A Determination of the Structure of the Intramolecular Charge Transfer State of 4-Dimethylaminobenzonitrile (DMABN) by Time-Resolved Resonance Raman Spectroscopy

Abstract

Picosecond time-resolved resonance Raman spectra of the A (intramolecular charge transfer, ICT) state of DMABN, DMABN-d 6 and DMABN-15 N have been obtained. The isotopic shifts identify the ν s (ph-N) mode as a band at 1281 cm -1 . The ∼96 cm -1 downshift of this mode from its ground state frequency rules out the electronic coupling PICT model and unequivocally supports the electronic decoupling TICT model. However, our results suggest some pyramidal character of the A state amino conformation.

Direct Observation of a Hydrogen‐Bonded Charge‐Transfer State of 4‐Dimethylaminobenzonitrile in Methanol by Time‐Resolved IR Spectroscopy

“…The spectral region between 2065 and 2235 cm À1 (Figure 1) covers the CN bands of the ground, LE, and ICT states. [3,4,6,16] The LE band is observed by Raman but not by TRIR spectroscopy. [6,16] The ICT state CN IR absorption band at 2104 cm À1 in MeCN and the ground state depletion (bleach) at 2213 cm À1 are clearly seen (Figure 1 a).…”

“…In nonpolar solvents, a single fluorescence band is observed from a locally excited (LE) state. In polar solvents, the initially populated LE state reacts further to produce a stable intramolecular charge-transfer (ICT) state, which gives rise to a second fluorescence band that overlaps with, but is abnormally red-shifted from, the LE emission.[1] Results of experiments using aprotic solvents are well described by models in which polarity is the only solvent property that affects the charge transfer reaction activation energy and the relative stabilization of the ICT and LE states.[2] Whilst much work continues to concentrate on determining the structures of the LE and ICT states, [3][4][5][6][7] the precise nature of the difference between the properties of the excited state in protic and aprotic solvents is little understood. For example, the fluorescence quantum yield of DMABN in protic solvents is lower and the fluorescence spectrum is further red-shifted and broadened, relative to measurements in aprotic solvents of the same polarity, [8,9] and the fluorescence decay kinetics are difficult to interpret.…”

“…The charge-separation step is thus seen to be necessary to favor DMABN-MeOH configurations that form bonds having lifetimes of a few picoseconds so that the majority of the DMABN excited state population becomes hydrogenbonded, despite the strong MeOH-MeOH interaction. We consider that the increased negative charge density on the cyano electron-acceptor region of the ICT state of DMABN [3,4] facilitates bonding at this site in the presence of a proton donor, such as the -OH group in alcohol, the electron acceptor group in turn becoming a proton acceptor.…”

Do fluorescence and transient absorption probe the same intramolecular charge transfer state of 4-(dimethylamino)benzonitrile?

“…5 Moreover, picosecond time-resolved resonance Raman spectra of the ICT state, obtained by using probe ͑Raman inducing͒ wavelength of 330 nm, exhibit several characteristic modes that are very similar to the modes observed for the benzonitrile radical anion. 6 An alternative ICT-state structure, known as PICT ͑P for planar͒, 7 is not supported by time-resolved laser spectroscopies ͑vide infra͒ or by a great majority of quantum chemical calculations. 8,9 The second issue, concerning the mechanism of the ICT reaction, has mostly been addressed by investigating the temporal characteristics of the fluorescence from the S 1 ͑ ‫ء‬ ͒, or the so-called locally excited ͑LE͒ state, with those of the longer-wavelength fluorescence from the ICT state.…”

Extreme cross-peak 2D spectroscopy

“…This kind of question lends itself to tools that can probe structure. Time-resolved infrared (16) and Raman (17,18) experiments have therefore provided essential insights.…”