Tetracyanoquinodimethane bithiophene (QOT2) has a long-lived (57 μs) photoinduced excited state that may correspond to triplets resulting from intramolecular singlet fission (SF). Since SF usually occurs through intermolecular processes, a detailed description of the excited states involved and their evolution is needed to verify this hypothesis. The photoresponse of QOT2 is investigated using high-level electronic structure methods and quantum dynamics simulations, which show ultrafast passage through a conical intersection from the bright 1 1 B u state to the dark 2 1 A g surface. Characterization of QOT2's 2 1 A g wave function found it to be composed of two strongly coupled triplets, leading to the first detailed electronic structure description of an intramolecular 1 (TT) state. The population of such a state upon excitation of QOT2 raises the possibility of SF through conformational changes that decouple the triplets. However, reaching an appropriate geometry for decoupled triplets appears unlikely given the energy cost of 1.76 eV. Consequently, the hypothesis that the long-lived excited state corresponds to 2 1 A g , a spin singlet, strongly interacting double triplet, was explored. Transition moment calculations to assign excited-state absorption signals and investigations into internal conversion and intersystem crossing decay pathways indicate that a long-lived 2 1 A g state with 1 (TT) character is consistent with the available experimental data.

The use of nonclassical states of light to probe organic molecules has received great attention due to the possibility of providing new and detailed information regarding molecular excitations. Experimental and theoretical results have been reported which show large enhancements of the nonlinear optical responses in organic materials due to possible virtual-electronic-state interactions with entangled photons. In order to predict molecular excitations with nonclassical light, more detailed investigations of the parameters involved must be carried out. In this report we investigate the details of the state-to-state parameters important in calculating the contribution of particular transitions involved in the entangled two-photon absorption process for diatomic molecules. The theoretical discussion of the entangled two-photon process is described for a set of diatomic molecules. Specifically, we provide detailed quantum chemical calculations which give accurate energies and transition moments for selection-rule allowed intermediate states important in the entangled nonlinear effect for the diatomic molecules. These results are used to estimate in a more accurate manner the nonmonotonic behavior of the entangled two-photon absorption cross-section. We also derive accurate approximations that can be used to predict the period between entanglement-induced transparencies without needing exact values of the transition dipole moments. These results suggest that with the additional parameters allotted by the entangled two-photon absorption (in comparison to the classical case), it may be possible to predict and later control the nonlinear absorption and transparency of a molecule at a constant incident photon frequency.

The use of alternate coordinate systems as a means to improve the efficiency and accuracy of anharmonic vibrational structure analysis has seen renewed interest in recent years. While normal modes (which diagonalize the mass-weighted Hessian matrix) are a typical choice, the delocalized nature of this basis makes it less optimal when anharmonicity is in play. When a set of modes is not designed to treat anharmonicity, anharmonic effects will contribute to inter-mode coupling in an uncontrolled fashion. These effects can be mitigated by introducing locality, but this comes at its own cost of potentially large second-order coupling terms. Herein, a method is described which partially localizes vibrations to connect the fully delocalized and fully localized limits. This allows a balance between the treatment of harmonic and anharmonic coupling, which minimizes the error that arises from neglected coupling terms. Partially localized modes are investigated for a range of model systems including a tetramer of hydrogen fluoride, water dimer, ethene, diphenylethane, and stilbene. Generally, partial localization reaches ∼75% of maximal locality while introducing less than ∼30% of the harmonic coupling of the fully localized system. Furthermore, partial localization produces mode pairs that are spatially separated and thus weakly coupled to one another. It is likely that this property can be exploited in the creation of model Hamiltonians that omit the coupling parameters of the distant (and therefore uncoupled) pairs.

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Localized orbitals are representations of electronic structure, which are easier to interpret than delocalized, canonical orbitals. While unitary transformations from canonical orbitals into localized orbitals have long been known, existing techniques maximize localization without regard to coupling between orbitals. Especially in conjugated π spaces, orbitals are collapsed by unitary localization procedures into nonintuitive, strongly interacting units. Over-localization decreases interpretability, results in large values of interorbital coupling, and gives unmeaningful diagonal Fock energies. Herein, we introduce orbitals of intermediate localization that span between canonical and fully localized orbitals. To within a specified error, these orbitals preserve the diagonal nature of the Fock matrix while still introducing significant locality. In systems composed of molecular fragments, π spaces can be localized into weakly coupled units. Importantly, as the weakly coupled orbitals separate, highly coupled orbitals maintain their expected structure. The resulting orbitals therefore correspond well to chemical intuition and maintain accurate orbital energies, making this procedure unique among existing orbital localization techniques. This article focuses on the formation and physical analysis of orbitals that smoothly connect the known fully delocalized and fully localized limits.