Thesis Colloquium: On the formation and evolution of the correlated triplet pair state in singlet fission: Insights from polarization-controlled impulsive optical spectroscopy

Singlet exciton fission (SEF) converts an optically excited singlet exciton into a spin-entangled correlated triplet pair state, (TT₁)¹,which can subsequently yield spatially separated triplets for photovoltaic applications and/or spin-polarized multiexcitonic states for quantum technologies¹. Despite extensive work², the mutual role of electronic couplings and nuclear motions in facilitating (TT₁)¹formation remains unclear.  A second unresolved question concerns the subsequent evolution of the (TT₁)¹ state: although significant electronic reorientation is expected during conversion from the initially excited singlet state to a triplet-pair species, this reorientation has rarely been directly probed. Tracking such reorientation can provide direct information about the time-evolving electronic character of (TT₁)¹ and its eventual decorrelation. This thesis addresses these questions using vibronic exciton models and polarization-controlled impulsive optical spectroscopic experiments and simulations.

The first part of this thesis examines how molecular vibrations promote non-adiabatic electronic-state mixing during singlet fission³. Using a numerically exact SEF dimer model, we show that high-frequency modes tune inter-excitonic energy gaps in both correlated and anti-correlated fashion, giving rise to coupled vibronic resonances that mix reactant and product states in exothermic and endothermic systems. Unlike the narrow vibronic resonances in photosynthetic proteins⁴, these resonances are robust and non-selective because acenes exhibit nearly ten-times larger Franck–Condon displacements, which are further broadened by low-frequency vibrations. However, strongly Franck–Condon-active modes also imply that prominent vibrational coherences may be spectroscopically visible without being functionally relevant. To isolate functional vibronic couplings, we simulate polarization-resolved two-dimensional electronic spectroscopic quantum beat maps and identify anisotropic quantum beats that directly report on inter-state vibronic mixing, distinguishing promoter vibrations from spectator modes that merely accompany ultrafast internal conversion.

The second part of this thesis investigates the evolution of the electronic character of the spin-entangled correlated triplet pair state, (TT₁)¹, in a library of covalently linked conformationally flexible pentacene dimers⁵ . Using two-dimensional electronic spectroscopy⁶, which correlates detection energy with excitation energy as 2D contour maps which evolve with waiting time, we find that (TT₁)¹ formation is specific to near-planar conformations, as revealed by two-dimensional kinetic rate maps. This formation is accompanied by large nuclear reorganization in the (TT₁)¹ photoproduct, evident from enhanced vibrational quantum beats in the photoproduct. Although (TT₁)¹, the subsequent high-spin species and the free triplets exhibit substantial spectral overlaps which complicates time-resolved analysis, the (TT₁)¹ species in strongly coupled pentacene dimers exhibits a unique near-IR excited state absorption.

Introducing polarization-selective pump–probe and anisotropy measurements to track the electronic character during the (TT₁)¹ evolution, we find that the transitions contributing to this band are of intermediate polarization – between the short axis polarized to S1-Sn and long-axis polarized similar to weakly correlated or free triplets T₁-T_n transitions – suggestive of singlet-triplet electronic mixing. Interestingly, the intermediate polarization persists throughout the triplet-pair lifetime, as reported by the negligible changes in the electronic anisotropy.

These observations show that, once the triplet pair is strongly bound, neither large nuclear reorganization nor structural fluctuations are fast enough to suppress persistent singlet–triplet electronic mixing, such that triplet-pair decorrelation is outcompeted by decay. The mechanism of triplet-pair decorrelation through THz vibrational motions⁷ is evidently slower than the rotational diffusion timescales of 0.2 ns and longer.