High Triplet Quantum Yield Research Articles

Unraveling the mechanisms of triplet state formation in a heavy-atom free photosensitizer.

Triplet excited state generation plays a pivotal role in photosensitizers, however the reliance on transition metals and heavy atoms can limit the utility of these systems. In this study, we demonstrate that an interplay of competing quantum effects controls the high triplet quantum yield in a prototypical boron dipyrromethene-anthracene (BD-An) donor-acceptor dyad photosensitizer, which is only captured by an accurate treatment of both inner and outer sphere reorganization energies. Our ab initio-derived model provides excellent agreement with experimentally measured spectra, triplet yields and excited state kinetic data, including the triplet lifetime. We find that rapid triplet state formation occurs primarily via high-energy triplet states through both spin-orbit coupled charge transfer and El-Sayed's rule breaking intersystem crossing, rather than direct spin-orbit coupled charge transfer to the lowest lying triplet state. Our calculations also reveal that competing effects of nuclear tunneling, electronic state recrossing, and electronic polarizability dictate the rate of non-productive ground state recombination. This study sheds light on the quantum effects driving efficient triplet formation in the BD-An system, and offers a promising simulation methodology for diverse photochemical systems.

Understanding remarkably high triplet quantum yield in thione analogs of perylenediimide: A detailed theoretical study.

The diverse and tunable electronic structures of perylenediimide (PDI), together with its high thermal and chemical stability, make the compound suitable for applications in bioimaging, electrical, and optical devices. However, a large singlet-triplet gap (ΔES-T) and almost zero spin-orbit coupling (SOC) between the lowest excited singlet (S1) and triplet (T1) restrict intersystem crossing (ISC) in highly fluorescent pristine PDI, yielding a near zero triplet quantum yield (ΦT). Interestingly, a thione analogs of PDI with varied S content (mS-PDIs, m = 1-4) was experimentally shown to yield ΦT ∼ 1.0 through efficient ISC. Time-dependent optimally-tuned range-separated hybrid calculations are performed to rationalize the experimentally observed red-shifted optical absorption and also the remarkably high ISC with almost zero radiative fluorescence reported for these mS-PDIs. To this end, the relative energies of low-lying excited singlets Sn (n = 1, 2) and a few triplets Tn(n = 1-3), along with their nature (nπ* or ππ*), are assessed for each of the mS-PDIs studied in chloroform. To our surprise and contrary to the earlier reports, both S1 and T1 are found to be of the same ππ* character, originating from the highest occupied to lowest unoccupied orbital transition, which, therefore, leads to a still large ΔES-T and vanishingly small SOC, as expected from the identical wavefunction symmetry. Increasing S content lowers S1(ππ*) due to a greater extent of π-delocalization, which well complements and supports the observed red-shift. More importantly, the T2 (or T3) closely lying to the S1 is of nπ* and, therefore, produces a relatively smaller ΔES-T and larger SOC. Detailed kinetics analysis suggests S1(ππ*) → T2(nπ*) is the primary ISC channel for all mS-PDIs, which is responsible for the remarkably high ΦT observed. In addition, comparable SOC and ΔES-T produce similar ISC rates for all mS-PDIs.

Tailoring intersystem crossing of perylenediimide through chalcogen-substitution at bay-position: A theoretical perspective.

Molecular-scale design strategies for promoting intersystem crossing (ISC) in small organic molecules are ubiquitous in developing efficient metal-free triplet photosensitizers with high triplet quantum yield (ΦT). Air-stable and highly fluorescent perylenediimide (PDI) in its pristine form displays very small ISC compared to the fluorescence due to the large singlet-triplet gap (ΔES-T) and negligibly small spin-orbit coupling (SOC) between the lowest singlet (S1) and triplet state (T1). However, its ΦT can be tuned by different chemical and mechanical means that are capable of either directly lowering the ΔES-T and increasing SOC or introducing intermediate low-lying triplet states (Tn, n = 2, 3, …) between S1 and T1. To this end, herein, a few chalcogen (X = O, S, Se) bay-substituted PDIs (PDI-X2) are computationally modeled aiming at introducing geometrical-strain at the PDI core and also mixing nπ* orbital character to ππ* in the lowest singlet and triplet excited states, which altogether may reduce ΔES-T and also improve the SOC. Our quantum-chemical calculations based on optimally tuned range-separated hybrid reveal the presence of intermediate triplet states (Tn, n = 2, 3) in between S1 and T1 for all three PDI-X2 studied in dichloromethane. More importantly, PDI-X2 shows a significantly improved ISC rate than the pristine PDI due to the combined effects stemming from the smaller ΔES-T and the larger SOC. The calculated ISC rates follow the order as PDI-O2 < PDI-S2 < PDI-Se2. These research findings will be helpful in designing PDI based triplet photosensitizers for biomedical, sensing, and photonic applications.

