Spontaneous Isomerization of a Hydroxynaphthalene-Containing Spiropyran in Polar Solvents Enhanced by Hydrogen Bonding Interactions

Abstract

The synthesis of spiropyran dyes exhibiting solvent-driven isomerization even in the dark condition is an important subject for the design of optical materials. A conventional synthesis strategy involves the conjugation of indoline moieties with electron-deficient aromatic moieties. Herein, we report that a spiropyran conjugated with a hydroxynaphthalene moiety (1) is a new member exhibiting solvent-driven isomerization, even bearing an electron-donating −OH moiety. The dye exists as a colorless spirocyclic (SP) form in nonpolar media. It, however, shows a blue color in polar media, especially in aqueous media, due to the formation of ring-opened merocyanine (MC) forms, where the isomerization terminates in 10 s even at room temperature. The spontaneous SP → MC isomerization originates from the MC forms stabilized by the highly delocalized π-electrons on the hydroxynaphthalene moiety. The solvation in polar media and the hydrogen bonding interaction with water molecules decrease the ground-state energy of the MC forms, triggering spontaneous isomerization. The dye exhibits two MC absorption bands assigned to the trans–trans–cis (TTC) and cis–trans–cis (CTC) isomers. The absorbance of the CTC band increases more significantly with an increase in the water content, and the increase exhibits a linear relationship with a hydrogen-bond donor acidity of solvents. The phenolate oxygen of the CTC form has larger hydrogen-bond acceptor basicity, resulting in stronger stabilization by the water molecule.

Introduction

Spiropyrans are a class of organic photochromes that have been studied extensively for half a century.^1,2 These dyes usually exist in solutions as colorless ring-closed spirocyclic (SP) forms (Scheme 1). They isomerize upon absorbing ultraviolet (UV) light to the colored ring-opened merocyanine (MC) forms and revert to the SP forms upon absorbing visible light.^3,4 Based on the distinctive color change associated with the photochromic behaviors, various optical materials such as switches,^5,6 memories,^7,8 and sensors⁹⁻¹¹ have been proposed. The reversible SP ⇌ MC isomerization can also be promoted by other external stimuli such as the pH,¹² temperature (thermochromism),¹³ cationic species,^14,15 anionic species,¹⁶ viscosity,¹⁷ and mechanical forces (mechanochromism).¹⁸ These stimuli successfully trigger the SP ⇌ MC isomerization even without light stimuli. The design of new spiropyrans that undergo reversible isomerization by external stimuli is therefore an important subject for wider applications.

Scheme 1. Reversible Isomerization of a Spiropyran Dye.

“Solvent” is one of the most basic and easy-to-use stimuli triggering the SP ⇌ MC isomerization. Early reports describe that, as summarized in Scheme 2, some spiropyran-bearing electron-withdrawing groups, such as carboxylic acid (2),¹⁹ its methyl ester (3),²⁰ sulfonic acid (4),²¹ and nitro (5) groups,²² when dissolved in polar organic or aqueous media, are isomerized to the MC forms even in the dark condition; although they exist as the SP forms in nonpolar media. As shown in Scheme 3a, in nonpolar media, the ground-state energy of the MC forms lies at a level higher than that of the SP form;²³ therefore, SP → MC isomerization does not occur. The MC forms have a hybrid structure between the zwitterionic and quinoidal forms.²² Polar solvents stabilize the zwitterionic forms by solvation, and the stabilization is enhanced by the hydrogen-bonding interaction with water molecules. These interactions decrease the ground-state energies of the MC forms to lower than those of SP (Scheme 3b), thus promoting spontaneous SP → MC isomerization even in the dark. However, in these systems,¹⁹⁻²² the equilibrium MC amounts are at most ∼50% even in aqueous media. For achieving a wider change in the MC amounts (strong color change) along with a change in solvents, the spiropyrans with highly stable MC forms must be synthesized.

Scheme 2. Reported Spiropyran Dyes (2–12) Exhibiting Solvent-Driven SP → MC Isomerization.

Scheme 3. Energy Diagrams for Isomerization of Spiropyrans.

