Vibrational Quantum Beat Amplitudes Implicate Coherent Intramolecular Singlet Fission in a Series of Pentacene Dimers

Singlet exciton fission (SEF) is initiated by ultrafast internal conversion of a singlet exciton into a correlated triplet pair (TT)¹. The ‘reaction coordinates’ for ultrafast SEF even in archetypal acene systems remain unclear with molecular design principles relying on tailoring electronic couplings to achieve new templates for efficient SEF materials. Covalently linked pentacene molecules provide a tunable template where the effects of molecular geometry, torsion, planarity and extent of conjugation on intramolecular SEF (iSEF) rates can be systematically investigated. Recently Majumdar, Patil and Musser et.al.² have reported narrowband pump-probe experiments on covalently linked tri-isopropyl silyl pentacene (TIPS-Pc) dimers which are connected by a fused thiophene ring linker and undergo iSEF. Charge transfer (CT) states are expected to minimally mix with the Frenkel exciton (FE) states in such systems and a role for direct mixing between multi-excitonic triplet pair (ME) and the FE states has been proposed ³.
Here we investigate the mechanistic aspects of iSEF that are operative in these systems through white-light pump-probe (PP) and two-dimensional electronic spectroscopy (2DES)¹ over a ~200 nm visible bandwidth with sub-10 fs pulses that creates a coherent superposition of states as the initial condition distinct from narrowband experiments. 2DES spectra and the resulting 2D decay associated spectra (2D-DAS) resolve a distinct non-emissive species (apart from the emissive species) along the excitation axis where the iSEF occurs with the largest amplitude. No excitation wavelength dependence for iSEF rates is found. A near-IR excited state absorption (NIR-ESA) signature is also distinctly seen along the excitation axis for both species at the earliest pump-probe waiting times hinting at a mixed FE-TT species that is initially excited and leads to iSEF.
Acenes undergo large change in geometry upon photoexcitation and therefore large amplitude vibrational quantum beats (wave packets) are expected even without vibronic coupling between pentacene units ⁴. Using impulsive PP experiments to resolve quantum beats from 200-1500 cm^-1 we find a strong suppression of quantum beats in the dimers which directly confirms the presence of excitonic delocalization in the dimers. Interestingly, this suppression of quantum beats not uniform and strongly coupled dimers show enhanced (suppressed) excited (ground) state beats, where the former coincide with the location of the NIR-ESA band that is specific to the formation of ME state. Together these observations provide comprehensive evidence for a direct iSEF mechanism that drives iSEF and spectroscopically manifests as an FE-ME intermediate at the earliest waiting times with enhanced vibrational beating amplitudes on the ME triplet pair state.

1. Thomas AS, Bhat VN, Tiwari V. Rapid scan white light two-dimensional electronic spectroscopy with 100 kHz shot-to-shot detection. J Chem Phys. 2023;159(24):244202. doi:10.1063/5.0179474
2. Majumder K, Mukherjee S, Panjwani NA, et al. Controlling Intramolecular Singlet Fission Dynamics via Torsional Modulation of Through-Bond versus Through-Space Couplings. J Am Chem Soc. 2023;145(38):20883-20896. doi:10.1021/jacs.3c06075
3. Fuemmeler EG, Sanders SN, Pun AB, et al. A Direct Mechanism of Ultrafast Intramolecular Singlet Fission in Pentacene Dimers. ACS Cent Sci. 2016;2(5):316-324. doi:10.1021/acscentsci.6b00063
4. Bhattacharyya A, Sahu A, Patra S, Tiwari V. Low- and high-frequency vibrations synergistically enhance singlet exciton fission through robust vibronic resonances. Proc Natl Acad Sci. 2023;120(49):e2310124120. doi:10.1073/pnas.2310124120