Abstract
The use of fluorophores for specific applications is determined to a large extent by their optical and electronic properties. These properties, however, are controlled by their sensitivity to microenvironment as well as their intra- and intermolecular interactions. This research effort explores how molecular interactions affect the optoelectronic properties of selected fluorophores and, thus, guides their use as fluorescent probes or photoactive materials in organic electronic applications. A combined spectroscopic and computational approaches was utilized in the first part to examine the photophysical properties of two different classes of probes: the highly responsive multisensing 4,6-diphenylpyrimidones and 4-[2-(6-hydroxy-2-naphthalenyl)-ethenyl]-1-methyl-pyridinium (HNEP+) and its deprotonated form (NEP), a benzo-fused derivative of Brooker’s merocyanine (BM). Our data indicate that the emission of the diphenylpyrimidones can be controlled by the identity of the electron-donating auxochrome, protonation state, solvent viscosity and polarity. Our investigation of HNEP+/NEP establishes that HNEP+/NEP and BMH+/BM differ in the extent of charge delocalization in the ground and the excited states inspite of their similar pKa values and structural homology. HNEP+ shows larger Stokes shifts compare to BMH+ and NEP exhibits relatively enhanced solvatochromism to BM. In the second part, we assess the relative contributions of geometric size, energy gap and frontier molecular orbital (FMO) features to pi-stacking interactions in a series of donor-acceptor molecular constructs. These factors were correlated with the magnitude of charge-transfer association constants determined through spectroscopic (1H NMR, UV-visible and steady state fluorescence) techniques. The results demonstrate that high molar absorptivity enhanced high charge transfer (CT) formation, while the energy difference between the HOMO-LUMO, pi-surface area, symmetry, shape and size of FMO dictate the molecular orientations or ordering and hence the strength of CT complex formation. These studies provide insight into photophysical behavior of these fluorophores and how they can be used as probes or sensors or photoactive materials.