Effects of organic and aqueous solvents on the electronic absorption and fluorescence of chloroaluminum (III) tetrasulphonated naphthalocyanine

The effects of various solvents on the ground and excited states of chloroa luminum (III) tetrasulphonated naphthalocyanine (AlNcS(4)) were studied. Bo th the absorbance and fluorescence spectra were found to be influenced by t he hydrogen bond donating ability of various solvents. As the hydrogen bond donating ability of the solvent increased, hypsochromic and bathochromic s hifts in the absorbance and fluorescence spectra were observed in protic an d aprotic solvents respectively. Plots of the absorbance and fluorescence m axima versus the E-T(30) solvent parameter showed linear relationships in b inary mixtures of protic-protic (methanol-H2O) and aprotic-protic (DMSO-H2O ) solvents. Aggregation was indicated by a broad band in the ground state a bsorption spectra and a low quantum efficiency 0.04 relative to the efficie ncy observed in organic solvents. A face-to-face conformation of the monome ric subunits of the dimer is suggested due to the red-shifted absorbance ba nd. The acid-base properties of the dye were studied and were indicative of a multi-step process. In acidic conditions (pH 1), protonation of the brid ging nitrogen atoms was identified by a broad band appearing red-shifted to those obtained at higher pH values. Under slightly acidic conditions a pKa value of 6.7 was determined for one of the meso-nitrogen. In alkaline cond itions a pKa of 11.5 was determined for another meso-nitrogen and a second fluorescence band emerged at 804 nm, red-shifted to the emission maxima. (C ) 1999 Elsevier Science B.V. All rights reserved.