Solvatochromic Effects on the Absorption Spectrum of 2-Thiocytosine

Abstract

The solvatochromic effects of six different solvents on the UV absorption spectrum of 2-thiocytosine have been studied by a combination of experimental and theoretical techniques. The steady-state absorption spectra show significant shifts of the absorption bands, where in more polar solvents the first absorption maximum shifts to higher transition energies and the second maximum to lower energies. The observed solvatochromic shifts have been rationalized using three popular solvatochromic scales and with high-level multireference quantum chemistry calculations including implicit and explicit solvent effects. It has been found that the dipole moments of the excited states account for some general shifts in the excitation energies, whereas the explicit solvent interactions explain the differences in the spectra recorded in the different solvents.

Photochemical and Photodynamical Properties of Sulfur‐Substituted Nucleic Acid Bases,

“…The tail redshifts with decreasing solvent polarity, suggesting that it originates from an n S π* transition (B. Ashwood, M. Pollum, C. E. Crespo‐Hernández, unpublished results) . Vertical excitation energies obtained using single‐reference and multiconfigurational computational methods predict that the lowest‐energy absorption maximum is due to a S 2 (π S π*) transition in the thiouracil and thiothymine derivatives , while it has been assigned primarily to a S 4 (π S π*) transition in 2‐thiocytosine . Similarly, the absorption tail has been assigned to a S 1 (n S π*) transition in the thiothymine and thiouracil derivatives, while it has been assigned to a combination of S 2 (π S π*) and S 1 (n S π*) transitions in 2‐thiocytosine .…”

“…Besides the primary p S p* transitions characteristic of lowenergy absorption band, the thiopyrimidine derivatives also exhibit a small-intensity absorption tail that extends deep into the UVA region (17,46,55,91,93,95,96). The tail redshifts with decreasing solvent polarity, suggesting that it originates from an n S p* transition (B. Ashwood, M. Pollum, C. E. Crespo-Hern andez, unpublished results) (55,93,96,98). Vertical excitation energies obtained using single-reference and multiconfigurational computational methods predict that the lowest-energy absorption maximum is due to a S 2 (p S p*) transition in the thiouracil and thiothymine derivatives (54,74,92,96,98,103,(106)(107)(108)(109)(110)(111)(112)(113)(114)(115)(116), while it has been assigned primarily to a S 4 (p S p*) transition in 2-thiocytosine (54).…”

“…For 2-thiocytosine, the calculations predict that UVB excitation populates primarily the S 2 (p S p*) state (54,98). The population in this state then internally converts to two minima in the lowest-energy potential energy surfaces that have 1 p S p* and 1 n S p* character, respectively (54).…”

“…Vertical excitation energies obtained using single-reference and multiconfigurational computational methods predict that the lowest-energy absorption maximum is due to a S 2 (p S p*) transition in the thiouracil and thiothymine derivatives (54,74,92,96,98,103,(106)(107)(108)(109)(110)(111)(112)(113)(114)(115)(116), while it has been assigned primarily to a S 4 (p S p*) transition in 2-thiocytosine (54). Similarly, the absorption tail has been assigned to a S 1 (n S p*) transition in the thiothymine and thiouracil derivatives, while it has been assigned to a combination of S 2 (p S p*) and S 1 (n S p*) transitions in 2-thiocytosine (98,103,106). The localization of the np* transition on the sulfur atom is responsible for its significant redshift relative to analogous transitions in the canonical nucleobases (98,103,106).…”

Solvation effects alter the photochemistry of 2-thiocytosine

“…The stationary absorption spectrum of 2tCyt in water solution shows a broad band with maxima at 270 nm (4.59 eV), 240 nm (5.17 eV) and a shoulder at about 220 nm (5.64 eV) [63]. Based on the results of ab initio calculations, the first peek was assigned by Mai et al to S 4 and S 2 states having similar 1 π S π * character (centered around the thiocarbonyl group but associated with different π * orbitals), and the remaining features to the S 6 and S 8 states, respectively, arising from 1 ππ * transitions within the aromatic ring [63].…”