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 absorption, polarized pump−probe fluorescence, electron paramagnetic resonance (EPR), electroabsorption (Stark), and transient triplet−triplet absorption spectroscopic methods. Pump−probe fluorescence anisotropy experiments show that the presence of meso-ethynyl substituents drives a reorientation of orthogonal x- and y-polarized singlet excited states in the macrocycle frame of reference with respect to that evinced for conventional free-base porphyrin chromophores. Analogous experiments in conjugated bis(porphyrin) species bis[(5,5',-10,20-bis[3' ',5' '-(di-tert -butyl)phenyl]porphinato)zinc(II)]ethyne, 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 substantial energetic splittings of the x- and y-polarized S1 states. The magnitude of this energetic gap results in the complete suppression of population exchange between excited states having orthogonal polarizations on the time scale of these measurements, and gives rise to singly degenerate emitting states polarized exclusively along the axis defined by the ethyne moiety. Stark 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-polarized S2 and S1 states. The EPR spectra of the low-lying photoexcited triplet excited states of these conjugated bis(porphyrin) compounds and their ethyne-substituted porphyrinic building blocks show an evolution in the |D| and |E| ZFS parameters with augmented conjugation consistent with a progressing oblate-to-prolate spin transition that causes the direction of largest dipolar interaction to align along the vector defined by the conjugated ethyne moiety. Conjugated arrays based on meso-ethyne elaborated porphyrin and (porphinato)zinc(II) precursors thus constitute an unusual class of oligomeric porphyrin species in that once a threshold level of conjugation is reached, the optical and magnetic principal axis systems become coincident.