Photophysics of a Nucleoside Analogue: 4-Cyanoindole-2'-deoxyribonucleoside

It has been shown that 4-cyanoindole-2'-deoxyribonucleoside (4CNI-NS) is a versatile spectroscopic probe of DNA structure and dynamics, as it can pair with all four natural DNA bases. However, its photophysics have not been examined in detail. Herein, we employed multiple techniques, including static fluorescence spectroscopy, time-resolved fluorescence spectroscopy, transient infrared spectroscopy, and theoretical calculations, to assess the photophysical properties of this nucleoside analogue in a series of solvents. We found that (1) its fluorescence has a large quantum yield (0.85 ± 0.10) and a relatively long decay lifetime (10.3 ± 1.0 ns) in H₂O, both of which become smaller in other solvents examined, (2) its maximum fluorescence emission frequency exhibits a linear dependence on the relative polarity index of the solvent, (3) the oscillator strength of its C≡N stretching vibration is increased by a factor of >10 upon transition to its excited electronic state (S₁), (4) besides solvent relaxation, a rotational motion around the single bond connecting the pentose group and the indole ring is also present in the S₁ state, and (5) in aprotic solvents both processes lead the C≡N stretching frequency (ν_ESA) to shift toward higher frequencies, whereas in protic solvents the effects of these processes are more complex. Taken together, these findings provide a molecular basis for interpreting the spectroscopic signals of this nucleoside analogue in practical applications.