The phenolate moiety of the zwitterionic MC forms possesses a partial negative charge (Scheme 1), meaning that stabilizing the negative charge is the key strategy to decreasing the ground-state energies of the MC forms. As shown in Scheme 2, introducing multiple electron-withdrawing groups into the aromatic ring (6,²⁴7²⁵) or replacing the aromatic ring with the electron-deficient rings such as quinoline (8,²⁶9, 10,²⁷ and 11(28)) as well as N-methylpyridinium (12)²⁹ has been proposed. They successfully promote SP → MC isomerization in polar organic or aqueous media with equilibrium MC amounts being more than 50%. Introducing electron-deficient aromatic groups has therefore been considered as a natural approach for the design of spiropyrans exhibiting solvent-driven isomerization.

In the present work, we synthesized a new spiropyran-containing 5-hydroxynaphthalene (1, Scheme 4). We found that the dye exhibits a clear solvent-driven SP ⇌ MC isomerization at room temperature even bearing an electron-donating −OH group. In nonpolar media, 1 shows almost no color due to the formation of the SP form. The increase in the solvent polarity exhibits gradual coloration, and the addition of water significantly enhances the coloration. This produces 70% of the MC forms, where the time to achieve the SP ⇌ MC equilibrium is less than 10 s even at room temperature. Equilibrium analysis and density functional theory (DFT) calculations indicated that the ground-state energy of the MC form of 1 is highly stabilized even bearing an electron-donating 5-hydroxynaphthalene moiety. The hydrogen bonding interaction further stabilizes the MC form, thus facilitating enhanced SP → MC isomerization.

Scheme 4. Structure of Spiropyrans Used in This Work.

Results and Discussion

Synthesis and Absorption Properties

Dye 1 was synthesized via the condensation of 1,3,3-trimethyl-2-methyleneindoline and 1,5-dihydroxy-2-naphthaldehyde³⁰ with 14% yield (see the Experimental Section). FAB(+)-MS analysis of 1 (Figure S1, Supporting Information) shows its molecular ion (m/z 344.1). As noted hereafter, ¹H NMR analysis of 1 in deuterated solvents revealed that it exists as a mixture of the SP and MC forms and does not confirm its purity. Therefore, 1 dissolved in DMSO-d₆ was transformed into the ring-opened protonated form [1 + H⁺] by the addition of a few drops of H₂SO₄.³¹¹H, ¹³C NMR, and ¹H–¹H COSY spectra of the resulting sample (Figures S2–S4, Supporting Information) identified the [1 + H⁺] species, confirming the purity of 1. Spiropyran dyes bearing a phenol (13) and a naphthalene moiety (14) (Scheme 4) were also synthesized by a procedure similar to that of 1 (see the Experimental Section), and their purity was confirmed by the NMR and MS analyses (Figures S5–S10, Supporting Information). These dyes, and a popular spiropyran (5) bearing a nitrobenzene moiety (Scheme 2), which shows solvent-driven isomerization,^10,16,17,22 were also used to compare the isomerization properties with 1.

UV–vis absorption spectra of 1 (25 μM) were measured at 20 °C in several organic solvents or water/DMSO mixtures with different ratios in the dark condition. As shown in Figure 1, 1, when dissolved in nonpolar solvents such as toluene, THF, and EtOAc, shows almost no absorption in the visible region, where the solutions are almost colorless. This indicates that, in nonpolar media, 1 exists as the SP form. However, 1 dissolved in polar organic solvents, such as 1,4-dioxane, CH₂Cl₂, MeCN, DMSO, EtOH, and MeOH, increases absorbance at 500–700 nm assigned to the MC forms, and the solution color becomes pale blue, indicating that 1 undergoes spontaneous isomerization in polar organic media. Interestingly, the MC absorption increases significantly by the addition of water: the absorbance increases with an increase in the water content of the water/DMSO mixtures, suggesting that water enhances the SP → MC isomerization, as observed for several spiropyrans.¹⁹⁻²² As shown in Figure S11 (Supporting Information), dye 5 (Scheme 2), which also shows solvent-driven spontaneous isomerization,²² shows only a small increase in the MC absorbance in DMSO, and the absorbance is much lower than that of 1 even in a DMSO/water (5/5 v/v) mixture. These results suggest that 1 undergoes highly efficient SP → MC isomerization even bearing an electron-donating hydroxynaphthalene moiety.

Figure 1.

Absorption spectra of 1 (25 μM) measured at 20 °C in organic solvents or water/DMSO mixtures with different ratios.