The effect of PAMAM dendrimers on the excited states of little-explored xanthene dyes. In search of eco-friendly photoinitiating systems

A complete photophysical and photochemical study on the partially halogenated xanthene dyes 2’,7’-dichlorofluorescein; 4, 5, 6, 7- tetrachlorofluorescein; 4’, 5’-dibromofluorescein and 4’, 5’-diiodofluorescein was carried out in aqueous solution. The effect of three PAMAM dendrimers of low generations (G0, G1 and G3) on their excited states was also evaluated. Absorption and fluorescence measurements showed that although the ground and singlet-excited states of the dyes interact weakly with the dendrimers, the binding constants obtained correlated with dendrimer size. The triplet-excited states of the dyes were studied by laser flash photolysis including those for the chlorinated fluoresceins; which, to the best of our knowledge, had not been characterized until now. Iodo- and bromo-fluoresceins showed high triplet quantum yields, whereas chlorinated ones showed values in the order of 0.10, indicating that, although low, the intersystem crossing is not a negligible process. Efficient electron transfer reactions were observed between amino moieties of PAMAM and the triplet states of the dyes. The triplet quenching rate constants also correlated with dendrimer size and the semi-reduced species of the dyes were characterized by their transient spectra. Radical formation efficiencies similar to those obtained with a typical tertiary amine were estimated, so the present results suggest that these dye/dendrimer couples might be promising as photoinitiating systems for polymerizations with low ecological impact.

Revisiting the Dynamics of Triplet Formation in Anthraquinones.

Triplet formation pathways in 9,10-anthraquinone (AQ) and its hydroxy derivative, 1-hydroxyanthraquinone (HAQ), are studied theoretically. Dynamics simulations on the model singlet-triplet potential energy surfaces within the linear vibronic coupling framework are performed to elucidate possible internal conversion (IC) and intersystem crossing (ISC) pathways in these molecules. An ultrafast IC decay from the "bright" S4 to S1 followed by efficient ISC via S1-T4 and S1-T5 pathways fosters a high triplet quantum yield (ΦT = 0.90) in AQ. In HAQ, a new nonradiative channel of "barrierless" excited-state intramolecular proton transfer (ESIPT) opens up and competes with the IC decay to S1 upon photoexcitation to the "bright" S2. Extremely fast ESIPT on S2 reduces the efficiency of triplet formation via possible ISC pathways involving S1 and S2, resulting in a low ΦT (=0.17).

Synergistic effects of photoinduced electron transfer and heavy atom effect based on BODIPY for efficient triplet photosensitizers

Achieving a long triplet lifetime and a high triplet quantum yield for efficient triplet sensitizers are crucial for commercializing triplet–triplet annihilation–upconversion (TTA-UC). Although many donor-acceptor (D-A) or heavy atom-based triplet sensitizers have been extensively studied, each strategy suffers from major drawbacks, such as low triplet quantum yield of D-A and short triplet lifetime of heavy atoms. Herein, we propose the integration of a donor-acceptor with a heavy atom, which improves both triplet lifetime and quantum yield because of the simultaneous utilization of photoinduced electron transfer and the heavy atom effect. To prove this, we synthesized Triphenylamine(D)-BODIPY(A)-bromine(H) (AM6, AM7, and AM8), which were compared with standards such as Triphenylamine(D)-BODIPY(A) (AM2) and BODIPY-Bromine (AM9). The donor-acceptor-heavy atom (D-A-H) compounds (AM6, AM7, AM8) exhibited excellent TTA-UC emission efficiency (3.9–6.5 %), which were high compared to AM2 (1.5 %) and AM9 (1.7 %). Moreover, threshold intensity (Ith) of AM6, AM7 and AM8 were lower (176–465 mW/cm2) than AM9 (1,916 mW/cm2), even though they had heavy atom. This demonstrates that D-A-H is an effective molecular design for triplet sensitizers with a synergistic effect.

