PH sensitive photophysical and photochemical properties of a pentaazadentate porphyrin-like gadolinium (III) complex

Abstract

The electronic absorption spectra, photobleaching property, emission, and triplet excited state characteristics of a pentaazadentate porphyrin-like gadolinium(III) complex have been investigated at acidic and basic conditions. The electronic absorption spectrum of this complex exhibits a Soret band at ca. 449 nm and a Q-like band at ca. 880 nm when the pH value of the solution is lower than 6.6. At basic conditions, the Q-like band blue shifts to ca. 624 nm, while the Soret band only shows an approximate 20 nm hyposochromic shift. The acidic solution is relatively stable upon exposure to ambient light, but the basic solution photobleached to colorless in approximately 3 hours. Irradiation of the Soret band of basic solutions at 420 nm causes faster photobleaching than irradiation of the Q-like band (624 nm). The emission of this complex at pH = 6.5 appears at ca. 915 nm (max.) and 1016 nm, which is a mirror image of the Q-like band, indicating the nature of the emitting state being the lowest singlet excited state. At pH = 9.0, the emission band shifts to ca. 585 nm (max.) and 630 nm (shoulder). The triplet transient difference absorption spectrum of the solution at pH = 6.5 exhibits a bleaching band at ca. 460 nm, a narrow positive band at ca. 450 nm, and a broad, moderately intense absorption band from 480 nm extending to the near-IR region (700 nm). The triplet excited state lifetime deduced from the decay of the transient absorption is approximately 81 ns.