Fluorescence from metalloporphyrins

Spectral Characteristics of the Fluorescence from Metalloporphyrin 2 States at Direct So > S2 Excitation (PF) and Cooperative Triplet Excitation (ADF)... 

The S2 state fluorescence of metalloporphyrins was first noticed by Bajema et al. ( ) and later the photophysical parameters concerned with the S2 state were determined on several metalloporphyrins (9,10). S2 - Sq emission from large molecules in condensed phases has also been recognized in many other organic compounds, e.g., azulene (11), thiocarbonyl compounds (12), and several polyenes (13). However, in some metalloporphyrins one can observe the S2 state fluorescence even after the excitation to the state (14-17), that is, the blue... 

The existence of this TTA bimolecular process makes it possible to carry out cooperative excitation of energy rich molecular states by low energy quanta. In particular, formation of upper excited singlet states as a result of TTA has been observed for several aromatic hydrocarbons (1) and metalloporphyrins ( ) by the demonstration of annihiTation-induced delayed fluorescence (AOF) from upper singlet states. 

We have carried out a comparative study of state fluorescence emitted from several metalloporphyrins Zn -tetraphenylporphin (Zn TPP), Zn - tetrabenzporphin (Zn T6P) and Cd TBP for direct one-quantum excitation and for annihilating two-quantum excitation. Low concentration (C = 1 - 2 X lO- M)... 

In principle, fluorescence spectra from shortlived upper metalloporphyrin states in solution can be shifted in the longwave direction with vibrational energy relaxation at the cost of short-wave excitation (7). However, in our case the energy difference of annihilatTng and direct excitation was slight ( <... 

Photodyncimics of metalloporphyrins have been extensively investigated on account of its importance in the understanding of photosynthesis and other processes of biological importance ( ). Particular atten-sion has been paid to the reason why the excited metalloporphyrins possess unique characteristics from the viewpoint of redox (2-4), energy transfer ( ), and other photodynamical processes (6,7). In comparison with the considerable knowledge accumulated on the photochemical properties of the lowest excited states, little has been known on the S2 - Sq fluorescence and Si Sq internal conversion processes which can also be regarded as unusual characters of metalloporphyrins. 

Zinc-5,10,15,20-tetraphenylporphyrin (ZnTPP) has been used as a coating material in ammonia sensors by immobilizing it on the surface of silicone rubber. Absorbance and fluorescence emission were the modes of detection. A spectral change is caused by the coordination of NH3 molecules to the Zn11 ion in the immobilized metalloporphyrins. Sensing films made from the ZnTPP immobilized in silicone rubber were found to be the most sensitive for NH3 sensing (20). 

Emission. The fluorescence properties of metalloporphyrin and porphyrin-like compounds depend on the character of central metal ion. Fluorescence of porphyrins is generally observed from the free-base or closed-shell metal complexes. The... 

In the LC box, metalloporphyrins can coordinate with 5CB [10,11]. Tetraphenylporphyrinatozinc have special property that the coordination interaction between ZnTPP and 5CB change, in other words, porphyrinatozinc changes from 5-coordinate (or 6-coordinate) to 4-coordinate because the transitions in the fluorescence spectra exhibit the corresponding absorption peaks [9,12]. We attribute this to that the excitation electron transfers from ZnTPP to 5CB and 5CB occurs optical-induced Fr edericksz transition so as to uncoordinate with ZnTPP (as shown in Figure 6). 

Considerable interest continues in the photochemistry and photophysics of metalloporphyrins and related species containing zinc and other metals. Details will be found in Part II. The biosynthesis of haem can be partially diverted to produce a zinc protoporphyrin in iron-deficiency anaemia and lead poisoning measurement of fluorescence emission from this species has been recommended... 

Metalloporphyrins constitute an important family of complexes which frequently emit from porphyrin IL states [13, 126, 184]. The characterization of emitting IL states outlined above also applies to metalloporphyrins and related complexes. For example, since zinc exerts only a moderate heavy atom effect, zinc porphyrins show an IL fluorescence at r. t. and a relatively long-lived phosphorescence at 77 K [185,186],e.g.,... 

In contrast, the luminescence of metalloporphyrin dyes originates from the tetrapyrrolic macrocycle, but it is greatly affected by the nature of the central metal ion coordinated by the porphyrin ring and by axial ligands. Pt " - and Pd -porphyrins are particularly interesting because of their bright short-decay phosphorescence at room temperatures, and practically no fluorescence. They... 

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