meso-t-Bu-BODIPY produces unusual spectroscopic and photophysical characteristics in comparison to those of related alkylated BODIPY dyes.

Absorption and emission spectra arising from the lowest energy transition in BODIPY have been simulated in the gas phase and water using a quantum mechanics/molecular mechanics (QM/MM) approach. Kohn-Sham density functional theory (DFT) is used to calculate both ground (So) and first excited (S1) states using the maximum overlap method to obtain the S1 state. This approach gives ground and excited state structures in good agreement with structures found using multiconfigurational perturbation theory (CASPT2). Application of a post-self-consistent field spin-purification relationship also yields transition energies in agreement with CASPT2 and available experimental data. Spectral bands were simulated using many structures taken from ab initio molecular dynamics simulations of the ground and first excited states. In these simulations, DFT is used for BODIPY, and in the condensed phase simulations the water molecules are treated classically. The resulting spectra show a blue shift of 0.3 eV in both absorption and emission bands in water compared to the gas phase. A Stokes shift of about 0.1 eV is predicted, and the width of the emission band in solution is significantly broader than the absorption band. These results are consistent with experimental data for BODIPY and closely related dyes, and demonstrate how both absorption and emission spectra in solution can be simulated using a quantum mechanical treatment of the electronic structure of the solute.

A note on versions:The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription. The electronic structure and photoinduced electron transfer processes in a K + fluorescent sensor that comprises a 4-amino-naphthalimide derived fluorophore with a triazacryptand ligand is investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT) in order to rationalise the function of the sensor. The absorption and emission energies of the intense electronic excitation localised on the fluorophore are accurately described using a ∆SCF Kohn-Sham DFT approach, which gives excitation energies closer to experiment than TDDFT. Analysis of the molecular orbital diagram arising from DFT calculations for the isolated molecule or with implicit solvent cannot account for the function of the sensor and it is necessary to consider the relative energies of the electronic states formed from the local excitation on the fluorophore and the lowest fluorophore→chelator charge transfer state. The inclusion of solvent in these calculations is critical since the strong interaction of the charge transfer state with the solvent lowers it energy below the local fluorophore excited state making a reductive photoinduced electron transfer possible in the absence of K + , while no such process is possible when the sensor is bound to K + . The rate of electron transfer is quantified using Marcus theory, which gives a rate of electron transfer of k ET =5.98 x 10 6 s −1 .

The binding within the ethene-argon and formaldehyde-methane complexes in the ground and electronically excited states is studied with equation of motion coupled cluster theory (EOM-CCSD), second-order Møller-Plesset perturbation theory (