Ultrafast Excited-State Dynamics of Hydrogen-Bonded Cytosine Microsolvated Clusters with Protic and Aprotic Polar Solvents

Abstract

Microsolvation effects on the ultrafast excitedstate deactivation dynamics of cytosine (Cy) were studied in hydrogen-bonded Cy clusters with protic and aprotic solvents using mass-resolved femtosecond pump−probe ionization spectroscopy. Two protic solvents, water (H 2 O) and methanol (MeOH), and one aprotic solvent, tetrahydrofuran (THF), were investigated, and transients of Cy•(H 2 O) 1−6 , Cy•(MeOH) 1−3 , and Cy•THF microsolvated clusters produced in supersonic expansions were measured. With the aid of electronic structure calculations, we assigned the observed dynamics to the lowenergy isomers of various Cy clusters and discussed the microsolvation effect on the excited-state deactivation dynamics. With the protic solvents only the microsolvated clusters of Cy keto tautomer were observed. The observed decay time constants of Cy•(H 2 O) n are 0.5 ps for n = 1 and ∼0.2−0.25 ps for n = 2−6. For Cy•(MeOH) n clusters, the decay time constant for n = 1 cluster is similar to that of the Cy monohydrate, but for n = 2 and 3 the decays are about a factor of 2 slower than the corresponding microhydrates. With the aprotic solvent, THF, hydrogen-bonded complexes of both keto and enol tautomers are present in the beam. The keto-Cy•THF shows a decay similar to that of the keto-Cy monomer, whereas the enol-Cy•THF exhibits a 2-fold slower decay than the enol-Cy monomer, suggesting an increase in the barrier to excited-state deactivation upon binding of one THF molecule to the enol form of Cy.

Hydrogen‐Bonded Complexes of Fluorophenylacetylenes: To Fluoresce or Not?

“…[5] However, in the case of cytosine and its methylated analogue, hydrogen bonding with water, methanol, and tetrahydrofuran does not alter the excited state dynamics, significantly. [6] Similar results were also reported for adenine and its hydrogen-bonded complex with water. [7] In a nutshell, it has been suggested that the nature of coupling (or lack of it) of the initially excited 1 ππ* bright state to one or more low-lying excited states largely determines the nature of excited state dynamics.…”

Formation of Long-Lived Dark States during Electronic Relaxation of Pyrimidine Nucleobases Studied Using Extreme Ultraviolet Time-Resolved Photoelectron Spectroscopy

“…Since the photophysics of nucleobases is strongly influenced by hydrogen bonding, it is important to compare the dynamics of aqueous nucleobases with those of isolated molecules in the gas phase. However, since nucleosides and nucleotides are vulnerable to evaporation at high temperatures, , we studied gaseous Thy and Ura alternatively. ,,− Thy and Ura are known to have keto forms in the gas phase almost exclusively; the minute amount of keto-enol tautomers, which have higher energy than the keto form by more than 9 kcal/mol, is practically negligible. On the other hand, the dominant tautomer of gaseous cytosine (Cyt) has a noncanonical form so that Cyt was excluded in the present study.…”

Tautomer‐Specific Resonant Photoelectron Imaging of Deprotonated Cytosine Anions

“…[1] Other rare tautomers have also been observed by various spectroscopic techniques. [5][6][7][8][9][10][11] Additionally,t he effect of water molecules on the structural dynamics and tautomerism of cytosine have been investigated, [12,13] showing that water can facilitate tautomerization from the canonical form to the rare tautomers by lowering the activation barriers of the prototropic process.T he high stability of the keto-amino tautomers in solution is derived from their large dipole moments. [3,14] Early thermodynamic and kinetic studies showed that the major KAN1H and minor KAN3H forms (Scheme S1 a) are in equilibrium in aqueous solution.…”