Chiral Photochemistry of Achiral Molecules: The Emblematic Case of Stilbene and Stiff-Stilbene

Chirality is a fundamental molecular property governed by the topography of the potential energy surface (PES). The barrier height between two enantiomers relative to the thermal energy dictates the timescale for their interconversion (and thus whether this can be observed). Thermally achiral molecules interconvert rapidly when the interconversion barrier is comparable to or lower than the thermal energy, in contrast to thermally stable chiral configurations. In principle, a change in the PES topography on the excited electronic state may diminish interconversion, leading to electronically prochiral molecules that can be converted from achiral to chiral by electronic excitation. We demonstrate that this is the case for two prototypical examples – cis-stilbene and cis-stiff stilbene. Both systems exhibit unidirectional photoisomerization for each enantiomer as a result of their electronic prochirality. We propose and simulate an experiment to demonstrate this effect in cis-stilbene based on its interaction with circularly polarized light. Our results highlight the drastic change in chiral behavior upon electronic excitation, opening up the possibility for asymmetric photochemistry from an effectively nonchiral starting point.

The Effect of the Mixture Composition of BmimBF4-Acetonitrile on the Excited State Relaxation Dynamics of a Solar Cell Dye D149: An Ultrafast Transient Absorption Study

It has been recognized that the understanding of the photo physic of the dyes used in solar cells in an important step in improving their efficiency. Certainly using ionic liquid as an electrolyte is a good solution as it stabilizes the excited state of the dye, however, because of the high viscosity, the diffusion of the components of the solar cell (dye, electrolyte, the chosen redox couple) is very low and has consequences on the other processes (Forward and backward processes). One of the ideas, is to modulate the viscosity of the ionic liquid by mixing the ionic liquid with a solvent. The goal then of this work is to quantify the mixture composition dependence of the excited state relaxation times. Other studies should be carried out to quantify the mixture dependence on the time characteristics of other processes (charge injection, collection etc.) to optimize the working optimal conditions of the solar cell. Following this goal, the present study is devoted to characterize the relaxation time of in the whole mixture composition of BmimBF4 and acetonitrile and in the neat components. For the first time, the decay relaxation times of the first excited electronic state of D149 dye, as obtained by transient absorption spectroscopy (TAS). These relaxation times are monitored by a gradual change of the local structure around a dye, from the one dominated by the interionic interactions, high viscosity and low polarity (as quantified by the static dielectric constant) in BmimBF4 to the one that is dominated by dipole-dipole interactions, low viscosity and high polarity in acetonitrile.<br>

The Effect of the Mixture Composition of BmimBF4-Acetonitrile on the Excited State Relaxation Dynamics of a Solar Cell Dye D149: An Ultrafast Transient Absorption Study

It has been recognized that the understanding of the photo physic of the dyes used in solar cells in an important step in improving their efficiency. Certainly using ionic liquid as an electrolyte is a good solution as it stabilizes the excited state of the dye, however, because of the high viscosity, the diffusion of the components of the solar cell (dye, electrolyte, the chosen redox couple) is very low and has consequences on the other processes (Forward and backward processes). One of the ideas, is to modulate the viscosity of the ionic liquid by mixing the ionic liquid with a solvent. The goal then of this work is to quantify the mixture composition dependence of the excited state relaxation times. Other studies should be carried out to quantify the mixture dependence on the time characteristics of other processes (charge injection, collection etc.) to optimize the working optimal conditions of the solar cell. Following this goal, the present study is devoted to characterize the relaxation time of in the whole mixture composition of BmimBF4 and acetonitrile and in the neat components. For the first time, the decay relaxation times of the first excited electronic state of D149 dye, as obtained by transient absorption spectroscopy (TAS). These relaxation times are monitored by a gradual change of the local structure around a dye, from the one dominated by the interionic interactions, high viscosity and low polarity (as quantified by the static dielectric constant) in BmimBF4 to the one that is dominated by dipole-dipole interactions, low viscosity and high polarity in acetonitrile.<br>

The Effect of the Mixture Composition of BmimBF4-Acetonitrile on the Excited State Relaxation Dynamics of a Solar Cell Dye D149: An Ultrafast Transient Absorption Study

It has been recognized that the understanding of the photo physic of the dyes used in solar cells in an important step in improving their efficiency. Certainly using ionic liquid as an electrolyte is a good solution as it stabilizes the excited state of the dye, however, because of the high viscosity, the diffusion of the components of the solar cell (dye, electrolyte, the chosen redox couple) is very low and has consequences on the other processes (Forward and backward processes). One of the ideas, is to modulate the viscosity of the ionic liquid by mixing the ionic liquid with a solvent. The goal then of this work is to quantify the mixture composition dependence of the excited state relaxation times. Other studies should be carried out to quantify the mixture dependence on the time characteristics of other processes (charge injection, collection etc.) to optimize the working optimal conditions of the solar cell. Following this goal, the present study is devoted to characterize the relaxation time of in the whole mixture composition of BmimBF4 and acetonitrile and in the neat components. For the first time, the decay relaxation times of the first excited electronic state of D149 dye, as obtained by transient absorption spectroscopy (TAS). These relaxation times are monitored by a gradual change of the local structure around a dye, from the one dominated by the interionic interactions, high viscosity and low polarity (as quantified by the static dielectric constant) in BmimBF4 to the one that is dominated by dipole-dipole interactions, low viscosity and high polarity in acetonitrile.<br>

