Susanne Salzmann

UV excitation and radiationless deactivation of imidazole

The vertical spectrum and the radiationless decay of imidazole have been investigated theoretically. Benchmark calculations were performed employing different methods and levels. Four different conical intersections were characterized and the reaction paths connecting the Franck-Condon region to them were computed. Two of the conical intersections show puckered structures while the other two show NH and ring dissociation patterns. The pipi( *)/S(0) N(1)-puckered conical intersection is connected to the planar pisigma( *)/S(0) ring-opened conical intersection by a branch of the crossing seam. After excitation into the first (1)pipi( *) state, the internal conversion can occur either in this branch of crossing seam or along the NH-dissociation path.

Influence of the LOV Domain on Low-Lying Excited States of Flavin: A Combined Quantum-Mechanics/Molecular-Mechanics Investigation

The ground and low-lying excited states of flavin mononucleotide (FMN) in the light, oxygen, and voltage sensitive (LOV) domain of the blue-light photosensor YtvA of Bacillus subtilis were studied by means of combined quantum-mechanical/molecular-mechanical (QM/MM) methods. The FMN cofactor (without the side chain) was treated with density functional theory (DFT) for the geometry optimizations and a combination of DFT and multireference configuration interaction (MRCI) for the determination of the excitation energies, while the protein environment was represented by the CHARMM force field. In addition, several important amino acid side chains, including the reactive cysteine residue, were included in the QM region in order to probe their influence on the spectral properties of the cofactor in two protein conformations. Spin-orbit coupling was taken into account employing an efficient, nonempirical spin-orbit mean-field Hamiltonian. Our results reveal that the protein environment of YtvA-LOV induces spectral shifts for the (pi pi*) states that are similar to those in aqueous solution. In contrast, the blue shifts of the (n pi*) states are smaller in the protein environment, enabling a participation of these states in the decay processes of the optically bright S(1) state. Increased spin-orbit coupling between the initially populated S(1) state and the T(1) and T(2) states is found in YtvA-LOV as compared to free lumiflavine in water. The enhanced singlet-triplet coupling is brought about partially by configuration interaction with (n pi*) states at the slightly out-of-plane distorted minimum geometry. In addition, an external heavy-atom effect is observed when the sulfur atom of the nearby cysteine residue is included in the QM region, in line with experimental findings.

Photophysical Properties of Structurally and Electronically Modified Flavin Derivatives Determined by Spectroscopy and Theoretical Calculations

Four different riboflavin (RF) derivatives, two electronically modified compounds (1- and 5-deazariboflavin, 1DRF and 5DRF) and two sterically modified compounds (7,8-didemethyl- and 8-isopropylriboflavin, DMRF and iprRF), were subjected to a combination of time-resolved measurements (absorption and fluorescence) and high-level quantum chemical investigations. Both alkyl-modified flavins showed similar fluorescence properties as the parent compound, yet 5DRF had a larger quantum yield of fluorescence (PhiF = 0.52) than RF (PhiF = 0.27). Interestingly, 1DRF did not show fluorescence at all under these steady state conditions. The triplet quantum yield was different for the modified flavins such that no triplet formation was found for 1DRF, whereas the other compounds all formed triplet states (PhiTR for 5DRF of 0.64 and 0.50 and 0.23 for iprRF and DMRF, respectively). The triplet states of the two alkyl-modified flavins decayed with similar time constants as the parent compound, whereas a shorter lifetime was measured for 5DRF (tauTR = 15 micros, compared to tauTR = 29 micros for RF). In the calculations, the flavin derivatives were modeled as lumiflavins, that is, without the ribityl chain. We conclude that for aqueous solutions of DMRF, iprRF, and 5DRF intersystem crossing (ISC) takes place from the S1 1(pipi*) to the T2 3(pipi*) state by a vibronic spin-orbit coupling mechanism, a process common to most flavins, whereas ISC is slow in excited 1DRF due to the absence of a close-by triplet state.

