Triplet state of 2-nitrothiophen. A laser flash-photolysis study

Abstract

Nanosecond laser flash photolysis (347.1 nm) of 2-nitrothiophen (NT) in several polar and non-polar solvents at 293 K shows a transient absorption centred at 545 ± 5 nm with ε= 9.8 × 10³ dm³ mol^–1 cm^–1, which is assigned to the lowest triplet excited state of NT, denoted ³NT. Whilst λ_max of ³NT does not shift with change of solvent polarity, its lifetime depends strongly on the solvent, being < 20 ns in toluene and 300 ns in acetonitrile. Triplet–triplet energy transfer experiments enabled the triplet energy, E_T, to be determined as 247 ± 2 kJ mol^–1.

³ NT is a reactive species, abstracting a hydrogen atom from diphenylmethanol with k₂=(6.70 ± 1.09)× 10⁷ dm³ mol^–1 s^–1 and an electron from 1,3,5-trimethoxybenzene with k₂=(5.12 ± 0.16)× 10⁹ dm³ mol^–1 s^–1, in acetone and acetonitrile solution, respectively. The standard reduction potential of ³NT is estimated to be 2.17 V vs. the standard hydrogen electrode (SHE), in aqueous solution. The observed pattern of reactivity suggests an increasing stabilisation of a π–π* triplet state in polar solvents relative to a n–π* triplet state, which is considered to be of lower energy in non-polar solvents.