Intersystem Crossing to Excited Triplet State of Aza Analogues of Nucleic Acid Bases in Acetonitrile

Abstract

Excited state characteristics of aza analogues of nucleic acid bases, 8-azaadenine (8AA), 5-azacytosine (5AC), 8-azaguanine (8AG), and 6-azauracil (6AU), in acetonitrile solution were comprehensively investigated with steady state absorption and emission spectra, transient absorption measurements, emission measurements for the singlet oxygen molecule, and time-dependent density functional theory (TD-DFT) calculations. The triplet-triplet absorption spectrum of 8AA whose peak was 455 nm was observed for the first time. Sensitized singlet oxygen formation of 8AA was also observed in O(2)-saturated acetonitrile with quantum yields of 0.15 +/- 0.02. It was concluded that there were two kinds of aza analogues of nucleic acid bases: type A had substantial quantum yield for the intersystem crossing and potential of O2 (1Delta(g)) formation (8AA and 6AU), and type B did not (5AC and 8AG). TD-DFT calculations indicated that type A molecules had a dark 1npi* state below the first allowed 1pipi* state, while both S1 and S2 states for type B molecules had a pipi* character. It strongly suggested that the dark 1npi* state below the 1pipi* state would play an important role in the ISC process of aza analogues of nucleic acid bases.

Photochemistry of Nucleic Acid Bases and Their Thio- and Aza-Analogues in Solution

“…However, photosensitivity to these drugs has also been reported [63,64]. As observed in the thiobases and other nucleobase analogues, the photochemical properties of the azabases are noticeably different from those of the natural bases [12,[14][15][16]. Therefore, it is both fundamentally informative and important to examine how a single nitrogen substitution affects the photophysical properties and excited-state dynamics observed in the nucleic acid bases.…”

“…10 À2 to 10 À3 was measured in 8-azaadenine, 8-azaguanine, and 6-azauracil (Table 18). Fluorescence emission in 8-azaguanine was assigned to the N8H tautomer, whereas it was assigned to the N9H tautomer in 8-azaadenine [14]. In the case of 6-azauracil [12], the fluorescence emission is excitation-wavelength-dependent.…”

“…On the other hand, the photophysics of the azabase derivatives can be significantly simplified by using acetonitrile as the solvent. Suzuki and co-workers [12,[14][15][16] have recently performed the first steady-state and time-resolved experiments with azabase analogues in acetonitrile solution. These studies will be reviewed briefly below.…”

“…The steady-state absorption and emission properties of the azabases in acetonitrile, including 6-azauridine and 6-aza-2-thiothymine, were investigated by Suzuki and co-workers [12,[14][15][16]. In general, the absorption spectra of the azabase derivatives shift slightly to the red compared to those of the natural bases, with the largest redshift observed in 6-aza-2-thiothymine, presumably because of the additional sulfur substitution (Table 18).…”

A general method to describe intersystem crossing dynamics in trajectory surface hopping

“…Examples of molecules where ISC is important range from aldehydes [37] and small aromatic compounds like benzene, [38] naphthalene, anthracene, and their carbonylic derivatives [39][40][41][42][43][44][45][46][47][48][49][50][51] to nitrocompounds. [43,[52][53][54][55][56][57][58][59][60][61] Furthermore, ISC has been reported for thio-substituted, [62][63][64][65][66][67][68] aza-substituted, [69] bromo-substituted, [70] and canonical nucleobases. [71][72][73] From a theoretical point of view, ISC can be explained by the interaction of states of different multiplicity by spin-orbit coupling (SOC), which is a relativistic effect.…”