Directly Connected AzaBODIPY–BODIPY Dyad: Synthesis, Crystal Structure, and Ground- and Excited-State Interactions

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.