Unravelling the enigma of ultrafast excited state relaxation in non-emissive aggregating conjugated polymers

Abstract

We investigate a class of non-emissive conjugated polymers with very short excited state lifetimes believed to undergo singlet fission and relaxation to mid-gap forbidden excited states. Poly(3-decylthieneylenvinylene) (P3DTV) and its heavy atom analog, poly(3-decylseleneylenvinylene) (P3DSV), are strongly aggregating conjugated polymers that experience large excited state displacements along multiple vibrational modes. We demonstrate this Franck-Condon vibrational activity effectively disperses excitation energy into multiple non-radiative channels that can be explained using a simple, two-state potential energy surface model. Resonance Raman spectroscopy is sensitive to early Franck-Condon vibrational activity and we observe rich harmonic progressions involving multiple high frequency CC backbone symmetric stretching motions (∼1000-1600 cm-1) in both systems reflecting mode-specific excited state geometrical displacements. Transient absorption spectra confirm that efficient non-radiative processes dominate excited state relaxation dynamics which are confined to π-stacked aggregated chains. Surprisingly, we found little influence of the heteroatom consistent with efficient vibrational energy dissipation. Our results highlight the importance of aggregation and multi-dimensional Franck-Condon vibrational dynamics on the ability to harvest excitons, which are not usually considered in materials design and optimization schemes.

Charge Transfer Doping of Conjugated Polymers with Large Vibrational Activities: Insights into the Regime of Partial Charge Transfer

“…The partially resolved progression interval represents the coalescence of many displaced vibrational modes ranging from ∼500 to 1600 cm −1 involving CC and CH stretching and bending motions with the full details of the Franck−Condon vibrational composition described in detail earlier. 33,35 Absorption contributions from amorphous (unaggregated) chains appear at higher energy (∼0.5 eV from the aggregate electronic origin), much weaker and devoid of a resolved structure. Similar to P3HT, contributions from aggregated or unaggregated chains may be discerned by decomposing their respective lineshapes from the overall absorption lineshape.…”

“…Specifically, on-resonance excitation selects lesser ordered segments corresponding to either unaggregated chains or aggregates with low intra-chain order. 33,35 On the other hand, pre-resonant excitation conditions preferentially excite ordered, aggregated chain segments. 40 Asterisks denote new transitions arising from the presence of F 4 -TCNQ consistent with appreciable CT interactions, and their specific nature can be illuminated using variable excitation conditions.…”

Large Excited-State Conformational Displacements Expedite Triplet Formation in a Small Conjugated Oligomer

“…However, Hu et al demonstrated that fluorescence emission could be restored by dispersing a PTV derivative in a solid inert host, causing aggregates to dissociate . Our group also recently observed rich Franck–Condon vibrational dynamics in aggregating PTVs, which promotes efficient nonradiative relaxation . While a comprehensive photophysical model describing the interdependence on molecular structure is still lacking, minimizing complications from aggregation and heterogeneity effects is essential to obtain unambiguous views of photophysical pathways .…”

“…17 Our group also recently observed rich Franck− Condon vibrational dynamics in aggregating PTVs, which promotes efficient nonradiative relaxation. 18 on molecular structure is still lacking, minimizing complications from aggregation and heterogeneity effects is essential to obtain unambiguous views of photophysical pathways. 19 To this end, we demonstrate that dTV can serve as a model system for resolving vibrational contributions to intersystem crossing events in related molecules 20 and nonaggregating polymeric counterparts.…”

An excited state dynamics driven reaction: wavelength-dependent photoisomerization quantum yields in [Ru(bpy)2(dmso)2]2+

“…This approximation has led to the development of useful models to explain and predict the excited-state reactivity of many chromophores and photochromes. [1][2][3][4][5] Recently, new interpretations of bifurcating transition states as well as molecules with large excited-state nuclear displacements have emerged to challenge the notion that a single reaction coordinate trajectory dominated by thermodynamics is capable of adequately describing complex excited-state reactivity. 4,6,7 The origin of these changes in interpretation is found in reactions where a transition state is located near a stationary point (location on the potential energy surface where energy gradients with respect to the nuclear position are small or zero) or a conical intersection thus enabling non-equilibrium molecular dynamics to dictate reaction outcomes over thermodynamics.…”