Theoretical Investigation on the Control of Macrocyclic Dihydroazulene/Azobenzene Photoswitches

In this work, we focus on macrocyclic structures comprised of two dihydroazulene (DHA) units and one azobenzene (AZB) unit and the possibility for photoisomerizing one unit selectively by tuning the excitation energies of each individual unit. An unfortunate overlap between the absorption bands of DHA and AZB as well as trans- and cis-AZB prevents us to have a full control on these macrocyclic structures, and their absorption bands need to be separated. By means of time-dependent density-functional theory calculations, we investigate the effects of ortho substitutions of the AZB unit by fluorine and chlorine atoms on the absorption spectra of the DHA/AZB macrocycles. The calculations on the isolated AZB show that substitutions lead to distortion of the planar molecular structure because of the repulsive interactions between halogen atoms and a systematic blue shift of the pi pi* bands between 25 and 50 nm. Moreover, separations between 10 and 48 nm, depending on the substituent, are observed in the n pi* bands. The results from the calculations on the substituted AZB-DHA-DHA macrocycles reveal significant separations of the DHA/trans-AZB and trans-/cis-AZB absorption bands by values of 46-73 and 15-52 nm, respectively, for different substitutions. We realize that ortho substitutions with mixed fluorine-chlorine atoms can provide the best separations in both pi pi* and n pi* bands of AZB-DHA-DHA photoisomers. The results of this work offer a guideline for designing and synthesizing new, efficient, and highly controllable materials applicable in devices for optical data storage and molecular electronics.