Photostability of indium phthalocyanines in organic solvents

doi:10.21203/rs.3.rs-3407051/v1

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Photostability of indium phthalocyanines in organic solvents

https://doi.org/10.21203/rs.3.rs-3407051/v1

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The effects of the macrocycle structure and the medium nature on photodecomposition under UV irradiation of different Indium phthalocyanines: the diindium tripledecker phthalocyanine, In₂Pc₃ (1), sandwich indium phthalocyanine, InPc₂ (2) and iodoindium phthalocyanine, InPcI (3) in benzene, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), dichloromethane (DCM) and 1-chloronaphtalene has been studied. The rate of decay of absorption is explained by a decomposition reaction which is of first-order kinetics with respect to the phthalocyanines concentration. In general, in the present of ligand I⁻ in phthalocyanine InPcI enhance the rate of decomposition.

Indium phthalocyanines

UV irradiation

photodecomposition

spectra UV-Vis

Phthalocyanines (Pcs), having an extensive delocalized 18π electron system on an aromatic macrocycle, have attracted great attention due to their unique spectroscopic, catalytic and structural properties [1]. Indium phthalocyanines with a heavy In(III) ion into the central cavity can have different structural features i.e.: the diindium tripledecker phthalocyanine, In₂Pc₃ (1), sandwich (or double decker) indium phthalocyanine, InPc₂ (2) or iodoindium phthalocyanine, InPcI (3) (Pc = C₃₂H₁₆N₈²⁻ phthalocyanine ligand) (Fig. 1) [2–5].

The electronic absorption spectra of In₂Pc₃, InPc₂ and InPcI solid compounds after dissolvation in not polar and pH neutral benzene as also in polar and basic pyridine (py) has been published, recently. In benzene, the molecules of the diindium tripledecker phthalocyanine, In₂Pc₃, undergoes transformation to ionic couples [InPc]⁺ [InPc₂]⁻, whereas the molecules of the indium sandwiches, InPc₂, remain relatively stable in the solvent. When the InPc₂, compound is dissolved in py, the Pc(•-) rings of the compound molecules undergo disjunction and rejection, what leads to formation three different chromophores in solution i.e.: In₂Pc₃ and ([InPc(2-)])₂ ⇔ 2InPc(3-) couple. Spectra for InPcI compound, are taken for comparison, at benzene and at py, show no changes of the chromophore [6]. The solvents used in experiment play a crucial role.

A series of new tetra-substituted, water solubility, indium phthalocyanine (InPc) complexes were used in the photodynamic antimicrobial activities against Staphylococcus aureus and Escherichia coli [7]. In the other side, indium phthalocyanines, tetrasubstituted at the non-peripheral and peripheral positions and octasubstituted at the peripheral position with the 4-benzyloxyphenoxy group or axially substituted are good candidates for nonlinear optical devices. These materials are important industrial dyes [8, 9].

Photostability of metallophthalocyanine (MPc) complexes is important for their applications as photocatalysts [10,11], in medicine as photosensitizers for photodynamic therapy of cancer [12–14] or for application in photooxidation of organic and inorganic impurities occurring in the natural environment, including phenol derivatives, alcohols and sulphur compounds [15–17].

Photodegradation is an oxidative degradation process performed to determine the stability of metallophthalocyanines by photoirradiation. In other words, it is the resistance of the molecules to light. Photodegradation of the macrocycle in MPc has usually been attributed to oxygen-mediated reactions that may be prompted due to absorption of photonic energy by both the phthalocyanines substrate and molecular oxygen. The energy of the excited states in metal phthalocyanines is quite enough to sensitize other molecules like molecular oxygen [18]. Since excited phthalocyanine may sensitize O₂ species to give rise to the formation of singlet molecular oxygen (¹Δ_g) along with some different reactive oxygen species (ROS), it is frequently referred to as the principal reason for the photo-bleaching of phthalocyanines [19–24]. Photobleaching mechanism is usually related to the attack of ¹O₂ or other ROS to the ring. In this sense, a compound with higher production of singlet oxygen or reactive oxygen species is expected to be less photostable due to the higher probability of ¹O₂ shocks with the Pc macrocycle. Photochemical decay of phthalocyanines in solution is considered to be a complex process due to the possible formation of different intermediate radical compounds, which may interact with each other and also with the system components. Photostability of metal phthalocyanines in solution is generally depends on the nature of the central atom and the structure of the macrocycle as well as from used organic solvent in experiment [22], especially since oxygen is more soluble in organic solvents than in water [25]. Solvent like dimethylformamide (DMF), frequently used in phthalocyanine studies, readily absorb oxygen from air [26]. Under normal experimental conditions, oxygen does not seem to influence the phthalocyanineʼs chemistry, unless exposed to light. The main product of the photodegradation process of metallophthalocyanines is phthalimide [14,22]. The formation of phthalimide can be related to molecular oxygen once adsorbed by the complex of phthalocyanines from air. Organic solutions of phthalocyanines have absorbed more oxygen from air than does pure solvents. Phthalimide appeared also when the solutions where deaerated [22]. Probably because some molecular oxygen may chemically combine with the macrocycle. The interaction of metallophthalocyanines with molecular oxygen may lead to the formation of different crystalline forms of phthalocyanines even [27,28].