¹H NMR Analysis

¹H NMR analysis of 1 was performed in several deuterated solvents to confirm the formation of the MC forms, where the full spectra are provided in Figures S12–S17 (Supporting Information). As shown in Figure 2a, the two methyl groups on the indoline moiety of the SP form are not equivalent due to its unsymmetrical structure and show two peaks (H_SP). In contrast, the two methyl groups of the MC forms are equivalent due to the symmetrical structure and show a single peak (H_MC). The spectrum obtained in pure DMSO-d₆ showed both H_SP and H_MC peaks and their integration determined the SP/MC ratio to be 85/15. The addition of 20% toluene-d₈ to the solution decreases the H_MC peak and the SP/MC ratio becomes 88/12, indicating that nonpolar solvents favorably produce the SP form. In contrast, the addition of D₂O to the solution increases the H_MC peak. In a DMSO-d₆/D₂O (5/5 v/v) mixture, the SP/MC ratio becomes 30/70, indicating that, as shown in Figure 1, water is a crucial factor promoting SP → MC isomerization.

Figure 2.

(a) ¹H NMR charts (400 MHz, 30 °C) of 1 (19 mM) measured in the respective solvents. The SP/MC ratio determined by the integration of the H_SP/H_MC peaks are shown in the figure. (b) Plots of the MC amount (%) against the Kamlet–Taft parameter (π*α).

Equilibrium Analysis

The spontaneous SP → MC isomerization of 1 is promoted by the stabilization of the MC forms, as is the case for the related spiropyrans.^{19−22,24−29} We performed the equilibrium absorption experiments to clarify the thermodynamic equilibrium constants between the SP and MC forms (K_eq) and the standard enthalpy for isomerization (Δ_rH). The relationship between K_eq and the SP and MC concentrations is as follows³²

1

2

The [SP]_eq and [MC]_eq are the equilibrium concentrations of the respective forms, C_T is the total concentration of 1, and A_MC and ε_MC are the absorbance and molar extinction coefficients of the MC form, respectively. It must be noted that 1 shows two absorption bands in the visible region (Figure 1), indicating the formation of two MC species. We therefore used the average absorbance of the two peaks as A_MC. The ε_MC values in DMSO and a DMSO/water (5/5 v/v) mixture were determined using the following Lambert–Beer equation

3

l is the optical path length of the cell and the [MC]_eq in the respective solvents can be determined by the NMR analysis (Figure 2a). The equilibrium absorption experiments were carried out by stirring the solutions containing different concentrations of 1 (C_T) for 4 h in the dark at different temperatures. Plots of C_T against A_MC provided a linear relationship. The K_eq values can be determined with the ε_MC values and the slopes. The Δ_rH values were determined with the K_eq values using the van’t Hoff equation (eq 4).²² Figure S18 (Supporting Information) shows the van’t Hoff plots of the equilibrium data, and Table 1 summarizes the obtained K_eq and Δ_rH values.

4

Table 1. Equilibrium Absorption Data for SP → MC Isomerization of the Dyes (1 and 5) in DMSO and DMSO/Water (5/5 v/v) Mixtures Determined in the Dark.

dye	solvent	εMC/L mol–1 cm–1a	temperature/°C	Keqb	ΔrH/kJ mol–1c
1	DMSO	2.43 × 104	30	0.1246	–1.50 ± 0.17
			40	0.1232
			50	0.1226
			60	0.1223
	DMSO/water (5/5 v/v)	8.05 × 104	20	1.7159	–9.15 ± 0.32
			30	1.5423
			40	1.3704
			50	1.2106
5	DMSO	3.77 × 104	30	0.0284	2.77 ± 0.28
			40	0.0294
			50	0.0304
			60	0.0314
	DMSO/water (5/5 v/v)	2.50 × 104	40	0.3121	–3.43 ± 0.52
			45	0.3036
			55	0.2927
			60	0.2879

^aDetermined at 20 °C using eq 3.

^bDetermined by stirring the solutions containing different concentrations of the dyes at the designated temperature for 4 h (1) and 24 h (5).

^cDetermined by the van’t Hoff plots (Figure S18, Supporting Information).

