Revealing the biradicaloid nature inherited in the derivatives of thieno[3,4-c][1,2,5]thiadiazole: a computational study

Abstract

Computational inquiries were made on non-classical thieno[3,4-c][1,2,5] thiadiazole and its pi donor derivatives (TT dyes) so as to delineate the factors responsible for their Near-Infrared (NIR) absorptions. For all dyes...

Computational Study on the Effect of Thienyl π-Donor on the Optical Response of Nonclassical Oligo-Pyrazinothienothiadiazole Biradicaloids

“…The long-wavelength electronic absorption feature was speculatively credited to the presence of hypervalent bonding and termed as super acceptors . This contradicts our earlier finding, and we have hypothesized that NIR absorption of these nonclassical dyes is the result of the interplay between charge transfer (CT) and BRC. , Similarly, TT 2 and TT 3 are the structures where the central pyrazine ring of TT 0 is replaced with pyrazino­[2,3- b ]­pyrazine and dipyrazino­[2,3- b :2′,3′- e ]­pyrazine ring, respectively. It is included in this study even though TT 2 and TT 3 have not yet been synthesized.…”

“… ,, The success of TDDFT may be a fortuitous error–error cancellation due to considerable CT in the BBT/TT dyes (which does not exist in the case of oxyallyl dyes) cannot be completely ruled out. M06-2x functional, in comparison with BH&HLYP, is less prone to symmetry breaking. , Furthermore, it is noteworthy that Grotjahn et al in their studies of capto-dative stabilized biradicaloids have estimated the vertical excitation energies nearly accurately with respect to CC2/CBS by the TD-M06 method . Because of these promising results, we have begun this part of the study using the results of the TD-M06.…”

“…Thus, the molecules chosen for this study are nonclassical nitrogen-rich oligothiadizolothiophenes TT n (where n = 0–3) and the four π-D-substituted derivatives of each one. Our earlier valence bond and molecular orbital studies have revealed a BRC in the experimentally synthesized near infrared (NIR) absorbing donor-substituted thienothiadiazole ( TT 0 ) and thiadiazolothienopyrazine ( TT 1 ) derivatives, which are earlier believed to be closed-shell species. , On the contrary, the experiments later confirmed that a positional isomer of TT 1 has a biradicaloid nature. , Hence, the validity of DFT predictions has been established through experimental studies in this class of molecules. The long-wavelength electronic absorption feature was speculatively credited to the presence of hypervalent bonding and termed as super acceptors .…”

“…Along with that, Fabian and Zahradník have qualitatively proposed a less conventional alternative approach to achieve a small highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap (HLG) (in line with that the excitation energy also decreases) by removing the orbital degeneracy of frontier non-bonding molecular orbitals of a biradical . To get rid of the degeneracy, one has to do heteroatom substitutions and/or allow the spins to effectively delocalize via lengthening the conjugated π-framework and ultimately lead to stable biradicaloids. − Bhanuprakash and co-workers have shown that in many cases, excitation energy shrinks as the biradicaloid character (BRC) proliferates. , In the meantime, Nakano and colleagues found a very interesting and important relationship between BRC and NLO activity . Their experimental and theoretical studies have confirmed that the magnitudes of second hyperpolarizability values are larger for the molecules having moderate BRC when compared with analogues closed-shell and extreme biradical systems. , It was also observed that the outspreading of conjugation by fused rings also increases the BRC as well as second hyperpolarizability .…”

Environmental effects on the singlet fission phenomenon: a model Hamiltonian-based study

“…Theoretical inspections greatly concentrate on the HOMO and LUMO frontier orbitals. 38–41 The S 0 –S 1 transition in popular class I singlet fission chromophores is reasonably described by a HOMO–LUMO electronic transition. 42 Thus, the model Hamiltonian is constructed on local spin states of a pair of interacting H 2 -like molecules ( i.e.…”