Excited State Dynamics of Zinc–Phthalocyanine Nanoaggregates in Strong Hydrogen Bonding Solvents

Abstract

2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine} (ZnPC) exists as monomeric species in DMSO and is reasonably strong fluorescent. But ZnPC forms H-aggregates in water and hexafluoroisopropanol, which are strong hydrogen bonding solvents. Nanoaggregates are nearly nonemissive. Transient absorption spectroscopic technique has been used to investigate the excited state relaxation processes in both monomeric and aggregated forms of ZnPC. The lifetime of the S 1 state of the monomoric form in DMSO is long (τ = 3.4 ns) but the excited states of ZnPC nanoaggregates show much faster ground state recovery (within 100 ps). The longest lifetime component, τ 3, which is independent of excitation density, has been assigned to the unimolecular decay of the S 1 -exciton in the absence of annihilation reaction, while τ 1 and τ 2 are the lifetimes obtained by the two-component fit of the nonexponential decay arising due to the time-dependent decay rates of the S 1 -excitons because of diffusive migration controlled exciton -exciton annihilation reaction. Rates of the annihilation reaction (2.0 × 10 −6 cm 3 s −1 ) and exciton migration (4.3 × 10 −5 m 2 /s) as well as diffusion length (about 85 nm) of the S 1 -exciton created in the ZnPC nanoaggregates in HFIP have been determined.

Exciton Dynamics in Phthalocyanine Molecular Crystals

“…However, it is preferable over surface hopping methods 63 when a quasi continuum of electronic states is present, as in this case. 64,65 The initial nuclear positions, () (0) derived from exciton-exciton annihilation measurements 70 although larger than other 26 exciton diffusion coefficients of Pc-based amorphous materials. 71 However, the diffusion coefficient of the ZnPc aggregate was estimated by assuming an incoherent regime with an exciton transfer rate of ~1.0 × 10 13 s −1 , which is close to the limit of validity of a first order rate process in a molecular system.…”

Kinetic model for the molecular system of Zinc(II)-2,9,16,23-phthalocyanine tetracarboxylate

“…It has the general spectra feature as those of other MPcs. [1][2][3][4][5][6][7] Compared to metal-free phthalocyanines, absorption spectrum of ZnPc(COONa) 4 consists of Soret band located in the blue region (positioned at B 1 = 341 nm and B 2 = 279 nm) and pronounced splitting of the Q bands (positioned at Q 1 = 691 nm and Q 2 = 641 nm). For unsubstituted ZnPc molecule in DMSO, Q maximum of the band is located at 673 nm.…”

“…Steady-state fluorescence of ZnPc(COONa) 4 was determined with JASCO-FP 6500 spectroscopy using fluorescence quartz cell (1 cm × 1 cm optical path length). Radiative (fluorescence) quantum yield of ZnPc(COONa) 4 (Φ ZnPc(COONa)4 r ) was determined with respect to that of PPME (Φ PPME r = 0.21) as a reference.…”

“…21 The reason for this behavior is attributed to the fact that higher order electronic state with n ≥ 2 is subject to very fast internal conversion from |T n to |T 1 state and from |S n to |S 1 state. 21,23,24 Sodium zinc(II)-2,9,16,23-phthalocyanine tetracarboxylate (ZnPc(COONa) 4 ) is a promising second-generation molecule has absorption band extended to about 800 nm. 25 Its far-red absorption and the convenient singlet oxygen quantum yield 25 nominate it to be an excellent anticancer drug from the point of view of photodynamic therapy.…”

“…25 Its far-red absorption and the convenient singlet oxygen quantum yield 25 nominate it to be an excellent anticancer drug from the point of view of photodynamic therapy. 26 The fusion of Na on the macrocycle of the periphery of the molecule makes ZnPc(COONa) 4 well soluble in water:NaOH solution. 25 Also, it will be probably soluble when injected in the blood during photodynamic treatment due to the existence of water and ions in the blood.…”