The K_eq values of 1 in DMSO are ∼0.12 and agree with the MC ratio determined by the NMR analysis (∼10%, Figure 2a), verifying the accuracy of the equilibrium analysis. In this case, Δ_rH is negative (−1.5 kJ mol^–1), suggesting that the MC form of 1 is indeed thermodynamically more stable than the SP form. This indicates that the stabilization of the MC form of 1 promotes spontaneous isomerization. In a water/DMSO (5/5 v/v) mixture, the K_eq values are ∼1.7, which also agree with the MC ratio determined by the NMR analysis (70%). The Δ_rH value (−9.5 kJ mol^–1) is more negative than that in DMSO, clearly indicating that water further stabilizes the MC form and enhances the SP → MC isomerization. In the case of dye 5, Δ_rH decreases with water addition, but the value (−3.4 kJ mol^–1) is more positive than that of 1. These data indicate that the MC forms of 1 are very stable.

Effect of Polarity and Hydrogen Bonding Interaction

The above results indicate that the MC form of 1 is stabilized in polar media, especially in aqueous media, promoting spontaneous SP → MC isomerization. This stabilization is affected by the solvent polarity and the hydrogen bonding interaction between the MC forms and solvent molecules, as also indicated for the related spiropyrans.^{19−22,24−29} This is confirmed by the Kamlet–Taft solvent parameters that can differentiate the contribution of the respective parameters, such as dipolarity/polarizability (π*), hydrogen-bond donor acidity (α), and hydrogen-bond acceptor basicity (β).³³⁻³⁶ The maximum absorbances of the shorter wavelength band for the MC form of 1 in different solvents (Figure 1) were plotted against each of the respective solvent parameters, where all parameters used are summarized in Table S1 (Supporting Information). As shown in Figure 3a, the data in aprotic solvents (toluene, acetone, THF, MeCN, DMSO, and dichloromethane) showed a relatively linear relationship with π*, but the data in protic solvents (MeOH, EtOH, and water/DMSO mixtures) do not have any relation. As shown in Figure 3b, the data in DMSO and the water/DMSO mixtures showed a good relationship with α, but other data were out of the relation. In the case for β, no relationship was observed (Figure 3c). These findings imply that π* and α parameters affect the SP → MC isomerization of 1. To further study this, these two parameters were hybridized by the following equation.³⁷

5

Figure 3.

Relationship between the absorbance of 1 in different solvents (maximum absorbance of the short-wavelength band for the MC form) and the Kamlet–Taft parameters such as (a) π*, (b) α, (c) β, and (d) hybridized parameter (π*α).

As shown in Figure 3d, the new parameter (π*α) exhibited the best relationship with all of the absorption data. The results indicate that the dipolarity/polarizability (π*) and hydrogen-bond donor acidity (α) of the solvents are the crucial factors promoting the isomerization of 1. As shown in Figure 2b, the MC ratio of 1 in the respective solvents determined by the NMR analysis (Figure 2a), when plotted against the π*α parameter, also showed a good relationship. This strongly supports the polarity and hydrogen-bond donor acidity of the solvents as the critical factor for isomerization.

DFT Calculations

DFT calculations were performed to confirm the strong stabilization of the MC forms of 1. The SP → MC isomerization of spiropyrans usually involves three-step reactions^23,38 via the CCC and CTC intermediates, where C and T denote the cis and trans forms, respectively. As shown in Scheme 5, the isomerization occurs as follows: (i) the spiro C₁–O bond cleavage of the SP form produces the CCC intermediate via the TS1 transition state; (ii) cis → trans isomerization around the C₂=C₃ bond of the intermediate produces a CTC form via the TS2 transition state; and (iii) cis → trans isomerization around the C₁–C₂ bond of CTC results in the formation of the MC form with a TTC structure via the TS3 state. During the isomerization, reaction (ii) is the rate-determining step, and the TTC is the most stable structure among the ring-opened MC forms. DFT calculations were performed within the Gaussian 16 program. Geometry optimization of the ground states was performed using the B3LYP function with the 6-31+G* basis set, where the polarizable continuum model (PCM) was used with DMSO as a solvent.³⁹ The transition states were optimized with the TS Berny method, where the nature of stationary points was checked by means of frequency calculations, and the transition states were verified by the intrinsic reaction coordinate (IRC) calculations.⁴⁰

Scheme 5. Proposed Pathway for SP → MC Isomerization of 1.

Figure 4a summarizes the optimized structures of the ground and transition states of 1 on the potential surface along with the relative energies with respect to the SP form. Comparison of the transition energies revealed that TS2 (step ii) has the highest energy and is the rate-determining step for SP → MC isomerization, as is the case for the related spiropyrans.^16,23,38 It must be noted that, as shown in Figure 4b, the TTC of dye 5 lies at a level (−21.8 kJ mol^–1) more positive than the TTC of 1 (−30.5 kJ mol^–1), which agrees with a lower K_eq of 5 (Table 1). This again confirms that the low ground-state energy of the MC forms of 1 promotes efficient SP → MC isomerization.

