Triplet state of 5-nitro-2-furoic acid by laser flash photolysis. Spectrum, lifetime and reactivity

Abstract

Nanosecond laser flash photolysis (347.1 nm) of 5-nitro-2-furoic acid (NFA) in both polar and non-polar solvents at 295 K shows a transient absorption centred at 490 ± 5 nm with εca. 2 × 10⁴ dm³ mol^–1 cm^–1 which is assigned to the lowest triplet excited state of NFA, denoted ³NFA. While λ_max of ³NFA is independent of the solvent polarity, its lifetime is strongly solvent-dependent, being 22.7 ns in water and 274 ns in MeCN. Triplet–triplet energy transfer experiments showed the triplet energy E_T to be 240 ± 4 kJ mol^–1. ³NFA is extremely reactive, thus it abstracts a hydrogen atom from diphenyl-amine with k₂=(8.64 ± 0.44)× 10⁹ dm³ mol^–1 s^–1 and from diphenylmethanol with k₂=(6.94 ± 0.55)× 10⁷ dm³ mol^–1 s^–1, both in acetone solution. Representative of electron-transfer rates to ³NFA are those of CO^2–₃ and NNN′ N′-tetramethyl-p-phenylenediamine with k₂=(1.56 ± 0.10)× 10⁸ dm³ mol^–1 s^–1 and k₂=(5.98 ± 0.20)′× 10⁹ dm³ mol^–1 s^–1, respectively. The logarithm of the rate constant of oxidation of a series of electron donors by ³NFA correlates well with the corresponding electrode potentials up to the diffusion limit, although the figure for F^– notably fails to fit the plot. The reduction potential of ³NFA is estimated to be 2.17 V. Both the lack of spectral shift with increasing solvent polarity and the reactivity towards hydrogen-atom donors indicate that ³NFA is an nπ* state which retains its nπ* character even in polar solvents.