Relevance of πσ* states in the photoinduced processes of adenine, adenine dimer, and adenine–water complexes

Abstract

Ab initio calculations and time-resolved photoionization spectroscopy were carried out to characterize the role of the lowest two pi sigma* excited states for the photoinduced processes in the adenine monomer, adenine dimer, and adenine-water clusters. The calculations show--with respect to the monomer--a stabilization of 0.11-0.14 eV for the pi sigma* states in different isomers of adenine dimer and an even bigger stabilization of 0.14-0.36 eV for isomers of adenine-(H2O)1 and adenine-(H2O)3. Hence, the stabilized pi sigma* states should play an important role in the excited-state relaxation of partially or fully solvated adenine. This conclusion is supported by experimental results: In the adenine monomer, strong n pi* state signals are observed. Those signals are reduced in adenine dimer and vanish in water clusters due to the competing relaxation via the pi sigma* states.

Electronic Structure of Adenine and Thymine Base Pairs Studied by Femtosecond Electron−Ion Coincidence Spectroscopy

“…Photochemical excitation of the S 2 (ππ*) state is followed by rapid internal conversion on a picosecond to femtosecond time scale. 3,5,[8][9][10][11]15,16 Internal conversion can populate the S 1 (nπ*) state, as proposed by theory 21 and spectroscopic and dynamic measurements. 3,6,22 The strong mixing of ππ* and nπ* states was confirmed by timeresolved photoelectron spectroscopy 8,9 and by rotational band contour analysis.…”

“…21,[24][25][26] A conical intersection to a quasi-dissociative πσ* may also play a role, 8,11 especially at higher excitation energies or in polarizable environments. 16,[27][28][29][30] Time-resolved photoelectron spectroscopy (TRPES) experiments comparing adenine with 9-methyl-substituted adenine 11 found the same biexponential time constants, 0.1 and 1.2 ps, as ion yield studies. 10,18 But different decay-associated photoelectron spectra of the 0.1 ps component were assigned to the participation of a πσ* state based on calculations of the FranckCondon spectra.…”

“…Possibly, the stabilization of the πσ* state in the dimer leads to a coupling with the ππ* state at the expense of the nπ* state. 16 The results of FEICO spectroscopy at pump-probe delays of 0 and 500 fs for the adenine monomer and dimer are shown in Figure 4. As in the time-resolved ion traces (Figure 3), we used a three photon 800 nm ionization process in these experiments.…”

“…Small molecular clusters in the gas phase can mimic important structural elements of DNA and thus help to extend the power of gas-phase spectroscopic methods to increasingly complex systems. [13][14][15][16][17][18] Here we report a direct characterization of the electronic structure for photoexcited states in DNA base pairs of adenine and thymine and correlate the structure with corresponding dynamic processes using femtosecond time-resolved electron-ion coincidence (FEICO) spectroscopy. 19,20 The photophysics of the isolated adenine monomer has been investigated in some detail.…”

“…In the dimers, an additional quenching of the nπ* state was assigned to a relaxation via the πσ* state. 16 TRPES is a powerful technique that can follow excited-state nonadiabatic processes in real time and often allows for the unambiguous assignment of excited-state character along a reaction coordinate. [40][41][42] Hence, TRPES should be the method of choice to identify changes in the electronic character between base monomers and dimers.…”