Figure 4.

Potential energy surfaces for SP → MC isomerization of (a) 1, (b) 5, and (c) 13 determined by DFT calculations (PCM: DMSO). The red bars show the data for the H₂O adducts. The numbers in parentheses are the relative energies (kJ mol^–1) with respect to those of the respective SP forms. The gray, blue, and red parts denote C, N, and O atoms, respectively. The blue texts are the C₂=C₃ bond length, and the green texts are the Mulliken charge for the phenolate oxygens.

The stabilization of the MC forms of 1 may originate from the highly delocalized π-electrons on the naphthalene moiety by the electron donation from two −OH groups on different rings (extra delocalization).⁴¹ This enhances charge migration within the fused rings and decreases the ground-state energies. This is confirmed by the absorption spectra of dye 13 bearing a phenol moiety (Scheme 3): it shows almost no formation of the MC form even in aqueous media (Figure S11, Supporting Information). This is because, as reported,⁴² the extra delocalization does not occur on the single benzene ring and does not lower the ground-state energy. As shown in Figure 4c, the ground-state energy of the TTC form of 13 is similar to that of the SP form, confirming the less-stabilized MC form. In addition, as shown in Figure S11 (Supporting Information), dye 14 bearing a naphthalene moiety (Scheme 3) also shows a weak absorption even in aqueous media, indicating that the substitution of the −OH group is necessary for the strong stabilization of the MC form. These data suggest that the highly delocalized π-electrons on the hydroxynaphthalene moiety lowers the ground-state energy of the MC forms of 1 and, hence, promotes efficient isomerization.

Two MC Forms

Spiropyrans usually exhibit a single MC absorption band associated with the formation of stable TTC forms.^16,23,38 However, as shown in Figure 1, 1 exhibits two absorption bands at ∼570 and ∼610 nm, indicating that 1 produces two MC forms. As shown in Figure 4a, the CTC form of 1 also lies at a negative level, which is similar to that of the TTC form of 5. This suggests that the two absorption bands for the MC forms of 1 correspond to the TTC and CTC forms. Time-dependent DFT (TD-DFT) calculations were performed to identify that either of the forms corresponds to either of absorption bands. As shown in Figure 5 (right), singlet electronic transition of the TTC form mainly consists of the HOMO → LUMO (S₀ → S₁) transition (Table S2, Supporting Information). Its calculated transition energy (2.39 eV, 519 nm) is close to the observed maximum of the shorter wavelength band of 1 (Figure 1). In contrast, as shown in Figure 5 (left), the singlet electronic transition of the CTC mainly consists of the HOMO → LUMO (S₀ → S₁) transition (Table S2, Supporting Information). Its transition energy (2.07 eV, 600 nm) is smaller than that of TTC (2.39 eV, 519 nm) and is close to the observed maximum of the longer wavelength band of 1 (Figure 1). These data indicate that the TTC and CTC forms are assigned to the shorter and longer wavelength absorption components, respectively.

Figure 5.

Energy diagrams and interfacial plots of main molecular orbitals of the TTC and CTC forms of 1.

It must be noted that, in the present neutral aqueous media, protonation or deprotonation of the TTC and CTC forms does not occur. As shown in Figure S19 (Supporting Information), the addition of HCl or NaOH to the water/DMSO (1/1 v/v) mixture containing 1 exhibits a blue-shifted (∼490 nm) or red-shifted (∼640 nm) absorption band, which are assigned to the protonated and deprotonated forms, respectively. As summarized in Table S3 and Figure S20 (Supporting Information), the TD-DFT calculation results indicated that the calculated transition energies of these species are 2.72 eV (456 nm) and 1.61 eV (771 nm), respectively, and are close to the observed maxima of the blue-shifted and red-shifted absorption bands, confirming these species as the protonated and deprotonated species, respectively. These findings indicate that these species do not exist in the present neutral aqueous media.

