Origin of Large Near-Infrared Solvatochromism of 18π-Electron Aromatic Monohydroxybenziphthalocyanine

Abstract

Near-IR (NIR) organic dyes have been widely utilized in life sciences and materials science. Herein we report an unusually large NIR solvatochromism of monohydroxybenziphthalocyanine, an analogue of 18π-electron aromatic phthalocyanine in which a single isoindoline unit is replaced with a phenol ring. The solvatochromism is attributed to deprotonation of the phenol moiety in highly polar solvents, leading to the generation of a strongly NIR-absorptive 18π-electron aromatic quinoidal monoanion.

Introduction

Phthalocyanine is a representative aromatic macrocycle with strong absorption and fluorescence in the red to near-IR (NIR) region, and has been widely used in various fields, for example, in bioimaging, photodynamic therapy, and solar cells.^1,2) Benziphthalocyanine is a phthalocyanine analogue in which a single isoindoline unit is replaced with a benzene ring^3–13) (Chart 1a). Benziphthalocyanines exhibit NIR absorption that is responsive to various external stimuli, such as protecting/deprotecting reagents and reductants/oxidants.^9,10,12,13) Notably, the introduction of hydroxyl groups into the benzene moiety of benziphthalocyanine affords two tautomers: a 6π-electron aromatic structure (phenol form) and an 18π-electron aromatic structure (quinoidal form)^9–12) (Chart 1b). We previously reported that dihydroxybenziphthalocyanine 1 shows characteristic NIR solvatochromism,⁹⁾ which is quite different from the weak solvatochromism of regular phthalocyanines,¹⁴⁾ and was tentatively ascribed to the existence of the two tautomers. Although solvatochromism in the UV-visible region is a well-known phenomenon in organic chemistry,¹⁵⁾ molecules that are strongly solvatochromic in the NIR region are rare.^16,17) Herein, we present an unusual NIR solvatochromism of monohydroxybenziphthalocyanine 2, whose absorption properties are drastically dependent on solvent polarity (Chart 1c). Experimental findings and computational analyses indicate that an anionic 18π-electron aromatic quinoidal structure, which is generated in highly polar solvents such as N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), is responsible for the intense absorption in the NIR region.

Chart 1. (a) Structures of Phthalocyanine and Benziphthalocyanine; (b) Reported Tautomerism of Dihydroxybenziphthalocyanine 1; (c) This Work: Solvatochromism of Monohydroxybenziphthalocyanine 2 Induced by Deprotonation

Results and Discussion

Monohydroxybenziphthalocyanine 2 was prepared according to the literature.¹²⁾ We measured the electronic absorption spectra of 2 in various solvents with different polarity (Fig. 1 and Table 1). A weak NIR absorption band was observed at 861 nm (ε = 1210 M^–1cm⁻¹) in CH₂Cl₂. This absorption was previously attributed to the quinoidal form, since methylation at the hydroxyl group fixed the structure in the phenol form, which has no NIR absorption beyond 750 nm.¹²⁾ This weak NIR band was also observed in less polar solvents such as hexane and toluene (hexane: 848 nm, ε = 370 M^–1cm⁻¹; toluene: 854 nm, ε = 540 M^–1cm⁻¹), indicating that the dominant species in these solvents is not the NIR-active quinoidal form, but the NIR-silent phenol form. Based on the dielectric constants of the solvents (hexane, 1.9; toluene, 2.4; CH₂Cl₂, 8.9), it appears that higher solvent polarity shifts the equilibrium towards the quinoidal form, resulting in slightly stronger absorption in the NIR region.

Fig. 1. (Left) Electronic Absorption Spectra of 2 (1 × 10⁻⁵ M) in (a) Hexane, (b) Toluene, (c) CH₂Cl₂, (d) EtOH, (e) DMF, and (f) DMSO at Room Temperature; Dielectric Constants: Hexane, 1.9; Toluene, 2.4; CH₂Cl₂, 8.9; EtOH, 24.6; DMF, 36.7; DMSO, 46.7; (Right) Expanded Spectra in the NIR Region

Table 1. Absorption Peaks (>600 nm) of 2 in Various Solvents

Solvent	Absorption λ [nm] (ε [M–1cm−1])
Hexane	848 (370), 652 (9460)
Toluene	854 (540), 648 (8840)
CH2Cl2	861 (1210), 632 (10500)
EtOH	859 (890), 772 (2860), 637 (9120)
DMF	758 (80900), 695 (27900), 659 (47700)
DMSO	764 (71300), 698 (21400), 660 (38600)
CH2Cl2a)	770 (80000), 701 (23800), 655 (42000)
DMFb)	831 (570), 635 (9100)

a) In DBU/CH₂Cl₂ (1 : 200, v/v). b) In TFA/DMF (1 : 1000, v/v).