Nanoscale Metal-Organic Layer Isolates Phthalocyanines for Efficient Mitochondria-Targeted Photodynamic Therapy.

Zinc-phthalocyanine (ZnPc) photosensitizers (PSs) have shown great potential in photodynamic therapy (PDT) owing to their strong absorption at long wavelengths (650-750 nm), high triplet quantum yields, and biocompatibility. However, the clinical utility of ZnPc PSs is limited by their poor solubility and tendency to aggregate in aqueous environments. Here we report the design of a new nanoscale metal-organic layer (nMOL) assembly, ZnOPPc@nMOL, with ZnOPPc [ZnOPPc = zinc(II)-2,3,9,10,16,17,23,24-octa(4-carboxyphenyl)phthalocyanine] PSs supported on the secondary building units (SBUs) of a Hf12 nMOL for PDT. Upon irradiation, SBU-bound ZnOPPc PSs absorb 700 nm light and efficiently sensitize the formation of singlet oxygen by preventing aggregation-induced self-quenching of ZnOPPc excited states. With intrinsic mitochondria-targeting capability, ZnOPPc@nMOL showed exceptional PDT efficacy with >99% tumor growth inhibition and 40-60% cure rates on two mouse models of colon cancer.

Synthesis, photophysics, and photochemistry of ball-type phthalocyanines

Phthalocyanines (Pcs) have numerous potential applications, including use as photosensitizers for photodynamic cancer therapy. These applications depend on specific photodynamic properties; therefore, new Pc compounds with unique and well characterized properties are in constant demand. In this study, four novel ball-type Pc compounds were synthesized with 1,2-benzenediol as a substituent, including one non-metal complex (H4Pc2) and three metal-containing complexes (Mg2Pc2, Co2Pc2, and Zn2Pc2). The structures of the synthesized compounds were characterized using common spectroscopic methods. Quantum yields for fluorescence (ΦF), triplet state (ΦT), singlet oxygen (ΦΔ), and photodegradation (Φd) were measured except Co2Pc2 in dimethylsulfoxide. All the three compounds exhibited low fluorescence quantum yield, high triplet quantum yield, and good stability in organic solvent, suggesting acceptable photostability. Complexes with H4Pc2 and Mg2Pc2 demonstrated differences in the Q band in their Uv–Vis spectra. Singlet oxygen quantum yield was 0.35 for Zn2Pc2. Fluorescence lifetimes (τF) for the complexes ranged from 6.67 to 2.95 ns, while triplet state lifetimes (τT) ranged from 50 to 440 μs. This novel group of Pcs possesses properties suitable for photodynamic therapy, particularly ZnPc.

Photophysics of BODIPY-Based Photosensitizer for Photodynamic Therapy: Surface Hopping and Classical Molecular Dynamics.

Halogenated BODIPY derivatives are emerging as important candidates for photodynamic therapy of cancer cells due to their high triplet quantum yield. We probed fundamental photophysical properties and interactions with biological environments of such photosensitizers. To this end, we employed static TD-DFT quantum chemical calculations as well as TD-DFT surface hopping molecular dynamics on potential energy surfaces resulting from the eigenstates of the total electronic Hamiltonian including the spin-orbit (SO) coupling. Matrix elements of an effective one-electron spin-orbit Hamiltonian between singlet and triplet configuration interaction singles (CIS) auxiliary wave functions are calculated using a new code capable of dealing with singlets and both restricted and unrestricted triplets built up from up to three different and independent sets of (singlet, alpha, and beta) molecular orbitals. The interaction with a biological environment was addressed by using classical molecular dynamics (MD) in a scheme that implicitly accounts for electronically excited states. For the surface hopping trajectories, an accelerated MD approach was used, in which the SO couplings are scaled up, to make the calculations computationally feasible, and the lifetimes are extrapolated back to unscaled SO couplings. The lifetime of the first excited singlet state estimated by semiclassical surface hopping simulations is 139 ± 75 ps. Classical MD demonstrates that halogenated BODIPY in the ground state, in contrast to the unsubstituted one, is stable in the headgroup region of minimalistic cell membrane models, and while in the triplet state, the molecule relocates to the membrane interior ready for further steps of photodynamic therapy.

Picolyl Porphyrin Nanostructures as a Functional Drug Entrant for Photodynamic Therapy in Human Breast Cancers.