Effect of Hydrogen Bonding Interactions

As shown in Figure 1, the absorbance for the both MC forms (TTC and CTC) of 1 increases with an increase in polarity and water content of the solvents. A notable feature is that the CTC absorbance increases more significantly than the TTC absorbance with an increase in the water content, implying that the hydrogen bonding interaction is involved in this behavior. Figure 6a shows absorption spectra of 1 in protic solvents (alcohols and alcohol/water mixtures), which were normalized based on the TTC absorbance (570 nm). Figure 6b plots the relative absorbance (CTC/TTC) against the hydrogen-bond donor acidity (α) of solvents (Table 1, Supporting Information). The linear correlation indicates that the hydrogen-bond donor acidity of solvents is a crucial factor for the increased absorbance of the CTC form.

Figure 6.

(a) Normalized absorption spectra of 1 (50 μM) in different protic solvents in the dark at 20 °C. (b) Relationship between the absorbance ratio (A_CTC/A_TTC) and the Kamlet–Taft parameter (α).

It is reported that the phenolate oxygen of the MC forms behaves as a Brønsted base and interacts with acidic species such as silanol groups (Si–OH).⁴³ Figure S21 (Supporting Information) summarizes the Mulliken charges of the respective atoms on the TTC and CTC forms of 1 determined by the DFT calculations. The phenolate oxygens of both forms have negative charges, indicating that they indeed act as Brønsted bases. As shown in Figure 4a, the phenolate oxygen of CTC has a charge (−0.764) more negative than that of TTC (−0.632), indicating that the CTC has stronger hydrogen-bonding acceptor basicity. Therefore, a stronger hydrogen bonding interaction between the phenolate oxygen of CTC with water molecules may result in the stronger stabilization of the CTC form. To clarify this, the CTC and TTC forms, when interacted with one water molecule, were also subjected to DFT calculations. As shown by the red bars in Figure 4a, the ground-state energies of both forms are decreased by the interaction with water molecules. In this case, the energy for the CTC form decreases more significantly (Δ−14.2 kJ mol^–1) than that for the TTC form (Δ−13.4 kJ mol^–1). These findings indicate that the CTC form is highly stabilized by a stronger hydrogen bonding interaction with the water molecule, due to the higher hydrogen-bonding acceptor basicity, and, therefore, results in the increased CTC absorbance with an increase in the water content (Figure 1).

Isomerization Kinetics and Photoresponse

Another noticeable feature of 1 is the rapid SP → MC isomerization within only 10 s even at room temperature. Figure S22 (Supporting Information) shows the time-dependent change in absorption spectra of 1 and 5 in a MeCN/water (5/5 v/v) mixture at 20 °C. In that, each of the dyes was dissolved in MeCN, and water was added to the solutions. The MC absorbance of 1 rapidly increased and reached equilibria within 10 s. In contrast, the MC absorbance of 5 increased very slowly and reached equilibria after >20 h. The slow equilibrium of 5 is because the activation energy for the isomerization is ∼110 kJ mol^–1;²² therefore, heating the solution is necessary for rapid isomerization. The rapid isomerization of 1 originates from the lower activation energy. Although the actual activation energy cannot be determined experimentally by the Arrhenius plots due to its rapid absorption change, DFT calculations confirm this. As shown in Figure 4b, the TS2 transition energy of 5 (102.0 kJ mol^–1) is similar to the experimental value, verifying the accuracy of the calculated activation energy. In contrast, the TS2 transition energy for 1 (54.2 kJ mol^–1) is much smaller than that of 5. The TS2 transition involves the rotation around the C₂=C₃ bond (Scheme 5); therefore, the C₂=C₃ bond strength is the crucial factor for this reaction. As shown in Figure 4, the calculated C₂=C₃ length of 1 is 1.476 Å, while the length of 5 is 1.469 Å, which is indicative of a weaker C₂=C₃ bond of 1. This is probably because the electron-donating property of the hydroxynaphthalene moiety suppresses the charge transfer from the indoline moiety and weakens the C₂=C₃ bond.

Spiropyrans usually undergo SP ⇌ MC isomerization by the irradiation of UV and visible light, respectively (Scheme 1).^3,4 It must be noted that 1 does not show photoisomerization behavior. As shown in Figure S23 (Supporting Information), photoirradiation of 365 nm light to the DMSO/water (5/5 v/v) mixture containing 1 scarcely increases the MC absorbance. This is probably because its excited-state losses energy via the fluorescence emission⁴⁴ (Figure S24, Supporting Information). The photoirradiation of 254 nm light significantly decreases the MC absorbance. FAB(+)-MS analysis of the resulting sample confirmed the fragmentation of 1, indicating that photoexcitation by shorter wavelength light leads to C₂=C₃ beaching, as observed in the related spiropyrans.⁴⁵ In contrast, the photoirradiation of 610 nm light does not decrease the MC absorption. This is probably because, as observed in a similar system,⁴⁶ the excited-state MC forms of 1 are also highly stabilized in solvents and require high activation energy for isomerization on the excited-state potential surface. These properties of 1 may suppress the SP ⇌ MC photoisomerization.

