Cooperative Action of Laser-Induced Thermal Effects and Ionic Coordination on the Order of TPPA0 Porphyrin Derivatives Self-Assembled Interface Probed via Real-Time Second Harmonic Generation

We performed real-time second harmonic generation (SHG) to study the cooperative action of laser-induced thermal effects and ionic coordination on the order/disorder of TPPA0 (5,10,15,20-(tetrakis((ethoxycarbonyl)methoxy)phenyl)porphyrin) monolayers on the interface of three aqueous solutions. Time dependence curves of SH signals of TPPA0 monolayers on pure water and ZnCl2 aqueous and CuCl2 aqueous interfaces were detected under the polarization combinations of p-in/p-out, s-in/p-out and 4.5 degrees-in/s-out. All the SH signals changed with time at the beginning and then reached the equilibrium state. For the pure water and ZnCl2 aqueous interfaces, SH signals under the polarization of p-in/p-out declined with time; while on the CuCl2 aqueous interfaces, the SH signals raised with time. To explain the various change of the SH signals with time, we also measured these time dependence curves under the polarizations of the p-in/p-out, s-in/p-out, and 45 degrees-in/s-out, all of which can be used to determine the orientation angle and corresponding orientation distribution width. It is found that the orientation angle with delta distribution cannot explain the ratios of the SH signal change; thus, the gauss orientational distribution width has to be considered instead of delta distribution, which means the TPPA0 monolayers undergone the change process of the orientational order until reaching equilibrium. Furthermore, using a rotating trough, the SH signals of TPPA0 monolayer on the three interfaces under the p-in/p-out polarization are nearly unchanged during the measurement. These results indicated that all of the SH signal changes with time are induced by local accumulated heat with repetitive laser excitation at 82 MHz. At last, we propose a molecular mechanism to explain the changes of the SH signal on TPPA0 monolayers. The cooperative action of laser-induced thermal effects and the ionic coordination cause the signal change with time, which means the order of the porphyrin derivatives self-assembled interface is affected by the combination of ions and heat. The finding in this work is important to the bioscience, as well as the functional molecular devices.