In contrast to the weak NIR absorption at 800–900 nm in the less polar solvents, a new peak appeared at 772 nm (ε = 2860 M^–1cm⁻¹) in EtOH, suggesting the generation of a new molecular species different from the above two forms. This peak was dominant in DMF and DMSO, in which the absorption peak around 800–900 nm disappeared completely. The molar absorption coefficients are very large in these solvents (DMF: 758 nm, ε = 80900 M^–1cm⁻¹; DMSO: 764 nm, ε = 71300 M^–1cm⁻¹). To get insight into the structure of the strongly NIR-active species, we examined the effects of acid and base on the spectra of 2 (Fig. 2). The addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to the solution in CH₂Cl₂ generated a new, intense absorption band at 770 nm (ε = 80000 M^–1cm⁻¹), which seems very similar to that in DMF. In contrast, the addition of trifluoroacetic acid (TFA) to the solution in DMF resulted in a similar spectrum to that of the solution in CH₂Cl₂, displaying a peak at 831 nm (ε = 570 M^–1cm⁻¹). These results indicate that an anionic species is generated in highly polar solvents, which have large dielectric constants (EtOH, 24.6; DMF, 36.7; DMSO, 46.7). Such deprotonation in polar solvents has been reported for dilute solutions of some phenol derivatives.^18–20) Considering that the basicity of DBU (pK_b = 12) is sufficient to deprotonate phenol (pK_a = 10) but not pyrrole (pK_a = 15), a monoanionic species lacking the phenolic proton seems to be responsible for the current NIR solvatochromism.

Fig. 2. (a) Electronic Absorption Spectra of 2 (1 × 10⁻⁵ M) in CH₂Cl₂ (Blue) and DBU/CH₂Cl₂ (1 : 200, v/v) (Red); (b) Electronic Absorption Spectra of 2 (1 × 10⁻⁵ M) in DMF (Red) and TFA/DMF (1 : 1000, v/v) (Blue)

In order to investigate the aromaticity of the monoanionic species, the ¹H-NMR spectrum of 2 was measured in CD₂Cl₂ in the presence of DBU (5 equivalents (equiv.)) (see the Supplementary Materials). The internal C–H and N–H protons within the macrocycle were observed upfield at 0.46 and 3.72 ppm, respectively, indicating the existence of a strongly aromatic diatropic ring current. This is in contrast to the weak 18π-electron aromaticity observed in CD₂Cl₂ without DBU (internal C–H: 6.24 ppm; internal N–H: 9.46 ppm), where the phenol form is predominant over the quinoidal form. Density functional theory (DFT) calculations were conducted to shed further light on the electronic structure of the monoanionic species. We computed the neutral phenol form A, the neutral quinoidal form B, and the monoanionic form C of monohydroxybenziphthalocyanine 3 without any peripheral substituents (Chart 2). We found that C takes a quinoidal structure that resembles B, judging from the bond lengths in the optimized structures (see the Supplementary Materials). A is calculated to be 6.2 kcal/mol more stable than B in the neutral form, whereas loss of the phenolic proton greatly increases the electron density of the benzene moiety and shifts the equilibrium completely to the quinoidal form. The NICS(1) value of C at the ring center is –10.73 ppm, indicating strong aromaticity comparable to that of the neutral 18π-quinoidal form B (NICS(1): –9.94 ppm). This is also consistent with the NMR results. Therefore, it can be concluded that the monoanionic species has a strongly aromatic 18π-quinoidal structure.

Chart 2. Calculated NICS(1) Values (ppm) of Monohydroxybenziphthalocyanine 3 without Any Peripheral Substituents at the M06–2X/6–31+G(d) Level

Chart 3 shows the proposed mechanism of the solvatochromism. In less polar solvents such as hexane and CH₂Cl₂, the tautomeric equilibrium of 2 is shifted towards the phenol form due to the stability of the 6π-electron aromatic benzene ring. In highly polar solvents such as DMF and DMSO, deprotonation generates the strongly aromatic 18π-electron quinoidal monoanion, which exhibits intense NIR absorption at around 750 nm. The formation of anionic species may also be involved in the previously reported solvatochromism of dihydroxybenziphthalocyanine 1.⁹⁾

Chart 3. Proposed Tautomerism of Monohydroxybenziphthalocyanine 2

Conclusion

We have investigated the unusual solvatochromic behavior of monohydroxybenziphthalocyanine 2. Experimental results and DFT calculations indicate that the solvatochromism involves not only the two neutral tautomeric phenol (6π) and quinoidal (18π) forms, but also the monoanionic quinoidal structure, which exhibits intense absorption in the NIR region owing to its strong 18π-electron aromaticity. Work to develop NIR imaging probes utilizing solvatochromic/halochromic benziphthalocyanines is underway in our laboratory.

