Anomalous fluorescence and low-temperature absorption spectra of naphtho[1,8-cd][1,2,6] thiadiazine and its derivatives: an example of second excited state emission (S2 → S0)

An anomalous fluorescence near 380 nm is detected from heptane solutions on naphtho[1,8-cd][1,2,6] thiadiazine (1), 6,7-dihydroacenaphtho[5,6-cd][1,2,6]thiadiazone (2), its unsaturated derivative (4), and the tetrachloro derivative of 1 (3), when these substances are excited into the S₀→S₂ absorption band near 350 nm. Excitation spectra verify that S₂→S₀ fluorescence is occurring. The measured lifetimes of each fluorescence are 2.2, 3.3, 0.3, 0.4 ns respectively for 1,2,3, and 4. Radiative and non-radiative rate constants are presented. Compounds 1 and 2 exhibit unusual low-temperature absorption spectra at 77 K in 3-methylpetane matrix. The disappearance of the long-wavelength transition at 650 nm is accompanied by a new transition near 450 nm at 77 K. It is theorized that a ground-state dimer can explain these observations.