Excited state substituent constants: to Hammett or not?

Abstract

In this study, an answer has been sought to several questions: Can substituents' impact on the electronic properties of molecules in the excited singlet states be summed up by a set of substituent constants? Are the well known, ground state Hammett r constants practical for this purpose? To answer these questions, the potentials and charges on atoms of the functional groups in two classes of compounds, p-substituted benzoic acids and p-substituted nitrosobenzenes, were regressed against r p . It appeared that all correlations found in the ground state of the molecules also hold in the excited state, with lower correlation coefficients in the latter. An attempt has also been made to find a molecular characteristic which could be useful in a comparison of the acidic properties of benzoic acids (in gas phase) in the first excited and ground states.

Activating Intersystem Crossing and Aggregation Coupling by CN-Substitution for Efficient Organic Ultralong Room Temperature Phosphorescence

“…The first strong absorption band is a result of the benzene-dominated π → π* transition, while the second weak band, which is absent in benzene, is due to the CN-activated n → π* transition. The three cyano-substituted compound TCNPh shows that the weakest absorption should be due to its high molecular symmetry. , Therefore, the n → π* transition is significantly enhanced with blue-shifted π → π* absorption after the CN substitution due to the electron-withdrawing effects of CN.…”

“…The three cyano-substituted compound TCNPh shows that the weakest absorption should be due to its high molecular symmetry. 37,38 Therefore, the n → π* transition is significantly enhanced with blue-shifted π → π* absorption after the CN substitution due to the electronwithdrawing effects of CN.…”

Substituent Effect in the First Excited Singlet State of Monosubstituted Benzenes

“…The need to construct substituent effect descriptors of the first excited π singlet state was investigated by Sadlej-Sosnowska and Kijak by adopting Sadlej-Sosnowska’s substituent active region (SAR) approach (see Methods). − They analyzed the computationally derived potentials and charges on atoms of the functional groups in both para-substituted benzoic acids and nitrosobenzenes and C ipso atoms.…”

Unraveling substituent effects on frontier orbitals of conjugated molecules using an absolutely localized molecular orbital based analysis

“…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. Also noteworthy are several other empirical constants, including analogs of the Hammett parameter25,26 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.…”