Mechanism of Spin-Exchange Internal Conversion: Practical Proxies for Diabatic and Nonadiabatic Couplings

International audienceSpin-exchange internal conversion (SEIC) is a general class of reactions having singlet fission and triplet fusion as particular cases. Based on a charge-transfer (CT)-mediated mechanism and analytical derivation with a model Hamiltonian, we propose proxies for estimating the coupling strength in both diabatic and adiabatic pictures for general SEIC reactions. In the diabatic picture, we demonstrated the existence of a bilinear relationship between the coupling strength and molecular orbital overlap , which provides a practical way to predict diabatic couplings. In the adiabatic picture, we showed that nonadiabatic couplings can be approximated by simple functions of the wave function CT coefficients. These approaches were verified through the investigation of singlet oxygen photosensitization, where both 1 Δg and 1 g oxygen states can be competitively generated by a triplet fusion reaction. The interplay between the CT-mediated mechanism, the spatial factors of the bimolecular complex, and the electronic structure of the oxygen molecule during the reaction explains the curiously small coupling to the 1 g state along specific incidence directions. The results from both the diabatic and adiabatic pictures provide a comprehensive understanding of the reaction mechanism, which applies to general SEIC problems

New Insights into the State Trapping of UV-Excited Thymine

Ljiljana Stojanović, Shuming Bai, and Mario Barbatti thank the support of the Aix-Marseille Initiative d’Excellence (A*MIDEX) grant (No. ANR-11-IDEX-0001-02) funded by the French Government “Investissements d’Avenir” program supervised by the Agence Nationale de la Recherche. This work was granted access to the HPC resources of Aix-Marseille Université financed by the project Equip@Meso (ANR-10-EQPX-29-01) also within the “Investissements d’Avenir” program. Artur F. Izmaylov acknowledges funding from a Sloan Research Fellowship and the Natural Sciences and Engineering Research Council of Canada (NSERC) through the Discovery Grants Program

Ultrafast Excited-state Proton Transfer Processes: Energy Surfaces and On-the-fly Dynamics Simulations

The excited-state intramolecular proton transfer (ESIPT) is reviewed for several benchmark systems [o-hydroxybenzaldehyde (OHBA), salicylic acid and 2-(2′-hydroxyphenyl)-benzothiazole (HBT)] in order to verify the applicability of the time-dependent density functional theory (TDDFT) and the resolution-of-the-identity approximate second-order coupled cluster (RI-CC2) methods. It was found that these approaches are very well suited for the description of ESIPT processes. A comparative investigation of previous and new excited-state dynamics simulations is performed for HBT, 10-hydroxybenzo[h]quinoline (HBQ), and [2,2′-bipyridyl]-3,3′-diol (BP(OH)2). The time scale for the ESIPT process in these systems ranges in the time interval of 30−40 fs for HBT and HBQ and amounts to about 10 fs for the first proton transfer step in BP(OH)2. The dynamics simulations also show that the proton transfer in HBT is strongly supported by skeletal modes and the proton plays a rather passive role, whereas in HBQ a semipassive mechanism is found due to its increased rigidity in comparison to HBT. The special role of the double proton transfer in BP(OH)2 is discussed as well

Understanding the Impact of Symmetrical Substitution on the Photodynamics of Sinapate Esters Using Gas-Phase Ultrafast Spectroscopy

Two model biomimetic systems, ethyl sinapate (ES) and its symmetrical analogue, diethyl 2-(4-hydroxy-3,5-dimethoxybenzylidene)malonate (or diethyl sinapate, DES), are stripped to their core fundamentals through gas-phase spectroscopy to understand the underlying photophysics of photothermal materials. Following photoexcitation to the optically bright S1(ππ*) state, DES is found to repopulate the electronic ground state over three orders of magnitude quicker than its non-symmetrical counterpart, ES. Our XMS-CASPT2 calculations shed light on the experimental results, revealing crucial differences in the potential energy surfaces and conical intersection topography between ES and DES. From this work, a peak conical intersection, seen for DES, shows vital importance for the non-radiative ground state recovery of photothermal materials. This fundamental comparative study highlights the potential impact that symmetrical substitution can have on the photodynamics of sinapate esters, providing a blueprint for future advancement in photothermal technology

Modulating Electron Transfer in an Organic Reaction via Chemical Group Modification of the Photocatalyst

