The lowest triplet states of bridged cis-2,2′-bithiophenes – theory vs. experiment

Abstract

Theoretical methods that were previously used to give a good quantitative description of the 3(1)Bu state of trans-2,2'-bithiophene are applied to characterize the lowest triplet states of three bridged cis-2,2'-bithiophenes: 3,3'-cyclopentadithiophene (CPDT), 3,3'-dithienylpyrrole (DTP), and 3,3'-dithienylthiophene (DTT). By obtaining highly accurate reproductions of the phosphorescence spectra of all three compounds, we rationalize the observed vibronic activity, further explore the performance of the applied theoretical methods, and address the quality of the reported experimental spectra. Over the course of this study we have, first, characterized the changes in the electronic structures between the ground state and the lowest triplet state and, second, expressed the related geometrical differences in terms of the Huang-Rhys factors. The Huang-Rhys factors have then been used to generate theoretical emission spectra with vibronic resolution. The applied procedure has yielded quantitative reproductions of the previously reported experimental phosphorescence spectra of DTT and DTP. The experimental spectrum of CPDT, on the other hand, turned out to be considerably narrower and intensity-deficient in its low energy region when compared with the theoretical results. Our experimental reinvestigation of the CPDT phosphorescence has given a refined spectrum that is significantly wider than the previously reported one, and is in nearly quantitative agreement with the theoretical prediction. This enabled us to attribute the observed discrepancy to an experimental artifact associated with the sensitivity characteristics of the commonly used photomultipliers.

Electron Delocalization in Planar Metallacycles: Hückel or Möbius Aromatic?

“…Although these electron populations significantly differ from the ED‐based ones, they seem to be quite reasonable as long as EDDB is considered a quantitative criterion of aromaticity. In fact, metallacycles are commonly known to be less aromatic than their classical counterparts, and it has recently been shown using the EDDB method that in the case of benzene about 90 % of electrons are delocalized within the π‐system . In this context, the cyclic delocalization of electrons in [Fe(CH) 6 H 2 ] is at least 20 % less effective than in C 6 H 6 .…”

The role of the long‐range exchange corrections in the description of electron delocalization in aromatic species

“…The resulting EDDB k population does not account for the cross‐ring electron delocalization and as such can be regarded as a counterpart of KMCI. The EDDB g ( r ) function, in turn, considers conjugations between all the chemical bonds in a molecule, and, as such, it can be used to evaluate global aromatic stabilization in large molecular systems or, for example, to study the nonlocal resonance effects in conjugated aromatic rings (in both ground‐ and excited states) …”

The electron density of delocalized bonds (EDDB) applied for quantifying aromaticity

“…without specified O), the EDDB(r) function considers conjugations between all the chemical bonds in a molecule, and, as such, it can be used to evaluate global aromatic stabilization 38 or to study the nonlocal resonance effects in conjugated aromatic rings (in both ground-and excited states). 59 It has to be emphasized that EDDB is far more efficient than MCDI, especially in the case of highly accurate wavefunctions of large molecular systems. For instance, the calculation of MCDI takes from a dozen of seconds to several hours depending on the size of the ring and the computational method used, while the EDDB calculation takes less than 1 s for all the aromatic rings considered in this study regardless of their size.…”