Abstract
Nucleoside-based diarylethenes are emerging as an especial class of photochromic compounds that have potential applications in regulating biological systems using noninvasive light with high spatio-temporal resolution. However, relevant microscopic photochromic mechanisms at atomic level of these novel diarylethenes remain to be explored. Herein, we have employed static electronic structure calculations (MS-CASPT2//M06-2X, MS-CASPT2//SA-CASSCF) in combination with non-adiabatic dynamics simulations to explore the related photoinduced ring-closing reaction of a typical nucleoside-based diarylethene photoswitch, namely, PS-IV. Upon excitation with UV light, the open form PS-IV can be excited to a spectroscopically bright S1 state. After that, the molecule relaxes to the conical intersection region within 150 fs according to the barrierless relaxed scan of the C1–C6 bond, which is followed by an immediate deactivation to the ground state. The conical intersection structure is very similar to the ground state transition state structure which connects the open and closed forms of PS-IV, and therefore plays a crucial role in the photochromism of PS-IV. Besides, after analyzing the hopping structures, we conclude that the ring closing reaction cannot complete in the S1 state alone since all the C1–C6 distances of the hopping structures are larger than 2.00 Å. Once hopping to the ground state, the molecules either return to the original open form of PS-IV or produce the closed form of PS-IV within 100 fs, and the ring closing quantum yield is estimated to be 56%. Our present work not only elucidates the ultrafast photoinduced pericyclic reaction of the nucleoside-based diarylethene PS-IV, but can also be helpful for the future design of novel nucleoside-based diarylethenes with better performance.
Highlights
Photochromism in molecular systems refers to a photoinduced reversible transformation between two isomers having distinct absorption spectra
As the first step to clarifying the photochemistry of PS-IV upon excitation, we optimized the ground state structures of the simplified PS-IV in both open and closed forms using the M06-2X/6-31G* method [89,90,91,92], which are denoted as S0-O and S0-C, respectively, in Figure 2 (see Figure S3, Supplementary Materials for S0 structures optimized using the SA-CASSCF(12,10)/6-31G* method)
At the MSCASPT2//M06-2X level, the potential energy of S0-C is about 5.6 kcal/mol lower than S0-O, indicating that S0-C is slightly more stable than S0-O. We use notation, such as MS-CASPT2//M06-2X, to represent that the single point energy is refined with the MS-CASPT2/6-31G* method using the geometry optimized at the M06-2X/6-31G* level
Summary
Photochromism in molecular systems refers to a photoinduced reversible transformation between two isomers having distinct absorption spectra. In addition to the color change, such a reaction is often accompanied with changes in some other physical/chemical properties such as fluorescence emission, geometry structure as well as chemical reactivity, et cetera, which gives it various applications in the fields of optical materials, memories, photoswitches and biological systems [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18] Motivated by their wide applications, several photochromic families such as azobenzenes, furylfulgides, spiropyrans, diarylethenes, et cetera, have been developed, in which the azobenzenes and furylfulgides change structures via Z/E isomerization while spiropyrans and diarylethenes transform between their open and closed forms via ring closing and opening reactions [9,10,11,12,13,14,15,16,17,18].
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