In the current work has been decided to measure of photostability of indium phthalocyanines using the UV-Vis spectra of In₂Pc₃, InPc₂ and InPcI in solution, for better comparison both organic solvents and structure of phthalocyanines. However, there is not much literature concerning the photostability of the double sandwich (or triple-decker) phthalocyanine, In₂Pc₃.

2.1. Materials and equipment

The indium of 99.998% purity and analytically pure phthalonitrile (Pht), benzene, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), dichloromethane (DCM) and 1-chloronaphtalene (1-Cl-NAPHTA), were purchased from commercial supplier and were used as received.

JASCO V-670 UV-Vis-NIR Spectrophotometer (Spectra Manager V.2 software) and quartz cuvettes (1 cm optical path length, V = 4 mL) were used to measure electronic absorption spectra.

2.2. Indium phthalocyanines

The In₂Pc₃ (1), InPc₂ (2) and InPcI (3), in crystalline forms, were obtained from indium fillings and phthalonitrile powder. The syntheses of In₂Pc₃ and InPc₂ were performed by thermal processing of both reactants in degassed ampoules as described elsewhere [3–5]. In case of the InPcI preparation, the thermal processing of indium and phthalonitrile was performed under iodine vapour [29]. The indium phthalocyanines 1–3 concentration (the order of 10^− 5 M, mol/dm³ depending on the compound) was fixed to achieve initial absorbance values between 0.8 and 1.3. A 10-ml sample of the indium phthalocyanines solution was prepared 1 day before used in experiment.

2.3. Photostability Studies

To assess the photostability a similar procedure was applied, as in previously reported work [19]. The tested solutions were exposed to UV radiation (500 µW/cm²) in a thermostated quartz cuvette (at 20° C). For this purpose, a mercury medium pressure lamp (Heraeus St-46) characterized by narrow emission lines of similar spectral irradiance at λ = 313, 366, and 436 nm was used. Irradiance was measured at the cuvette wall facing the light source. The explored solutions were not de-aerated prior to testing. Kinetic curves, A = f(t), reflecting the phthalocyanine degradation were determined from time-correlated absorbance measurements, i.e., from the peak absorption value of the Q-band. The effective photolysis rate constants, k, were computed directly from kinetic data (CurveExpert v.1.34 software, Hyams Development, Huntsville, AL, USA)

The spectra of the solutions prepared by dissolvation of the In₂Pc₃, InPc₂ and InPcI compound crystals at organic solvents are collected in Fig. 2a-2c and peak position of λ_max in Table 1, respectively. UV–Vis spectra are especially fruitful to establish the structure of the phthalocyanines. Generally, the indium phthalocyanines exhibit typical electronic spectra with two strong absorption regions, one of them is in the visible region at 600–700 nm (Q band) and the other one is in the UV region at about 300–350 nm (B band or Soret band ), both bands correlate to π–π*transitions, but the spectrum of In₂Pc₃ is little different, its Q band is split in all organic solvents, expect for DMSO.

Table 1

Peak position (λ_max, nm) of the B, CT (charge transfer) and Q bands in the UV-Vis spectra of indium phthalocyanines; MLCT = metal-to-ligand charge transfer, LMCT = ligand-to-metal charge transfer
Phthalocyanine/ Solvent	In₂Pc₃		InPc₂	InPcI
Benzene	B	338	343	362
	MLCT	455	623, 653, 690	622, 660, 690
	Q	622, 652, 687
	LMCT	933
DMF	B	336	355	355
DMF	Q	610, 682, 725	615, 654, 682	615, 650, 682
DMSO	B	355	355	355
DMSO	Q	618, 653, 686	618, 653,686	618, 653, 686
2CM	B	355	358	358
	MLCT	456	622, 658, 688	621, 659, 688
	Q	592, 621, 654, 688
	LMCT	823, 943
1-chloronaphtalene	B	341	362	364
	MLCT	428	628, 661, 697	628, 665, 696
	Q	591, 625, 655, 694
	LMCT	938

To better illustrate the effect of the solvent, the phthalocyanines under study are presented in supplementary (see sec. 1).

