Light-intensity dependence in the photochromism of dibenzo[2.2](4,4′)-azobenzenophane

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Abstract
Reduction of 4′,4″-dinitro-o-terphenyl under conditions of high dilution, produces a mixture of macrocyclic poly(azobenzene) compounds. From the mixture, three kinds of macrocycle, namely, dimer 2{dibenzo[2.2](4,4′)azobenzenophane}, trimer 3 and tetramer 4 have been isolated pure. In toluene, absorption peaks due to the π→π* transition for the macrocycles undergo dramatic hypsochromic shifts as the ring size of the macrocycles decreases from the trimer 3 to the dimer 2; this is an indication that benzene rings at both ends of the dimer 2 are fixed perpendicularly to two azobenzene planes in the macrocycle. The photostationary state of the dimer 2 in toluene at λ= 366 nm contained the EZ isomer which undergoes thermal isomerization to the more stable EE isomer at the first-order rate of 0.768 s–1. The reaction of 2 is much faster than that of the dimer 5, which was described in our last paper. Due to the fast thermal EZ to EE isomerization, the efficiency of the photoisomerization from the EE isomer to the ZZ isomer is light-intensity dependent. This dependence is observed at higher light intensities for the dimer 2 than for the dimer 5.