TD-DFT analysis of excitation of the closed-form spironaphthopyran in methanol solution: The contribution of vibronic transitions and intermolecular hydrogen bonds

A set of forty hybrid functionals, combined with a 6-31++G(d,p) basis set and an IEFPCM solvent description, was applied to calculate the S0→S1 excitation of the spironaphthopyran (SNP) photochromic compound. None of the hybrid functionals resulted in Cspiro-O bond cleavage upon optimization of the S1 excited state of the SNP, in contrast to the spiropyran BIPS which we studied earlier. At the same time, the deformations of the SNP during excitation turned out to be very small (in contrast to those for BIPS) which made it possible to calculate its vibronic absorption spectrum. The best agreement between the calculated and experimental absorption bands was demonstrated by the SOGGA11X functional. The main result of this work is confirmation of the vibration participation in the breaking of the Cspiro-O bond in the S1 excited state of the SNP. This assumption based on experimental data was previously put forward by other authors. Strong H-bonds between the SNP and two methanol molecules enhance the photoinduced distortions of the latter and are slightly weakened upon excitation.

Kinetics of photochemical reactions of biphotochromic compounds based on spironaphthopyran and enamine--conjugation effect

A novel biphotochromic compound (BPC) with two photochromic fragments, namely spironaphthopyran and hydroxyazomethine, was synthesized and studied by nanosecond laser flash photolysis using the excitation wavelengths of 337, 430, 470 and 500 nm in methanol and toluene. The photoexcitation of BPC results in the formation of at least two colored transients. The first one is the merocyanine form B (the maximum in the absorption spectrum is near 600 nm and the lifetime is 0.1 and 0.05 s in methanol and toluene, respectively) due to the spiro-bond break followed by isomerization. The second one is the trans-keto form A(Kt) (the maximum in the absorption spectra is near 480 nm and the lifetime is 0.1 and 5 ms in methanol and toluene, respectively) as a result of cis-enol or cis-keto tautomer transformations. The relative yields of B and A(Kt) depends essentially on the wavelength of excitation. The form A(Kt) is the key transient formed under excitation with the visible light, whereas its yield under excitation with UV light is comparable with that of B. The specific solvation by methanol molecules favors the spirocycle opening even under visible light excitation. The results obtained for novel BPC were compared with those for other BPC where the same fragments are combined in such a way that the form B is the major one under excitation with UV light whereas it virtually is not observed under visible light excitation. The difference in both BPC is discussed in terms of conjugation (π-coupling) between photochromic fragments.