On the photochemical properties of the organometallic trans cyrhetrenyl chalcone (E). Deceleration of the photoisomerization and quenching of the luminescence effected by the photogenerated cis isomer (Z)

Abstract

The photoprocesses of two organometallic chalcone complexes: one trans (E) [(η5-C5H4C(O)CH=CH-Ph)Re(CO)3], (1a), and the other functionalized with a macrocyclic trans (E) [(η5-C5H4C(O)CH=CH-4-benzo-15-crown-5)Re(CO)3], (1b), were investigated in fluid solution. Isomerization from the trans (E isomer) to the cis (Z isomer) and a weak luminescence from (1b) were observed when deaerated solutions of (1a) and (1b) were respectively irradiated at 300nm and 350nm. A marked non-linear dependence of the product Z concentration on the irradiation time was observed. Similarly the intensity of the (1b) luminescence decreased non-linearly with the irradiation time. These departures form linearity have been rationalized on the basis of a mechanism incorporating a weak inner filter effect and an energy transfer reaction from an excited state of E to Z. Other effects on the photolysis regime were of a structural origin. The smaller isomerization quantum yield of (1b) relative to (1a) (ϕisom=0.59 for (1b) vs. ϕisom=0.69 for (1a)) was attributed, to a large extent, to steric hindrance resulting from the appended macrocycle. Complexation of Ca2+ by the macrocycle in (1b) rises the quantum yield of isomerization to ϕisom=0.62, an effect associated with an inductive displacement of π electronic density that weakens the chalcone’s double bond and defeats the macrocycle’s steric hindrance.