The major challenge in photodynamic therapy (PDT) is to discover versatile photosensitizers (PSs) that possess good solubility in biological media, enhanced singlet oxygen generation efficacy, and photodynamic activity. Working in this direction, we synthesized a picolylamine-functionalized porphyrin conjugate, compound 1, and its zinc complex compound 2. Compound 1 forms spherical structures in methanol, whereas compound 2 exhibited vesicular structures. Compared to the existing PSs like foscan and photofrin, compound 2 exhibited a high singlet oxygen generation efficiency and triplet quantum yield. The complex also showed good water solubility, and its PDT activity was demonstrated through in vitro studies using MDA-MB 231 breast cancer cells. The mechanism of biological activity evaluated using various techniques proved that the active compound 2 induced predominantly singlet oxygen-triggered apoptosis-mediated cancerous cell death. Our results demonstrate that zinc insertion in the picolyl porphyrin induces an enhanced triplet excited state, and the singlet oxygen yields quantitatively and imparts excellent in vitro photodynamic activity, thereby demonstrating their pertinence as a nanodrug in future photobiological applications.

In-situ synthesis of gold nanoparticles on graphene quantum dots-phthalocyanine nanoplatforms: First description of the photophysical and surface enhanced Raman scattering behaviour

Owing to the need for new low-dimensional molecular assemblies with tailored electronic properties, the current study presents a facile approach for the synthesis and assembly of gold nanoparticles (AuNPs) onto functional graphene quantum dots (GQDs)-phthalocyanines (Pcs) arrays and the investigation of their photophysical and surface enhanced Raman scattering (SERS) properties. The GQDs were functionalized with L-glutathione (GSH) (to form GQDs@GSH) in order to assist coupling to the low symmetry Zn tris–(tert–butyl) mono carboxyphenoxy (propionic acid) phthalocyanine (complex 1) to form 1@GQDs. The affinity of gold (Au) to sulphur (S) was exploited for the assembly of the AuNPs onto 1@GQDs platform to form 1@GQDs-AuNPs. Transmission electron microscopic investigations confirmed the formation of monodispersed, spherical Pc/GQDs@GSH/AuNPs hybrids. The nanocomposite displayed high triplet quantum yields, which translated into high singlet oxygen quantum yield as high as 87%. Furthermore, the formed composites demonstrated strong surface enhanced Raman scattering (SERS) properties with an unprecedented intrinsic maximal enhancement factor of more than 30-fold. These nanostructures also retain more than 90% of their original SERS intensities after a week of storage, displaying superb stability under ambient conditions. These results highlight the remarkable potential of this composite as a unique Raman-based PDT dosimetric agent.

Effect of central metals and peripheral substituents on the third-order nonlinear optical properties of tetra-benzimidazole and benzothiazole substituted phthalocyanines

A series of tetra substituted phthalocyanines bearing 2-mercaptobenzimidazole or 2-mercaptobenzothiazole units with different central metals (Zn and In) were synthesized. Their photophysical properties including the parameters of fluorescence and triplet state were studied. The benzothiazole substituted phthalocyanines exhibit stronger third-order nonlinear optical properties (NLO) absorption effects, higher triplet quantum yield (ΦT) and faster intersystem crossing rate compared to the benzimidazole substituted phthalocyanines. With the same central metals, benzothiazole substituted phthalocyanine complexes have the lower fluorescence quantum yield (ΦF), the higher triplet quantum yield (ΦT) and the larger third-order NLO properties as a result of larger conjugated structure and spin-orbit coupling induced by sulphur atoms. The influences of the central metal atoms and the peripheral substituents on the third-order NLO properties are discussed using photophysical parameters.

Insights into the Complex Photophysics and Photochemistry of the Simplest Nitroaromatic Compound: A CASPT2//CASSCF Study on Nitrobenzene

Nitrobenzene is the simplest nitroaromatic compound and yet is characterized by a challenging and rich photophysics and photochemistry. In the present contribution, the main decay paths undertaken by the system after UV absorption from both the brightest 1(Laππ*) and the lowest 1(nAπ*) singlet excited states have been characterized by means of CASPT2//CASSCF computations. The obtained results match with the main photophysical properties experimentally reported: the lack of fluorescence and phosphorescence emission is justified by the presence of accessible conical intersections and intersystem crossing regions between, respectively, the 1(nAπ*) and 3(nAπ*) states and the ground state, while the high triplet quantum yield is attributable to the strong coupling between the 1(nAπ*) and 3(πOπ*) states along the main decay path of the former. Two not previously reported singlet-triplet crossing regions, termed (T1/S0)stc-NO and (T1/S0)stc-ep, have been here documented, from which the ground state can decay toward NO and phenoxy radical production and toward the formation of an epoxide ring structure, respectively. A possible mechanism leading to the photoisomerization of the nitro into the nitrite group, believed to be a key step in the photodegradation of nitrobenzene, has been proposed, based on the geometrical deformation recorded along the decay path leading from the 1(nAπ*) state back to the original ground state through a conical intersection characterized by a significant shortening of the carbon-nitrogen bond.

