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

Abstract

Benzophenone (BP) despite its relatively simple molecular structure is a paradigmatic sensitizer, featuring both photocatalytic and photobiological effects due to its rather complex photophysical properties. In this contribution we report an original theoretical approach to model realistic, ultra-fast spectroscopy data, which requires describing intra- and intermolecular energy and structural relaxation. In particular we explicitly simulate time-resolved pump-probe spectra using a combination of state-of-the art hybrid quantum mechanics/molecular mechanics dynamics to treat relaxation and vibrational effects. The comparison with experimental transient absorption data demonstrates the efficiency and accuracy of our approach. Furthermore the explicit inclusion of the solvent, water for simulation and methanol for experiment, allows us, despite the inherent different behavior of the two, to underline the role played by the H-bonding relaxation in the first hundreds of femtoseconds after optical excitation. Finally we predict for the first time the two-dimensional electronic spectrum (2DES) of BP taking into account the vibrational effects and hence modelling partially symmetric and asymmetric ultrafast broadening.

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

“…It is worth noting that solvation increases the brightness of these near-IR transitions, as reported elsewhere. 115 As reported in Table 2, five bright S 1 ¦S n vertical transitions are found in the broad energy range from 1.8 to 4.5 eV above the S 1 state energy when employing both multireference and TD-DFT methods: single excitations are associated to the first two ESAs, namely S 1 -ESA1 and S 1 -ESA2, while double excitations contribute to three high-lying ESAs, namely S 1 -ESA3-5.…”

Intermolecular Hydrogen Bonding Controlled Intersystem Crossing Rates of Benzophenone

“…10 Ultraviolet (UV) excited benzophenone is known to undergo ultrafast conversion from singlet to triplet states, a property which is exploited in some of the above applications, but the intersystem crossing (ISC) mechanism remains the subject of experimental and computational investigations. [11][12][13][14][15][16][17][18][19][20][21] Here, we examine how the ISC is modified by hydrogen bonding in a protic solvent.…”

COBRAMM 2.0 — A software interface for tailoring molecular electronic structure calculations and running nanoscale (QM/MM) simulations

“…A focus is taken in the Vis probing window as it provides fingerprints that can also be registered with linear-response-based method. [90] After the first 100 fs, this main fingerprint is red-shifted towards the 12-14k cm -1 range while blue-shifting the other intense transitions registered at the FC region. This strong shift is encompassed by pronounced carbonyl stretchings occurring along the S1(nOπ*) relaxation path that stabilise (red-shift) the excited state absorption signal while blue-shifting other higher-lying nOπ* contributions.…”

“…Two water molecules H-bonded to the oxygen of BP were included in the QM part of the calculation. Geometry data was obtained from a previous Amber ff99 equilibration run with periodic boundary conditions [90]. All optimizations were started from the same, OM2/Amberpre-optimized ground state geometry.…”

“…Fig 12 displays a simulated two-dimensional excited state (2DES) spectrum, based on one excited S1 state trajectory computed with the TD-B3LYP method (see ref. [90] for details). The spectrum itself was created by using the RASPT2 method to obtain estimates for vertical excitation energies and associated transition dipole moments on top of the DFT computed geometries out of the trajectory.…”