PHOTOPHYSICAL PROPERTIES OF NUCLEIC ACID COMPONENTS—1. THE PYRIMIDINES: THYMINE, URACIL, N, N‐DIMETHYL DERIVATIVES AND THYMIDINE

Abstract

Abstract— Low‐temperature (and some room temperature) absorption and emission, fluorescence and phosphorescence, data including quantum yields and lifetimes have been obtained from the title pyrimidine bases as a function of the nature of the solvent environment. Modest vibrational resolution has been observed for the first time in the absorption spectra, particularly for thymine and uracil. The excitation spectra also show structure. The quantum yields of fluorescence (φF) and phosphorescence are independent of the excitation wavelength. Thymine, thymidine and uracil have profoundly different photophysical properties in polar‐aprotic vs polar‐protic solvents. The N, N‐dimethyl substitution of thymine and uracil produces photophysical changes comparable to the solvent change for the unsubsti‐tuted bases. The species involved in the emission processes is the keto (lactam) form. It is probable that 1,3(n,π*) state(s) has(have) changed order relative to a lowest 1(π,π*) state as a consequence of both the solvent change and N, N‐dimethyl substitution. The lowest triplet state is assigned as 3(n π*). We propose that an important factor contributing to the previously reported excitation wavelength dependence of φF and φT1 (φisc) for nucleic‐acid components is the equilibrium coexistence of H‐bonded and non‐H‐bonded forms each having different photophysical properties. Consideration is given of the impact of the significantly different photophysical properties of nucleic‐acid bases as a function of the nature of the solvent upon the photochemical properties.

Electronic and Vibrational Spectroscopy of 1‐Methylthymine and its Water Clusters: The Dark State Survives Hydration

“…[15] The triplet states of uracil and thymine have been characterized in considerable detail. Weak phosphorescence of U at 77 K was observed with a maximum at 450 nm (2.76 eV) [4] which compares favorably with the computed vertical T 1 !S 0 deexcitation energy of % 2.73 eV. [12] The adiabatic transition energy is about 3.2 eV.…”

“…Phosphorescence is weak because the T 1 -S 2 and T 1 -S 0 spin-orbit coupling matrix elements are small whereas the significant T 1 -S 1 coupling does not lead to enhanced emission because the S 1 state is dark. [12] A triplet lifetime of 75 ms was observed for T at 77 K via phosphorescence [4] in contrast to a unimolecular decay time of 32 ms in deaerated aqueous solutions as measured by nanosecond transient absorption spectroscopy (ns-TAS). [16] Triplet state formation has a low quantum yield < 0.02 in water.…”

“…The pyrimidine bases uracil (U) and thymine (T) feature broad absorptions in the gas phase and in solution [4][5][6][7][8][9][10] with an onset at around 275 nm in the vapor spectrum. This first absorption band has been assigned to a 1 pp* S 2 !…”

TDDFT investigation on nucleic acid bases: Comparison with experiments and standard approach

“…These predictions are in accordance with the experimental data that in an aprotic solvent, the uracil and thymine have the n* state as the lowest singlet electronic excited state while in protic solvent the singlet * state lies lowest. 9,22 It is evident from the data shown in Tables 4 and 5 that the first * type Rydberg transition, the * orbital being localized at the N1H and C6H bonds, would be located in between the lowest two singlet * transitions. It is about 0.5 eV higher in energy than the lowest singlet vertical * state.…”

Solvent Effect on the Singlet Excited-state Dynamics of 5-Fluorouracil in Acetonitrile as Compared with Water

“…This is the most stable form in the electronic ground state, 24,25 and the only tautomer identified in solution and in the gas phase. 4,26 Results and discussion. Steady-state absorption and fluorescence spectra of 5FU in acetonitrile and aqueous solution are shown in Figure 1.…”