Photoreactivity of a push-pull merocyanine in static electric fields: a three-state model of isomerization reactions involving conical intersections

From a one-mode to a multi-mode understanding of conical intersection mediated ultrafast organic photochemical reactions

This review discusses how ultrafast organic photochemical reactions are controlled by conical intersections, highlighting that decay to the ground-state at multiple points of the intersection space results in their multi-mode character.

State-averaged CASSCF with polarizable continuum model for studying photoreactions in solvents: Energies, analytical nuclear gradients, and non-adiabatic couplings

This paper presents state-averaged complete active space self-consistent field in polarizable continuum model (PCM) for studies of photoreactions in solvents. The wavefunctions of the solute and the PCM surface charges of the solvent are optimized simultaneously such that the state-averaged free energy is variationally minimized. The method supports both fixed weights and dynamic weights where the weights are automatically adjusted based on the energy gaps. The corresponding analytical nuclear gradients and non-adiabatic couplings are also derived. Furthermore, we show how the new method can be entirely formulated in terms of seven basic operations, which allows the implementation to benefit from existing high-performance libraries on graphical processing units. Results demonstrating the accuracy and performance of the implementation are presented and discussed. We also apply the new method to the study of minimal conical intersection search and photoreaction energy pathways in solvents. Effects from the polarity of the solvents and different formulas of dynamic weights are compared and discussed.

A systematic model study quantifying how conical intersection topography modulates photochemical reactions

Despite their important role in photochemistry and expected presence in most polyatomic molecules, conical intersections have been thoroughly characterized in a comparatively small number of systems. Conical intersections can confer molecular photoreactivity or photostability, often with remarkable efficacy, due to their unique structure: at a conical intersection, the adiabatic potential energy surfaces of two or more electronic states are degenerate, enabling ultrafast decay from an excited state without radiative emission, known as nonadiabatic transfer. Furthermore, the precise conical intersection topography determines fundamental properties of photochemical processes, including excited-state decay rate, efficacy, and molecular products that are formed. However, these relationships have yet to be defined comprehensively. In this article, we use an adaptable computational model to investigate a variety of conical intersection topographies, simulate resulting nonadiabatic dynamics, and calculate key photochemical observables. We varied the vibrational mode frequencies to modify conical intersection topography systematically in four primary classes of conical intersections and quantified the resulting rate, total yield, and product yield of nonadiabatic decay. The results reveal that higher vibrational mode frequencies reduce nonadiabatic transfer, but increase the transfer rate and resulting photoproduct formation. These trends can inform progress toward experimental control of photochemical reactions or tuning of molecules' photochemical properties based on conical intersections and their topography.

The origin of the solvent dependence of fluorescence quantum yields in dipolar merocyanine dyes

An increasing activation energy barrier to a conical intersection was identified as the reason for higher fluorescence lifetimes and quantum yields for merocyanines in polar solvents.

Fluorophores with high quantum yields are desired for a variety of applications. Optimization of promising chromophores requires an understanding of the non-radiative decay channels that compete with the emission of photons. We synthesized a new derivative of the famous laser dye 4-dicyanomethylen-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM), i.e., merocyanine 4-(dicyanomethylene)-2-tert-butyl-6-[3-(3-butyl-benzothiazol-2-ylidene)1-propenyl]-4H-pyran (DCBT). We measured fluorescence lifetimes and quantum yields in a variety of solvents and found a trend opposite to the energy gap law. This motivated a theoretical investigation into the possible non-radiative decay channels. We propose that a barrier to a conical intersection exists that is very sensitive to the solvent polarity. The conical intersection is characterized by a twisted geometry which allows a subsequent photoisomerization. Transient absorption measurements confirmed the formation of a photoisomer in unpolar solvents, while the measurements of fluorescence quantum yields at low temperature demonstrated the existence of an activation energy barrier.

Spectroscopic and DFT investigation on the photo-chemical properties of a push-pull chromophore: 4-Dimethylamino-4'-nitrostilbene

4-Dimethylamino-4'-nitrostilbene (DANS), a π-conjugated push-pull molecule, has been investigated by means of a combined spectroscopic and computational approach. When the Raman excitation is close to the visible electronic transition of DANS, vibrational bands not belonging to DANS appear in the spectra, increasing with the laser power. These bands are observed at room temperature in the solid phase, but not at low temperature or in solution, and we interpret them as due to a thermally-activated photoreaction occurring under laser irradiation in the visible spectral region. Density-functional calculations correctly reproducing the electronic and vibrational spectra of DANS, describe the charge-transfer process, indicate that an azo-derivative is the product of the photoreaction of DANS and provide a reasonable interpretation of this process.

The role of the dipolar neighborhood on the relaxation dynamics of multichromophoric merocyanines

The interactions of neighboring, highly dipolar merocyanine dyes lead to stabilization of the intramolecular charge-transfer state.

Discrimination of nuclear spin isomers exploiting the excited state dynamics of a quinodimethane derivative

Despite the concept of nuclear spin isomers (NSIs) exists since the early days of quantum mechanics, only few approaches have been suggested to separate different NSIs. Here, a method is proposed to discriminate different NSIs of a quinodimethane derivative using its electronic excited state dynamics. After electronic excitation by a laser field with femtosecond time duration, a difference in the behavior of several quantum mechanical operators can be observed. A pump-probe experimental approach for separating these different NSIs is then proposed.

Defining a polymethine dye for fluorescence anisotropy applications in the near-infrared spectral range

Fluorescence anisotropy in the near-infrared (NIR) spectral range is challenging because of the lack of appropriate NIR fluorescent labels. We have evaluated polymethine fluorescent dyes to identify a leading candidate for NIR anisotropy applications. The NIR dye LS601 demonstrated low fluorescence anisotropy values (r) as a result of its relatively long fluorescent lifetime 1.3 ns. The r value of LS601 unbound and coupled to biological macromolecules was found to have a sufficient dynamic range from 0.24 to 0.37, demonstrating the feasibility of fluorescence anisotropy in the NIR. The viability of fluorescence anisotropy using a NIR label was demonstrated by characterization of dye-protein conjugates. These results open the door to a number of applications in drug discovery, fluorescence anisotropy imaging and contrast agent development.

Metal-organic charge transfer can produce biradical states and is mediated by conical intersections

The present paper illustrates key features of charge transfer between calcium atoms and prototype conjugated hydrocarbons (ethylene, benzene, and coronene) as elucidated by electronic structure calculations. One- and two-electron charge transfer is controlled by two sequential conical intersections. The two lowest electronic states that undergo a conical intersection have closed-shell and open-shell dominant configurations correlating with the 4s(2) and 4s(1)3d(1) states of Ca, respectively. Unlike the neutral-ionic state crossing in, for example, hydrogen halides or alkali halides, the path from separated reactants to the conical intersection region is uphill and the charge-transferred state is a biradical. The lowest-energy adiabatic singlet state shows at least two minima along a single approach path of Ca to the π system: (i) a van der Waals complex with a doubly occupied highest molecular orbital, denoted Φ(2)(1), and a small negative charge on Ca and (ii) an open-shell singlet (biradical) at intermediate approach (Ca...C ≈2.5-2.7 Å) with molecular orbital structure (1)(2), where (2) is an orbital showing significant charge transfer form Ca to the π-system, leading to a one-electron multicentered bond. A third minimum (iii) at shorter distances along the same path corresponding to a closed-shell state with molecular orbital structure has also been found; however, it does not necessarily represent the ground state at a given CaC distance in all three systems. The topography of the lowest adiabatic singlet potential energy surface is due to the one- and two-electron bonding patterns in Ca-π complexes.