Resolving the Singlet Excited State Manifold of Benzophenone by First-Principles Simulations and Ultrafast Spectroscopy

Abstract

Accurate characterization of the high-lying excited state manifolds of organic molecules is of fundamental importance for the interpretation of the rich response detected in time-resolved nonlinear electronic spectroscopies. Here, we have characterized the singlet excited state manifold of benzophenone (BP), a versatile organic photoinitiator and a well-known DNA photosensitizer. Benchmarks of various multiconfigurational/multireference (RASSCF/PT2) and time-dependent density functional theory (TD-DFT) approaches allowed assignments of experimental linear absorption signals of BP in the ultraviolet (UV) region, with unprecedented characterization of ground state absorptions in the far UV. Experimental transient absorption spectra obtained by UV-vis pump-probe spectroscopy at very short time delays are shown to be directly comparable to theoretical estimates of excited state absorptions (from the low-lying nπ* and ππ* singlet states) in the Franck-Condon region. Multireference computations provided reliable interpretation of the PP spectra, with TD-DFT results yielding a fair agreement as long as electronic transitions featuring double excitations contributions are not involved. These results lay the groundwork for further computational studies and interpretation of experimental nonlinear electronic spectra of benzophenone in more complex systems, such as BP/DNA adducts.

The effect of solvent relaxation in the ultrafast time-resolved spectroscopy of solvated benzophenone

“…This bright band can be assigned to transitions taking place from S 1 to relatively high-energy states. As also shown in a previous contribution 44 the transitions involves the π* system of the aromatic ring and the excited n level of the carbonyl oxygen. However, while the experimental spectrum in methanol presents a slight red shift most likely attributable to intramolecular vibrational relaxation, the simulated results in water show a significant blue-shift of about 4000 cm -1 (0.5 eV) from 100 to 300 fs.…”

“…It is also noteworthy that other approaches such as DFT/multi-reference configuration interactions (DFT/MRCI) also yields a very good agreement with the experimental steady state spectrum as reported in ESI. As reported elsewhere 44 it is also remarkable that TD-DFT, especially using the B3LYP functional, provides a good and balanced description of the various excited states as compared to wave function based RASPT2 results.…”

“…We exploit the results of the simulations to predict in particular UV-Vis transient absorption (TA) spectra as well as 2DES data. We assess the accuracy of this approach by (i) bench marking with high-level, computationally very expensive, modelling 44 and (ii) by comparing with experimental TA data obtained for BP in methanol, which has a similar polarity but weaker Hbonding character as compared to water, and where BP is much more soluble. While the predicted dominant spectral features match with the experimentally observed ones, our theoretical results anticipate an ultrafast spectral relaxation of the surrounding water molecules and may be interpreted in terms of relaxation of the solvent H-bonding interactions with BP in the excited state.…”

Deactivation of Excited States in Transition‐Metal Complexes: Insight from Computational Chemistry

“…The theoretical study of ESA can be very useful to interpret the resultso ft he widely used pump-probe experimentsi nt ime-resolved spectroscopy.R ecently,apromising approach has been published based on the RASSCFm ethod considering al arge number of roots. [153] Good results are reported for the organic chromophore benzophenone and it would be very interesting to investigate the performance of this approach for transition-metal complexes. The second new aspectc ould offer am ulticonfigurationalg eometry optimizer that is competitive with the standard DFT schemes, both in accuracy and speed.…”

Ultrafast and radiationless electronic excited state decay of uracil and thymine cations: computing the effects of dynamic electron correlation

“…The transitions were broadened with Gaussian functions having full width at half maximum of 0.3 eV, as used in similar organic systems, 65 while the IR signals were broadened with Gaussian functions having full width at half maximum of 10 cm −1 . 66 This is an approximation: we have not computed rovibrational broadenings explicitly.…”