DFT-based methods in the design of two-photon operated molecular switches

Conjugated organic molecules with photochromic properties are being extensively studied as prospective optical switching and data storage materials. Among different photochromic compounds, diarylethenes demonstrate thermal stability, fatigue resistance, and high quantum yield. The mechanism of photoswitching in diarylethenes involves a symmetry-allowed conrotatory electrocyclic reaction, initiated by UV light. Replacement of one UV photon with two near-IR ones would offer a number of practical advantages, including drastic increase in storage capacity via three-dimensional multilayer design. For this purpose we designed a prototype molecule with a two-photon absorbing (2PA) pendant substituent, attached to the photochromic diarylethene moiety. However, this molecule was experimentally shown to have lost the photoswitching properties. We analyze reasons for this loss using quantum chemistry tools. Analysis of the nodal structure of the frontier Kohn-Sham orbitals, allowed us to trace the route of the problem to the lone pair orbital of the 2PA substituent falling within the HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) gap of the photoreactive diarylethene moiety. We suggest a chemical modification of the 2PA substituent in order to restore the order of the orbitals. Potential energy plots along the reaction coordinate at the M05-2X/6-31G* theory level for the prototype 2PA photochromic molecule before and after the modification confirm the predictive capability of the proposed orbital approach. The Slater transition state method was used to obtain geometries along the reaction pathway by the constrained optimization of excited states, whereas potential energy curves were plotted using the recently proposed (Mikhailov, I. A.; Tafur, S.; Masunov, A. E. Phys. Rev. A 2008, 77 (1), 012510) a posteriori Tamm-Dancoff approximation to the time-dependent density functional theory in second order of the external field. We show that this combination is able to produce accurate potential surfaces for 1B and 2A excited states, as compared to available experimental data and results of high-level multireference wave function theory methods.