Accurate Calculation of Excited-State Absorption for Small-to-Medium-Sized Conjugated Oligomers: Multiconfigurational Treatment vs Quadratic Response TD-DFT

1,6-Diazapyrene: A Novel, Well-Defined, Small-Size Prototype System for Nitrogen-Containing PAHs

The quest for nitrogen-doped (N-doped) polycyclic aromatic hydrocarbons (PAHs) requires well-defined prototype systems to understand the relationship between the structure and the resulting photophysical and photochemical properties. To this end, a novel, simple, and small compound, 1,6-diazapyrene, was synthesized. In-depth analysis, employing optical spectroscopy and (time-dependent) density functional theory, (TD-)DFT, elucidates the optical excitations on the basis of MO symmetry, energy, and topology considerations; the study further unveils the photophysical and photochemical deactivation kinetics after photoexcitation, revealing extreme changes against pyrene as well as against the well-known 2,7-diazapyrene isomer. The high sensitivity of the aza-substitution position to generate such changes is considered as highly relevant for the targeted design of N-doped PAHs in general.

Excited-State Absorption: Reference Oscillator Strengths, Wave Function, and TDDFT Benchmarks

Excited-state absorption (ESA) corresponds to the transition between two electronic excited states and is a fundamental process for probing and understanding light-matter interactions. Accurate modeling of ESA is indeed often required to interpret time-resolved experiments. In this contribution, we present a dataset of 53 ESA oscillator strengths in three different gauges and the associated vertical transition energies between 71 excited states of 21 small- and medium-sized molecules from the QUEST database. In a few cases, we additionally investigated the effect of geometry relaxation on excited-state geometries. The reference values were obtained within the quadratic response (QR) CC3 formalism using eight different Dunning basis sets. We found that the d-aug-cc-pVTZ basis set is always adequate while its more compact double-ζ counterpart, d-aug-cc-pVDZ, performs well in most cases. These QR-CC3 data allow us to assess the performance of QR-TDDFT, with and without applying the Tamm-Dancoff approximation, using a panel of global and range-separated hybrids (B3LYP, BH&HLYP, CAM-B3LYP, LC-BLYP33, and LC-BLYP47), as well as several lower-order wave function methods, i.e., QR-CCSD, QR-CC2, EOM-CCSD, ISR-ADC(2), and ISR-ADC(3). We show that QR-TDDFT delivers acceptable errors for ESA oscillator strengths with CAM-B3LYP showing particular promise, especially for the largest molecules of our set, and in the Franck-Condon (FC) region. We also find that ISR-ADC(3) exhibits excellent performance in this region. When using excited-state optimal geometries, the relative performance of wave function-based approaches remains consistent with trends observed in the Franck-Condon region. However, for TD(A)-DFT, the accuracy varies more significantly, as the performance of different exchange-correlation functionals significantly depends on the chosen geometry.

Discriminating Clockwise and Counterclockwise Photoisomerization Paths in Achiral Photoswitches by Excited-State Electronic Circular Dichroism

Despite the numerous investigations of photoisomerization reactions from both the computational and experimental points of view, even in complex environments, to date there is no direct demonstration of the direction of rotation of the retinal chromophore, initiating the vision process in several organisms, occurring upon light irradiation. In the literature, many proposals have been formulated to shed light on the details of this process, most of which are extracted from semiclassical simulations. Although high hopes are held in the development of time-resolved X-ray spectroscopy, I argue in this work that simpler but less known techniques can be used to unravel the details of this fascinating photochemical process. In fact, chiroptical spectroscopy would unambiguously prove the direction of the rotatory motion of the chromophore during the photoisomerization process by probing excited state chirality, a piece of information that, so far, has been exclusively extracted from atomistic simulations. I demonstrate this statement by computing the expected chiroptical response along photoisomerization pathways for several models of the retinal chromophores that are found in nature bound to rhodopsins, including nuclear ensemble spectra from semiclassical dynamics simulations, that can be compared with time-resolved experiments.

Near and vacuum UV polarization spectroscopy of 1,4-distyrylbenzene

The UV absorbance bands of 1,4-distyrylbenzene (1,4-Bis[(E)-2-phenylethenyl]benzene, DSB) are investigated by Synchrotron Radiation Linear Dichroism (SRLD) spectroscopy using stretched polyethylene as an anisotropic solvent. The observed polarization data provide information on the transition moment directions of the observed spectral features. The investigation covers the range 15,000-58,000 cm^-1 (667-172 nm), thereby providing new information on the transitions of DSB in the vacuum UV region. The observed spectrum is characterized by four main band systems centered at 27,600, 41,000, 49,800, and 57,500 cm^-1 (362, 244, 201, and 174 nm). In general, the observed bands and their polarization directions are well predicted by the results of quantum chemical calculations using Time-Dependent Density Functional Theory (TD-DFT) with the functional CAM-B3LYP, and with the semiempirical all-valence-electrons method LCOAO.