A theoretical study of low-lying singlet and triplet excited states of quinazoline, quinoxaline and phthalazine: insight into triplet formation

Quinazoline, quinoxaline and phthalazine are nitrogen containing heterocyclic aromatic molecules which belong to the class diazanaphthalenes. These isomers have low-lying nπ* and naphthalene-like ππ* states that interact via spin-orbit coupling. In this contribution, we study their structure and electronic states by means of a coupled-cluster method. The computed properties are compared to those of cinnoline which were obtained in our previous study [Etinski et al., Phys. Chem. Chem. Phys., 2014, 16, 4740]. The excited state features of these isomers are dependent on the position of the nitrogen atoms. We find that quinazoline and quinoxaline exhibit similarities in the ordering and character of the excited states. In contrast, a marked difference in the electronic and geometric structures of the lowest excited triplet states of cinnoline and phthalazine is noticed, although both are orthodiazanaphthalenes. Our findings suggest that the S₁ [radiolysis arrow - arrow with voltage kink] T₁ channel is responsible for the rapid intersystem crossing in quinazoline and quinoxaline, whereas the S₁ [radiolysis arrow - arrow with voltage kink] T₂ pathway is active in phthalazine.

Nonadiabatic photodynamics and UV absorption spectrum of all-trans-octatetraene

The short-time molecular quantum dynamics of all-trans-octatetraene after electronic excitation to the first bright valence state is theoretically investigated.

Solvent tunable photophysics of acridone: a quantum chemical perspective

High-level electronic structure methods and quantum chemistry programs have been employed for a thorough investigation of the photophysics of acridone in isolated and solvated states.

Quantum-Chemical Studies on Excitation Energy Transfer Processes in BODIPY-Based Donor-Acceptor Systems

BODIPY-based excitation energy transfer (EET) cassettes are experimentally extensively studied and serve as excellent model systems for the investigation of photophysical processes, since they occur in any photosynthetic system and in organic photovoltaics. In the present work, the EET rates in five BODIPY-based EET cassettes in which anthracene serves as the donor have been determined, employing the monomer transition density approach (MTD) and the ideal dipole approximation (IDA). To this end, a new computer program has been devised that calculates the direct and exchange contributions to the excitonic coupling (EC) matrix element from transition density matrices generated by a combined density functional and multireference configuration interaction (DFT/MRCI) calculation for the monomers. EET rates have been calculated according to Fermi's Golden Rule from the EC and the spectral overlap, which was obtained from the calculated vibrationally resolved emission and absorption spectra of donor and acceptor, respectively. We find that the direct contribution to the EC matrix element is dominant in the studied EET cassettes. Furthermore, we show that the contribution of the molecular linker to the EET rate cannot be neglected. In our best fragment model, the molecular linker is attached to the donor moiety. For cassettes in which the transition dipole moments of donor and acceptor are oriented in parallel manner, our results confirm the experimental findings reported by Kim et al. [J. Phys. Chem. A 2006, 110, 20-27]. In cassettes with a perpendicular orientation of the donor and acceptor transition dipole moments, dynamic effects turn out to be important.

Carotenoids and light-harvesting: from DFT/MRCI to the Tamm-Dancoff approximation

Carotenoids are known to play a fundamental role in photosynthetic light-harvesting (LH) complexes; however, an accurate quantum-mechanical description of that is still missing. This is due to the multideterminant nature of the involved electronic states combined with an extended conjugation which limits the applicability of many of the most advanced approaches. In this study, we apply a multireference configuration interaction extension of density functional theory (DFT/MRCI) to describe transition energies and densities as well as the corresponding excitonic couplings, for the three lowest singlet excited states of nine carotenoids present in three different LH complexes of algae and plants. These benchmark results are used to find an approximated computational approach, which could be used to quantitatively reproduce the key quantities at a reduced computational cost. To this end, we tested the Tamm-Dancoff approximation (TDA) to time-dependent density functional theory in combination with different functionals. By analyzing the errors with respect to DFT/MRCI-TDA results for the full set of electronic properties, we conclude that TDA-TPSS with small basis sets indeed represents an effective approach to investigate LH processes that involve carotenoids.

Towards an understanding of the singlet–triplet splittings in conjugated hydrocarbons: azulene investigated by anion photoelectron spectroscopy and theoretical calculations

Electron density overlaps are correlated with singlet triplet splittings: azulene as a test case.

Internal heavy atom effects in phenothiazinium dyes: enhancement of intersystem crossing via vibronic spin–orbit coupling

The effect of substituting the intra-cyclic sulphur of thionine by oxygen (oxonine) and selenium (selenine) on the intersystem crossing (ISC) efficiency has been studied using high level quantum mechanical methods.

