Substitution can be used to efficiently tune the photophysical properties of chromophores. In this study, we examine the effect of substituents on the absorption and fluorescence properties of anthracene. The effects of mono-, di-, and tetrasubstitution of electron-donating and -withdrawing functional groups were explored. In addition, the influence of a donor-acceptor substituent pair and the position of substitution were investigated. Eleven functional groups were varied on positions 1, 2, and 9 of anthracene, and on position 6 of 2-methoxyanthracene and 2-carboxyanthracene. Moreover, the donor-acceptor pair NH/COH was added on different positions of anthracene for additional studies of doubly substituted anthracenes. Finally, we looked into quadruple substitutions on positions 1,4,5,8 and 2,3,6,7. Vertical excitation energies and oscillator strengths were computed using density functional theory with the hybrid CAM-B3LYP functional and 6-311G(d) basis set. Correlations between the excitation energies or oscillator strengths of the low-lying bright L state and the Hammett sigma parameter, σ, of the substituents were examined. The energy is red-shifted for all cases of substitution. Oscillator strengths increase when substituents are placed along the direction of the transition dipole moment of the bright L excited state. Substitution of long chain conjugated groups significantly increases the oscillator strength in comparison to the cases for other substituents. In addition, the results of quadruply substituted geometries reveal symmetric substitution at the 1,4,5,8 positions significantly increases the oscillator strength and can lower the band gap compared to that of the unsubstituted anthracene molecule by up to 0.5 eV.

Modeling the optical spectra of molecules in solution presents a challenge, so it is important to understand which of the solvation effects (i.e., electrostatics, mutual polarization, and hydrogen bonding interactions between solute and solvent molecules) are crucial in reproducing the various features of the absorption and fluorescence spectra and to identify a sufficient theoretical model that accurately captures these effects with minimal computational cost. In this study, we use various implicit and explicit solvation models, such as molecular dynamics coupled with non-polarizable and polarizable force fields, as well as Car–Parrinello molecular dynamics, to model the absorption and fluorescence spectra of indole in aqueous solution. The excited states are computed using the equation of motion coupled cluster with single and double excitations combined with the effective fragment potential to represent water molecules, which we found to be a computationally efficient approach for modeling large solute–solvent clusters at a high level of quantum theory. We find that modeling mutual polarization, compared to other solvation effects, is a dominating factor for accurately reproducing the position of the peaks and spectral line shape of the absorption spectrum of indole in solution. We present an in-depth analysis of the influence that different solvation models have on the electronic excited states responsible for the features of the absorption spectra. Modeling fluorescence is more challenging since it is hard to reproduce even the correct emitting state, and force field parameters need to be re-evaluated.

The absorption and fluorescence of indole and ncyanoindole derivatives are modeled in the gas phase and aqueous solution using high-level quantum mechanical methods and implicit solvation. These molecules have been experimentally examined as fluorescent probes for studying the structure, function, and hydration status of proteins and it is found that substitution of the cyano group on different positions of indole has diverse effects on the absorption and fluorescence spectra in water solvent. Our calculations predict that in absorption the L b excited state is lower in energy than the L a state for all positional isomers in the gas phase and in solution. In fluorescence, however, water solvent causes level inversion leading to emission from the L a excited state for indole and n-cyanoindole derivatives with the cyano on the six-membered ring. However, when cyano substitution is on the five-membered ring, L a is not stabilized enough and emission occurs from the L b excited state. In addition, we predict that the relatively high fluorescence intensity of 4cyanoindole in aqueous solution results from minimization of radiationless decay pathways since both absorption and fluorescence occur from the lowest excited state (unlike the other derivatives).

The effective fragment potential (EFP) approach is a sophisticated hybrid approach that allows the inclusion of solvation effects when describing properties and reactivity in the condensed phase, without using empirical parameters. This work examines the performance of the EFP method when describing microsolvation in electronically excited states of neutrals and anions. The examples selected include both localized valence states, as well as diffuse nonvalence states, which represent greater challenges to conventional electronic structure methods. The equation-of-motion coupled cluster with singles and doubles (EOM-XX-CCSD) methodology has been used to provide the quantum chemical description of both the full microsolvated clusters, and the chromophoric moiety in mixed quantum/EFP calculations. We find that, when averaging over multiple configurations of microsolvated clusters, the differences between QM/EFP and full quantum results are minimal, although individual configurations often have larger errors. As expected, diffuse states have somewhat larger errors, although not significantly so. The close proximity of states leading to mixing can make QM/EFP less accurate because a change of ordering of states can occur. Other properties, such as photoelectron images and lifetimes of metastable states, are very well described for the monohydrated clusters investigated.

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