We present a theoretical study of the solvent-induced three-photon absorption cross section of a highly conjugated fluorene derivative, performed using density functional (DFT) cubic response theory in combination with the polarizable continuum model. The applicability of the often used two-state model is examined by comparison against the full DFT response theory results. It is found that the simplified model performs poorly for the three-photon absorption properties of our symmetric charge-transfer molecule. The dielectric medium enhances the three-photon absorption cross section remarkably. The effects of solvent polarity and geometrical distortions have been carefully examined. A detailed comparison with experiment is presented.

One- and two-photon circular dichroism spectra of R-(+)-3-methyl-cyclopentanone, a system that has been the subject of recent experimental studies of (2+1) resonance-enhanced multiphoton ionization circular dichroism, have been calculated with an origin-invariant density functional theory approximation in the region of the lowest electronic excited states, both for the gas phase and for a selection of solvents. A polarizable continuum model is used in the calculations performed on the solvated system. Two low-lying conformers are analyzed, and a comparison of the intensities and characteristic features is made with the corresponding two-photon absorption for each species, also for the Boltzmann-averaged spectra. The effect of the choice of geometry, basis set, and exchange-correlation functional is carefully analyzed. It is found that a density functional theory approach using the Coulomb attenuating method variant of Becke's three-parameter exchange and the Lee-Yang-Parr correlation functionals with correlation-consistent basis sets of double-zeta quality can reproduce the experimental electronic circular dichroism spectra very well. The features appearing in experiment are characterized in terms of molecular excitations, and the differences in the response of each state in the one- and two-photon processes are highlighted.

Most biological processes in nature are marked with a distinctive fingerprint: they only involveleft-handed molecules. This is one of the reasons why a thorough understanding of the chemical,physical and optical properties of chiral molecules and structures is of great significance for thescientific community. Chiroptical activity is commonly quantified in terms of one-photoncircular dichroism measurements. However, to uncover unusual chiroptical effects, polarizationdependent two-photon absorption becomes a useful spectroscopic alternative. The ability tocontrol at will the polarization of the photons that mediate the transition can increase thefundamental understanding regarding light-matter interactions in chiral systems. Herein wereport on the first experimental measurement of the two-photon absorption circular dichroismspectra and its theoretical support. The finding of new nonlinear fingerprints in chiral moleculesoffers new opportunities in the recognition and understanding of optically active systems inregions where CD present strong limitations.

Both the electronic and the vibronic contributions to one- and two-photon absorption of a D-pi-D charge-transfer molecule (4-dimethylamino-4(')-methyl-trans stilbene) are studied by means of density functional response theory combined with a linear coupling model. Vibronic profiles of the first four excited states are fully explored. The dominating vibrational modes for both Franck-Condon and Herzberg-Teller contributions are identified. The Franck-Condon contribution dominates the spectra of first, second, and fourth excited states. The Herzberg-Teller contribution is on the other hand of comparable size for the third excited state, where its inclusion leads to a blueshift with respect to the vertical transition. A similar vibronic coupling behavior is found for both one- and two-photon absorptions

A computational study of the vibronically resolved electronic circular dichroism (ECD) spectra of R-(+)- 3-methyl-cyclopentanone, including both Franck-Condon and Herzberg-Teller contributions, shows how the latter can introduce a change of sign on the chiral response of an electronic excited state. This sign inversion within the vibronically resolved electronic band, which can be interpreted as a change of the chirality of the system, has in principle important consequences in comparisons of theoretical and experimental ECD spectra employed for the assignment of absolute configurations.

The vibrationally resolved electronic circular dichroism (ECD) spectra of the two dominant conformers of (R)-(+)-3-methylcyclopentanone in gas phase are computed by density functional response theory, with a full account of Franck-Condon and Herzberg-Teller vibrational contributions at the harmonic level. Proper inclusion of the latter contributions was made possible by the recent implementation of effective-scaling computations of vibrational overlaps and of analytical gradients of time dependent DFT. The Coulomb-attenuated Becke three parameters Lee-Yang-Parr (CAM-B3LYP) functional reproduces both the position and the intensity of the experimental peaks, providing a remarkable improvement over the spectra obtained with the popular hybrid B3LYP functional, and allowing a confident assignment of the CD fine vibrational structure. Franck-Condon and Herzberg-Teller contributions are discussed in detail. The Computed decrease of the CID intensity in the gas phase upon increase of the temperature of the sample follows the trend observed experimentally in different solvents.

A harmonic adiabatic approach in combination with density functional response theory for computing twophotonvibronically resolved circular dichroism spectra of chiral molecules is presented. It includes bothFranck-Condon and Herzberg-Teller contributions and it takes fully into account frequency changes andDuschinsky effects. Model calculations have been performed for two dominant conformers of (R)-(+)-3-methylcyclopentanone in the gas phase. It is found that the Herzberg-Teller contribution can introduce asign change in two-photon circular dichroism of a single excited electronic state of a conformer. The changesurvives after Boltzmann averaging, and it might be amenable to experimental verification. Interestinginterference effects between Franck-Condon and Herzberg-Teller contributions are revealed and analyzedin detail. Results obtained within the more approximate and less computationally intensive linear couplingvibronic model are also given for comparison.