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Characterization of the thermal and photoinduced reactions of photochromic spiropyrans in aqueous solution.

Hammarson M, Nilsson JR, Li S, Beke-Somfai T, Andréasson J - J Phys Chem B (2013)

Bottom Line: The experimental studies on the hydrolysis reaction mechanism were supplemented by calculations using quantum mechanical (QM) models employing density functional theory.The results show that (1) the substitution pattern dramatically influences the pKa-values of the protonated forms as well as the rates of the thermal isomerization reactions, (2) water is the nucleophile in the hydrolysis reaction around neutral pH, (3) the phenolate oxygen of the merocyanine form plays a key role in the hydrolysis reaction.Hence, the nonprotonated merocyanine isomer is susceptible to hydrolysis, whereas the corresponding protonated form is stable toward hydrolytic degradation.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical and Biological Engineering, Physical Chemistry, Chalmers University of Technology , 412 96 Göteborg, Sweden.

ABSTRACT
Six water-soluble spiropyran derivatives have been characterized with respect to the thermal and photoinduced reactions over a broad pH-interval. A comprehensive kinetic model was formulated including the spiro- and the merocyanine isomers, the respective protonated forms, and the hydrolysis products. The experimental studies on the hydrolysis reaction mechanism were supplemented by calculations using quantum mechanical (QM) models employing density functional theory. The results show that (1) the substitution pattern dramatically influences the pKa-values of the protonated forms as well as the rates of the thermal isomerization reactions, (2) water is the nucleophile in the hydrolysis reaction around neutral pH, (3) the phenolate oxygen of the merocyanine form plays a key role in the hydrolysis reaction. Hence, the nonprotonated merocyanine isomer is susceptible to hydrolysis, whereas the corresponding protonated form is stable toward hydrolytic degradation.

No MeSH data available.


Reactionprofile for hydrolysis of the MC isomer obtained usingthe QM(1w) model. Energy values for the minimum energy pathway viathe phenolate mediated proton transfer (solid line), and the reactionpathway with direct protonation (dashed line) were obtained consideringsolvent effects of water at the IEFPCM-B3LYP/6-31+G(d,p) level oftheory.
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fig8: Reactionprofile for hydrolysis of the MC isomer obtained usingthe QM(1w) model. Energy values for the minimum energy pathway viathe phenolate mediated proton transfer (solid line), and the reactionpathway with direct protonation (dashed line) were obtained consideringsolvent effects of water at the IEFPCM-B3LYP/6-31+G(d,p) level oftheory.

Mentions: The structure of the hydrolysis productsshows that the nucleophilic attack of the water molecule takes placeon the double-bond between the nitro-phenolate and the indoleniumfragments, with the water oxygen (OW) coupling to carbonatom CA (for labeling of the atoms see Scheme 1 and Scheme 3). Accordingly,in the initial reactant state MC, for both QM(1w) andONIOM(6w) models, the water molecule is coordinated on the phenolateoxygen OPh close to the central CA–CB double bond as shown in Figure 7 andFigure S7 in the Supporting Information. The reaction in QM(1w) then proceeds via the first transition state(TSI) with a barrier of 26.5 kcal/mol into the firstintermediate (I), in which OPh becomes protonatedand the OH group from the water molecule (OW–H)forms a bond with CA. Judging from the structure of thehydrolysis products, the protons from the two OH groups eventuallyhave to transfer to the opposite carbon, CB, which willfinally result in a methyl group on the indolenium fragment. Fromintermediate I, the overall reaction could in principleproceed with a double proton transfer, where OW–Hprotonates CB, together with simultaneous OW protonation by OPh–H (see Scheme 3). However, the corresponding TS energy shown in Figure 8 is above 40 kcal/mol, which renders this reactionpath very unlikely. Instead, the structure of intermediate I allows for a direct protonation of CB from OPh–H, with a barrier height of 28.9 kcal/mol at TSII (see Table 3 and Figures 7 and 8). From this point, intermediate II, the reaction proceeds with the breakup of the bond betweenCA and CB via TSIII, with ΔG = 26.5 kcal/mol. In intermediate III thefragments are coordinated with OW–H on CB, which is in sp2 hybridization. Although at this pointthe proton is still bonded to the oxygen, the extended conjugationwith the ring system renders the molecule in a planar conformation.Finally the protonation of CB takes place with a smallbarrier from III (19.4 kcal/mol) through TSIV (20.7 kcal/mol), and the hydrolysis products are formed (IV and HP). Considering reaction energetics of the minimumenergy path, there are three TSs: TSI, TSII, and TSIII, which have similar relative energies wherethe highest barrier corresponds to the transfer of the first protonto CB from OPh (TSII) with a freeenergy of 27.2 and 28.9 kcal/mol at the B3LYP/6-31+G(d,p) and B3LYP/6-311++G(2d,2p)levels of theory, respectively. Considering the reactant and the productstates, MC and HP, the small relative energydifference, −0.6 and 0.3 kcal/mol for the B3LYP/6-31+G(d,p)and B3LYP/6-311++G(2d,2p) levels, respectively, are in line with theratios of kh and k–h displayed in Table 2. Thisis also in accordance with other similar hydrolysis reactions, wherereversibility was observed.68 Note thatafter the breakup of the CA–CB bond at TSIII, there are two molecular fragments, and the followingsteps in the reaction involve a shallow TS in our investigation. Consequently,the formation of the final two hydrolysis product molecules couldin principle also be achieved by proton transfers with other solventmolecules that the present quantum chemical calculations do not consider.


