Spectroscopic, photochromic and kinetic properties of 5'-benzothiazolyl derivatives of spiroindolinenaphthopyrans: An experimental and theoretical study

• A series of 5-spironaphthopyran-derivatives was obtained and investigated.
• Electron withdrawing substituents decrease the merocyanine stability.
• Thermal back reaction barrier is primarily determined by merocyanine stabilization.
• 5-Nitro substituted spironaphthopyran has an intense CT absorption band.
• 5-Nitro group bathochromically shifts the merocyanine long wavelength absorption.

The effects exerted by the substituents (H, Cl, OCH3, CN, NO2, benzo) in the 5-position of the indoline moiety on spectroscopic and kinetic properties of photochromic 5'-(benzothiazol-2-yl)-spiro[indoline-2,3'-[3Н]naphtho[2,1-b]pyrans], known as efficient chelators of transition metal ions, have been investigated by means of electronic absorption spectroscopic and DFT PBE0/6-311G** methods. Stabilization of the merocyanine isomeric form of spiropyrans decreases with the increase in electron withdrawing properties of the substituents giving rise to increase of the thermal decoloration rates. In contrast to other substituents, 5-nitro group significantly enhances absorptivity of the cyclic form in the visible spectroscopic region and provides for the bathochromic shift of the longest wavelength absorption band of the merocyanine isomer. The origin of the electronic transitions determining the UV/Vis absorption spectra of the spironaphthopyrans is discussed based on the data of TD DFT calculations.

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