A spectroscopic and computational study of the singlet and triplet excited states of synthetic β-functionalized chlorins

This paper presents a comparative investigation of the absorption, fluorescence, electron paramagnetic resonance (EPR), and transient triplet-triplet absorption spectroscopic properties and triplet state dynamics of two functionalized, synthetic, meso-phenylchlorins. The chromophores investigated are the novel 2-hydroxy-3-oxa-5,10,15,20-tetrakisphenylchlorin (3) and the known 2,3-dioxo-5,10,15,20-tetrakisphenylchlorin (4). In these chromophores, one peripheral -CH=CH- bond of the parent porphyrin meso-tetrakisphenylporphyrin (TPP, 1) was formally replaced by a -CH(OH)O- (lactol) or a beta-diketone moiety. The spectroscopic data are compared with results from investigations on the parent porphyrin TPP studied here and the parent chlorin 5,10,15,20-tetrakisphenylchlorin (TPC, 2) from the literature. The spectroscopic observables are examined both qualitatively within the framework of the four orbital model and quantitatively using MNDO-PSDCI methods. The results delineate the role of beta-lactol and beta-dicarbonyl moieties in controlling the electronic and spectroscopic properties of these chromophores. This investigation serves as the foundation from which to derive a general understanding of the effects of beta-functionalization on the electronic properties of chlorin-type chromophores. This knowledge is required for the design and understanding of long-wavelength absorbing and fluorescing chromophores to be used in light harvesting systems and photomedicine. (C) 2003 Elsevier B.V. All rights reserved.