Conclusions

We found that a hydroxynaphthalene-containing spiropyran (1) exhibits spontaneous SP → MC isomerization in the dark at room temperature. This is the first example exhibiting solvent-driven SP → MC isomerization even bearing an electron-donating moiety. 1 exists as a SP form in nonpolar media but is isomerized to the MC forms in polar organic media. The isomerization is further enhanced in aqueous media achieving the formation of ∼70% MC forms. These properties originate from the MC forms stabilized by the highly delocalized π-electrons on the hydroxynaphthalene moiety. The solvation in polar media and the hydrogen bonding interaction with water molecules decreases the ground-state energy of the MC forms and triggers spontaneous isomerization. 1 shows two MC absorption bands assigned to the CTC and TTC isomers, where the former band increases more significantly with an increase in the water content. The phenolate oxygen of the CTC isomer has stronger hydrogen-bond acceptor basicity and interacts strongly with water molecules. This thus decreases the ground-state energy of CTC more significantly and shows a larger absorption increase. Another notable feature of 1 is the low activation energy for isomerization due to the electron-donating hydroxynaphthalene moiety; this leads to rapid SP → MC isomerization (within 10 s) even at room temperature. The rapid and strong coloration of 1 affected by solvent properties may open a new strategy for application to optical materials. The simple molecular design presented here based on a hydroxynaphthalene moiety may contribute to the creation of new solvatochromic molecules as well as new functional spiropyran dyes.

Experimental Section

General

All of the reagents used were supplied from Wako, Aldrich, and Tokyo Kasei and used as received. Water was purified by a Milli-Q system. Dye 5 was synthesized according to the procedure reported.¹⁰ 1,5-Dihydroxy-2-naphthaldehyde was prepared according to the literature procedure,³⁰ and the product was used without purification. Absorption spectra were measured on an UV–vis spectrophotometer (JASCO; FS-110) equipped with a temperature controller (ETCS-761) using a 10 mm path length quartz cell. All of the spectral measurements were performed under aerated conditions for the data reproducibility. ¹H NMR, ¹³C NMR, and ¹H–¹H COSY charts were obtained using a JEOL JNM-ECS400 spectrometer. FAB-MS analysis was performed on a JEOL JMS 700 mass spectrometer. Photoirradiation was carried out with a Xe lamp (300 W; Asahi Spectra Co. Ltd.; Max-302) equipped with band-pass filters.^47,48

Synthesis of 1 [1′,3′,3′-Trimethylspiro[benzo[h]chromene-2,2′-indolin]-7-ol]

1,3,3-Trimethyl-2-methyleneindoline (883 mg, 5.1 mmol) and 1,5-dihydroxy-2-naphthaldehyde (941 mg, 5.0 mmol) were refluxed in EtOH (15 mL) for 18 h under an Ar gas atmosphere. The resultant was concentrated by evaporation, and the residue was purified through silica gel column chromatography with n-hexane/ethyl acetate (10/1 v/v) as an eluent, affording 1 as a dark green solid (240 mg, 14%). ¹H NMR for the protonated form (400 MHz, DMSO with H₂SO₄, TMS): δ (ppm) 8.77 (1H, d, J = 16.8 Hz), 8.07 (1H, d, J = 9.6 Hz), 7.78–7.85 (3H, m), 7.69 (1H, d, J = 9.6 Hz), 7.51–7.61 (3H, m), 7.34 (1H, t, J = 8.4 Hz), 7.03 (1H, d, J = 7.6 Hz), 4.09 (3H, s), 1.76 (6H, s). ¹³C NMR for the protonated form (400 MHz, DMSO with H₂SO₄, TMS): δ (ppm) 181.9, 157.7, 154.0, 148.1, 143.7, 142.4, 129.5, 129.4, 128.0, 127.4, 127.2, 123.2, 122.3, 117.8, 115.4, 115.3, 115.1, 113.2, 111.3, 52.3, 34.6, 26.5. FAB-MS m/z: calcd for C₂₃H₂₁NO₂, 344.1646; found, 344.1652.

