The effect of dimerization on the excited state behavior of methylated xanthine derivatives: a computational study

Abstract

The behavior of monomers and dimers of methylated xanthine derivatives in their excited states is investigated by means of the ADC(2), CASSCF, and CASPT2 methods. The results of the calculations of stationary points in the ground and excited states, minima on the S0/S1 crossing seams and the relaxation pathways are used to provide the interpretation of experimental observations of the monomer xanthine derivatives. The effect of dimerization on the excited state properties is studied for various relative orientations of the monomers in the dimer complexes in comparison with the relevant monomer species. A significant stabilization in the excited state minima of dimers is observed. These can act as trapping sites. Various types of conical intersections, with both localized and delocalized characters of wavefunctions, have been found, mainly energetically above the lowest bright excited state in the FC region. In addition, structures with the bonds formed between the two monomers were also found on the crossing seams. The possibility of ultrafast relaxation via these conical intersections is discussed.

Spectroscopic signatures and structural motifs in isolated and hydrated caffeine: a computational study

“…1,6 In this regard photo dynamics of nucleic acid analogue, xanthine (3,7-dihydro-purine-2,6-dione), plays an important role with the goal of transferring the experience obtained in these simpler systems to more complex situation found in DNA. 9 Nachtigallova et al 10 studied the excited state behavior of methylated xanthines and their computed relaxation paths from the lowest excited state S 1 minima towards conical intersections indicates that at least part of the population will be retained in the excited state so that it will be detected in the R2PI experiment as sharp vibrationally resolved spectra, which is in agreement to earlier report. Resonant two photon ionization (R2PI) experiment performed for the methylated derivatives of xanthine (caffeine, theophylline and theobromine) in the gas phase 8 were all sharp and vibronically resolved, suggesting a long excited state lifetime.…”

“…However in the case of smaller (or non) conjugated system, p-p* transition may exhibit opposite behavior, as reported recently by Boni et al 43 Therefore, a blue shi for a p-p* transition is attributed to the reduction in the p-electron conjugation length 49 of the O 2 -bonded caffeine monohydrates (involving isolated carbonyl) in contrast to the O 1 -bonded monohydrates. The missing of the most stable O 1 -bonded clusters in the R2PI spectra can be explained by the assumption that water at conjugated carbonyl alters the relaxation prole by opening additional relaxation channels 1,2,9,10,44 for ultrafast decay to the ground state. As seen in Table 2, oscillator strength increases in O 1 bonded caffeine clusters, whereas it decreases in O 2 bonded caffeine clusters.…”

Enhanced semiempirical QM methods for biomolecular interactions

“…The very good accuracy of SQM-DH methods for biomolecular interactions in comparison to high-level CCSD(T)/CBS references begs the question of whether these methods might not be good enough to replace DFT-D methods for the accurate computation of such interactions in medium sized systems. Accordingly many studies on host/guest or inclusion complexes were published over the last years [100–104] , as well as on molecular tweezers [105] , non-covalent complexation in general [106–110] , and supramolecular chemistry [111,112] , most of them with very promising results at SQM-DH level. Muddana and Gilson presented a study on 29 CB7 host-guest systems with PM6-DH +/COSMO based on their ‘minima mining’ (M2) approach for predicting binding affinities and found good agreement with experiment [113] .…”

Spectroscopic signatures and structural motifs in isolated and hydrated theophylline: a computational study

“…In the isolated theophylline, the lowest excited state S 1 exhibits ππ* character and primary photo-excitation involves this optically bright state S 1 ( 1 ππ* ). 17 It is common knowledge that hydration can shift electronic states and hence, it can modify the excited state dynamics.…”