Enhanced intersystem crossing in carbonylpyrenes.

Ultrafast intersystem crossing of carbonylpyrenes in chloroform was investigated by femtosecond pump-probe spectroscopy. When compared to the dominant fluorescence decay pathway in pyrene, carbonyl functionalized pyrenes display near-unity triplet formation upon photoexcitation. The excited singlet state (Sp) undergoes rapid intersystem crossing (kISC) concomitantly with internal conversion (kIC) to lower excited singlet states (Sn) within a timescale of 5-11 ps (1/τ2 = kIC + kISC). Furthermore, intersystem crossing from lower excited singlet states (Sn) proceeds through coupling with receiver triplet states, eventually leading to high triplet quantum yields (ΦT = 97%; tetraacetylpyrene). Followed by internal conversion in the triplet manifolds, phosphorescence decay on a microsecond timescale is observed from the emitter triplet state.

Studying the intersystem crossing rate and triplet quantum yield of meso-substituted porphyrins by means of pulse train fluorescence technique

Excited state dynamics, particularly intersystem crossing, of a set of meso-substituted porphyrins bearing different electron–donor and acceptor groups was studied by pulse train fluorescence technique. Triplet quantum yield was found to be critically dependent on the nature of meso-substituents in the porphyrin system. Porphyrins with meso methoxyphenyl groups were found to show high triplet quantum yields ([Formula: see text] between 0.70 and 0.81). Moreover, the quantity of methoxyphenyl groups and the substitution pattern directly influence [Formula: see text]. Alternatively, porphyrins attached to nitrophenyl group possess low triplet quantum yield values (~0.3). The observed structure-properties relationships suggest new ways for tuning the optical properties of porphyrins via chemical modification.

Internal heavy atom effects in phenothiazinium dyes: enhancement of intersystem crossing via vibronic spin-orbit coupling.

The effect of substituting the intra-cyclic sulphur of thionine by oxygen (oxonine) and selenium (selenine) on the intersystem crossing (ISC) efficiency has been studied using high level quantum mechanical methods. The ISC rate constants are considerably increased when going from O towards Se while the fluorescence rate constants remain unchanged. For the three dyes, all accessible ISC channels are driven by vibronic spin-orbit coupling (SOC) between ππ* states. The interplay between the ground and low-lying excited states has been investigated in order to determine the dominant relaxation pathways. In oxonine the relaxation to the ground state after photoexcitation in water proceeds essentially via fluorescence from the S1(πHπL*) bright state (kF = 2.10 × 10(8) s(-1)), in agreement with the high experimental fluorescence quantum yield. In aqueous solution of thionine, the ISC rate constant (kISC ∼ 1 × 10(9) s(-1)) is one order of magnitude higher than fluorescence (kF = 1.66 × 10(8) s(-1)) which is consistent with its high triplet quantum yield observed in water (ϕT = 0.53). Due to a stronger vibronic SOC in selenine, the ISC rate is very high (kISC ∼ 10(10) s(-1)) and much faster than fluorescence (kF = 1.59 × 10(8) s(-1)). This suggests selenine-based dyes as very efficient triplet photosensitizers.

Nonlinear absorption and scattering of C_60 colloids in water with Triton X-100 surfactant

New synthesis techniques for highly concentrated colloidal C60 suspensions were developed. The nonlinear absorption and nonlinear scattering behavior of colloidal C60 suspensions and benchmark materials (carbon black suspension and C60 solution) were studied with an apparatus that simultaneously measured the total scattered and transmitted energy, inferring absorbance. These experimental results were compared to simple thermodynamic and reverse saturable absorption models, as well as a hybridized model proposed for the nonlinear optical behavior of C60 colloids. All samples followed an attenuation pattern in the nonlinear scattering regime that was fit by a single extinction coefficient, indicating that the energy in excess of that required to reach the sublimation threshold does not significantly affect the size of the induced scattering centers. C60 colloids evidenced strong quenching of the first excited singlet band, leading to weak intersystem-crossing to the triplet manifold. The degree of quenching was morphology dependent. Tighter crystalline packing led to stronger quenching. Samples with higher triplet quantum yield evidenced less efficient heating of the particles. Consequently, for otherwise similar C60 colloids, stronger nonlinear absorption response was found to diminish the nonlinear scattering response. Large, crystalline C60 colloids had a stronger nonlinear optical response than benchmarks.