Synthesis of 13 [1′,3′,3′-Trimethylspiro[chromene-2,2′-indolin]-6-ol]

1,3,3-Trimethyl-2-methyleneindoline (883 mg, 5.1 mmol) and 2-hydroxybenzaldehyde (621 mg, 5.0 mmol) were refluxed in EtOH (15 mL) for 18 h under Ar. The resultant was concentrated by evaporation and purified through silica gel column chromatography with n-hexane/ethyl acetate (6/1 v/v) as an eluent, affording 13 as an off-white solid (528 mg, 36%). ¹H NMR (400 MHz, DMSO, TMS): δ (ppm) 8.84 (1H, s), 7.00–7.07 (2H, m), 6.86 (1H, d, J = 10.0 Hz), 6.70 (1H, J = 7.2 Hz), 6.53 (1H, d, J = 2.4 Hz), 6.42–6.50 (3H, m), 5.68 (1H, d, J = 10.4 Hz), 2.58 (3H, s), 1.16 (3H, s), 1.03 (3H, s). ¹³C NMR (400 MHz, DMSO, TMS): δ (ppm) 151.1, 148.5, 147.3, 137.0, 129.9, 127.9, 121.9, 120.2, 119.5, 119.2, 116.8, 115.3, 113.3, 107.2, 103.7, 51.6, 29.1, 26.2, 20.5. FAB-MS m/z: calcd for C₁₉H₁₉NO₂, 293.1416; found, 293.1416.

Synthesis of 14 [1′,3′,3′-Trimethylspiro[benzo[h]chromene-2,2′-indoline]]

1,3,3-Trimethyl-2-methyleneindoline (467 mg, 2.7 mmol) and 1-hydroxy-2-naphthaldehyde (430 mg, 2.5 mmol) were refluxed in EtOH (15 mL) for 18 h under Ar. The resultant was concentrated by evaporation and purified through silica gel column chromatography with n-hexane/CH₂Cl₂ (1/1 v/v) as an eluent, affording 14 as a purple solid (90 mg, 11%).¹ H NMR (400 MHz, DMSO, TMS): δ (ppm) 7.74 (1H, d, J = 8.0 Hz), 7.60 (1H, d, J = 8.4 Hz), 7.49 (1H, t, J = 14.4 Hz), 7.45 (1H, t, J = 10.8 Hz), 7.35–7.39 (2H, m), 7.28–7.32 (3H, m), 6.78 (1H, t, J = 14.8 Hz), 6.55 (1H, d, J = 7.6 Hz), 5.76 (1H, d, J = 10.0 Hz), 2.59 (3H, s), 1.21 (3H, s), 1.11 (3H, s). ¹³C NMR (400 MHz, DMSO, TMS): δ (ppm) 149.2, 148.1, 136.9, 134.7, 130.1, 128.2, 128.0, 127.0, 126.1, 125.3, 123.3, 122.0, 121.1, 119.8, 119.6, 118.2, 113.2, 107.4, 105.3, 51.6, 29.2, 26.1, 20.5. FAB-MS m/z: calcd for C₂₃H₂₂NO, 328.1696; found, 328.1699.

DFT Calculations

The calculations were performed with tight convergence criteria within the Gaussian 16 package. Geometry optimizations of the ground states were performed using a B3LYP/6-31+G* basis set, where PCM was used with DMSO as a solvent.³⁹ The transition states were optimized with the TS Berny method, where the nature of stationary points was checked by frequency calculations, and the states were verified by the IRC calculations.⁴⁰ The excitation energies and oscillator strengths of the models were calculated by the TD-DFT at the same level of optimization. Cartesian coordinates are summarized in the Supporting Information.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c05400.

• Solvent parameters used; TD-DFT calculation results; characterization data of 1, 13, and 14; absorption spectra of 5, 13, and 14; ¹H NMR spectra of 1; van’t Hoff plots; absorption spectra of 1 under different conditions; energy diagrams; Mulliken charge distribution; time-dependent absorption spectrum change; absorption spectra of 1 after photoirradiation; fluorescence spectra of 1; Cartesian coordinates for 1, 5, and 13; and references (PDF)

Author Contributions

All authors contributed equally.

The authors declare no competing financial interest.