Absorption Band Shapes of a Push–Pull Dye Approaching the Cyanine Limit: A Challenging Case for First Principle Calculations

Abstract

The absorption band shapes of a solvent tunable donor-acceptor dye have been theoretically investigated by using Kubo's generating function approach, with minimum energy geometries and normal coordinates computed at the DFT level of theory. The adopted computational procedure allows us to include in the computation of Franck-Condon factors the whole set of normal modes, without any limitation on excitation quanta, allowing for an almost quantitative reproduction of the absorption band shape when the equilibrium geometries of the ground and the excited states are well predicted by electronic computations. Noteworthy, the functionals that yield more accurate band shapes also provide good prediction of the moment variations upon excitation; because the latter quantities are rarely available, theoretical simulation of band shapes could be a powerful tool for choosing the most appropriate computational method for predictive purposes.

Fine Structure in Electronic Spectra of Cyanine Dyes: Are Sub‐Bands Largely Determined by a Dominant Vibration or a Collection of Singly Excited Vibrations?

“…Consequently many quantitative calculations concerning dyes which incorporated the effect of Duschinsky rotation were performed, whose modus operandi are consistent with the conclusion that “…the shoulders are not determined by a unique, dominant, vibrational normal mode but rather by a collection of singly excited vibrations.”…”

“…Consequently many quantitative calculations concerning dyes which incorporated the effecto fD uschinsky rotation were performed, [37][38][39][40][41][42][43][44] whose modus operandi are consistent with the conclusion that "…the shoulders are not determined by au nique, dominant, vibrational normal mode but rather by ac ollection of singly excited vibrations." [35] In the following section we will discuss the "rightness"o f both models in light of spectroscopic data obtained from instrumental measurement.…”

“…Because variations in vibrational frequency pertaining to this part of the chromophorei nduced by differences in groups at each end of the cyanine dyes tructure are relativelys mall, it mayb ed educed that the sub-bands arise from the coupling of av ibration of the polymethine chain with the S 0 -S 1 electronic transition.It is noteworthy that in Refs. [35,[37][38][39][40][41][42][43][44],P ouradier's work wasn either cited, nor was his analysis of experimental spectrad iscussed.…”

“…Computational studies like that employed in 2007, which claim a proper theoretical analysis of the sub‐bands, continue to appear in the literature. Examples include papers dealing with the electronic spectra of cyanines, merocyanines and squaraines on the basis that the sub‐bands are composed of several peaks associated with different vibrations. As noted previously, this type of approach is far from one which relies on the role of a single dominant vibration in the electronic spectrum.…”

Synthesis, Crystal Structure, Spectroscopic Properties, and DFT Studies of 7,9-Dibromobenzo[h]quinolin-10-ol

“…As observed by the only slightly solvent polarity dependent shift of the emission (absorption) spectra in 1, the charge transfer character of tautomer emission for 1 is slim. Thus, solvent effects are not considered throughout these computations [37]. In addition to the unconstrained optimization, constrained calculations on the S 0 and S 1 potential energy curves of 1 were scanned, keeping the O−H bond lengths fixed at values in the range from 1.03 Å to 1.83 Å in steps of 0.1 Å.…”