Triplet‐Triplet Absorption Spectroscopy of Some Highly Dipolar Unsaturated Nitro Compounds

The triplet‐triplet absorption spectra of p ‐ N, N ‐dimethylnitroaniline, 4‐nitro‐ p ‐terphenyl, 1‐amino‐4‐nitrofluorene, 5‐nitroacenaphthene, trans ‐1‐(4‐methoxy‐phenyl)‐2‐nitroethylene (MeONS), and trans ‐1‐(4‐dimethylaminophenyl)‐2‐nitroethylene (DANS) in EPA glass at 77 K are reported, together with molar extinction coefficients and PPP‐SCF‐MO‐CI calculations. The two nitrostyrenes, MeONS and DANS, have been examined in fluid media at room temperature using nanosecond laser photolysis, and their triplet lifetimes found to increase substantially with increase in solvent polarity and charge‐transfer character of the compound. This is interpreted in terms of the diradical/zwitterionic nature of the triplet state affecting the T 1 –S 0 energy gap at the 90° twisted configuration of the olefinic linkage. The decrease in both the triplet yield and the fluorescence yield of DANS with increase in solvent polarity is explained by the intervention of an internal conversion process involving a rotation of the dimethylamino group in the lowest singlet excited state.