Role of Intermolecular Interactions in the Excited-State Photophysics of Tetracene and 2,2′-Ditetracene

We carried out a comparative study on the excited-state properties of tetracene, which is prone to singlet fission, and its 2,2 '-ditetracene derivative to analyze the dependence of such loss channels on molecular packing and, thus, intermolecular interactions. In neat single crystals, the absorption and emission spectra of 2,2 '-ditetracene are significantly red-shifted by about 150 meV from those of tetracene, while in the case of isolated molecules dispersed in crystalline anthracene matrixes, both dyes show virtually identical emission patterns. Unlike tetracene, the absence of delayed fluorescence in 2,2 '-ditetracene single-crystal photoluminescence (PL) and time-dependent density functional theory (DFT) calculations based on experimental single-crystal structures indicate a decline in the excited singlet-state energy below the E(S-1) = 2E(T-1) threshold. Simultaneously, time-resolved temperature-dependent PL data reveal an interplay of the molecular S1 state and an excited dimer state, which is efficiently populated above 10 K. Upon cooling, the photoluminescence of 2,2 '-ditetracene crystals outperforms that of tetracene by more than an order of magnitude and highlights the potential of molecular design, here the covalent linking of two tetracenes, to conserve the optical properties of the individual chromophores while decisively improving their photophysical properties in the crystalline aggregate.