Photophysics of Dibenzo[a,l]pentacene upon Monomer to Thin Film Transformation: Enhanced Visible Light Absorption and Ultrafast Singlet Fission

Abstract

Linear polyacenes and their derivatives are well-recognized organic semiconductors because of their fascinating singlet fission (SF) property. However, higher polyacenes (e.g., pentacene and hexacene) suffer from poor chemical stability. Herein, we present photophysics of a nonlinearly fused analogue of heptacene, namely, dibenzo[a,l]pentacene in monomer and thin film forms. Nonlinear fusion of benzene rings at two sides of the linear pentacene core results in a weaker and blueshifted absorption. Interestingly, thin film formation introduces a substantial red shift of absorption along with ∼5 times enhancement in absorption strength due to the cooperative intermolecular interaction in the solid state. Femtosecond and nanosecond pump−probe experiments reveal that the thin film of DBPn undergoes fast singlet fission (a time constant of ∼5.2 ps) to the triplet pair state, followed by dissociation to long-lived free triplets in hundreds of picoseconds with near-unity SF yield. Endowed with higher solubility and better photostability as compared to pentacene, observation of fast and efficient singlet fission makes DBPn a potential candidate for organic photovoltaic applications.

Photoluminescence spectroscopy of dibenzopentacene single-crystals: multiple emissive states across temperature, time, and magnetic field in a pursuit of exothermic singlet fission

“…The strongest PL intensity of state I with a narrow 0-0 peak below 10 K is in line with the enhanced intensity of the free exciton emission in other organic semiconductors at helium temperatures. At 5-10 K, the PL decay of state I at 600 nm is described by two lifetimes, 3.8 and 6.1 ns with equal weights, which are close to the fluorescence lifetime of 5.2-7.4 ns determined for isolated DBP molecules in different solvents [35][36][37] or 4.1-4.7 ns measured in polystyrene matrix in this work. The superradiance effect, if present, is not very pronounced as we do not observe a decrease of the lifetime upon cooling (Fig.…”

“…In toluene, dichlorobenzene or tetrahydrofuran solution, the lowest-energy absorption of DBP assigned to S 0 -S 1 excitation occurs at around 2.34 eV and is followed by 2-3 vibrational replica at 0.15-0.18 eV. [34][35][36] Very similar absorption spectra with S 0 -S 1 transition at 2.34 eV were also obtained for DBP diluted in a polystyrene matrix in ref. 36 and in this work (Fig.…”

“…Different to ref. 35, the room temperature S 1 lifetime in our work is still about 1 ns, but our time resolution does not allow detection of a fast process with a time constant of a few ps. Ref.…”

“…Fluorescence studies of DBP in solution were reported by Canuel et al, 37 Tovstopyat et al 36 and Manna et al 35 The S 1 -S 0 emission has the main peak at 2.33-2.38 eV and quantum yield of 30-50% (the values depend on the solvent polarity). The PL spectra of DBP diluted in polystyrene (Fig.…”

“…35 The PL spectrum in ref. 35 has the main peak at 600 nm and corresponds well to the free exciton emission in single crystals. Different to ref.…”