Modeling of Spiropyran: Calculation Of Absorption Spectra
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J. A. Young*, B. L. Farmer*, and W. W. Adams" *Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903-2442
"-Materials Directorate, Wright Laboratory, Wright-Patterson
Air Force Base, OH 45433-7734
ABSTRACT Semiempirical computational chemistry techniques were used to investigate the absorption spectra of l'-isopropyl-3',3'-dimethyl-indoline-spiro-6-nitro-8-methoxy-benzopyran. The spectra of both the closed (spiropyran) and open (merocyanine) forms were calculated using the INDO/Cl method. Spectroscopic calculations showed the effects, both electronic and steric, that substituents such as hydrogen, methoxy and thiophene have on the absorption spectrum. Knowledge of the absorption spectra as a function of the choice and placement of substituents and molecular geometry will potentially allow optical properties to be optimized for specific applications.
INTRODUCTION Photochromic compounds have attracted both scientific and commercial attention in the past two decades. The photochromic properties of spiropyrans were first noted by Fischer and
Hirshberg in 1952 [1]. Since then various spiropyrans have been synthesized with a variety of substituents to yield differing absorption characteristics. Currently, such chromophores are used in high resolution photography, optical devices, variable transmission devices, photovoltaic and holographic systems [1]. Spiropyrans represent a specific class of organic chromophores which change color upon irradiation. a result of increased conjugation due to the molecular rearrangement scheme shown in figure 1. Originally in the closed, spiro. form the 7relectron systems of the heterocycle and pyran fragments are orthogonal and essentially noninteracting. The primary absorption band of the spiropyran lies in the ultraviolet region of the electromagnetic spectrum. When the spiropyran is irradiated by ultraviolet light, photodissociation of the C-O bond occurs via the alpha mechanism [2]. The ensuing molecular rearrangement results in the open merocyanine form in which the heterocycle and pyran fragments are coplanar. The resultant increase in electron delocalization leads to the appearance of an absorption band in the visible range of the spectrum. This closed/open process is reversible by heat or light absorption.
45 Mat. Res. Soc. Symp. Proc. Vol. 374 01995 Materials Research Society
R R
N
N02
0O
Nhv2
N
hv
OCH3
OCH 3
Figure 1. Photochromism of the indoline-spiro-benzopyran yields the merocyanine form. R' denotes the substituent varied in this work. This research focuses on l'-isopropyl-3',3'-dimethyl-indoline-spiro-6-nitro-8-methoxybenzopyran (indoline-spiro-benzopyran). This particular compound was selected for study in concert with a parallel synthesis effort toward developing materials for various sensor applications. The absorption spectra of both the spiropyran and merocyanine forms may be tailored by the addition of substituents to the chromophore [1]. These changes in the spectrum are due to both the electronic and steri
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