Linear and nonlinear optical polarizabilities in supramolecular aggregates: Effects of hydrogen bonding and dipolar interactions

We consider linear hydrogen halide chains, (HX)n, X=F, Cl and Br, with varying chain length, n, to compare the role of hydrogen bonding strengths in controlling the optical response functions of linear aggregates. We find that while for weakly H-bonded linear chains, the 1st hyperpolarizabilities (β) increase with the increase in the number of monomers in the aggregates, for strong H-bonding species like HF chains, the β shows a remarkable contrast with a maximum at the small chain limit with reduction thereafter with increase in chain length. This behavior is compared with that of a pure dipolar aggregate like (CO)n. The weak H-bonded species follow the trends similar to that of such dipolar aggregates. Additionally, we have considered the H-bonded linear chains as bridges between two terminal components for charge-transfer molecular one-dimensional devices, NH2-(HF)n-NO2. In contrast to the traditional π-conjugated species, NH2-(CHCH)n-NO2, we find that such linear H-bonded chains have very poor charge-transfer abilities. We report that strong H-bonding by itself is a negative parameter for nonlinear optical response functions at the supramolecular level, although, the directional properties of the H-bonds can be utilized for material integration. We show that it is the long-range supramolecular forces together with H-bonding at the intermediate energy scale at certain special geometries, which brings in a favorable situation for better optoelectronic applications.