Lattice Vibrations and Time-Dependent Evolution of Local Phonon Modes during Exciton Formation in Conjugated Polymeric Molecules

Based on nonadiabatic molecular dynamics that integrate electronic transitions with the time-dependent phonon spectrum, this article provides a panoramic landscape of the dynamical process during the formation of photoinduced excitons in conjugated polymers. When external optical beam/pulses with intensities of 10 µJ/cm2 and 20 µJ/cm2 are utilized to excite a conjugated polymer, it is found that the electronic transition firstly triggers local lattice vibrations, which not only locally distort alternating bonds but change the phonon spectrum as well. Within the first 60 fs, the occurrence of local distortion of alternating bonds accompanies the localization of the excited-state’s electron. Up to 100 fs, both alternating bonds and the excited electronic state are well localized in the middle of the polymer chain. In the first ~200 fs, the strong lattice vibration makes a local phonon mode at 1097.7 cm−1 appear in the phonon spectrum. The change of electron states then induces the self-trapping effect to act on the following photoexcitation process of 1.2 ps. During the following relaxation of 1.0 ps, new local infrared phonon modes begin to occur. All of this, incorporated with the occurrence of local infrared phonon modes and localized electronic states at the end of the relaxation, results in completed exciton formation.

Intermolecular vibrations mediate ultrafast singlet fission

Singlet fission is a spin-allowed exciton multiplication process in organic semiconductors that converts one spin-singlet exciton to two triplet excitons. It offers the potential to enhance solar energy conversion by circumventing the Shockley-Queisser limit on efficiency. We study the primary steps of singlet fission in a pentacene film by using a combination of TG and 2D electronic spectroscopy complemented by quantum chemical and nonadiabatic dynamics calculations. We show that the coherent vibrational dynamics induces the ultrafast transition from the singlet excited electronic state to the triplet-pair state via a degeneracy of potential energy surfaces, i.e., a multidimensional conical intersection. Significant vibronic coupling of the electronic wave packet to a few key intermolecular rocking modes in the low-frequency region connect the excited singlet and triplet-pair states. Along with high-frequency local vibrations acting as tuning modes, they open a new channel for the ultrafast exciton transfer through the resulting conical intersection.

Arresting doubly excited electronic state mediating lightning speed ring-opening reaction of 1,3-cyclohexadiene

The photoinduced ring-opening reaction of 1,3-cyclohexadiene to produce 1,3,5-hexatriene is a well-known example of the Woodward-Hoffmann rule for stereochemical reactions governed by molecular orbital symmetry, and it plays an essential role in photobiological synthesis of vitamin D3 in the skin. Since the photoexcited 11B state of 1,3-cyclohexadiene is not electronically correlated to the ground state of 1,3,5-hexatriene, the reaction is expected to proceed via non-adiabatic transitions through a doubly excited 21A electronic state. However, spectroscopic observation of this elusive state has been difficult. Here we present the results of a photoelectron spectroscopy study using table-top ultrafast deep and extreme UV lasers, based on filamentation four-wave mixing and high-harmonic generation, which enabled us to arrest the 21A state. It is shown that the ring-opening reaction takes only 60 fs to complete, which is a considerably shorter time than previous experimental and theoretical estimates. The ballistic reaction creates vibrational coherence in the reaction products.

Singlet Oxygen in Plants: Generation, Detection, and Signaling Roles

Singlet oxygen (1O2) refers to the lowest excited electronic state of molecular oxygen. It easily oxidizes biological molecules and, therefore, is cytotoxic. In plant cells, 1O2 is formed mostly in the light in thylakoid membranes by reaction centers of photosystem II. In high concentrations, 1O2 destroys membranes, proteins and DNA, inhibits protein synthesis in chloroplasts leading to photoinhibition of photosynthesis, and can result in cell death. However, 1O2 also acts as a signal relaying information from chloroplasts to the nucleus, regulating expression of nuclear genes. In spite of its extremely short lifetime, 1O2 can diffuse from the chloroplasts into the cytoplasm and the apoplast. As shown by recent studies, 1O2-activated signaling pathways depend not only on the levels but also on the sites of 1O2 production in chloroplasts, and can activate two types of responses, either acclimation to high light or programmed cell death. 1O2 can be produced in high amounts also in root cells during drought stress. This review summarizes recent advances in research on mechanisms and sites of 1O2 generation in plants, on 1O2-activated pathways of retrograde- and cellular signaling, and on the methods to study 1O2 production in plants.