exciton localization or trapping (~100 fs), localized nuclear reorganization or excitonic energy transfer (observed as a spectral redshift on ~1-10s of ps, Figure 3), and singlet-exciton decay (observed as overall intensity decay on a

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400-500 ps lifetime, Figure 3). More recent exploration of polymer photophysics has utilized novel higher-order spectroscopies to gain deeper insights on the energetic relaxation of singlet excitons;12,67 results from these experiments suggest that relaxation in P3HT is dominated by local structural relaxation associated largely with evolution in inter-ring torsional relaxation. An initial study in our lab was the first to use time-resolved Raman spectroscopy to interrogate singlet-exciton spectroscopy and dynamics in polymers, with a focus on the structural evolution that underlies relaxation of transient electronic spectra that occurs on picosecond timescales.40 Resonant enhancement via the singlet-exciton absorption band in the near infrared (far from the polymer’s ground-state absorption below 500 nm) was used to ensure large signal contrast between excited- and ground-state Raman features. Time-resolved spectra are shown in Figure 4. The nominal local-mode assignments of excited-state Raman features were made through comparisons with the assigned Raman spectra of ground-state and doped P3HT (referenced to the dimer structure shown in the inset). All excited-state features were observed to diminish with electronic decay of the singlet exciton through inter-system crossing (ISC) and radiative decay. However, the time-dependent intensities of these features were observed to decay with different amplitudes on a 9-ps timescale, reflecting mode-specific changes to resonant enhancements as a result of excited-state relaxation.

These enhancements are

determined in part by Franck-Condon overlaps of the photoprepared nuclear wave-packet in the excited electronic state and vibrational states of the higher-lying state involved in the resonant Raman transition;68 thus, mode-specific changes in enhancement reflects the degree to which the displacements between these potential surfaces along each Raman active coordinate evolves through local conformational relaxation.

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Figure 4. Time-resolved Raman spectra of regio-regular (RR) poly(3-hexylthiophene) in chlorobenzene. Vibrational assignments are based on comparisons to ground-state and oxidized polymer, with local mode character referenced to the dimer structure in the inset. (Adapted from Ref. 37 with author permission.)

Previous work with transient electronic spectroscopies has shed little light on correlations between local structure and photophysical properties of transient states in polymers, as these methods generally have had limited ability to expose structural heterogeneities within an ensemble of photoexcited polymer. The Raman frequency dispersion of conjugated oligomers as a function of length (Figure 1) indicates that the C=C stretching frequency could be used as a probe of effective conjugation length within a heterogeneous ensemble of excited polymer; this is because the local monomer aromaticity should be reduced in the excited relative to the ground state. As the effective conjugated length should likewise correlate with electronic absorption and emission, resonant Raman excitation as a function of wavelength could be used to probe structural heterogeneities that underlie broad electronic responses.

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Figure 5. Raman spectra of excited RR-P3HT singlets measured with various Raman-excitation wavelengths 1 ps after photoexcitation. Inset: Time-dependent peak position measured at each Raman-excitation wavelength. Variation in C=C peak frequency reflects structural variation underlying the inhomogeneous breadth of the exciton absorption band. (Adapted from Ref. 38 with author permission.) Figure 5 shows the stimulated Raman spectra of photoexcited RR-P3HT in the C=C/CC stretching region collected at various Raman excitation wavelengths at a fixed delay (1 ps) after photoexcitation.42 The most intense feature near 1470 cm-1 corresponds with the C=C symmetric stretch and exhibits a nearly 80 cm-1 redshift in peak position as the Raman-excitation pulse is tuned from 860 to 950 nm. The magnitude of this shift is comparable to shifts observed with weakly aromatic oligomers (e.g. oligoenes) for a change in conjugation length of only a few monomer units and is consistent with a lower monomer aromaticity in the polymer’s excited state. The peak position of the C=C symmetric stretch exhibits no correlated trends with time and is essentially constant, as can be expected based on our studies with quaterthiophenes. In total this illustrates that singlet excitons probed via lower Raman-excitation energies have greater effective conjugation than those probed at higher energies; notably, only weak shifts (<10 cm-1) are observed by tuning Raman-excitation energy across the breadth of the ground-state

