Kinetic analysis of photoinduced reactions in hydrogen-bonded complexes of anthracene-urea with anions

Transient absorption measurement of urea-anthracene species nPUA (n = 1, 2, 9; PUA = 1-anthracen-n-yl-3-phenylurea) in the presence of acetate anions (tetrabutylammonium acetate, TBAAc) gave relatively long-lived transient species with lifetimes of ∼100 μs, which were unaffected by O2. The lifetimes showed significant temperature dependence, which allowed us to determine ΔH‡ and ΔS‡. ΔS‡ had a large negative value, in contrast to ΔH‡, indicating that the dissipation process of the transient species was entropy-controlled. A linear relationship between TΔS‡ and ΔH‡ suggested entropy–enthalpy compensation, which is generally found in host–guest systems with hydrogen bonding. These findings allowed us to assign the transient species as X, which is the ground state responsible for longer wavelength emissions, and conclude that the dissipation of the transient species corresponded to recovery of the complex in the ground state. We propose a reaction scheme for the photochemical process applying to urea derivatives in the presence of anions.