New Developments of a Multifrequency Virtual Spectrometer: Stereo‐Electronic, Dynamical, and Environmental Effects on Chiroptical Spectra

Abstract

Computational spectroscopy has recently evolved from a field reserved for specialists toward a general tool allowing interpretations and analyses of experimental results. However, the current practice of providing tables of transitions for rigid geometries, possibly tuned by phenomenological broadening, is by far too naive. In order to improve this situation in the last few years we have been developing a general, robust, and user-friendly virtual spectrometer (VS) able to complement experimental studies for complex systems in condensed phases. The VS is based on flexible graphical pre- and postprocessing tools interfaced with general number-crunching software. This last tool is rooted in several electronic structure methodologies (DFT, TD-DFT, post-Hartree-Fock), powerful discrete/continuum models for describing environmental effects, and general vibrational and vibronic models. These last topics are the main focus of this work, which sketches our latest developments related to effective inclusion of anharmonic contributions, together with time-independent and/or time-dependent descriptions of vibronic transitions including Franck-Condon, Herzberg-Teller, and Duschinsky effects. Some test cases are described in some detail with the aim of showing the role of different effects in ruling vibrational (VCD) and electronic (ECD, CPL) chiral spectroscopies.

Aiming at an accurate prediction of vibrational and electronic spectra for medium‐to‐large molecules: An overview

“…Because of the sensitivity of chiral spectroscopies, vibrational contributions are often more critical that for the nonchiral ones. While HT effects can be less important than in the case presented here, with the experimental band having a single sign, they can still modulate the shape and improve the overall agreement . The situation can become even more complex when multiple electronic states are involved since vibronic band‐shapes are very likely to overlap, canceling or enhancing each other, so that a proper representation of their envelope asymmetry is still necessary .…”

“…An important difference, however, is that they depend on multiple properties. In addition to requiring some specific theoretical and algorithmic developments, mentioned before (see references and for details), a critical aspect of the simulation of chiral spectra is that band intensities and transition probabilities, related to the overlap integrals between the vibrational states, are often disconnected. For this reason, the reliability of the methods, used to detect resonances in anharmonic calculations or select the most intense vibronic transitions for instance, become critical as a validation done a posteriori on the results is generally not obvious.…”

“…Hence, to facilitate the simulation and interpretation of multiple spectra, several conditions need to be met. A first challenge is to provide a simple interface, lowering the barrier to the broad adoption of state‐of‐the‐art methods able to produce accurate data for the problems at hand . This can be achieved by automatizing most aspects of the calculations.…”

“…A second requirement is to be able to choose the most appropriate methodology based on the size and topology of the molecular system under study, the target accuracy, and the complexity of the environment and its possible effects on the system. In this article, we will focus on cost‐effective methods, suitable for the simulation of electronic or vibrational spectra of medium‐to‐large molecular systems, ranging from half‐a‐dozen to more than a 100 atoms, and how they can be integrated within a unified platform, that we will call the virtual spectrometer . While those methods are primarily intended for semirigid molecules, their extension to support also more flexible systems will be discussed.…”

Circularly Polarized Luminescence from Axially Chiral BODIPY DYEmers: An Experimental and Computational Study

“…In the present situation, it is also likely that the dependence of the magnetic and electric transition dipoles on the torsional motion cannot be disregarded. The full consideration of such an effect cannot be accomplished at a pure electronic level and it would require the inclusion of vibronic contributions beyond the Franck–Condon approximation …”