Interchromophoric homoconjugation effect and intramolecular charge-transfer transition of the triptycene system containing a tetracyanoquinodimethane chromophore

Abstract

The mechanism of the intramolecular charge-transfer (CT) transition of 2,2′-(9,10-dihydro-9,10-o-benzenoanthracene-1,4-diyliidene)bispropanedinitrile (1), a triptycene system containing a tetracyanoquinodimethane (TCNQ) chromophore, has been clarified by the π-electron SCF-Cl-dipole velocity (DV) MO calculation. The intramolecular CT transition of (1)(λ_max 535 nm, log ε 3.40) accepts absorption intensity from the intense π→π* transition of the TCNQ chromophore (λ_max 409 nm, log ε 4.66), as follows. The HOMO of the TCNQ group mixes with the HOMO of two benzene groups, by the interchromophoric homoconjugation of a through-space interaction, to build up the new HOMO of the combined system. The CT transition from the new HOMO to the LUMO of TCNQ moiety is partially allowed because the transition includes part of the allowed TCNQ transition. The intensity of the CT transition therefore directly reflects the degree of mixing between the HOMOs of TCNQ and two benzene moieties. The SCF-Cl-DV MO calculation corroborates the present mechanism. When the ratio of β(homoconjugation)/β(aromatic) is 35%, the calculated curve is in good agreement with the observed electronic spectrum: Calc., λ_max 537 nm (log ε 3.50), λ_max 406 nm (log ε 4.59); Found, λ_max 535 nm (log ε 3.40), λ_max 409 nm (log ε 4.66). As illustrated in Figure 5, the HOMO of the total system obviously contains the HOMO of the TCNQ group. From the results of the calculation the interchromophoric homoconjugation effect β(homo) of the triptycene system was estimated to be ca. 30% of the regular aromatic conjugation β(arom).