3.1. Photostability of indium phthalocyanines

A solution of indium phthalocyanine (InPc) in all organic solvent, normally very stable when stored in the dark, features a progressive loss in color intensity after being exposed to UV-light. The character of spectral changes observed in UV-photolyzed solutions of the different indium phthalocyanines evidently depends on nature of organic solvents. Generally, the changes observed in the spectra for the all investigated phthalocyanines were similar in benzene. The process of photodegradation of MPc has occurred in three steps. In the former case, three distinct reaction stages emerge from the spectra, featuring the formation of an intermediate product and consequently its degradation. Spectral changes have been identified, as shown in Fig. 3 for In₂Pc₃ and section 2 in supplementary, for others compounds (2.1 InPc₂ in benzene 2.2 InPcI in benzene).

The process of photodegradation of indium phthalocyanines in others organic solvents have occurred in one step, expect of process of In₂Pc₃ which will be discussed later in this article. There were significant changes observed of the B or Q-bands for indium phthalocyanines indicating that they are not photostable. Spectral changes observed during UV-illumination for solution of InPc₂ in DMF, DMSO, DCM and 1-chloronaphtalene are presented in Fig. 4. and in section 3 in supplementary, for others compounds (3.1 In₂Pc₃ and 3.2 InPcI in others organic solvents).

Photostability of In₂Pc₃ in 1-chloronaphtalene

Process of photodegradation of In₂Pc₃ has occurred in two steps in 1-chloro-naphthalene. Absorption spectra are presented on Fig. 5.

When the In₂Pc₃ compound is dissolved in 1-chloro-naphthalene, and next of UV- irradiation the inner Pc(2-) ring of diindium tripledecker phthalocyanine molecule undergoes disjunction and rejection. In the results of it in the solution the couple of chromophores, being in the dynamical balance ([InPc(2-)])2 = 2 InPc(3-) is formed. Similar changes spectral has been observed early when have investigated molecular balance forms of indium phthalocyanines in pyridine solution [6]. After 210 min of UV-irradiation of solution In₂Pc₃ in 1-choloro-naphthalene has formed new form with new absorption spectra at 663 nm and 698 nm. The observed spectral changes have indicated that process of transformation In₂Pc₃ occurred.

3.2. Process kinetics of photodegradation of indium phthalocyanines in organic solvents

Kinetic considerations were addressed mainly to the photolysis rate of the UV- irradiated investigated compounds, and where applicable, also to the decay of the above mentioned intermediate form produced during the first reaction step, which appeared quite stable. At this stage of investigation, however, it has not be identified other byproducts, which could be assigned any characteristic and reliable UV–vis spectrum. Hence, the calculations were based essentially on the changes observed in the spectral range of 350–500 nm. Generally, the photolysis induced by UV radiation has followed a first-order reaction

A = A₀ ⋅ e^− kt (1)

where A and A₀ are absorbances measured for the Q-band of the MPc complex at time t and t = 0, respectively, and k is the effective reaction rate constant and demonstrated similar performance on the time scale as in the example presented in Fig. 6. The effective photolysis rate constants, k, have been reported in Table 2. (For more extensive data see section 4 in supplementary, Fig. 4)

Kinetics of the degradation process has proved to depend on the molecular structure of the complex and seems to be controlled by interactions of the macrocycle bridging nitrogen atoms with the solvent molecules.

Effect of organic solvents

The influence of the solvent one may attribute either to different solvation effects or a possible chemical interaction between the phthalocyanine complex and the solvent molecules.

