Excited state dynamics of 3,6-diaryl-1,2,4,5-tetrazines. Experimental and theoretical studies

The photophysical behavior was studied of three symmetrical 1,2,4,5-tetrazine derivatives substituted with two aromatic substituents (phenyl: DPT; p-methoxyphenyl: DAT; thiophen-2-yl: DTT). The UV–visible absorption spectra of these compounds in cyclohexane and acetonitrile show two absorption maxima at 500–550 nm and 290–330 nm, as well as a shoulder at lower energies on the latter absorption band. The electronic transitions were assigned on the basis of TD-DFT calculations. In contrast with some other tetrazine derivatives, these compounds exhibit only weak fluorescence (Φf ≈ 10−4 to 10−3) from the S1 (nπ*) state. When the molecules are excited to a higher energy ππ* state, fluorescence from the nth excited state (n = 6 for DPT, n = 5 for DAT and n = 4 for DTT) is detected. Time-correlated single photon counting (TC-SPC) and femtosecond transient absorption (fs-TA) measurements showed that internal conversion from the Sn state to the S1 state is unusually slow, of the order of 30 ps for DTT and 20 ps for DAT.