Electronically excited states of tryptamine and its microhydrated complex

The lowest electronically excited singlet states of tryptamine and the tryptamine (H2O)1 cluster have been studied, using time dependent density functional theory for determination of the geometries and multireference configuration interaction for the vertical and adiabatic excitation energies, the permanent dipole moments, and the transition dipole moment orientations. All molecular properties of the seven experimentally observed conformers of tryptamine could be reproduced with high accuracy. A strong solvent reorientation has been found upon electronic excitation of the 1:1 water cluster of tryptamine to the L(a) and L(b) states. The adiabatically lowest excited singlet state in case of the tryptamine monomer is the L(b) state, while for the 1:1 water complex, the L(a) is calculated below the L(b) state.

Photophysics of phenalenone: quantum-mechanical investigation of singlet–triplet intersystem crossing

We have examined the electronic and molecular structure of 1H-phenalen-1-one (phenalenone) in the electronic ground state and in the lowest excited states, as well as intersystem crossing. The electronic structure was calculated using a combination of density functional theory and multi-reference configuration interaction. Intersystem crossing rates were determined using Fermis golden rule and taking direct and vibronic spin-orbit coupling into account. The required spin-orbit matrix elements were obtained applying a non-empirical spin-orbit mean-field approximation. Our calculated electronic energies are in good agreement with experimental data. We find the lowest excited singlet states to be of the npi* (S1) and pipi* (S2) type. Energetically accessible from S1 are two triplet states of the pipi* (T1) and npi* (T2) type, the latter being nearly degenerate to S1. This ordering of states is retained when the molecular structure in the electronically excited states is relaxed. We expect very efficient intersystem crossing between S1 and T1. Our calculated intersystem crossing rate is approximately 2 x 10(10) s(-1), which is in excellent agreement with the experimental value of 3.45 x 10(10) s(-1). Our estimated phosphorescence and fluorescence rates are many orders of magnitude smaller. Our results are in agreement with the experimentally observed behavior of phenalenone, including the high efficiency of 1O2 production.

Isolated and Solvated Thioxanthone: A Photophysical Study

Quantum chemical methods have been employed to study the photophysics of thioxanthone in vacuum and various solvents. Structurally, the solvation leads to a lengthening of the carbonyl bond, whereas the benzene skeleton is mostly unaffected. This is mirrored by the larger blue shift of the (n(O)π*) states as compared to the red shift which the (ππ*) states undergo. For a proper understanding of the radiative and radiationless processes occurring, the excitation energy profile along a linearly interpolated path has been determined in various cases. The interesting interplay of excited states thus revealed, has been investigated to qualitatively suggest the relaxation pathways available (or dominant) in the cases under study. Rates for these processes have also been computed wherever possible.

Spectroscopic and Theoretical Study on Electronically Modified Chromophores in LOV Domains: 8-Bromo- and 8-Trifluoromethyl-Substituted Flavins

Two chemically synthesized flavin derivatives, 8‐trifluoromethyl‐ and 8‐bromoriboflavin (8‐CF3RF and 8‐BrRF), were photochemically characterized in H2O and studied spectroscopically after incorporation into the LOV domain of the blue light photoreceptor YtvA from Bacillus subtilis. The spectroscopic studies were paralleled by high‐level quantum chemical calculations. In solution, 8‐BrRF showed a remarkably high triplet quantum yield (0.97, parent compound riboflavin, RF: 0.6) and a small fluorescence quantum yield (0.07, RF: 0.27). For 8‐CF3RF, the triplet yield was 0.12, and the fluorescence quantum yield was 0.7. The high triplet yield of 8‐BrRF is due to the bromine heavy atom effect causing a stronger spin–orbit coupling. Theoretical calculations reveal that the decreased triplet yield of 8‐CF3RF is due to a smaller charge transfer and a less favorable energetic position of T2, required for intersystem crossing from S1 to T1, as an effect of the electron‐withdrawing CF3 group. The reconstitution of the LOV domain with the new flavins resulted in the typical LOV photochemistry, consisting of triplet state formation and covalent binding of the chromophore, followed by a thermal recovery of the parent state, albeit with different kinetics and photophysical properties.