The rate of the photochemical degradation proved to depend on the solvent used. Each of the studied InPc compounds showed increasing stability in the order DCM < DMSO < DMF < Benzene ~ 1-chloronaphtalene. In particular benzene and 1-chloronaphtalene appeared to produce a substantial stabilizing effect upon the macrocycle, very pronounced also in the case of the least stable compound InPcI. Dichloromethane has appeared organic solvent which indium phthalocyanines are the most unstable under UV-irradiation. Upon extended broadband (λ > 320 nm) irradiation in unstabilized CH₂Cl₂ (radical-forming non-coordinating solvent). This organic solvent can proved reactive radicals species (RRS) or even HCl [30,31]. Surprising effect has been observed when used DMSO, electron acceptor solvent. In this solvent all indium phthalocyanines are very unstable during UV-irradiation. DMSO may degraded during process of illumination into sulfate (SO₄²⁻) through the formation of methansulfinate (CH₃SO₂⁻) and methansulfonate (CH₃SO₃⁻) as sulphur-containing intermediates [32].

Table 2

Photolysis effective rate constants, k, (min^− 1 ); I_UV = 500 µW/cm² of Indium phthalocyanines in organic solvents
MPc/ solvent	benzene*	DMF	DMSO	DCM	1-Cl-NAPHTA *
In₂Pc₃	0.0077	0.021	0.13	0.610	0.0033
InPc₂	0.012	0.054	0.15	0.700	0.037
InPcI	0.019	0.105	0.21	1.03	0.041
) I stage, in benzene all phthalocyanines, but in 1-chloro-naphthalene In₂Pc₃ only*

Mechanism of photodegradation

The indium phthalocyanine photodegradation rate depends on the kind of organic solvent used in experiments [ Tab.]. It has been discovered that photochemical behavior of phthalocyanines in coordinating solvent DMSO and in non-coordinating benzene is significantly different [33 ]. The photostability of MPcs in solution particular emphasis on their photobleaching in photooxidation processes. It is assumed that the photodecomposition consists of reactions of the complex with O₂, and/or the solvent. Two mechanisms with O₂, are possible. The excited triplet state of the MPc can interact with ground state molecular oxygen, generating radical ions, superoxide and hydroperoxyl radicals, which subsequently afford oxidation of the substrate by Type I mechanism or the MPc molecule is first excited to the singlet state and through intersystem crossing forms the triplet state, and then transfers the energy to ground state oxygen, or direct electron transfer leading to the formation of singlet dioxygen, ¹O, (¹Dg), from triplet dioxygen, ³O₂, (³Ʃ_g) could, in a second step, attack the macrocycle and oxidize it, for example to phthalimide [14,22, 34,35].

Effect of molecular structure of indium phthalocyanines

The indium phthalocyanines have different molecular structure (Fig. 1). The most stable is In₂Pc₃ in all organic solvent. In this compound are two phthalocyanines rings so under UV-irradiation undergoes transformation to ionic couples [InPc]⁺~[InPc₂]⁻, what were in solutions of benzene and 1-chloro-naphthalene. It should be added herein that similar experiments are known for some lanthanide analogs. In particular in Ref. [29] it has been reported that the La₂Pc₃ and Lu₂Pc₃ absorbance spectra in benzene show Q band position at wavelength characteristic for classical monophthalocyanines. On the other hand InPcI is the most unstable phthalocyanine. In general, in the present of ligand I⁻ in phthalocyanine InPcI enhance the rate of decomposition.

Summarizing this work, it must be emphasize the importance of stabilizing effects, which may expand the lifetime of photoexcited macrocycles such as indium phthalocyanines. As it has been proved, properly balanced distribution of the p-electrons involved in the conjugated bonding system produced higher strength of the phthalocyanine moiety. The chemistry of the studied photodegradation process is determined by the nature of interactions between the bridging nitrogen atoms of the macrocycle and the solvent molecules, which has been supposed most likely control the kinetics of the decay of photosensitized phthalocyanines. Thus, by combining structural features of the phthalocyanine compound and chemical properties of the applied solvent, it is possible to design the most optimized conditions for a photochemical reaction. It has been shown that in the present of ligand I^- in the indium phthalocyanine enhance the rate of photodecomposition, while In₂Pc₃ shows the best photostability.

The author declare that: I have no conflict of interest.

Acknowledgements

Thanks to Prof. R Kubiak from Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wroclaw for all indium phthalocyanines.

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Scheme 1 is available in the Supplementary Files section.

No competing interests reported.

Scheme1.jpg
Scheme 1. The process involves singlet dioxygen addition to the macrocyclic ring followed by cleavage at the meso-positions [eqn. (1)].
supplementaryDyrda.docx

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Photostability of indium phthalocyanines in organic solvents

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