Photophysics of flavin derivatives absorbing in the blue-green region: thioflavins as potential cofactors of photoswitches

The purpose of this study was to find flavin derivatives with absorption maxima in the blue-green region of the visible spectrum that might be used as alternative cofactors in blue-light photoreceptors. To this end, the vertical absorption spectra of eight lumiflavin-related compounds were calculated by means of quantum chemical methods. The compounds differ from lumiflavin by the subsitution of an S atom for an O atom at the 2- and/or 4-positions of the isoalloxazine core, the substitution of an N atom for a CH group in the 6- and/or 9-positions, or an extension of the π system at the 7- and 8-positions. For the three most promising compounds, 2-thio-lumiflavin, 4-thio-lumiflavin, and 2,4-dithio-lumiflavin, the quantum chemical investigations were extended to include geometry relaxations in the excited states, rates for spin-forbidden transitions and an estimate of spectral shifts brought about by polar protic environments. We find these thiocarbonyl compounds to have very promising excited-state properties. They absorb in the blue-green wavelength regime around 500 nm, i.e., substantially red-shifted with respect to lumiflavin that is the cofactor of natural blue-light photoreceptors. Their triplet quantum yields are predicted to be close to unity while their triplet lifetimes are long enough to enable bimolecular photochemical reactions. The combination of these properties makes the thioflavins potentially suitable candidates as cofactors in biomimetic photoswitches.

On the molecular mechanism of non-radiative decay of nitrobenzene and the unforeseen challenges this simple molecule holds for electronic structure theory

We present a complete mechanistic picture of the non-radiative relaxation of nitrobenzene via IC and ISC along three internal coordinates.

On the photophysics of carotenoids: a multireference DFT study of peridinin

We present a quantum-mechanical investigation of the photophysics of a specific carotenoid, peridinin, which is present in light-harvesting complexes. The fundamental role played by the geometry in determining the position and character of its low-lying singlet electronic states is investigated using a multireference DFT approach in combination with a continuum solvation model. The main photophysical properties of peridinin appear to be governed by the lowest two singlet excited states, as no evidence points to an intermediate S* state and the energies of the upper excited states are too high to allow their population with excitation in the visible range. These two excited states (S1, 2(1)A(g)(-) and S2, 1(1)B(u)(+)) are highly connected through the conjugation path here characterized by the value of the bond length alternation (BLA). The S1 and S2 states present distinct natures for small BLA values, whereas for larger ones they become more similar in terms of both brightness and dipolar character and their energies become closer. The geometrical issue is thus of fundamental importance for a correct interpretation of the spectroscopic signatures of peridinin.

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.

Electronic coherence provides a direct proof for energy-level crossing in photoexcited lutein and beta-carotene

We investigate femtosecond transient absorption dynamics of lutein and beta-carotene. Strong oscillations up to about 400 fs are observed, depending on excitation or detection wavelength and solvent. We propose electronic quantum beats as the origin of these oscillations. They provide direct proof for strong coupling of the 1B(u)(+) with another electronic "dark" state predicted by quantum chemical calculations to be the 1B(u)(-) state resulting in a crossing within a dynamic relaxation model. The overall dynamics can be described well by an optical Bloch equation approach.

Spin-forbidden transitions in flavone

The ground and low-lying excited electronic states of flavone were investigated by means of quantum chemical methods including spin-orbit coupling. Minimum structures were determined employing (time-dependent) Kohn-Sham density functional theory. Spectral properties were computed utilizing a combined density functional and multi-reference configuration interaction (DFT/MRCI) method. Intersystem crossing (ISC) rate constants for the S1-->T1 transition were computed using a discretized Fermi golden rule approach. For the evaluation of phosphorescence lifetimes a multi-reference spin-orbit configuration interaction procedure (DFT/MRSOCI) was invoked. According to the calculations the phenyl ring is twisted out of the benzopyrone plane by 28 degrees in the electronic ground state whereas the nuclear frame is nearly planar in the lowest excited (npi*)1 (S1) state and is slightly V-shaped in the (pipi*)3 (T1) and (pipi*)1 (S2) states. The calculations clearly show that the T1 state has mainly pipi* character. The large spin-orbit coupling of the S1 and T1 states and their small energy gap explain the high S1-->T1 ISC rate for which a value of kISC approximately 3x10(11) s is computed, in good agreement with experimental build-up times of the Tn<--T1 absorption. In the absence of collisions and other nonradiative processes, the T1 state of flavone is predicted to be long-lived with a pure phosphorescence lifetime of tauP approximately 4 s, in qualitative agreement with low-temperature measurements. The much faster decay of triplet flavone observed in fluid solutions is ascribed to nonradiative processes.