The extent of charge transfer: A qualitative computational study on electronic transitions of unsymmetrical squarylium dyes

Abstract

The nature of lowest energy electron transitions of unsymmetric squarylium (USQ) dyes has been studied through ab initio and DFT methods with Le Bahers’s diagnosing indexes. Soliton structure of atomic charge alternation and bond length alternation data indicates the asymmetric nature of charge distribution with respect to geometry. Unlike in symmetrical counterparts, occurrence of ground state dipole moment along with large transition dipole moments associated with electron transitions point up to the experimentalist’s hypothesis of greater charge-transfer (CT) character in USQ derivatives. However, poor net charge transferred (∼0.5e), negligible CT distance (0–1.2Å), maximum ratio of charge increment and depletion barycenters (above 90%) and large H-index illustrates the poor CT behavior of lowest energy electron transitions in USQ derivatives. No correlation has been seen for net charge transferred and charge transfer distance with lowest energy electron transition. The present work offers a design principle that instead of USQ derivatives, symmetrical analogs which are easy to synthesize and workup, and hence low cost derivatives can be preferred as sensitizers in DSSC.