Responsive Fluorophore Aggregation Provides Spectral Contrast for Fluorescence Lifetime Imaging.

Fluorophores experience altered emission lifetimes when incorporated into and liberated from macromolecules or molecular aggregates; this trend suggests the potential for a fluorescent, responsive probe capable of undergoing self-assembly and aggregation and consequently altering the lifetime of its fluorescent moiety to provide contrast between the active and inactive probes. We developed a cyanobenzothioazole-fluorescein conjugate (1), and spectroscopically examined the lifetime changes caused by its reduction-induced aggregation in vitro. A decrease in lifetime was observed for compound 1 in a buffered system activated by the biological reducing agent glutathione, thus suggesting a possible approach for designing responsive self-aggregating lifetime imaging probes.

Resolving population dynamics and interactions of multiple triplet excitons one molecule at a time.

Resolving the population dynamics of multiple triplet excitons on time scales comparable to their lifetimes is a key challenge for multiexciton harvesting strategies, such as singlet fission. We show that this information can be obtained from fluorescence quenching dynamics and stochastic kinetic modeling simulations of single nanoparticles comprising self-assembled aggregated chains of poly(3-hexylthiophene) (P3HT). These multichromophoric structures exhibit the elusive J-aggregate type excitonic coupling leading to delocalized intrachain excitons that undergo facile triplet formation mediated by interchain charge transfer states. We propose that P3HT J-aggregates can serve as a useful testbed for elucidating the presence of multiple triplets and understanding factors governing their interactions over a broad range of time scales. Stochastic kinetic modeling is then used to simulate discrete population dynamics and estimate higher order rate constants associated with triplet-triplet and singlet-triplet annihilation. Together with the quasi-CW nature of the experiment, the model reveals the expected amounts of triplets at equilibrium per molecule. Our approach is also amenable to a variety of other systems, e.g., singlet fission active molecular arrays, and can potentially inform design and optimization strategies to improve triplet harvesting yields.

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

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.

Effect of a heavy heteroatom on triplet formation and interactions in single conjugated polymer molecules and aggregates.

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.

Population dynamics of multiple triplet excitons revealed from time-dependent fluorescence quenching of single conjugated polymer chains.

The advent of multiple exciton harvesting schemes and prolonging exciton lifetimes to improve performance attributes of solar cells based on conjugated organic materials presents some interesting challenges that must be overcome in order to realize the full potential of these strategies. This is especially important for applications involving multi-chromophoric conjugated polymers where interactions between multiple spin-forbidden triplet excitons can be significant and are mediated by chain conformation. We use single molecule spectroscopic techniques to investigate interactions between multiple triplet excitons and emissive singlets by monitoring time-dependent fluorescence quenching on time scales commensurate with the triplet lifetime. Structurally related conjugated polymers differing by heteroatom substitution were targeted and we use a stochastic photodynamic model to numerically simulate the evolution of multi-exciton populations following photoexcitation. Single chains of poly(3-hexylthiophene) (P3HT) exhibit longer-lived triplet dynamics and larger steady-state triplet occupancies compared to those of poly(3-hexylselenophene) (P3HS), which has a larger reported triplet yield. Triplet populations evolve and relax much faster in P3HS which only becomes evident when considering all kinetic factors governing exciton population dynamics. Overall, we uncover new guidelines for effectively managing multi-exciton populations and interactions in conjugated polymers and improving their light harvesting efficiency.

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

Intersystem crossing in conjugated organic molecules is most conveniently viewed from pure electronic perspectives; yet, vibrational displacements may often drive these transitions. We investigate an alkyl-substituted thienylene-vinylene dimer (dTV) displaying efficient triplet formation. Steady-state electronic and Raman spectra display large Stokes shifts (∼4000 cm-1) involving high-frequency skeletal symmetric stretching modes (∼900-1600 cm-1) in addition to large displacements of low-frequency torsional motions (∼300-340 cm-1). Transient absorption spectroscopy reveals the emergence of distorted singlet (S1) and triplet signatures following initial vibrational relaxation dynamics that dominate spectral dynamics on time scales > 100 ps, with the latter persisting on time scales up to ca. 7 µs. Potential energy surfaces calculated along the dominant displaced out-of-plane torsional mode reveal shallow energy barriers for entering the triplet manifold from S1. We propose that dTV is a good model system for understanding vibrational contributions to intersystem crossing events in related polymer systems.