Spectral and Photophysical Behavior of Cytisine in n-Hexane. Experimental Evidence for the S 1 (n,π) → S 0 Fluorescence

Spectral and photophysical properties of (-)-cytisine (the compound used as a smoking cessation aid and a potential drug in Alzheimer's and Parkinson's diseases) were investigated. The two conformers of cytisine, whose presence in the S state has been earlier proved by the NMR and IR methods as well as in theoretical calculation, in nonpolar n-hexane show a rarely observed prompt fluorescence from the S (n,π*) excited state. This observation is unambiguously evidenced by very small radiative rate constants of these two emitting conformers, k = 7.4 × 10 and 3.0 × 10 s . Their lifetimes in the S (n,π*) state are relatively long, τ = 1.9 and 6.7 ns; therefore, their fluorescence quantum yield is relatively high ϕ ∼ 10 . The long-wavelength band in the cytisine absorption originates from the excitation to the S (π,π*) state, while the S (n,π*) state is not observed in this spectrum. Thus, the excited state S (π,π*) is manifested only in the absorption spectrum, while the excited state S (n,π*) is seen only in the fluorescence spectrum, so cytisine in n-hexane is characterized by close lying (n,π*) and (π,π*) excited singlet states.