Singlet and triplet excited states of emissive, conjugated bis(porphyrin) compounds probed by optical and EPR spectroscopic methods

The nature of the singlet and triplet excited states of a series of meso-to -meso ethyne-linked bis(porphyrin) compounds was probed by electronic absor ption, polarized pump-probe fluorescence, electron paramagnetic resonance ( EPR), electroabsorption (Stark), and transient triplet-triplet absorption s pectroscopic methods. Pump-probe fluorescence anisotropy experiments show t hat the presence of meso-ethynyl substituents drives a reorientation of ort hogonal x- and y-polarized singlet excited states in the macrocycle frame o f reference with respect to that evinced for conventional free-base porphyr in chromophores. Analogous experiments in conjugated bis(porphyrin) species bis[(5,5',-10,20-bis[3 ",5 "-(di-tert-butyl)phenyl]porphinato)zinc(II)]eth yne 5-[10,20-bis[3 ",5 "-(di-tert-butyl)phenyl]porphinato)zinc(II)]-5'-[10' ,20'-bis[3 ",5 "- (di-tert-butyl)phenyl]porphyryl]ethyne, and bis[5,5',-10, 20-bis[3 ",5 "-(di-tert-butyl)phenyl]porphyryl]ethyne demonstrate substanti al energetic splittings of the x- and y-polarized S-1 states. The magnitude of this energetic gap results in the complete suppression of population ex change between excited states having orthogonal polarizations on the time s cale of these measurements, and gives rise to singly degenerate emitting st ates polarized exclusively along the axis defined by the ethyne moiety. Sta rk spectroscopic experiments show that the electronically symmetric meso-to -meso ethyne-bridged bis[(porphinato)zinc(II)] complex exhibits changes in dipole moment with respect to the ground state in its respective x-polarize d S-2 and S-1 states. The EPR spectra of the low-lying photoexcited triplet excited states of these conjugated bis(porphyrin) compounds and their ethy ne-substituted porphyrinic building blocks show an evolution in the \D\ and \E\ ZFS parameters with augmented conjugation consistent with a progressin g oblate-to-prolate spin transition that causes the direction of largest di polar interaction to align along the vector defined by the conjugated ethyn e moiety. Conjugated arrays based on meso-ethyne elaborated porphyrin and ( porphinato)zinc(II) precursors thus constitute an unusual class of oligomer ic porphyrin species in that once a threshold level of conjugation is reach ed, the optical and magnetic principal axis systems become coincident.