Photophysics of Diphenylacetylene: Light from the “Dark State”

A weak band at the tail of the known tolane (diphenylacetylene, DPA) fluorescence spectrum in several solvents is assigned to the forbidden 1¹A_u → 1¹A_g transition on the basis of its lifetime (∼200 ps) and its fluorescence excitation spectra. The 1¹A_u state, generally called the dark state, is not truly dark. We report the temperature (<i>T</i>) dependence of DPA fluorescence quantum yields (ϕ_f) in methylcyclohexane (MCH) solution and the fluorescence and phosphorescence quantum yields of DPA in glassy MCH at 77 K. Significant differences between fluorescence and phosphorescence excitation spectra reveal that, in addition to the 1¹B_1u ← 1¹A_g transition, the first DPA absorption band includes a transition to another excited state, most probably the 1¹B_2u state, from which intersystem crossing is more efficient. The <i>T</i> dependence of ϕ_f values in MCH solution is shown to be consistent with the previously reported <i>T</i> dependence of the lifetimes of transient DPA singlet excited state absorptions in the picosecond time scale. Transient absorption decay rate constants in hexane, methylcyclohexane and decalin as a function of <i>T</i> are retreated. Application of the medium enhanced barrier model shows that the medium is fully engaged with the molecular motion that is involved in the activated nonradiative decay path of the 1¹B_1u state. In accord with theoretical calculations and experimental observations, that process is assigned to the diabatic internal conversion of the short-lived linear fluorescent π,π* (1¹B_1u) state, over a low intrinsic energy barrier, to the longer lived weakly fluorescent <i>trans</i>-bent π,σ* (1¹A_u), which is the precursor of the DPA triplet state. Absorption and fluorescence measurements in several solvents show that the 1¹B_1u–1¹A_g energy gap decreases linearly with increasing medium polarizability. Our results allow a more definitive state order assignment for DPA