A New Approach for Studying Bond Rupture/Closure of a Spiro Benzopyran Photochromic Material: Reactivity Descriptors Derived from Frontier Orbitals and DFT Computed Electrostatic Potential Energy Surface Maps

Abstract

This paper focuses on computations technique within the framework of the TD-DFT theory for studying the relationship between structure-properties of reversible conversion of photochromic materials. Specifically, we report on 1′,3′-dihydro-8-methoxy-1′,3′,3′-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-(2H)-indole] (SP) and its isomers. TD-DFT calculated UV-Vis electronic spectra of the closed and open isomers of this photochromic material are in excellent agreement with the experimental results. Moreover, this paper reports on the results of theoretical investigations of reactivity indices that may govern the conversion between spiropyrans and its isomers. In addition, the solvent and rigidity of the medium significantly control the thermal bleaching of the photogenerated colored isomers and hence the switch ability pattern of the photochromic material. The effect of molecular structure computed by DFT in gas-phase and solvents onCspiro-Obond length has been shown to correlate with photochromic properties. For this compound, DFT optimized geometry could be used to predict photochromism. Furthermore, in an attempt to predict the driving force for MC → SP, this work explores, for the first time, profitable exploitation of the calculated and visualized mapped electrostatic potential energy surfaces (ESP map). Interestingly, it seems that the electrostatic potential forces over the molecular fragments govern spirobond rupture/closure reactions. Thermodynamically, all-trans-colored isomer (CTT) is the most stable merocyanine-like form.

Spiropyran/Merocyanine Amphiphile in Various Solvents: A Joint Experimental–Theoretical Approach to Photophysical Properties and Self-Assembly

“…All the measurements are conducted at pH 7. In the last paragraph we would like to mention that calculated and measured photophysical spectra (mainly positions of the absorption bands and solvatochromism) are consistent with the reported in the literature [26,27,30,32,45,53,67,69,73,[77][78][79]104,111,112] findings for the positive photochromic spiropyran systems, as well as for still rare data concerning photoisomerization of SP/MC-containing surfactants, exhibiting negative photochromism.…”

“…Upon irradiation with UV light or after dissolution in water, the TMAB-C3-SP form is transformed into the opened TMAB-C3-MC form, accompanied by a solution color change. The visualization of the excitations in MC isomer (Figure 9b) illustrates delocalized conjugated nature of the molecule, which is supported by the majority of theoretical studies [67,77,79,97,110]. The excitations at 504-516nm are attributed to the π − π * transition of the opened form [43].…”

“…for merocyanines [78]. The combination B3LYP/6-311G * has also shown a good agreement with measured spectra [79]. The electronic spectra are simulated employing Gaussian functions with the half-width at half height of 0.333 eV to build a continuous spectrum from a collection of transition peaks corresponding to the time-dependent DFT transition energies and oscillator strengths.…”

“…This property has been calculated for the optimized geometries in implicit water. The analysis of the ESP surfaces is a common approach to describe the ring-opening reaction of spiropyrans [79,98], to calculate the group charges accumulated on the acceptor and donor groups of the MC form as useful indicators to get insight into the charge separated state of the molecule [67,69], to shed light on the polarity of the protonated MC forms upon hydrolysis [70], for studying crystal packings of various spiropyrans [93]. The mentioned properties are assuredly important, but here we would like to emphasize that the charge redistribution appears also as a decisive factor of hydrophobicity change upon light stimulus, that has been anticipated in experiments [27,30,44].…”

“…In literature it has been reported that bands lying at shorter λ correlate to the π − π * transitions in indoline [43,108]. In some publications, for instance by Abdel-Mottaleb and Ali [79], the electronic transitions involved charge transfer from donor indoline to acceptor chromene (with nitro substituent) molecular fragment. Costa Duarte et al [109] has reported that the excitation at λ max =280nm includes HOMO→LUMO π − π * transition, and both orbitals are delocalized over the whole molecule.…”