Quantum mechanically, the substituent effect on the photophysical properties of conjugated molecules can be investigated using either qualitative molecular orbital (MO) theory or quantitative quantum chemistry calculations. Also noteworthy are several other empirical constants, including analogs of the Hammett parameter 25,26 and quantities defined based on charge population analysis, 27,28 as descriptors exclusively for excited-state properties, while their general applicability and usefulness remain unclear. Despite numerous efforts, 16–24 Hammett constants still lack a rigorous theoretical basis, especially in the context of analyzing substituent effects on the photophysical properties of conjugated molecules. 8–13 Most recently, researchers have observed some correlation between the Hammett constants and photophysical properties of oxyluciferin analogs, 14,15 curcumin derivatives, 5 and other chromophores/fluorophores. Ever since, Hammett constants have also been employed in many other structure–property analyses. 6,7 These constants were developed originally to quantify the electron-donating and electron-withdrawing capabilities of different substituent groups in affecting the dissociation of benzoic acid and other reactions. 6Įmpirically, the substituent effects on conjugated systems have been explained using classical descriptors, especially Hammett constants ( e.g. In synthetic chemistry, it has been common practice over the last several decades to introduce EDGs/EWGs onto conjugated organic molecules to modulate their chemical reactivity. In the design of organic light-emitting/solar-cell materials and fluorescent probe molecules, 1–5 it is routine to attach electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) to dye molecules to tune their frontier orbital energies and absorption/emission wavelengths. 1 Introduction Substituent effects on conjugated molecules have been extensively explored in chemical, biochemical, and materials research. We expect this ALMO-based analysis to bridge the gap between concepts from qualitative orbital interaction analysis and quantitative electronic structure calculations. Specifically, an out-of-phase mixing of high-lying occupied orbitals on the naphthalene core and the dimethylamino group leads to an elevated HOMO, whereas an in-phase combination of LUMOs on the naphthalene core and the propionyl group lowers the LUMO energy of the entire molecule. For the example case of prodan (a typical dye molecule), it is found that inter-fragment orbital mixing plays a key role in narrowing the HOMO–LUMO gap of the naphthalene core. permanent electrostatics/Pauli repulsion, mutual polarization, inter-fragment orbital mixing). This provides a bottom-up avenue towards quantification of effects from distinct physical origins ( e.g. In this work, the absolutely localized molecular orbitals (ALMO) based analysis is extended to analyze the effects of substituent groups on the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of a given system. However, there lacks a generally applicable tool that can unravel the underlying interactions between orbitals from a substrate molecule and those from its substituents in modern electronic structure calculations, despite the long history of qualitative molecular orbital theory. It is common to introduce electron-donating or electron-withdrawing substituent groups into functional conjugated molecules (such as dyes) to tune their electronic structure properties (such as frontier orbital energy levels) and photophysical properties (such as absorption and emission wavelengths).

Homo and lumo organic chemistry