Electronic spectra and trans-cis photoisomerism of carbocyanines. A theoretical (CS INDO CI) and experimental study

Two carbocyanine dyes, DOC(3,3’-diethyloxacarbocyanine) and DTC (3,3’-diethylthiacarbocyanine), were studied in detail by a combined experimental and theoretical treatment previously adjusted on simple streptocyanines. The BPPC (bisphenylaminopentamethine cyanine) dye was also studied to better correlate the properties of DOC and DTC with those of their parent streptocyanine BMPC (bisdimethylaminopentamethinecyanine). In the experimental part of the work, carried out on methanol solutions of BPPC+, Cl-, DOC+, I- and DTC+, I-, we determined the separate spectra of the stable forms and the photoisomers produced by irradiation into the visible absorption region. The theoretical part, based on CS INDO S+ D + T- CI calculations, was especially devised to provide as thorough an interpretation as possible of the electronic spectra (both S,-.S, and S,-T,,) of BPPC, DOC and DTC in terms of molecular subunits and to try to identify each observed species as a specific geometrical isomer (all-rrans or mono&). In all cases theproperties of the stable forms were consistent with the molecules assuming the all-trans structure. The lowest singlet excited state (S,), responsible for the colour band, retained nearly pure cyanine character (‘E in Platt’s notation), while the states falling in the second (medium UV) absorption region had prevailing aromatic (‘H) or charge transfer (‘G) character. This provided an explanation for the absence of a definite “cis peak” effect inthese compounds. BPPC was found to give rise to the same photoisomer as BMPC (3-4 ci.r) and an additionalphotoisomer clearly identified as the l-2 cis form. On the other hand, in DOC and DTC, where the spectral changes caused by the irradiation were far weaker, the single observed photoisomer could be better assigned as 2-8 cis, i.e. corresponding to the 2-3 cir form of BMPC.