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absorption band, consistent with the frequency dispersion behaviors of oligothiophenes.17,19 We have recently demonstrated that the observed frequency dispersion for excited polymer is supported by correlations in electronic spectroscopy (absorption vs. stimulated emission) and timescales for conformational fluctuations as interrogated with transient hole burning of an ensemble of P3HT singlet excitons.42 Notably, energies of exciton absorption and emission are positively correlated; conformational fluctuations from motifs with longer to motifs with shorter effective conjugation lengths occur more slowly than the reverse, which is consistent with nonequilibrium exciton relaxation towards greater overall conjugation. We have observed similar correlations between structure, electronic spectroscopy, and electronic relaxation dynamics of singlet excitons of poly(3-cyclohexyl,4-methylthiophene) (PCMT).41 Calculations of the ground-state geometries for the corresponding tetramer reveal that the 3,4 di-substitution of the monomer gives rise to a ring-to-ring dihedral angle of nearly 90 degrees; consequently, the ground-state absorption of PCMT falls in the near UV (peak ~320 nm) and the C=C stretching frequency is higher than that of amorphous P3HT. TAS shows that relaxation of the singlet exciton is dominated by efficient ISC, as reflected by the appearance of a broad absorption characteristic of triplet oligothiophenes on a 26-ps rise-time. Such fast ISC dynamics are characteristic of short or non-planar oligothiophenes.55,69,70 The Raman spectra of the singlet and triplet excitons of PCMT can be examined directly by applying a FSRS probe: Raman excitation at 840 nm (i.e. on the red edge of the singlet absorption) selectively interrogates the singlet and its decay predominantly via fast ISC, whereas 600 nm is resonant with both the singlet and triplet absorption bands and can directly trace interconversion of singlet and triplet vibrational features. Time-resolved Raman spectra of the C=C/C-C stretching region reveals that the singlet C=C stretch has a higher frequency when

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probed with the higher Raman-excitation energy (1490 vs. 1475 cm-1); this is similar to our observations with P3HT and reflects a preferential probing of different effective conjugation lengths or local conformations that underlie the breadth of the singlet (electronic) absorption band. There is also a sizable discrepancy in the singlet lifetime as determined from the evolution in the time-resolved Raman spectra collected in resonance at the red vs. blue edges of the singlet absorption band (39 vs. 18 ps, respectively, based on the lifetime of the singlet vibrational features). The singlet lifetime is predominantly determined by ultrafast ISC, which is known to be sensitive to conjugation length and conformation in thiophene-based systems.55,69,70 Hence, this discrepancy in lifetime further supports that wavelength-selective interrogation of the PCMT singlet exciton is sensitive to structural heterogeneity of the polymer that underlies the breadth of the excited-state absorption band. In addition to our work Iwata and Takaya recently developed a FSRS instrument that enables Raman interrogation either preresonantly or via resonance enhancement on the red edge of transient electronic absorption features in the near infrared, which they have used to interrogate relaxation of poly(3-dodecylthiophene) (P3DDT) dissolved in toluene.71 The spectral features obtained are qualitatively similar to those obtained with P3HT (Figure 4) and PCMT, with key differences that can be attributed to a more desirable resonance condition in the nearIR, less influence from broad background signal subtractions, and interrogation of more structural ordered regions (longer conjugation lengths) of the amorphous polymer chains in solution using a lower Raman-excitation energy. These authors note a slow (130 ps) red-shift in the C=C stretching frequency that they attribute to a slow extension of the conjugation length following excitation. The magnitude of this shift (20 cm-1) is comparable with the variation in

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C=C frequency observed for P3HT with variation in Raman excitation energy and that is associated with heterogeneity in the distribution of exciton conjugation lengths (Figure 5). In addition to efforts to use FSRS to explore the link between local structure and vibrational spectroscopy of localized polymer excitons, Fazzi, Soci, and coworkers recently published a comprehensive computational and experimental study identified differences in the excited-state vibrational spectroscopy of RRa and RR-P3HT polymer films that are associated with different degrees of structural disorder.72

Specifically, these authors determined that

infrared-active (not Raman-active) C-S stretches, as well as Raman-active C=C stretches, may be sensitive markers in the excited-state vibrational spectroscopy of polythiophenes that could be used to assess relative structural order and structure-properties correlations within these materials.

Microstructure and Charge Separation in Aggregated Polymer Materials Conjugated materials as used in device applications are aggregated or assembled; this raises the question about how local intermolecular interactions impact the nature of transient and charge-separated states in these materials.

