Effect of Peripheral Substitution on the Electronic Absorption and Fluorescence Spectra of Metal‐Free and Zinc Phthalocyanines

Abstract

The effect of substituents on the position and intensity of the electronic absorption and fluorescence spectra of phthalocyanines (Pcs) was examined for 35 Pc compounds. When electron-releasing groups are bound to four alpha-benzo positions of the Pc skeleton, the B and Q bands shift to longer wavelength. Relative to this shift, the effect of introducing the same electron-releasing groups at the other four alpha positions amounts to about 1.6-2.0. Although the effect is not always clearly seen, introduction of electron-releasing groups in the beta-benzo positions of the Pc skeleton generally shifts the Q band to shorter wavelength. The effect of electron-withdrawing groups is exactly the opposite with respect to the alpha and beta positions. These effects can be reasonably explained by considering the magnitude of the atomic orbital coefficients of the carbon atoms derived from molecular orbital (MO) calculations. In addition, the following intriguing phenomena were observed in the experiments, although not all were explained theoretically: 1) the splitting of the Q band of metal-free Pcs decreases with increasing wavelength of the Q band, 2) the ring currents of Pcs with Q bands at longer wavelength are generally smaller, and 3) the absorption coefficients of the Q band of Pc compounds with 16-electron-releasing substituents are larger than those of the corresponding tetra- and octasubstituted Pcs by several tens of percent. 4) Our PPP calculations suggested that the absorption coefficient of the Q band of Pcs with more strongly electron releasing substituents is larger. 5) The second HOMO of the Pcs with the Q band at longer wavelength has b(1u) symmetry, as opposed to the a(2u) symmetry of normal Pcs. 6) Pcs showing S1 emission maxima at wavelengths longer than about 740 nm generally have quantum yields of less than 0.1.

Synthesis and Isolation of 2,9,17,23-Tetrakis[(3,5-dimethyl-1H-pyrazol-1-yl)phenoxy]phthalocyanine and Its Magnesium(II) and Zinc(II) Complexes

“…These properties make them important candidates as potential agents for fluorescence imaging and photodynamic therapy of cancer. [12][13][14][15][16] Unsubstituted phthalocyanines are known to have low solubility in most organic solvents that restricts of their application. Introduction of aryl or heteroaryl substituents in phthalocyanine core enhances solubility of macroheterocycles in many organic solvents and gives them new functional properties.…”

Preparation, structure, and electrochemistry of porphyrinato titanium (IV) benzenedithiolates with a trithiole ring, a dithiin ring, and two 2-cyanoethylthio groups

“…Since the benzenedithiolate coordinates to the titanium atom of porphyrin on one side, the molecule has C 2v symmetry; therefore, the broadening of the Soret band would be related to the electronic transition of unsymmetric porphyrin [14]. The Soret band absorption of 4a and 4b is more broadened than that of 4c, suggesting that the aggregation property of 4c is weaker than that of 4a and 4b.…”

Pyridone substituted phthalocyanines: Photophysico-chemical properties and TD-DFT calculations

“…Michl has described how a consideration of the inductive and mesomeric effects of peripheral substituents can be used to rationalize trends observed in the optical spectra of aromatic cyclic polyenes [22], and the application of this approach has been reviewed in the context of porphyrins [23] and phthalocyanines [21]. The relative destabilizations of the pMOs that lie immediately above the 1a 2u MO of 1 are significantly smaller than those that were reported previously for phthalocyanines with electron donating alkoxy and thioalkyl groups [24], which have more pronounced shoulders of absorbance to the red of the B band [25]. It is noteworthy that the introduction of the 4(6methyl 3nitro2oxo1,2dihydropyridin4yloxy) substituents is predicted to have a strongly electron withdrawing inductive effect on the entire stack of pMOs of 1 (Fig.…”