Strategical Designing of Donor-Acceptor-Donor Based Organic Molecules for Tuning Their Linear Optical Properties

Low-energy linear absorption spectrum of a series of 48 donor acceptor donor (D-A-D) scheme based thiophone benzo(bis-)X-diazole-molecules with X = 0, S, Se, or Te are calculated using time dependent density functional theory in order to propose strategical design of molecules that can efficiently absorb light in the infrared and visible region of the solar spectrum. Our study establishes that optical properties of the D-A-D based organic molecules significantly depend on the donor-to-acceptor (D/A) ratio and the strength of the acceptor moiety. Thus, by choice of a suitable D/A ratio and type of the acceptor moiety, the linear absorption spectrum can be largely shifted, in general, while the optical gap can be engineered over a wide energy range of similar to 0.2-2.3 eV, in particular. It is also noticed that the increase in acceptor units (i.e., when D/A <= 1) leads to increase in steric hindrance in between them. This, in turn, disrupts the effective conjugation length and increases the optical gap. However, this effect is found to dominate strongly in the bis-configurations of the molecules as compared to the nonbis compositions. In order to reduce this effect for rational designing of effective D-A-D type chromophores with less steric hindrance, the role of pi-conjugated ethylene (-CH=CH-) linkage/spacer between the A-A units is explored further. Here, it is found that introduction of such linkage substantially decreases the steric hindrance and, thereby, the optical gap as well. Besides this, our study also highlights and explains the impact of the acceptor moiety in improving the absorption capabilities of these molecules in the low-energy region.