Nature of electron transitions in polymethine dyes-derivatives of indeno[2,1-b]chromene

The nature of the first and the three higher electron transitions in symmetrical and unsymmetrical polymethine dyes containing indeno[2,1-b]chromene residues as end groups is discussed. Diagrams of electron density redistribution upon excitation are used for determination of the electron transition localization. It is shown that the first transition in symmetrical dyes and unsymmetrical compounds with pyrilium, thiapyrilium, benz[c,d]indolium and flavilium residues is localized in the polymethine chain and accompanied by electron transfer at neighboring atoms, whereas the two higher transitions are associated with electron density transfer from the terminal groups to the polymethine chains and hence could be considered as quasi-local transitions. In contrast, in the dyes containing benzothiazolium and indolenium residues as end groups with noticeably small effective lengths, inversion of the polymethine and quasi-local transitions occurs.