To date this has been addressed largely by

interrogating the transient electronic spectroscopies of controlled preparations of specific polymer microstructures or material morphologies.65,73,74 Vibrational probes of transient electronic and charge distributions in material environments promise further insights about how the properties of transient states are linked with local material structure.75 In our work we have applied time-resolved Raman spectroscopy to interrogate molecular packing motifs and charge distributions associated with transient charge-separated states in polymer aggregates, specifically probing the dynamics of polaron-pair formation in nanoparticle

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aggregates of P3HT. Like P3HT films prepared for device applications through spin-coating, nanoparticles prepared by polymer reprecipitation (50-200 nm in size) have mixed microstructure that can be recognized readily from their steady-state absorption and Raman spectra.76,77 Additionally, Clafton and Kee demonstrated that nanoparticles and films have qualitatively similar photoinduced dynamics.78

Given these photophysical similarities with

films, suspensions of nanoparticles are convenient samples for multi-pulse laser experiments (such as time-resolved FSRS), as they have increased optical density compared to films and are highly robust to interactions with laser pulses with high peak powers.

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Figure 6. Transient electronic (a) and Raman (b) spectra associated with polaron-pair formation in reprecipitated P3HT nanoaggregates. (c) Comparison of the ground-state Raman (GSR) and pure-transient Raman (PTR) spectra of the polymer aggregate and polaron pairs, respectively. (Adapted from Ref. 40 with author permission.)

The formation of transient polaron pairs (putative local charge pairs) has been associated with the appearance of a transient absorbance band at 650 nm following excitation of polymer films.

Guo et al demonstrated that this absorption feature increases super-linearly with

excitation fluence, and thus postulated that the underlying species can be formed through

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biexcitonic annihilation at high excitation densities.65 A mechanism that involves biexciton annihilation to create a single local charge-separated pair is supported by observation of a partial ground-state bleach recovery (signifying ground-state recovery of one of the two excitons) that is correlated with the appearance of transient absorption in the red; transient absorption spectroscopy of P3HT nanoparticles following high-fluence excitation is presented in Figure 6(a). A more recent study at low fluences and high time resolution has suggested that polaron pairs may form directly and coherently upon polymer excitation; thus formation of local chargeseparated pairs would represent a transient trapping mechanism for excitons in aggregated materials.79 Unfortunately, it is not straightforward to ascribe the electronic absorption transient directly to specific structural characteristics or charge separation within the material. In order to relate its formation/presence to local morphology and charge distribution we have used timeresolved FSRS pre-resonant with the putative polaron-pair absorption (Raman excitation at 643 nm) to interrogate vibrational characteristics of this short-lived transient.43 Figure 6(b) presents the time-dependent Raman spectroscopy of P3HT nanoparticles in the C-C/C=C stretching region (1330-1490 cm-1).

Time-dependent spectra exhibit features associated with stacked

regions of the ground-state polymer before sample photoexcitation (delays < 0 ps). These features are depleted upon photoexcitation at 500 nm (0 ps), with a delayed shift of the C-C and C=C features at slightly lower frequencies; both features subsequently shift back to the frequencies of the ground-state polymer with a concomitant intensity recovery on a timescale of a few picoseconds. Importantly the evolution in the Raman depletion and shift are correlated with the kinetics of the bleach in ground-state electronic absorption (<600 nm) and absorption of the putative polaron pair peaked at 635-650 nm, respectively. Hence both time-resolved Raman

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and electronic spectroscopies probe the same photoinduced kinetics of the sample, yet the vibrational sensitivity of the former offers greater insights about the local structure and charge distribution than the latter. Pure transient Raman features associated with the transient polaron-pair can be isolated through spectral decomposition of transient and (excitation) fluence-dependent Raman spectra. Comparison of the two component spectra, as plotted in Figure 6(c) reveals a few important conclusions: Firstly, spectral analysis reveals a ~12 cm-1 downshift in C=C/C-C stretches, which is consistent with experimental21,26,27,72 and computational72 spectra of charge-doped polymer. Secondly, the disappearance of the red-shifted Raman features attributable to putative polaron pairs is correlated with recovery of Raman features from ordered (stacked) phases of polymer. Thirdly, only modest changes are observed in the positions and intensities of other features in the Raman spectra. Together these results support that these species exhibit charge-pair character and are formed (or trapped) within ordered regions of aggregated polymer. These findings are consistent with observations that charge-pair species are metastable on highly ordered polymer stacks (H- and J-aggregates) prepared in solution.80 Increasing disorder in polymer stacking would be predicted to eliminate polaron pair formation pathways, although this comes at the price of reduced energy transfer rates and charge mobilities through polymer aggregates. Recent work from other labs has illustrated further the power of interrogating the vibrational spectroscopy of transient excited and charged states of polymer materials to probe exciton migration and charge separation at donor-acceptor interfaces that underlie photovoltaic performance and relate these processes to the local structure and dynamics of the photoactive polymer. For example, Provencher et al. used FSRS to capture unambiguous spectroscopic signatures of charge separation dynamics at donor-acceptor interfaces in bulk heterojunctions of

