Role of the Hydrogen-Bond in Porphyrin J-Aggregates

J-aggregates of conjugated molecules are known to be the result of π-π stacking and hydrophobic and electrostatic interactions with hydrogen bonding providing stabilization. The present study on porphyrin derivatives, thanks to the absence of chargeable groups, helps to focus the attention on the effect of the hydrogen-bonding interactions. Moreover, the aqueous medium, causing a competitive effect for the formation of H-bonds with porphyrins, strengthens the role of the inter-porphyrin hydrogen bonding contribution in the self-assembly. Herein it is proven, through UV-vis and time-resolved fluorescence measurements, that the presence of peripheral OH groups, able to form hydrogen bonds with the nitrogen atoms of the macrocycle, has an important role in driving towards the formation of J-type aggregates rather than having only a stabilization effect. Indeed, although the porphyrin derivative with peripheral OH-to-OCH2CH3 group substitution gives rise to aggregates with almost comparable size, as indicated by dynamic light scattering, in the resulting structures porphyrins take an H-type arrangement.