Substitution effects on the absorption spectra of nitrophenolate isomers.
Charge-transfer excitations highly depend on the electronic coupling between the donor and acceptor groups. Nitrophenolates are simple examples of charge-transfer systems where the degree of coupling differs between ortho, meta and para isomers. Here we report the absorption spectra of the isolated anions in vacuo to avoid the complications of solvent effects. Gas-phase action spectroscopy was done with two different setups, an electrostatic ion storage ring and an accelerator mass spectrometer. The results are interpreted on the basis of CC2 quantum chemical calculations. We identified absorption maxima at 393, 532, and 399 nm for the para, meta, and ortho isomer, respectively, with the charge-transfer transition into the lowest excited singlet state. In the meta isomer, this π-π* transition is strongly redshifted and its oscillator strength reduced, which is related to the pronounced charge-transfer character, as a consequence of the topology of the conjugated π-system. Each isomer's different charge distribution in the ground state leads to a very different solvent shift, which in acetonitrile is bathochromic for the para and ortho, but hypsochromic for the meta isomer.
Nano-Bio Spectroscopy Group and ETSF Scientific Development Centre, Departamento de Física de Materiales, UPV/EHU, Centro de Física de Materiales CSIC-UPV/EHU-MPC and DIPC, Av. Tolosa 72, E-20018 San Sebastián, Spain.
Marius
M
Wanko
Jørgen
J
Houmøller
Kristian
K
Støchkel
Maj-Britt
MB
Suhr Kirketerp
Michael Åxman
MÅ
Petersen
Mogens Brøndsted
MB
Nielsen
Steen Brøndsted
SB
Nielsen
Angel
A
Rubio
D005740:Gases / Q000737:chemistry; D062385:Hydroxybenzoates / Q000737:chemistry*; D007536:Isomerism; D008956:Models, Chemical; D011789:Quantum Theory; D012997:Solvents / Q000737:chemistry; D013053:Spectrophotometry
D005740:Gases; D062385:Hydroxybenzoates; D012997:Solvents; C017616:phenolic acid