Characterization of the thermal and photoinduced reactions of photochromic spiropyrans in aqueous solution.

Hammarson M, Nilsson JR, Li S, Beke-Somfai T, Andréasson J - J Phys Chem B (2013)

Reactionprofile for hydrolysis of the MC isomer obtained usingthe QM(1w) model. Energy values for the minimum energy pathway viathe phenolate mediated proton transfer (solid line), and the reactionpathway with direct protonation (dashed line) were obtained consideringsolvent effects of water at the IEFPCM-B3LYP/6-31+G(d,p) level oftheory.
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fig8: Reactionprofile for hydrolysis of the MC isomer obtained usingthe QM(1w) model. Energy values for the minimum energy pathway viathe phenolate mediated proton transfer (solid line), and the reactionpathway with direct protonation (dashed line) were obtained consideringsolvent effects of water at the IEFPCM-B3LYP/6-31+G(d,p) level oftheory.
Mentions: The structure of the hydrolysis productsshows that the nucleophilic attack of the water molecule takes placeon the double-bond between the nitro-phenolate and the indoleniumfragments, with the water oxygen (OW) coupling to carbonatom CA (for labeling of the atoms see Scheme 1 and Scheme 3). Accordingly,in the initial reactant state MC, for both QM(1w) andONIOM(6w) models, the water molecule is coordinated on the phenolateoxygen OPh close to the central CA–CB double bond as shown in Figure 7 andFigure S7 in the Supporting Information. The reaction in QM(1w) then proceeds via the first transition state(TSI) with a barrier of 26.5 kcal/mol into the firstintermediate (I), in which OPh becomes protonatedand the OH group from the water molecule (OW–H)forms a bond with CA. Judging from the structure of thehydrolysis products, the protons from the two OH groups eventuallyhave to transfer to the opposite carbon, CB, which willfinally result in a methyl group on the indolenium fragment. Fromintermediate I, the overall reaction could in principleproceed with a double proton transfer, where OW–Hprotonates CB, together with simultaneous OW protonation by OPh–H (see Scheme 3). However, the corresponding TS energy shown in Figure 8 is above 40 kcal/mol, which renders this reactionpath very unlikely. Instead, the structure of intermediate I allows for a direct protonation of CB from OPh–H, with a barrier height of 28.9 kcal/mol at TSII (see Table 3 and Figures 7 and 8). From this point, intermediate II, the reaction proceeds with the breakup of the bond betweenCA and CB via TSIII, with ΔG = 26.5 kcal/mol. In intermediate III thefragments are coordinated with OW–H on CB, which is in sp2 hybridization. Although at this pointthe proton is still bonded to the oxygen, the extended conjugationwith the ring system renders the molecule in a planar conformation.Finally the protonation of CB takes place with a smallbarrier from III (19.4 kcal/mol) through TSIV (20.7 kcal/mol), and the hydrolysis products are formed (IV and HP). Considering reaction energetics of the minimumenergy path, there are three TSs: TSI, TSII, and TSIII, which have similar relative energies wherethe highest barrier corresponds to the transfer of the first protonto CB from OPh (TSII) with a freeenergy of 27.2 and 28.9 kcal/mol at the B3LYP/6-31+G(d,p) and B3LYP/6-311++G(2d,2p)levels of theory, respectively. Considering the reactant and the productstates, MC and HP, the small relative energydifference, −0.6 and 0.3 kcal/mol for the B3LYP/6-31+G(d,p)and B3LYP/6-311++G(2d,2p) levels, respectively, are in line with theratios of kh and k–h displayed in Table 2. Thisis also in accordance with other similar hydrolysis reactions, wherereversibility was observed.68 Note thatafter the breakup of the CA–CB bond at TSIII, there are two molecular fragments, and the followingsteps in the reaction involve a shallow TS in our investigation. Consequently,the formation of the final two hydrolysis product molecules couldin principle also be achieved by proton transfers with other solventmolecules that the present quantum chemical calculations do not consider.

Bottom Line: The experimental studies on the hydrolysis reaction mechanism were supplemented by calculations using quantum mechanical (QM) models employing density functional theory.The results show that (1) the substitution pattern dramatically influences the pKa-values of the protonated forms as well as the rates of the thermal isomerization reactions, (2) water is the nucleophile in the hydrolysis reaction around neutral pH, (3) the phenolate oxygen of the merocyanine form plays a key role in the hydrolysis reaction.Hence, the nonprotonated merocyanine isomer is susceptible to hydrolysis, whereas the corresponding protonated form is stable toward hydrolytic degradation.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical and Biological Engineering, Physical Chemistry, Chalmers University of Technology , 412 96 Göteborg, Sweden.

ABSTRACT
Six water-soluble spiropyran derivatives have been characterized with respect to the thermal and photoinduced reactions over a broad pH-interval. A comprehensive kinetic model was formulated including the spiro- and the merocyanine isomers, the respective protonated forms, and the hydrolysis products. The experimental studies on the hydrolysis reaction mechanism were supplemented by calculations using quantum mechanical (QM) models employing density functional theory. The results show that (1) the substitution pattern dramatically influences the pKa-values of the protonated forms as well as the rates of the thermal isomerization reactions, (2) water is the nucleophile in the hydrolysis reaction around neutral pH, (3) the phenolate oxygen of the merocyanine form plays a key role in the hydrolysis reaction. Hence, the nonprotonated merocyanine isomer is susceptible to hydrolysis, whereas the corresponding protonated form is stable toward hydrolytic degradation.

No MeSH data available.