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.
Triplet formation and interactions with emissive singlet excitons are investigated in poly(3-hexylselenophene) (P3HS) using single molecule spectroscopy. P3HS is a heavy atom analog of the more commonly studied poly(3-hexylthiophene) (P3HT), a benchmark polymer for solar cells. P3HS tends to aggregate strongly which necessitates dilution to ultra-low levels within a solid inert host in order to resolve photophysical responses of single chains. Fluorescence excitation intensity modulation is performed on isolated P3HS chains using a sequence of rectangular pulses of varying intensities to probe the presence of spin-forbidden triplet excitons. Triplet population dynamics originating from singlet-triplet and triplet-triplet interactions appear as quenching of the initial fluorescence intensity to steady-state levels on characteristic time scales of ∼1-10 μs. Over 80% of all molecules studied display significant fluorescence intensity modulation (quenching depths >50%) indicative of efficient intersystem crossing and large triplet occupancies. Because triplets are highly localized and singlet-triplet and triplet-triplet annihilation rate constants are comparable to those of intersystem crossing, multiple triplets are present at any given time on single P3HS chains. Triplet lifetimes were estimated to be ∼4 μs (upper limit) determined from recovery to the ground electronic singlet state in the absence of light and, surprisingly, triplets vanish at the onset of P3HS aggregation. This result was unexpected since P3HS triplet formation takes place on time scales <30 ps making this process competitive with most accessible non-radiative deactivation pathways.
The pathways and dynamics of converting spin-allowed (S = 0) singlet excitons to spin-forbidden (S = 1) triplets have significant implications in determining performance metrics of conjugated polymers in optoelectronic devices. We study the effect of structural ordering factors on triplet formation in self-assembled aggregate π-stacked chains of poly(3-hexylthiophene) (P3HT) using single-molecule time-resolved intensity modulation and electric-field-dependent photoluminescence (PL) spectroscopy. Triplet generation is only efficient in P3HT aggregates of high purity, and formation yields are found to increase with nanofiber size. We propose that the high intrachain order in purified aggregates that extends exciton coherence lengths, leading to J-aggregate spectral signatures, is also important for populating interchain charge transfer (CT) states that, at longer times, recombine preferentially to triplets according to spin statistics. Electric-field-dependent PL decays of isolated P3HT aggregates show large modulation of a long-lived emitting state attributed to the delocalized intrachain exciton with substantial CT state admixture. Our results demonstrate the importance of dark CT states in mediating exciton relaxation and spin conversion processes that are usually obscured in conventional thin films by heterogeneity. We further demonstrate the utility of subtle structural factors for selecting photophysical outcomes by careful control of processing conditions.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.