A Complementary Theoretical Study on Six-Nitro BIPS Spiropyran Photoswitching Mechanism

A Complementary Theoretical Study on Six-Nitro BIPS Spiropyran Photoswitching Mechanism

ABSTRACT

The photochromic interconversion between spiropyran (SP) and merocyanine (MC) was investigated using density functional theory and its linear-response time-dependent extension. Equilibrium and transition-state geometries in both the ground and electronically excited states were optimized using the @B97X-D3BJ exchange-correlation functional, combined with empirical dispersion corrections, the conductor-like polarizable continuum model for solvation, and the def2-TZVPP triple-basis set. Conical intersection geometries were located using the spin-flip TDDFT approach. Key molecular configurations, including equilibrium, transition-state, and conical intersection geometries, were mapped along the SP-MC interconversion, and the energetically most favorable relaxation pathways were determined. The results also demonstrate that the relaxation pathway through the triplet manifold could be a possible alternative. The influence of the solvent environment on UV-vis absorption, excited-state relaxation, and SP-MC interconversion was emphasized. Finally, the efficiency of the full back-and-forth SP-MC transformation, induced by either light or thermal effects, was characterized in both vacuum and polar environments.

One | Introduction

Photoswitches are molecular systems that can reversibly change their structure in response to external stimuli, particularly when irradiated with light of a specific wavelength. Such systems enable the development of novel materials whose physical or chemical properties can be tuned through controlled geometrical configurations.

A representative example of this class is the photochromic interconversion between spiropyran (SP) and merocyanine (MC) (see Scheme One), which exhibits a characteristic photoswitching behavior. The SP-MC system has been widely employed in developing technological applications, including optical data storage, optical switches, nanoparticles, photonic crystals, light-driven smart materials, information encryption, and various biological systems.

Experimentally, the reversible photochemical process between SP and MC has been shown to occur through a sequence of distinct steps. Upon light absorption and subsequent excitation of

SP, the C-O bond is broken along the excited-state relaxation pathway, resulting in the opening of the Two-H-benzopyran ring. Next, a rotation around one of the C-C bonds leads to the formation of the MC conformation. The reverse transformation, which does not necessarily require photoexcitation but can proceed thermally, begins with a similar rotation about the C-C bond, reconstructing the ring structure and regenerating the SP form.

During this SP equals MC interconversion, several photochemical characteristics have been experimentally identified. For instance, the reaction rate and yield depend on the solvent, both for the MC to SP back transformation and the SP to MC ring-opening photoreaction. Moreover, a faster MC to SP conversion is observed under light irradiation than under purely thermal conditions. Finally, substituents attached to the nitrogen sites of the indoline ring can significantly alter the photochemical response of SP.

A precise theoretical description of the photochromic ring-opening process has long been the subject of active debate.

Earlier studies, employing either simplified benzopyran model systems or the full SP molecular structure, have proposed several possible relaxation mechanisms. However, no comprehensive theoretical studies have yet been published that also take radiationless relaxations into account in the interconversion mechanism between SP and MC. In these studies, the N O two substituent on the benzopyran ring was often omitted, or lower-level theoretical methods were applied, while some focused primarily on comparing the performance of different exchange-correlation functionals.

The aim of this work is to provide a detailed ab initio theoretical description of the complete SP right-left arrow MC interconversion and to elucidate the photochemical behavior of each stage, including UV absorption, radiative and nonradiative relaxation of excited states, and structural relaxation processes.