Photophysical properties of gallium hydroxyl tetratolylporphyrin and 132-demethoxycarbonyl-(gallium hydroxyl)–methyl-pheophorbide a
Introduction
Metal porphyrin derivatives are biologically important chromophores with a macrocyclic-conjugated system of π-electrons. The synthesis and photophysical characterization of metal porphyrins and especially the diamagnetic Ga-porphyrin-complexes were and still are a hot topic in chemistry and optical spectroscopy [1], [2], [3], [4]. Characteristic and partly intense B and Q absorption bands in the near-ultraviolet and visible regions, respectively, arise from the (π → π*) transitions of the electronic system of the porphyrin. In case of closed shell configuration of the central metal, the outer-shell electrons give only a minor perturbation [3], [5].
Specifically, several reports on Ga-porphyrin-complexes have appeared in the past, where both synthetic procedures and photophysical studies were in the center of interest [1], [2], [3], [4]. Insertion of gallium into porphyrins has been achieved by two different synthetic approaches in the past: reaction of the free base with gallium (III)-salts, e.g. chloride, in acetic acid [6] and by the solvent-free procedure by reacting the porphyrin with gallium (III)-acetylacetonate in a phenol melt [7].
Interestingly, no photophysical studies on gallium chlorins have been published to our knowledge, and preparative chemistry is largely unknown. Generally, synthetic procedures and photophysical properties of free base chlorins and of a large variety of metal chlorins, especially chlorophyll a derivatives, have been studied extensively for decades [8], [9], [10], [11], [12], [13], [14], [15]. Related gallium containing species, however, do not seem to have gained much interest, even though the additionally available axial position in gallium centered tetrapyrroles could be particularly useful as a versatile coupling site for the synthesis of novel donor–acceptor systems. Such novel dyads could be utilized in the investigation of photoinduced energy or electron transfer.
Moreover, on the way towards 2nd and 3rd generation photosensitizers for photodynamic therapy (PDT) [16], besides a number of porphyrins, certain chlorins were considered as prospective photosensitizing molecules [17]. Their characteristic absorption spectra with bands in the far red spectral region with large extinction coefficients combined with high quantum yields for singlet oxygen generation [18] suggest their potential efficiency in photodynamic activity.
In 1971, Bajema et al. [19] first observed S2-fluorescence of a porphyrin: zinc tetrabenzoporphyrin (zinc tetraphenylporphyrin) in octane and in argon matrices. Since that time emission from higher excited states was observed for a variety of diamagnetic metal porphyrins [3], [5], [19], [20], [21], [22], [23], [24]. The large energy gap (Ga-(OH)-TTP: 6800 cm−1) and the parallel energy surfaces of S1 and S2 state retard the internal conversion between the two states. Therefore a greater transition moment granted to S2 → S0 transition yields emission from the S2 state [5]. Ohno et al. investigated the S2-fluorescence of different metal porphyrins and also for a gallium porphyrin (Ga-Cl-TPP in benzene) [3].
The steady state S2-fluorescence can be used as a reference to determine the rates of photoinduced electron transfer (PET) in, e.g., thin layers like Langmuir–Blodgett-films, where the fluorescence intensity is very low and time resolved measurements become impossible. Due to the fact that only the S1 contributes to the PET, the ratio between the intensities of S1- and S2-fluorescence can be considered as measure of PET rates [25].
In a comparative study we investigated the photophysical properties of Ga-(OH)-TTP and a novel semi synthetic Ga-(OH)-chlorin derived from chlorophyll a, and evaluated their suitability as either electron donor in electron transfer systems or as efficient photosensitizer for PDT. This study is expected to re-focus attention on gallium containing tetrapyrroles and to stimulate further preparative-synthetic and photophysical investigations.
