Observation of Photophysical Processes of a Heptacene Derivative: Monomeric Behavior in Homogeneous Solution and...

A soluble heptacene derivative, 5,9,14,18-tetrakis(triisopropylsilylethynyl)-heptacene (TIPS-Hep), was synthesized in situ from an α-diketone precursor via the Strating-Zwanenburg reaction, and its photophysical properties were comprehensively characterized in both solution and thin-film states. In deoxygenated dilute tetrahydrofuran (THF) solution, TIPS-Hep exhibited monomeric behavior. Absorption peaks were observed at 671, 737, 820, and 970 nm, accompanied by a shoulder at 870 nm, while fluorescence peaks were observed at 894 and 961 nm with a fluorescence lifetime (τ_S) of <100 ps. Nanosecond transient absorption spectroscopy revealed a triplet lifetime (τ_T) of 19 μs and a triplet quantum yield (Φ_ISC) of ∼2%. The short excited-state lifetime in dilute solution was estimated to be 87 ps by femtosecond transient absorption spectroscopy and precludes efficient molecular encounters, indicating that singlet fission is unlikely and that intersystem crossing serves as the dominant pathway for triplet formation. This interpretation is supported by transient decay analysis, which revealed no kinetic signatures associated with SF. In contrast, solution-processed thin films of TIPS-Hep, generated in situ by photoirradiation of a spin-coated α-diketone precursor, exhibited faster and enhanced formation of the triplet excited states. Since multiple excited species are involved, several kinetic models were examined, and a sequential model based on a SF pathway (S₁ + S₀ → TT → 2T₁) was identified as the most consistent with the experimental results. Global analysis of femtosecond transient absorption data with the sequential SF model assuming three species revealed high yields of correlated triplet pairs (TT, 75% at the maximum value of 100%) and independent triplets (T₁, 20% at the maximum value of 200%). These results provide valuable mechanistic insights into the excited-state dynamics of higher acenes.