Proton transfer and n → π* transition in the photophysics of 1,N⁶-ethenoadenosine

The photophysical characteristics of 1,N⁶-enthenoadenosine (εAdo) show irregularities in terms of the expected photophysics from a pH equilibrium between two forms that absorb light at different wavelengths. Furthermore, a comparison between the absorption spectra of purine, adenine, and εAdo leads to the conclusion that εAdo does not follow the adenine, but rather has more in common with the purine. The adenine itself does not follow its parent compound, purine. We, therefore, reinterpret the absorption of εAdo, such as the unprotonated form has two absorption bands, the second of which is an n → π* transition, whereas the protonated form has only one π → π* absorption band, which overlaps with the first absorption band of the unprotonated form. The n → π* absorption "disappeared" upon protonation, apparently due to stabilization of the lone-pair electrons. Under these presumptions, the photophysics of εAdo is no longer peculiar. Transitions to and from both excited singlet states, S_ππ* and S_nπ*, along with the relative order of their respective triplets, are shown to play an active role in the photophysics of εAdo. Therefore, the reported multiple emissions from εAdo, at low temperature, are to be expected. The reported observations in the literature provide evidence for the multiple excited states of εAdo. In the present work, cyclodextrins provide a powerful tool in the photophysical study of εAdo. In particular, cyclodextrin host isolation matrix (CHIM) provides a unique environment that can be applied to mimic the photophysics of the isolated molecule in the gas phase or at low temperatures.