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the ‘push-pull’ polymer PCDTBT and the canonical electron acceptor phenyl-C61-butyric acid methyl ester (PCBM).81 These authors found that direct donor-to-acceptor electron transfer occurs faster than exciton relaxation in PCDTBT (within 300 fs vs. 3 ps, respectively), with no evidence found for charge-transfer intermediates. Secondly, the vibrational spectra of polarons formed did not exhibit spectral dynamics on timescales up to 100 ps after photoexcitation, which they interpret as a sign that the photogenerated hole is free from significant Coulombic interaction with the charge pair. In studies with bulk heterojunction of poly[2-methoxy-5-(3’,7’dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) mixed with PCBM, Mathies and coworkers isolated differences in the Raman spectroscopy of MDMO-PPV within pure-polymer and interfacial regions with the electron acceptor.82

Using the differences in vibrational

spectroscopy ascribed to different local structures in these regions, the authors were able to show that excitons diffuse to the donor-acceptor interface prior to energetic relaxation from the Franck-Condon region, giving rise to an increase in interfacial signal on a timescale of ~300 fs. Their findings indicate that facile exciton transport to an interface must occur prior to local relaxation in the bulk, pointing to the need for the design of donor polymers that have slow relaxation or small reorganization energy to improve the efficiencies for charge separation.

Conclusions and Perspectives on the Future Directions As outlined above, transient Raman (and vibrational spectroscopy in general) has great potential for interrogating the properties and dynamics of excited conjugated polymeric and oligomeric materials – not only through anticipated sensitivities to molecular conformation and structure, but also to electron delocalization and intermolecular packing.

Progress in its

application to conjugated polymeric and oligomeric materials will require further experimental

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and computational characterization of the vibrational signatures of excited states, both in small oligomers, which can be understood in great detail, and in local excited states in conjugated polymers. Insights on how intramolecular structure impacts the spectroscopy and properties of excited-states can be gained from studies with linear and cyclic oligomers that introduce or incorporate well-defined structural defects or constraints that can impact local structural characteristics.83 Such motifs could also facilitate the study of charge separation of protoexcitonic or polaron-pair states.79,84 A significant technical challenge for applying vibrational probes of structural relaxation is the direct characterization of ultrafast structural dynamics associated with localization of excited states in conjugated polymers.

The timescales associated with these processes are

expected to be considerably faster than typical vibrational dephasing timescales,45 such that spectroscopic signatures of this process remain elusive. We and others have observed vibrational dynamics (e.g. peak shifts and changes in feature bandwidths) on timescales that are thought to be relevant for exciton localization,41,85 but it is still not clear how to attribute these to specific structural dynamics associated with exciton trapping along polymer chains. Additional challenges are related to developing approaches for selectively interrogating specific morphologies within device-relevant materials (e.g. organic photovoltaics). Various solution-phase methods have been developed recently for precisely controlling polymer aggregation, and interrogating controlled materials with the structural sensitivity of vibrational spectroscopy provides a means for exploring how the characteristics of these domains determines properties and dynamics of excitonic and charge-separated states in aggregated or assembled materials. Recently Dasgupta and coworkers have demonstrated that the Ag symmetric C=C stretch of the electron acceptor PCBM may also provide clues about local material environments

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that could be exploited further in time-resolved measurements.36 However, a challenge for using transient Raman (or vibrational) spectroscopy to probe true material and device architectures is to develop in situ methods for probing material morphologies, such as time-resolved Raman imaging with spatial resolution adequate to differentiate between polymer and electron acceptor domains on a scale of nanometers. Recent progress in this direction has taken cues from the stimulated-emission depletion (STED) fluorescence imaging community to improve spatial resolution of FSRS.86 Further improvements in this area will enable direct mapping between transient states and material morphology, and such efforts will be facilitated by insights gained from pure spectroscopic studies of these conjugated materials similar to those described herein.

Acknowledgements W. Y. and J. Z. are grateful for support from the Kilpatrick and Langmuir-Cresap fellowships. A. B. gratefully acknowledges start-up support from JHU.

