Photoisomerization and optical properties of a subphthalocyanine–azobenzene–subphthalocyanine triad

Abstract

The synthesis and characterization of a subphthalocyanine–azobenzene–subphthalocyanine triad is reported. Evidence for trans ↔ cis isomerization of the linking azobenzene moiety is observed in the NMR and optical spectra when the subphthalocyanine rings are used as light-harvesting units. Significantly, a decrease in fluorescence intensity is observed on moving from trans → cis with a recovery in intensity observed on moving back from cis → trans that can be attributed to a change in the rate of non-radiative decay.

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Introduction

It is well known that azobenzene (AB) exhibits a reversible trans ↔ cis isomerization upon irradiation with UV and visible region light. It has previously been demonstrated that the AB moiety can be incorporated as a light triggered molecular switch into a variety of different structures, and numerous multi-functional materials have been synthesized and investigated for use in applications on this basis.¹ Heteroaromatic macrocycles, such as phthalocyanines and porphyrins,² show distinctive properties when they are incorporated into AB structures. For example, an AB-substituted zinc phthalocyanine exhibits photoresponsive J-aggregation behaviour,³ and the isomerization behaviour of an azo-based silicon phthalocyanine photoswitch can be modulated upon the absorption of an incident photon by the phthalocyanine ring.⁴ It has also been demonstrated that reversible isomerization of two axial AB ligands leads to the modulation of the fluorescence of phosphorus(V) porphyrin⁵ and similar properties have been reported for an AB-linked zinc diporphyrin.⁶

Porphyrin and fullerene moieties have been linked by an AB moiety to form a donor–acceptor system,⁷ and a similar triad was formed with a bowl-shaped subphthalocyanine (subPc) replacing the porphyrin as the donor group.⁸ SubPcs⁹ are comprised of three isoindoline moieties coordinating a tetrahedral boron atom with an axial ligand, linked by aza-nitrogen atoms (Scheme 1). Intense absorption (550–600 nm) and emission are observed in the visible region due to the heteroaromatic 14 π-electron system, which can act as an efficient light-harvesting unit.¹⁰ Intense luminescence is usually observed for subPcs, since there is little or no aggregation in solution due to the cone-shaped structure of the subPc ligand. Due to easy axial ligand exchange,¹¹ multi-functional subPcs can be readily prepared and their possible utility for use as organic photodetectors,¹² and in molecular cages,¹³ organic solar cells¹⁴ and organic light-emitting diodes (OLEDs),¹⁵ and also in photodynamic therapy (PDT)¹⁶ and as sensor probes,¹⁷ has been studied. Herein the synthesis and characterization of an axially conjugated subPc–AB–subPc triad (1) is reported, along with its trans ↔ cis photoisomerization properties. 1 has been fully characterized by ¹H, ¹³C and ¹¹B NMR, HR-MS, X-ray single crystal structure analysis, UV-visible absorption and fluorescence spectroscopy.

Scheme 1 The synthesis of the subphthalocyanine–azobenzene–subphthalocyanine triad 1.

Experimental

Materials and instrumentation

All reagents were obtained from commercial suppliers and used without further purification unless otherwise indicated. Air and moisture-sensitive reactions were carried out under an argon atmosphere. HR-MS data were collected using an LTQ-Orbitrap mass spectrometer XL MS (Thermofisher, Bremen, Germany) equipped with an electrospray ionization interface (ESI). The NMR spectroscopic measurements were recorded on a Bruker 500 or 400 MHz spectrometer with CDCl₃ or dimethyl sulfoxide-d₆ as the solvent.

Synthesis of 1

2 (86 mg, 0.2 mmol, 2 equiv.) was added to a solution of 3 (21.4 mg, 0.1 mmol, 1 equiv.) in dry toluene held in a 15 ml pressure vessel, and the mixture was heated at 180 °C for 5 days. The solvent was evaporated under reduced pressure. The crude products were purified by neutral alumina column chromatography with THF as the eluent to obtain a cardinal red solid (26 mg, yield: 26%). ¹H NMR (400 MHz, CDCl₃) δ 8.85–8.87 (m, 12H), 7.90–7.93 (m, 12H), 7.23 (d, J = 8.8 Hz, 4H), 5.41 (d, J = 8.8 Hz, 4H). ¹³C NMR (100 MHz, CDCl₃) δ 151.36, 130.94, 130.16, 129.92, 123.61, 122.41, 122.28, 119.18. ¹¹B NMR (400 MHz, CDCl₃) δ −14.81. HR-MS m/z: calcd for C₆₀H₃₂B₂N₁₄O₂: 1002.3019, found: 1003.3223 [M + H]⁺.

