Spectral detection of J-aggregates of cationic porphyrin and investigation of conditions of their formation

Formation of J-aggregates of a cationic 5,10,15,20-tetrakis(1-methyl-4-pyridiniumyl)porphine ([H2TMPyP]4+) mediated by ion-pair interaction with four BH4− anions was determined by electronic absorption and resonance Raman spectroscopies. The participance of tosylate anions present as counterions in the original porphyrin sample, as well as of [HTMPyP]3+ ion generated at strongly alkaline pH was excluded by a series of experiments employing the same spectroscopic methods. A partial desintegration of the aggregates on a time-scale of several days is attributed to hydrolysis of BH4− anions into borates. The specificity of BH4− anions in ion-pairing with [H2TMPyP]4+ and J-aggregate formation is tentatively ascribed to their small size and, consequently, minimal steric requirement within the aggregates.