Acknowledgments

This work was supported in part by Grants from JSPS Grants-in-Aid for Scientific Research on Transformative Research Areas (A) (No. 22H05125), JSPS KAKENHI (S) (No. 17H06173), JSPS KAKENHI (A) (No. 22H00320), JST CREST (No. JPMJCR19R2), NAGASE Science Technology Foundation, Naito Foundation, Chugai Foundation (to M. U.), Uehara Memorial Foundation (to M. U. and N. T.), JSPS KAKENHI (No. 22K15245) (to N. T.), and JSPS KAKENHI (No. 20H02728) (to A. M.).

Conflict of Interest

The authors declare no conflict of interest.

Supplementary Materials

This article contains supplementary materials.

References and Notes

• 1) McKeown N. B., “Phthalocyanine Materials: Synthesis, Structure and Function,” Cambridge University Press, Cambridge, 1998.
• 2) Lo P.-C., Rodríguez-Morgade M. S., Pandey R. K., Ng D. K. P., Torres T., Dumoulin F., Chem. Soc. Rev., 49, 1041–1056 (2020).
• 3) Elvidge J. A., Golden J. H., J. Chem. Soc., 1957, 700–709 (1957).
• 4) Çetin A., Sripothongnak S., Kawa M., Durfee W. S., Ziegler C. J., Chem. Commun., 43, 4289–4290 (2007).
• 5) Sripothongnak S., Pischera A. M., Espe M. P., Durfee W. S., Ziegler C. J., Inorg. Chem., 48, 1293–1300 (2009).
• 6) Durfee W. S., Ziegler C. J., J. Porphyr. Phthalocyanines, 13, 304–311 (2009).
• 7) Sripothongnak S., Barone N. V., Çetin A., Wu R., Durfee W. S., Ziegler C. J., J. Porphyr. Phthalocyanines, 14, 170–177 (2010).
• 8) Costa R., Schick A. J., Paul N. B., Durfee W. S., Ziegler C. J., New J. Chem., 35, 794–799 (2011).
• 9) Toriumi N., Muranaka A., Hirano K., Yoshida K., Hashizume D., Uchiyama M., Angew. Chem. Int. Ed., 53, 7814–7818 (2014).
• 10) Toriumi N., Muranaka A., Hashizume D., Uchiyama M., Tetrahedron Lett., 58, 2267–2271 (2017).
• 11) Verite P. M., Azarias C., Jacquemin D., Theor. Chem. Acc., 137, 61 (2018).
• 12) Toriumi N., Asano N., Ikeno T., Muranaka A., Hanaoka K., Urano Y., Uchiyama M., Angew. Chem. Int. Ed., 58, 7788–7791 (2019).
• 13) Yanagi S., Matsumoto A., Toriumi N., Tanaka Y., Miyamoto K., Muranaka A., Uchiyama M., Angew. Chem. Int. Ed., 62, e202218358 (2023).
• 14) Ogunsipe A., Maree D., Nyokong T., J. Mol. Struct., 650, 131–140 (2003).
• 15) Reichardt C., Chem. Rev., 94, 2319–2358 (1994).
• 16) Furuyama T., Uchiyama S., Iwamoto T., Maeda H., Segi M., J. Porphyr. Phthalocyanines, 22, 88–94 (2018).
• 17) Ashoka A. H., Ashokkumar P., Kovtun Y. P., Klymchenko A. S., J. Phys. Chem. Lett., 10, 2414–2421 (2019).
• 18) Fery-Forgues S., Lavabre D., Lozar J., New J. Chem., 19, 1177–1186 (1995).
• 19) Riedel F., Spange S., J. Phys. Org. Chem., 25, 1261–1268 (2012).
• 20) To rule out the possibility that the generation of the anionic species could be attributed to basic impurities present in the highly polar solvents, we added AcOH to EtOH, DMF, and DMSO (1 : 1000, v/v) solutions of 2. AcOH did not significantly change the absorption spectra, indicating these solvents did not contain strongly basic impurities (see the Supplementary Materials for the purification methods of the solvents).