International audienceTuning electron transfer (ET) rates from catalysts to substrates is important for modulating photocatalytic organic reactions. In this work, we have taken pyrene-based photocatalysts (Py) for photocatalytic hydrodefluorination of polyfluoroarenes (FA) as model systems, and conducted a first-principle study on modulating ET rates from Py to FA via chemical modification of Py with different electron donating/withdrawing groups (EDGs/EWGs). The computed spatial distributions of frontier Kohn-Sham orbitals suggest that ET is energetically more favorable for Py-EDGs than for Py-EWGs. The estimated ET rates by a simplified Marcus model show that they are appreciably enhanced by EDGs substitution and weakened by EWGs substitution. Noticeably, the associated Gibbs free energy change plays a dominant role. Our findings of tuning ET rates for Py-FA complexes via chemical group modifications cast new insight into the rational design of metal-free photocatalysts for organic transformations. TOC Graphi

Photodynamics Simulations of Thymine: Relaxation into the First Excited Singlet State

Ab initio nonadiabatic dynamics simulations are reported for thymine with focus on the S2 → S1deactivation using the state-averaged CASSCF method. Supporting calculations have been performed on vertical excitations, S1 and S2 minima, and minima on the crossing seam using the MS-CASPT2, RI-CC2, MR-CIS, and MR-CISD methods. The photodynamical process starts with a fast (\u3c100 fs) planar relaxation from the S2 ππ* state into the πOπ* minimum of the S2 state. The calculations demonstrate that two π-excited states (denoted ππ* and πOπ*) are actually involved in this stage. The time in reaching the S2/S1 intersections, through which thymine can deactivate to S1, is delayed by both the change in character between the states as well as the flatness of the S2 surface. This deactivation occurs in an average time of 2.6 ps at the lowest-energy region of the crossing seam. After that, thymine relaxes to the nπ* minimum of the S1state, where it remains until the transfer to the ground state takes place. The present dynamics simulations show that not only the πOπ* S2 trapping but also the trapping in the nπ* S1 minimum contribute to the elongation of the excited-state lifetime of thymine

Multireference approaches for excited states of molecules

Understanding the properties of electronically excited states is a challenging task that becomes increasingly important for numerous applications in chemistry, molecular physics, molecular biology, and materials science. A substantial impact is exerted by the fascinating progress in time-resolved spectroscopy, which leads to a strongly growing demand for theoretical methods to describe the characteristic features of excited states accurately. Whereas for electronic ground state problems of stable molecules the quantum chemical methodology is now so well developed that informed nonexperts can use it efficiently, the situation is entirely different concerning the investigation of excited states. This review is devoted to a specific class of approaches, usually denoted as multireference (MR) methods, the generality of which is needed for solving many spectroscopic or photodynamical problems. However, the understanding and proper application of these MR methods is often found to be difficult due to their complexity and their computational cost. The purpose of this review is to provide an overview of the most important facts about the different theoretical approaches available and to present by means of a collection of characteristic examples useful information, which can guide the reader in performing their own applications

Steady and Time-Resolved Photoelectron Spectra Based on Nuclear Ensembles

Semiclassical methods to simulate both steady and time-resolved photoelectron spectra are presented. These approaches provide spectra with absolute band shapes and vibrational broadening beyond the Condon approximation, using an ensemble of nuclear configurations built either via distribution samplings or nonadiabatic dynamics simulations. Two models to account for the electron kinetic energy modulation due to vibrational overlaps between initial and final states are discussed. As illustrative examples, the steady photoelectron spectra of imidazole and adenine and the time- and kinetic-energy-resolved photoelectron spectrum of imidazole were simulated within the frame of time-dependent density functional theory. While for steady spectra only electrons ejected with maximum allowed kinetic energy need to be considered, it is shown that to properly describe time-resolved spectra, electrons ejected with low kinetic energies must be considered in the simulations as well. The results also show that simulations based either on full computation of photoelectron cross section or on simple Dyson orbital norms provide results of similar quality

Photoinduced Formation of H-bonded Ion Pair in HCFC-133a

International audienceHigh-level multireference electronic structure calculations have been performed to study the Cl − yield from photoexcited CF3CH2Cl (HCFC-133a). The analysis of this process tells that it relates to an electron transfer from the carbon to the Cl atom, forming a highly polar contact ion-pair complex, CF3HCH + •••Cl − , in the excited (S3) state. This complex has a strong binding energy of 3.53 eV, from which 0.47 eV is due to an underlying hydrogen bond. Through comparison with the results obtained for the prototype CH3Cl system, where a similar ion-pair state associated with Cl − formation has also been observed, it is suggested that photodissociation of HCFC-133a can also yield Cl − through analogous process. This hypothesis is further supported by nonadiabatic dynamics simulations, which shows formation of the ion-pair complex in the sub-picosecond time scale.