The role of H bonding and dipole-dipole interactions on the electrical polarizations and charge mobilities in linear arrays of urea, thiourea, and their derivatives

Computational studies using density functional theory are carried out on linear chains of urea, N,N-'-dimethyl urea and N,N,N-',N-'-tetramethyl urea, and their sulfur analogs, viz., thiourea, N,N-'-dimethyl thiourea and N,N,N-',N-'-tetramethyl thiourea with varying chain length, to understand the effect of hydrogen bonding and dipolar interactions on the optoelectronic response properties of such linear aggregates. While molecules of urea, N,N-'-dimethyl urea, and the corresponding sulfur analogs, thiourea, N,N-'-dimethyl thiourea, are stabilized in linear chains by hydrogen bonding, the molecules of N,N,N-',N-'-tetramethyl urea and N,N,N-',N-'-tetramethyl thiourea in the linear chains are stabilized by purely dipolar interactions. To understand the contributions of electrostatic and polarization effects on such intermolecular interactions, we study the effect of an external electric field on the intermolecular interactions in these systems. We find that the strength of hydrogen bonding increases while that of dipolar interactions decreases with increase in external field strength. We account for such findings by decomposing the interaction terms into charge-transfer and electrostatic interaction terms. The effects of these interactions on the linear and nonlinear optical properties together with transport properties such as carrier mobilities are estimated to understand their suitability for device applications.