X-ray structure determination

The X-ray crystallographic data for 1 was carried out at 291 K on a Rigaku Saturn CCD spectrometer with graphite monochromated Mo Kα radiation (λ = 0.71070 Å). The structure was solved by direct methods and refined on F² by full-matrix least-squares using the Crystal Clear and (SHELXS-97) programs. 1: C₆₀H₃₂B₂N₁₄O₂; a yellow block-like crystal of approximate 0.27 × 0.25 × 0.22 mm³ dimensions was measured. Space group P1̄, a = 10.671(3) Å, b = 11.787(3) Å, c = 12.167(3) Å, α = 90.260(4)°, β = 94.290(5)°, γ = 105.427(4)°, V = 1470.6(7) ų, Z = 1, F(000) = 516, ρ = 1.13204 g cm⁻³, R₁ = 0.2085, wR₂ = 0.2973, GOF = 1.003. CCDC 1401329 contains the supplementary crystallographic data for this paper.†

Spectroscopic measurements

UV-visible absorption spectra were recorded with a Shimadzu UV-2550 spectrophotometer. Fluorescence measurements were carried out on a Hitachi F-4600 spectrofluorometer. Solutions for absorption and emission measurements were contained in 1 × 1 cm quartz cuvettes. For all measurements, the temperature was kept constant at 298 ± 2 K. Fluorescence lifetimes were measured using a time correlated single photon counting (TCSPC) setup (Single Photon Avalanche Diodes, PDM series, Picoquant GmbH). The excitation source was a diode laser (LDH-P-C-405 with 10 MHz repetition rate, 70 ps pulse width).

Computational details

Optimized geometries for the trans and cis conformers of 1 (trans-1 and cis-1) and the corresponding AB structures were obtained by using the B3LYP functional of the Gaussian 09 software package¹⁸ with 6-31G(d) basis sets in DMSO using the polarized continuum model with the default settings. TD-DFT calculations for B3LYP optimized geometries for the trans-1 and cis-1 carried out in a similar manner at the LC-ωPBE/6-31G(d) and B3LYP/6-31G(d) levels of theory.

Results and discussion

Synthesis

The synthetic strategy for the subphthalocyanine–azobenzene–subphthalocyanine triad 1 is shown in Scheme 1. Boron subphthalocyanine chloride (2)¹⁹ and 4,4′-bis-hydroxyazobenzene (3)²⁰ were prepared according to the reported procedures (the details are provided as ESI†). 1 was synthesized from 2 and 3 by using dry toluene as the solvent, and was obtained in 26% yield after purification by column chromatography.

Crystal structure

Single crystals of 1 were obtained through the slow diffusion of methanol into a chloroform solution. The crystal structure of 1 is shown in Fig. 1. As has been reported previously for a wide range of subPcs,⁹ the boron atom is coordinated in a tetrahedral geometry by three nitrogen and one oxygen atoms, resulting in a bowl-shaped conical conformation for the two subPc moieties that form an opposite configuration. The azobenzene moiety is highly planar in trans form, in which bond lengths of N=N and C–N are 1.134 Å and 1.512 Å respectively. And the B–O–C angle is 117.8°.

Fig. 1 The X-ray crystal structure of 1. Solvent molecules are omitted for clarity.

Spectroscopic properties of the photoisomerization process

In almost all cases, the boron atom gives rise to a sharp singlet NMR peak that lies between −13.8 and −19.6 ppm.^9,21 The observed peak at −14.81 ppm confirms that the boron atoms are tetra substituted (the details are provided as ESI†). The UV-visible absorption spectrum of triad 1 in DMF is similar to that of (Cl)BsubPc (2) spectral profile with a B band at ca. 300 nm and a Q band in the 550–600 nm region (Fig. 2). A shoulder of intensity corresponding to the main azobenzene band, is observed in the 357 nm region. The spectrum of 1 does not correspond to a summation of the spectra of 2 and 3 in the 2 : 1 stoichiometry of 1 (Fig. 2), since intramolecular interactions result in an increase in the intensity for the B and Q bands of the subPc moieties and a decrease in the intensity of the 357 nm band of the AB moiety. This provides clear evidence for the formation of the triad.

