Low- and high-frequency vibrations synergistically enhance singlet exciton fission through robust vibronic resonances.

Singlet exciton fission (SEF) is initiated by ultrafast internal conversion of a singlet exciton into a correlated triplet pair [Formula: see text]. The "reaction coordinates" for ultrafast SEF even in archetypal systems such as pentacene thin film remain unclear. Couplings between fast electrons and slow nuclei are ubiquitous across a range of phenomena in chemistry. Accordingly, spectroscopic detection of vibrational coherences in the [Formula: see text] photoproduct motivated investigations into a possible role of vibronic coupling, akin to that reported in several photosynthetic proteins. However, acenes are very different from chlorophylls with 10× larger vibrational displacements upon photoexcitation and low-frequency vibrations modulating intermolecular orbital overlaps. Whether (and if so how) these unique features carry any mechanistic significance for SEF remains a poorly understood question. Accordingly, synthetic design of new molecules aiming to mimic this process across the solar spectrum has broadly relied on tuning electronic couplings. We address this gap and identify previously unrecognized synergistic interplay of vibrations, which in striking contrast to photosynthesis, vitally enhances SEF across a broad, nonselective and, therefore, unavoidable range of vibrational frequencies. We argue that attaching mechanistic significance to spectroscopically observed prominent quantum beats is misleading. Instead, we show that vibronic mixing leads to anisotropic quantum beats and propose readily implementable polarization-based two-dimensional electronic spectroscopy experiments which uniquely distinguish vibrations which drive vibronic mixing and promote SEF, against spectator vibrations simply accompanying ultrafast internal conversion. Our findings introduce crucial ingredients in synthetic design of SEF materials and spectroscopy experiments aiming to decipher mechanistic details from quantum beats.

High-sensitivity fluorescence-detected multidimensional electronic spectroscopy through continuous pump-probe delay scan.

Fluorescence-detected multidimensional electronic spectroscopy (fMES) promises high sensitivity compared to conventional approaches and is an emerging spectroscopic approach toward combining the advantages of MES with the spatial resolution of a microscope. Here, we present a visible white light continuum-based fMES spectrometer and systematically explore the sensitivity enhancement expected from fluorescence detection. As a demonstration of sensitivity, we report room temperature two-dimensional coherence maps of vibrational quantum coherences in a laser dye at optical densities of ∼2-3 orders of magnitude lower than conventional approaches. This high sensitivity is enabled by a combination of biased sampling along the optical coherence time axes and a rapid scan of the pump-probe waiting time T at each sample. A combination of this approach with acousto-optic phase modulation and phase-sensitive lock-in detection enables measurements of room temperature vibrational wavepackets even at the lowest ODs. Alternative faster data collection schemes, which are enabled by the flexibility of choosing a non-uniform undersampled grid in the continuous T scanning approach, are also demonstrated.

Vibronic resonance along effective modes mediates selective energy transfer in excitonically coupled aggregates.

We recently proposed effective normal modes for excitonically coupled aggregates that exactly transform the energy transfer Hamiltonian into a sum of one-dimensional Hamiltonians along the effective normal modes. Identifying physically meaningful vibrational motions that maximally promote vibronic mixing suggested an interesting possibility of leveraging vibrational-electronic resonance for mediating selective energy transfer. Here, we expand on the effective mode approach, elucidating its iterative nature for successively larger aggregates, and extend the idea of mediated energy transfer to larger aggregates. We show that energy transfer between electronically uncoupled but vibronically resonant donor-acceptor sites does not depend on the intermediate site energy or the number of intermediate sites. The intermediate sites simply mediate electronic coupling such that vibronic coupling along specific promoter modes leads to direct donor-acceptor energy transfer, bypassing any intermediate uphill energy transfer steps. We show that the interplay between the electronic Hamiltonian and the effective mode transformation partitions the linear vibronic coupling along specific promoter modes to dictate the selectivity of mediated energy transfer with a vital role of interference between vibronic couplings and multi-particle basis states. Our results suggest a general design principle for enhancing energy transfer through synergistic effects of vibronic resonance and weak mediated electronic coupling, where both effects individually do not promote efficient energy transfer. The effective mode approach proposed here paves a facile route toward four-wavemixing spectroscopy simulations of larger aggregates without severely approximating resonant vibronic coupling.

Effective normal modes identify vibrational motions which maximally promote vibronic mixing in excitonically coupled aggregates.

Controlling energy transfer through vibronic resonance is an interesting possibility. Exact treatment of non-adiabatic vibronic coupling is necessary to fully capture its role in driving energy transfer. However, the exact treatment of vibrations in extended systems is expensive, sometimes requiring oversimplifying approximations to reduce vibrational dimensionality, and do not provide physical insights into which specific vibrational motions promote energy transfer. In this communication, we derive effective normal modes for understanding vibronically enhanced energy transfer in excitonically coupled aggregates. We show that the dynamics of the overall high-dimensional vibronic Hamiltonian can be better understood through one-dimensional Hamiltonians separable along these effective modes. We demonstrate this approach on a trimer toy model to analyze the role of an intermediate "trap" site in mediating energy transfer between electronically uncoupled sites. Bringing uncoupled sites into vibronic resonance converts the "trap" into a "shuttle" for energy transfer. By deconvolving the dynamics along the aggregate normal modes, our approach identifies the specific vibrational motions, which maximally promote energy transfer, against spectator modes, which do not participate in vibronic mixing.