Biographies Art Bragg received his Ph.D. from UC Berkeley, where he worked with Professor Daniel M. Neumark studying the electronic relaxation dynamics of molecular and cluster anions with timeresolved photoelectron imaging. He was a Post-Doctoral fellow with Professor Benjamin J. Schwartz at UCLA, where he investigated solvent-mediated ultrafast charge transfer dynamics in solution. In 2010 he joined the Chemistry faculty at Johns Hopkins, where he has established a research program focused on elucidating ultrafast dynamics of conjugated molecular materials with time-resolved electronic and Raman spectroscopies. Wenjian Yu obtained his B.S. in Chemistry from Fudan University in 2010. Currently he is a Ph.D. student in the Bragg lab at Johns Hopkins University. His research focuses on interrogating structure and dynamics in conjugated polymers with combinations of femtosecond stimulated Raman and transient electronic spectroscopies (time-resolved absorption and holeburning). Jiawang Zhou obtained his B.S. in Chemical Physics from the University of Science and Technology of China in 2010. Currently he is a Ph.D. student in the Bragg lab at Johns Hopkins

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University. His research focuses on interrogation of photoresponsive materials (e.g.  and hybrid - conjugated oligomers, polymers, and photoswitches) by means of femtosecond stimulated Raman and transient absorption spectroscopies. Timothy Magnanelli received his undergraduate degrees in Chemistry and Mathematics from University of Maryland Baltimore County in 2009. He joined the Bragg lab in 2011. His research focuses on studying structural dependencies of charge separation and energy transfer in aggregated and assembled conjugated organic materials using various ultrafast spectroscopic techniques.

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Table of Contents Image 52x52mm (300 x 300 DPI)

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Figure 1: Variation (or “dispersion”) in symmetric C=C stretching frequency of oligofurans (OF, Ref. 31), oligopyrroles (OP, Ref. 32), and oligothiophenes (OT, Ref. 30) with number of monomer units (n). Best-fit trend lines (vs. 1/n) are plotted for comparison, as are the C=C symmetric stretching frequencies for annealed and amorphous films of poly(3-hexylthiophene) (Refs. 17,19). The symmetric stretching mode is illustrated in the inset image of septithiophene (7T). Figure 1 760x604mm (96 x 96 DPI)

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Figure 2: Time-resolved Raman spectroscopy as a probe of relaxation dynamics in photoexcited quaterthiophenes. (a) Transient Raman spectra of 3,3′′′-dihexyl-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (DH4T). (b) Time-dependence in the peak frequency of the out-of-phase C=C stretch for DH4T and 2,2′:5′,2′′:5′′,2′′′-quaterthiophene (4T). (c) Out-of-phase and in-phase C=C stretching frequencies computed with bithiophene at various interring dihedral angles, illustrating greater sensitivity of the former to conformational relaxation; 0° corresponds with a planar, “trans-S” structure. (Adapted from Ref. 36 with author permission.) Figure 2 224x535mm (96 x 96 DPI)

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Figure 3. Time-resolved absorption spectra of regio-regular poly(3-hexylthiophene). Raman excitation wavelengths used to interrogate structural dynamics and heterogeneity underlying the near-IR singlet excited-state absorption band are shown. (Adapted from Ref. 37 with author permission.) Figure 3 660x475mm (150 x 150 DPI)

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Figure 4. Time-resolved Raman spectra of regio-regular (RR) poly(3-hexylthiophene) in chlorobenzene. Vibrational assignments are based on comparisons to ground-state and oxidized polymer, with local mode character referenced to the dimer structure in the inset. (Adapted from Ref. 37 with author permission.) Figure 4 189x129mm (96 x 96 DPI)

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Figure 5. Raman spectra of excited RR-P3HT singlets measured with various Raman-excitation wavelengths 1 ps after photoexcitation. Inset: Time-dependent peak position measured at each Raman-excitation wavelength. Variation in C=C peak frequency reflects structural variation underlying the inhomogeneous breadth of the exciton absorption band. (Adapted from Ref. 38 with author permission.) Figure 5 660x523mm (150 x 150 DPI)

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Figure 6. Transient electronic (a) and Raman (b) spectra associated with polaron-pair formation in reprecipitated P3HT nanoaggregates. (c) Comparison of the ground-state Raman (GSR) and pure-transient Raman (PTR) spectra of the polymer aggregate and polaron pairs, respectively. (Adapted from Ref. 40 with author permission.) Figure 6 78x146mm (300 x 300 DPI)

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