Fig. 2 (a) The UV-visible absorption spectra of 1 (red line) and 2 (black line) in DMF. (b) The UV-visible absorption spectra of 1 (red line) and a 2 : 1 molar ratio mixture of 2 and 3 (black line) in DMF.

The photoisomerization of 1 was studied in dilute DMF solutions. Conversion of the trans conformer to a cis rich photostationary state (PSS) was accomplished through irradiation with 365 nm light. The cis → trans back reaction was achieved in the visible region with a 570 nm laser, indicating that subPc can act as a light-harvesting unit in this context. In the dark, the isomerization occurs or followed under about two days dark conditions. A shoulder of intensity is observed at 357 nm (Fig. 3), which can be assigned to the main trans-AB π–π* transition, since this band loses intensity upon irradiation at 365 nm. The photoisomerization of AB 3 was studied using dilute DMSO solutions and significant spectral changes were observed (Fig. 4), which are consistent with this band assignment. Upon irradiation at 365 nm, 3 undergoes trans → cis photoisomerization, while the cis → trans back reaction was achieved through irradiation at 490 nm. After several reversible photoisomerization cycles,²² only limited photochemical degradation of 1 occurs when spectral changes are monitored at 357 nm (Fig. 3).²³ This means that 1 meets the basic requirements for use as a molecular photoswitch.

Fig. 3 (a) The UV-visible absorption changes upon the photoisomerization of 1 in DMF. The left figure shows the formation of the cis rich PSS through irradiation at 365 nm, while the right figure shows the regeneration of the trans conformer upon irradiation at 570 nm. (b) Spectral changes observed at 357 nm during multiple isomerization cycles after irradiation alternating at 365 and 570 nm.

Fig. 4 Photoisomerization of AB 3 in DMSO through irradiation at 365 nm results in the generation of a cis rich PSS (red line), while the trans conformer (black line) is regenerated after irradiation at 490 nm.

TD-DFT calculations were carried out for B3LYP optimized structures for trans-1 and cis-1 at the LC-ωPBE/6-31G(d) and B3LYP/6-31G(d) levels of theory (Fig. 5 and S6†). The B3LYP functional is known to significantly underestimate the energies of transitions with significant charge transfer character,²⁴ so calculations should be carried out with more than one functional to check the validity of band assignments. The spectra of the trans and cis conformers of most ABs contain weak forbidden n–π* bands at ca. 450 nm,²³ and this transition is predicted to lie slightly to the blue of the subPc Q band in the spectra of cis-1 and trans-1 in the envelope of weaker vibrational bands (Fig. 5 and S6†). The Q and B bands of the subPc moiety are predicted to be almost unaffected by the change in the conformation of the AB linker moiety, as is observed experimentally (Fig. 3) and lie in the spectral regions characteristic for these transitions in subPc spectra,^9,25 so can be assigned unambiguously based on the spectroscopic measurements. The main π–π* bands of cis-ABs typically lie in the 240–280 nm region, while those of trans-ABs lie at ca. 320 nm.²⁶ The TD-DFT calculations provide further evidence that the shoulder of intensity at 357 nm can be assigned to the trans-1 structure on this basis (Fig. 5 and S6†). The marked shift of this band to significantly higher energy in the spectrum of cis-1 (Fig. 5) explains the most significant spectral change that is observed on photoisomerization (Fig. 4).

Fig. 5 TD-DFT calculations for the B3LYP optimized geometries of trans-1 and cis-1 carried out at the LC-ωPBE/6-31G(d) level of theory in DMSO. The Q and B bands of the subPc moiety are highlighted with red diamonds, while green and black diamonds are used for bands associated with the subPc rings and AB linker, respectively. Gray diamonds denote charge transfer bands. The Chemcraft program³⁴ was used to generate simulated spectra with a band width at half height of 4000 cm⁻¹. Spectra calculated at the B3LYP/6-31G(d) level of theory are provided as ESI.†

