Spectral and Photophysical Behavior of Cytisine in n-Hexane. Experimental Evidence for the S1(n,π*) → S0 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 S0 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 S1(n,π*) excited state. This observation is unambiguously evidenced by very small radiative rate constants of these two emitting conformers, kF = 7.4 × 105 and 3.0 × 105 s–1. Their lifetimes in the S1(n,π*) state are relatively long, τS1 = 1.9 and 6.7 ns; therefore, their fluorescence quantum yield is relatively high ϕF ∼ 10–3. The long-wavelength band in the cytisine absorption originates from the excitation to the S2(π,π*) state, while the S1(n,π*) state is not observed in this spectrum. Thus, the excited state S2(π,π*) is manifested only in the absorption spectrum, while the excited state S1(n,π*) is see...