DFT-Based Methods in the Design of Two-Photon Operated Molecular Switches

Abstract

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.

Theory and computations of two-photon absorbing photochromic chromophores

“…We also demonstrated good accuracy of the ATDA, comparing one-and two-photon absorption spectra for several chromophores with experimental data. Below we will review the studies [30] and 1a 1d S1 particle S1 hole S4 particle S4 hole S1 particle S1 hole S3 particle S3 hole Figure 4. Structural formulas (top row), 1PA profiles (row 2), 2PA profiles (row 3), and isosurfaces for natural transition orbitals (bottom row) for studied conjugated chromophores.…”

“…Studies [30] and [160] used the Gaussian 2003 Rev. E1 suite of programs [112], and hybrid meta-GGA exchangecorrelation functional M05-2X from Truhlar's group [165].…”

“…Thus, the electronic structure of the reactant can be used to predict the nature of the lowest excited state of the molecule, potential energy surface for this excited state, and the photochemical kinetics of electrocyclization. Based on the electronic structure of the systems considered in this Subsection, a general design principle for the photochromic molecules was formulated [30]: substituents should not introduce any electron energy levels within the HOMO-LUMO gap between the essential orbitals. The theoretically predicted 1PA and 2PA spectra for the open form of molecules 2b and 2c are plotted in Figure 12.…”

“…However, TD-DFT was found to be somewhat less successful in description of PESs in the vicinity of a CIX [29]. The recent study [30] demonstrated that these difficulties may be routed in the failure of the restricted KohnSham formalism for the reference ground state close to the geometry of the pericyclic minimum, and introduced a possible solution, reviewed below in Section 3.…”

Two-Photon Absorbing Molecules as Potential Materials for 3D Optical Memory

“…In particular, we focus on molecular materials, and, thereby, topics of devices, for example, laser system, optical system, and so on, which have been introduced in some excellent reviews [2,34], are not included. The significant studies on theory and calculations mainly using a DFT (density functional theory) method, which are useful in the design of new two-photon absorbing molecules, have also been reported [35][36][37] and reviewed [38]. …”