Directly connected azaBODIPY–BODIPY Dyad: synthesis, crystal structure and ground- and excited-state interactions

Abstract

Directly connected, strongly interacting sensitizer donor–acceptor dyads mimic light-induced photochemical events of photosynthesis. Here, we devised a dyad composed of BF₂-chelated dipyrromethene (BODIPY) directly linked to BF₂-chelated tetraarylazadipyrromethene (azaBODIPY) through the β-pyrrole position of azaBODIPY. Structural integrity of the dyad was arrived from two-dimensional NMR spectral studies, while single-crystal X-ray structure of the dyad provided the relative orientation of the two macrocycles to be ∼62°. Because of direct linking of the two entities, ultrafast energy transfer from the 1BODIPY* to azaBODIPY was witnessed. A good agreement between the theoretically estimated Förster energy transfer rate and experimentally determined rate was observed and this rate was found to be higher than that reported for BODIPY–azaBODIPY analogues connected with spacer units. In agreement with the free-energy calculations, the product of energy transfer, ¹azaBODIPY* revealed additional photochemical events such as electron transfer leading to the creation of BODIPY^•+–azaBODIPY^•– radical ion pair, more so in polar benzonitrile than in nonpolar toluene, as evidenced by femtosecond transient spectroscopic studies. Additionally, the spectral, electrochemical, and photochemical studies of the precursor compound azaBODIPY–dipyrromethane also revealed occurrence of excited-state events. In this case, electron transfer from the ¹azaBODIPY* to dipyrromethane (DPM) yielded DPM^•+–azaBODIPY^•– charge-separated state. The study described here stresses the role of close association of the donor and acceptor entities to promote ultrafast photochemical events, applicable of building fast-response optoelectronic and energy-harvesting devices.