On the simulation of vibrationally resolved electronic spectra of medium-size molecules: the case of styryl substituted BODIPYs

Abstract

BODIPY dyes are used in a variety of applications because of their peculiar spectroscopic and photo-physical properties that vary depending on the stereochemistry of the functional groups attached to the boron-dipyrromethene core structure. In this work, we have applied several computational methods, adapted for semi-rigid molecules based on the Franck-Condon principle, for the study of the optical properties of BODIPY systems and for the understanding of the influence of functional groups on their spectroscopic features. We have analyzed the electronic spectra of two styryl substituted BODIPY molecules of technological interest, properly taking into account the vibronic contribution. For comparison with recently recorded experimental data in methanol, the vibrationally resolved electronic spectra of these systems were computed using both Time-Independent (TI) and Time-Dependent (TD) formalisms. The first step toward the analysis of optical properties of the styryl modified BODIPYs was a benchmark of several density functionals, to select the most appropriate one. We have found that all benchmarked functionals provide good results in terms of band shape but some of them show strong discrepancies in terms of band position. Beyond the issue of the electronic structure calculation method, different levels of sophistication can be adopted for the calculation of vibronic transitions. In this study, the effect of mode couplings and the influence of the Herzberg-Teller terms on the theoretical spectra has been investigated. It has been found that all levels of theory considered give reproducible results for the investigated systems: band positions and shapes are similar at all levels and little improvements have been found in terms of band shape with the inclusion of Herzberg-Teller effect. Inclusion of temperature effects proved to be challenging due to the important impact of large amplitude motions. Better agreement can be achieved by adopting a suitable set of coordinates coupled with a reduced-dimensionality scheme.

Theoretical Investigation of the Circularly Polarized Luminescence of a Chiral Boron Dipyrromethene (BODIPY) Dye

“…This begins from the choice of the electronic structure calculation method, but also includes the representation of the potential energy surfaces and their vibrational structures. The latter is often disregarded but plays an essential role in determining the shape and intensity of absorption and emission spectra (Le Guennic et al, 2012 ; Pedone et al, 2012 ; Avila Ferrer et al, 2013 ; Barone et al, 2014 ; Hodecker et al, 2016 ; Liu et al, 2016 ; Padula et al, 2016 ; Hu et al, 2017 ; Fortino et al, 2019 ). With these considerations in mind, an extensive study of the methods available to describe excited-states properties of molecular systems of this size can be highly valuable.…”

Triarylborane‐Based Helical Donor–Acceptor Compounds: Synthesis, Photophysical, and Electronic Properties

“…We note that the MN15 functional performed excellently for the lowest energy CT transitions when compared with the experimental results, which agrees with ap revious benchmark study on BODIPY dyes. [42] Using the optimized groundstate geometries as starting coordinates, TD-DFT geometry optimizations of the S 1 states were performed. The triplet states (T 1 )w ere optimized using the unrestricted DFT formalism and the MN15 functional with the 6-31G(d) basis set.…”

“…The lowest‐energy vertical transitions were calculated (singlets, ten states) by TD‐DFT using the MN15 functional in combination with the 6‐31G(d) basis set. We note that the MN15 functional performed excellently for the lowest energy CT transitions when compared with the experimental results, which agrees with a previous benchmark study on BODIPY dyes . Using the optimized ground‐state geometries as starting coordinates, TD‐DFT geometry optimizations of the S 1 states were performed.…”

Ab Initio Semiclassical Evaluation of Vibrationally Resolved Electronic Spectra With Thawed Gaussians

“…(78) and (79) with k 1 given by the Hessian of the ground surface at the equilibrium geometry, and thus avoid computing excited-state Hessians. In the adiabatic shift approximation, q 2 is given by the optimized excited-state geometry, whereas in the vertical gradient approximation, this optimization is avoided and the q 2 parameter in the model (79) is obtained by evaluating the gradient of the ab initio excited-state potential energy at the ground-state equilibrium geometry (Avila Ferrer and Santoro, 2012;Egidi et al, 2014;Fortino et al, 2019):…”