J-aggregation and photodimerisation behaviour of cinnamoyloxy side chains of hyper-branched polymers determined by higher-order derivative spectra

Abstract

Hyper-branched polyester polyols (HBPs) comprised of 2,2-dimethyolpropionic acid (DMPA) were treated with cinnamoyl chloride to give the corresponding polycinnamates (DCi20, DCi30 and DCi40) in order to compare their spectroscopic and photochemical properties with those of poly(vinyl cinnamate) (PVCi) as a linear counterpart, as well as low-mass cinnamates. λ_max of the cinnamate residues of DCi20, DCi30 and DCi40 suffered from marked red-shifting when compared with those of PVCi and the model compounds in solution. Fourth as well as eighth-order derivative spectra revealed that DCi20, DCi30 and DCi40 give rise to a new sub-peak ascribable to a J-aggregate at ca. 320 nm, causing the red-shifting of λ_max in the absorption spectra. On the other hand, higher-order derivative spectra of PVCi consisted solely of the sub-peaks which are close to those of the low-mass compounds in dilute solution, indicating that PVCi does not form any aggregate at all. The new sub-peak due to the J-aggregate was more distinctly observed in the fourth-order derivative spectra for a film of DCi40, while a PVCi film shows again no sub-peak ascribable to aggregation. Photoirradiation of a DCi40 film with 313 nm light resulted in the rapid and thorough disappearance of the sub-peak due to the J-aggregate as a result of exclusive photodimerisation, when compared with that of the sub-peaks due to monomeric cinnamate, which displays both photoisomerisation and photodimerisation. It is assumed that dicinnamated DMPA residues attached densely to the branched side chains of HBPs form dendritic units that are prone to head–tail stacking to give the J-aggregate.