Spectroscopic and Theoretical Study on Electronically Modified Chromophores in LOV Domains: 8‐Bromo‐ and 8‐Trifluoromethyl‐Substituted Flavins

Two chemically synthesized flavin derivatives, 8-trifluoromethyl- and 8-bromoriboflavin (8-CF(3)RF and 8-BrRF), were photochemically characterized in H(2)O and studied spectroscopically after incorporation into the LOV domain of the blue light photoreceptor YtvA from Bacillus subtilis. The spectroscopic studies were paralleled by high-level quantum chemical calculations. In solution, 8-BrRF showed a remarkably high triplet quantum yield (0.97, parent compound riboflavin, RF: 0.6) and a small fluorescence quantum yield (0.07, RF: 0.27). For 8-CF(3)RF, the triplet yield was 0.12, and the fluorescence quantum yield was 0.7. The high triplet yield of 8-BrRF is due to the bromine heavy atom effect causing a stronger spin-orbit coupling. Theoretical calculations reveal that the decreased triplet yield of 8-CF(3)RF is due to a smaller charge transfer and a less favorable energetic position of T(2), required for intersystem crossing from S(1) to T(1), as an effect of the electron-withdrawing CF(3) group. The reconstitution of the LOV domain with the new flavins resulted in the typical LOV photochemistry, consisting of triplet state formation and covalent binding of the chromophore, followed by a thermal recovery of the parent state, albeit with different kinetics and photophysical properties.

A theoretical study of thionine: spin—orbit coupling and intersystem crossing

A study of the possible intersystem crossing (ISC) mechanisms (S→T) in thionine (3,7-diamino-phenothiazin-5-ium), which is conducive to the efficient population of the triplet manifold, is presented. The radiationless deactivation channels {S(1),S(2)(π → π*) → T(1),T(2)(π → π*)} have been examined. Since the direct ISC does not explain the high triplet quantum yield in this system, attention has been centered on the vibronic spin-orbit coupling between the low-lying singlet and triplet (π → π*) states of interest. An efficient population transfer from the S(1)(π(H) → π(L)*) state to the T(2)(π(H-1) → π(L)*) state via this channel is confirmed. The calculated ISC rate constant for this channel is k(ISC) ≈ 3.35 × 10(8) s(-1), which can compete with the radiative depopulation of the S(1)(π(H) → π(L)*) state via fluorescence (k(F) ≈ 1.66 × 10(8) s(-1)) in a vacuum. The S(1)(π(H) → π(L)*) → T(1)(π(H) → π(L)*) and {S(2)(π(H-1) → π(L)*) → T(1),T(2)(π → π*)} ISC channels have been estimated to be less efficient (k(ISC) ≈ 10(5)-10(6) s(-1)). Based on the computed ISC rate constants and excited-state solvent shifts, it is suggested that the efficient triplet quantum yield of thionine in water is primarily due to the S(1)(π(H) → π(L)*) → T(2)(π(H-1) → π(L)*) channel with a computed rate constant of the order of 10(8)-10(9) s(-1) which is in accord with the experimental finding (k(ISC) = 2.8 × 10(9) s(-1)).

Synthesis and photophysical properties of peripherally and non-peripherally mercaptopyridine substituted metal free, Mg(II) and Al(III) phthalocyanines

The syntheses and photophysical properties of metallophthalocyanines containing Mg2+ and Al3+ as central metal ions and their unmetallated derivative (complexes 3–8) tetra- substituted at the non-peripheral (α) and peripheral (β) positions with 2-mercaptopyridine are reported. The trends in triplet and fluorescence quantum yields are described for these compounds. The complexes exhibited relatively high triplet quantum yields (ΦT) ranging from 0.33 to 0.54 and fluorescence quantum yields (ΦF) ranging from 0.02 to 0.29. Triplet state lifetimes of the complexes were long ranging from 120 to 460μs, indicating a potential use of these complexes as photosensitizers.