Singlet Fission in a Contorted Naphthalenediimide Dimer Proceeds Through a Coherently Coupled Triplet Pair State

Efficient generation of triplet pair excitons from a singlet exciton is a photophysical process of significant fundamental interest with applications across photovoltaic and quantum technologies. Understanding and controlling singlet exciton fission (SEF) has motivated several synthetic templates based on chromophores such as acenes, terrylenes, perylenes, subphthalocyanine, and metallopyrrins. Fast recombination of the correlated triplet pair (TT)¹, especially in intramolecular SEF systems, prevents the efficient generation of free triplets. Orbital interactions arising from the interchromophoric distance and relative orientation are one of the key factors that drive such recombination. To overcome this, a contortion of the chromophore skeleton in bridged dimers may be a distinct design strategy for preventing recombination losses through structural distortion. Recently, slow-time resolution pump-probe experiments reported[1] SEF in contorted naphthalenediimide (NDI) dimers where the distortion of the chromophore skeleton tunes the electronic coupling to favor the formation of (TT)¹ over excimer formation or charge separation.
Here, we apply pump-probe and two-dimensional electronic spectroscopy (2DES) to investigate the mechanistic principles that drive this process. Using a combination of impulsive excitation with polarization control, we elucidate the key role of a coherently mixed intermediate state that results from the coupling between the (TT)¹ and locally excited (LE) states in the dimer and forms within 200 fs. The evidence of coherent coupling is revealed by a strong positive upper cross-peak in the 2D spectra, even at early pump-probe delays. This is further confirmed by the electronic anisotropy of the system, which is already significantly lower than the single uncoupled dipole limit as early as 30 fs. This intermediate state then decays into a relaxed (TT)¹ state on picosecond timescale.
We then apply a pulse polarization sequence that selects signal pathways arising due to the mixing of transitions with different polarization directions. Isotropic versus polarization-controlled experiments show that the mixed LE-TT intermediate (that forms within 200 fs) is selected more in the latter case, further confirming the electronically mixed nature of this intermediate. Our study reveals the rich mechanistic details that underlie intramolecular SEF where conclusive evidence is presented for a mixed LE-TT intermediate. The evidence we provide goes beyond the qualitative rate-like description of intramolecular singlet fission and motivates a deeper theoretical understanding of the electronic reorientations involved during the conversion of a singlet into a correlated triplet pair.

[1] Bansal, D.; Kundu, A.; Singh, V. P.; Pal, A. K.; Datta, A.; Dasgupta, J.; Mukhopadhyay, P. A Highly Contorted Push–Pull Naphthalenediimide Dimer and Evidence of Intramolecular Singlet Exciton Fission. Chem. Sci. 2022, 13 (39), 11506–11512.