Laser Flash Photolysis of trans-1,2-Bis(4-pyridyl)ethylene in Aqueous Solution

Article abstract of DOI:10.1021/j100163a022

Reactions of trans-1,2-bis(4-pyridyl)ethylene (neutral form: M) and its conjugate acids, HM⁺ and H2M²⁺, following excitation by laser pulses, were studied in aqueous solution.The initial photoreaction of M (at pH > 7, λ_exc = 248 nm) is photoionization in a one quantum process, thereby forming the radical cation (λ_max = 325 nm, lifetime 80 ns) and the hydrated electron (e^-_aq) in low quantum yield (0.01).The H adduct radical (HM^., λ_max = 455 nm) is formed by addition of e^-_aq to M followed by protonation with water, as in pulse radiolysis.HM^. and its conjugate acid (H2M^.+, λ_max = 500 nm) are in equilibrium with pK_a = 9.8 in buffered solution.H2M^.+ is also formed (in low yield) via biphotonic photoionization of HM⁺ at pH 4-5 and of H2M²⁺ at pH < 3.6.For H2M²⁺ the photohydroxide (H2M⁺OH, λ_max = 400 nm) was observed.H2M⁺OH is the only transient in neat aqueous solution, when photoionization does not occur (λ_exc = 308 nm) and decays by protonation (rate constant 7 x 10⁷ dm³ mol^-1 s^-) to give the photohydrate, as is supported by time-resolved conductivity measurements.In 2-propanol/water (1:9 and 1:1, λ_exc = 248 or 308 nm) HM^. or H2M^.+, depending on pH, are mainly formed by H-atom abstraction from the alcohol involving the ¹(n,?^*) states of M and HM⁺.Excited H2M²⁺, however, yields H2M⁺OH, as it does in the absence of 2-propanol.