Resolving Anomalous Heavy Atom Effects from Discrete Triplet Mediated Photochemistry Events on Single Conjugated Polymer Chains

Abstract

Spin-forbidden triplet excited states of conjugated polymers have important ramifications for material performance and stability, yet triplet processes are difficult to understand and control at the bulk material level. We investigate the effect of a heavy heteroatom and chain conformation on triplet-mediated oxygen photochemistry events in poly(3-hexylthiophene) (P3HT) and poly(3-hexylselenophene) (P3HS) systems using high throughput single molecule spectroscopic imaging. Fluorescence intensity transients of both polymers exhibit discrete intermittency behavior (blinking) characteristic of collapsed conformations and efficient energy funneling. Although both systems have similar molecular weights (∼30 kDa), P3HS transients show unexpectedly longer average "on" times and larger average intensities that we attribute to shorter-lived triplets and faster ground electronic state recycling than in P3HT counterparts. This lowers the probability of sensitizing reactive oxygen species and residence times in "off" states. We use detailed statistical modeling incorporating a hidden two-state Markov chain with a transient bleach state to simulate irreversible photobleaching. Statistical distributions of "on" and "off" time distributions from simulated fluorescence intensity transients are in excellent agreement with experiment, consistent with lower average triplet occupancies in P3HS. Our findings offer new molecular-level insights of heavy atom effects on triplet occupancies and discrete photochemistry events that are difficult to resolve at the ensemble level because of averaging over all conformations and packing arrangements.

Triplet Population Dynamics of Single Conjugated Polymer Molecules and Nanoscale Assemblies

“…Comparing the expected triplet residence times to known diffusion behavior (or, encounter frequencies) of oxygen in polymer films indicates that P3HS chains are far less likely to sensitize reactive species. With experimental kinetic parameters, these behaviors were simulated in the limit of the three-level model using a hidden Markov chain approach (see Figure ) that shows good agreement with measured transients …”

“…Single molecule level studies of heavy atom effects in polymers are much rarer due to the diminished solubility of polymers containing large heteroatoms and, more importantly, drastically lower fluorescence yields. Datko et al studied isolated single chains of P3HT alongside those of poly­(3-hexylselenophene) (P3HS) of similar molecular weight on both short and long time scales to capture the full range of heavy atom-mediated triplet dynamics . Because triplets are inextricably tied to oxygen-related photochemistry, it is first useful to assess how typical triplet residence times impact photodegradation.…”

“…State occupancies are shown on the right axis of each graph represented by different levels accessed in the course of the transient (not to scale). Reproduced with permission from ref . Copyright 2018 American Chemical Society.…”

“…(c) Steady-state triplet populations ( P n (∞)) for each polymer. Reproduced with permission from ref . Copyright 2019 Springer Nature.…”

“…Datko et al studied isolated single chains of P3HT alongside those of poly(3-hexylselenophene) (P3HS) of similar molecular weight on both short and long time scales to capture the full range of heavy atom-mediated triplet dynamics. 56 Because triplets are inextricably tied to oxygenrelated photochemistry, it is first useful to assess how typical triplet residence times impact photodegradation. Fluorescence transients of single P3HS and P3HT chains dispersed in polystyrene were investigated while exposed to the ambient environment, and intermittency characteristics (i.e., "on/off" time distributions) can be used to compare the efficacies of triplet-induced oxygen sensitization causing longer time flickering behavior.…”