Section snippets
Sample preparation and analysis
Tetratolylporphyrin, gallium acetylacetonate, phenol and common laboratory solvents were obtained from Sigma–Aldrich, Taufkirchen, Germany. 132-Demethoxycarbonyl methylpheophorbide a was obtained by extracting and processing the blue–green algae Spirulina maxima following a procedure of Smith et al. [26]. NMR-spectra were recorded using a Bruker DRX-300 NMR spectrometer (300 MHz for 1H). Mass analysis was performed with a Finnigan MAT 90 spectrometer and UV/VIS spectra were recorded on a
Results and discussion
The structures of Ga-(OH)-TTP 1 and Ga-(OH)-chlorin 2 are shown in Fig. 1. The OH-group of 2 can be positioned either syn or anti to the 17-ester chain. Both isomers were not separated, since significant differences in the photophysical properties of both isomers are not expected. The absorption spectra of 1 and 2 are displayed in Fig. 2. Concerning the latter compound, it is well known that dihydrogenation of one of the peripheral cryptoolefinic double bonds of the porphyrin system causes
Conclusions
The photophysical properties of both compounds make them interesting for application as photosensitizers in PDT and as electron donors in artificial photosystems. Moreover the S2-fluorescence of the Ga-(OH)-TTP could be used as strong measure of the efficiency of light induced electron transfer in artificial photosystems. The Ga-(OH)-chlorin has suitable absorption bands and extinction coefficients as well as a high singlet oxygen quantum yield to be effectively utilized as photosensitizer in
Acknowledgments
The authors thanks Mr. Wöhlecke for the technical support.
References (30)
- et al.
Inorg. Comm.
(2003) - et al.
J. Mol. Spectrosc.
(1971) - et al.
Chem. Phys. Lett.
(1999) - et al.
Thin Solid Films
(2001) - et al.
Thin Solid Films
(1998) - et al.
Inorg. Chem.
(1987) - et al.
J. Chem. Phys.
(1985) - et al.
Inorg. Chem.
(1985) - et al.
Inorg. Chem.
(1978)
Liebigs Ann. Chem.
J. Am. Chem. Soc.
J. Chem. Soc.
Biochim. Biophys. Acta
Cited by (7)
Comprehensive review of photophysical parameters (ε, Φ<inf>f</inf>, τ<inf>s</inf>) of tetraphenylporphyrin (H<inf>2</inf>TPP) and zinc tetraphenylporphyrin (ZnTPP) – Critical benchmark molecules in photochemistry and photosynthesis
2021, Journal of Photochemistry and Photobiology C: Photochemistry ReviewsCitation Excerpt :Here, the literature searching that yielded Tables 1–6 relied on use of Reaxys, keyword searches in websites of selected journals, and PDF-file searching of large sets of downloaded manuscripts from selected journals. Altogether some ∼2000 papers from the literature were manually scrutinized to arrive at the set reported herein [1–871]. The organization of the discussion is as follows.
Geometric and electronic structures of 5,10,15,20-tetraphenylporphyrinato Palladium(II) and Zinc(II): Phenomenon of Pd(II) complex
2019, Journal of Molecular StructureCitation Excerpt :Porphyrins are of interest for their extreme stability and ability to coordinate to almost all of the metal ions found in the periodic table [9]. Indeed, the metals change the photophysical and spectroscopic characteristics of porphyrins, thereby modulating their stability, hydrophobicity, the ability to form aggregates in solutions, and intracellular distribution [10,11]. Thermodynamic stability of metal porphyrins depends on the electronegativity, the oxidation number and the effective ionic radius of a central metal ion.
Novel metal complexes of boronated chlorin e<inf>6</inf> for photodynamic therapy
2009, Journal of Organometallic ChemistryOrienting the heterocyclic periphery: A structural model for chloroquine’s antimalarial activity
2014, Chemical CommunicationsFluorescence and phosphorescence from higher excited states of organic molecules
2012, Chemical ReviewsPhotosensitisers in biomedicine
2009, Photosensitisers in Biomedicine