Ultrafast Spectroscopy and Dynamics of Excitons and Spin-Correlated Triplet-Pair Intermediates Generated in the Singlet Exciton Fission of Naphtho-[2,1,8-qra]-tetracene

The search for molecules that undergo efficient singlet (exciton) fission (SF) to generate two triplet excitons following absorption of a single photon and create the possibility of generation of a pair of charge carriers has been very intense during the last two decades. Tetracene and its derivatives have shown great potential to become efficient SF materials. Under this consideration, earlier workers predicted the potentiality of naphtho-[2,1,8-qra]-tetracene (NpTc), which nearly satisfies the energetic requirement of SF, to work as an efficient SF material. But its detailed photophysics and dynamics of the SF process are yet to be explored. This work provides a comprehensive account of a detailed investigation of the photophysical properties of the excited states of the molecule in polycrystalline mediums by using ultrafast fluorescence and transient absorption (TA) spectroscopic techniques to resolve the intricate mechanism of the SF process. Generation of a pair of free triplet (T₁) excitons in the SF process has been found to follow a mechanism of two sequential and one parallel step after the population of the singlet (S₁) exciton state. Two sequential steps consist of the ultrafast decay of the S₁ exciton populating the spin-correlated interacting triplet pair state (CTP1), in which two T₁ excitons reside on two molecules at neighboring sites in the crystal. In turn, the CTP1 undergoes the spatial separation of two triplets to create a spin-correlated but noninteracting spin correlated triplet pair, the CTP2 state, in which the triplet excitons are spatially separated beyond their neighboring sites. In the last step of the SF process, the disappearance of the CTP2 state follows a parallel step, in which two T₁ excitons either get separated into two free T₁ excitons or undergo triplet fusion to generate back one S₁ exciton and one molecule in the S₀ state. The overall lifetime of completion of the SF process (or generation of two free T₁ excitons) in 45 nm thin films is about 31 ps. However, the SF rate decreases upon increasing the thickness of the film, for which one of the possible reasons may be attributed to reduction in intermolecular electronic coupling energy in thicker films.