Lifetimes of the triplet states of partially deuterated trans-stilbenes and structurally related compounds

The triplet lifetimes of phenanthrene, phenanthrene-9-D1, phenanthrene-D10, 2-phenylinden, 5:10-dihydroindeno-(2:1-a)-inden, trans-stilbene, trans-perdeuterostilbene, and eight partially deuterated trans-stilbenes (α-D-stilbene, αα'-D2-stilbene, 2,2'-D2-stilbene, 3,3'-D2-stilbene, 4,4'-D2-stilbene, 2,3,4,5,6-D5-stilbene, α,2,3,4,5,6-D6-stilbene, and 2,2',3,3',4,4',5,5',6,6'-D10-stilbene) were measured by flash photolysis in EPA at 77 K. The triplet lifetime depends on the number and position of deuterium atoms in the molecule. Within the phenyl rings, the different positions in stilbene have only a minor effect on the triplet lifetime. Deuteration of the olefinic hydrogen atoms, on the other hand, results in a significant extension of the triplet lifetime. An olefinic hydrogen atom in trans-stilbene exerts a 16 times greater effect on non-radiative deactivation than a hydrogen atom in the phenyl ring. The effect of deuterium or hydrogen substitution is additive. CNDO calculations support the assumption that in trans-stilbene the deuterium effect is greatest when the highest π-electron amplitude coincides with a large local CH out-of-plane vibrational amplitude of the olefinic C=C bond. The results are corroborated by comparison with the decay times of some stilbene-related compounds, such as phenanthrene, phenanthrene-9-D1, phenanthrene-D10, 2-phenylinden, and 5:10-dihydroindeno-(2:1-a)-inden.