Triad 1 exhibits fluorescence from the S₁ state of the subPc moiety, due to deactivation of the AB excited state through the isomerization process.²⁷ Upon excitation at λ = 400 nm, a fluorescence emission band is observed at 587 nm. The decay curve can be fitted with a single exponential decay curve that has a lifetime (τ_F) of 2.52 ns (Fig. 6), which is close to the value reported for subPc 2.²⁸ A decrease in emission intensity is observed after trans → cis isomerization upon irradiation at 365 nm, while there is a recovery in the observed intensity upon irradiation at 570 nm after cis → trans isomerization (Fig. 6). In the absence of aggregation-induced emission,²⁹ dative bonding interactions,³⁰ hydrogen bonding²⁷ and metal binding,³¹ and the minor fluorescence changes that have been attributed to steric repulsion³² in AB derivatives, the most obvious explanation is a change in the rate of non-radiative decay. The AB moiety of 1 adopts a planar conformation in the trans conformer (Fig. 1). When isomerization takes place to form the cis conformer, there is greater scope for conformational flexibility based on changes to the dihedral angle around the N=N bond (Fig. 7), since the AB moiety no longer has a planar π-conjugation system. Conformational flexibility usually results in an enhanced rate of non-radiative decay and hence a decrease in emission intensity. In DMSO, the energy gap between the B3LYP optimized structures of the cis- and trans-1 structures was found to be 14.6 kcal mol⁻¹, only slightly higher than the value of 13.5 kcal mol⁻¹ obtained for the parent AB structures. When structures were optimized with one of the subPc rings aligned in the opposing orientation with respect to the B–O bonds from those shown in Fig. 7, there were negligible differences in the energies of the B3LYP optimized structures and in the predicted optical properties (the details are provided as ESI†).

Fig. 6 (a) Fluorescence spectra changes for the Q band upon the photoisomerization of 1 in DMF. The figure on the left shows the effect of irradiation at 365 nm due to the formation of the cis rich PSS, while that on the right shows the regeneration of the trans conformer through irradiation at 570 nm. (b) Fluorescence emission lifetime data for 1 and 2.

Fig. 7 The B3LYP optimized geometries for the trans and cis conformers of 1. The trans-1 structure is almost identical to the X-ray structure in Fig. 1.

NMR changes in photoisomerization process

NMR spectroscopy can also be used to investigate the isomerization process.^22d,33 The 2D ¹H–¹H COSY spectrum of 1 (Fig. 8) contains four peaks centered at 8.8, 7.9 ppm, 7.2 and 5.4 ppm. The two upfield peaks that coupled to each other can be readily assigned to the protons from AB linker since they are shielded by the aromatic ring current of two neighbouring subPcs, while two downfield signals are protons from subPcs. When a CDCl₃ solution of 1 is irradiated with UV region light (365 nm), two new peaks are observed at 5.24 (d, J = 8.8 Hz) and 6.15 (d, J = 8.7 Hz), and the doublet subPc proton peak at 8.80 ppm gradually change into multiple peaks, indicating that trans → cis conformation change cause the proton signals of AB linker further upfield shifted. This is consistent with the optimized geometry of cis conformer in which two subPc moieties are closer than in the trans conformer (Fig. 7). Integration of the ¹H NMR spectra indicates that 25% of triad 1 is present as the cis form in the PSS that is formed in this context. The appearance of the new NMR peaks indicates that there is a drastic conformational change upon isomerization.

Fig. 8 (a) The 2D ¹H–¹H COSY spectrum of 1 in CDCl₃. (b) ¹H NMR spectra of 1 in CDCl₃: (b₁) before photoirradiation, (b₂) irradiation at 365 nm for 1.5 h, (b₃) for 5.5 h, (b₄) for 7.5 h.

Conclusions

In conclusion, a novel subPc–AB–subPc triad has been synthesized and fully characterized. Evidence for multiple reversible trans ↔ cis isomerization cycles was obtained from the spectral changes that are associated primarily with the AB linker moiety. Additional evidence for the conformational change can be deduced from other minor spectral changes that are observed in the NMR spectroscopy. Significantly, a decrease in fluorescence intensity is observed on moving from trans → cis with a recovery in intensity observed on moving back from cis → trans that can be attributed to changes in the rate of non-radiative decay. Efforts to explore the suitability of other novel multi-functional subPc triads and their suitability for use as light-sensitive or photochromic materials are currently underway.

Acknowledgements

Financial support was provided by the Major State Basic Research Development Program of China (Grant No. 2013CB922101 & 2011CB808704), the National Natural Science Foundation of China (No. 21371090), the Natural Science Foundation of Jiangsu Province (BK20130054) to ZS and the China-South Africa joint research program (CS08-L07 and UID: 95421) to ZS and JM, and an NRF of South Africa CSUR grant (93627) to JM.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: X-ray analysis of 1, synthesis details, NMR spectra, HR-MS data and additional TD-DFT calculations. CCDC 1401329. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra11452k

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