Photophysical properties of phenylethyne-linked porphyrin and oxochlorin dyads

A set of porphyrin-porphyrin and oxochlorin-oxochlorin dyads has been prepared in which the constituent pigments are joined at the meso-positions by a phenylethyne linker. Attachment of an ethynyl substituent to the meso-position of a tetrapyrrolic macrocycle strongly perturbs the electronic properties of the ring. The inherent asymmetry of the phenylethyne linker affords the possibility of perturbing either end of the dyad. The porphyrin dyads include bis-Zn, Zn-free base (Fb), and bis-Zn species wherein 0, 1, or 2 of the three nonlinking aryl rings of one of the Zn porphyrins are perfluorinated. The two oxochlorin dyads are both ZnFb species in which the meso-ethyne substituent is located on either the Zn complex or the Fb. The dyads have been studied using static and time-resolved absorption and emission spectroscopy and electrochemical techniques. The optical and electrochemical properties of a series of monomeric reference compounds were also examined. The time-resolved optical studies reveal that energy transfer in the phenylethyne-linked dyads is faster by ∼ 10-fold or more than in analogous dyads joined by a diphenylethyne linker. In particular, energy transfer occurs with a rate constant of > (20 ps) ^-1 between the phenylethyne-linked oxochlorins and an efficiency of >98%, to be compared with a rate of (140 ps) ^-1 and an efficiency of 83% found previously for diphenylethyne-linked oxochlorins. Taken together, these results should be useful in the design of multipigment architectures that absorb in the red and undergo fast and efficient energy transfer, as required for light-harvesting applications.