Intramolecular charge transfer of 4-(dimethylamino)benzonitrile probed by time-resolved fluorescence and transient absorption: No evidence for two ICT states and a πσ∗ reaction intermediate

Abstract

For the double exponential fluorescence decays of the locally excited (LE) and intramolecular charge transfer (ICT) states of 4-(dimethylamino)benzonitrile (DMABN) in acetonitrile (MeCN) the same times tau(1) and tau(2) are observed. This means that the reversible LE<==>ICT reaction, starting from the initially excited LE state, can be adequately described by a two state mechanism. The most important factor responsible for the sometimes experimentally observed differences in the nanosecond decay time, with tau(1)(LE)<tau(1)(ICT), is photoproduct formation. By employing a global analysis of the LE and ICT fluorescence response functions with a time resolution of 0.5 ps/channel in 1200 channels reliable kinetic and thermodynamic data can be obtained. The arguments presented in the literature in favor of a pisigma( *) state with a bent CN group as an intermediate in the ICT reaction of DMABN are discussed. From the appearance of an excited state absorption (ESA) band in the spectral region between 700 and 800 nm in MeCN for N,N-dimethylanilines with CN, Br, F, CF(3), and C(=O)OC(2)H(2) p-substituents, it is concluded that this ESA band cannot be attributed to a pisigma( *) state, as only the C-C[Triple Bond]N group can undergo the required 120 degrees bending.

Intramolecular charge transfer in aminobenzonitriles and tetrafluoro counterparts: fluorescence explained by competition between low-lying excited states and radiationless deactivation. Part I: A mechanistic overview of the parent system ABN

“…One of the evidences that support the assignment to a LE band is that the decay time of this band is the same as that of the 450 nm band (also assigned to a LE transition). 26,34 Moreover Zachariasse has found similar bands in DMABN analogues without the cyano group, 57 ruling out the possibility of assignment to the RICT transition and supporting again the assignment to a LE transition. On the other hand, this band blue shifts as the polarity of the solvent increases, the fact related to solutes with a large dipole moment, which supports the assignment made by some experimentalist to a RICT transition.…”

The role of the πσ* state in intramolecular charge transfer of 4-(dimethylamino)benzonitrile

“…A very recent ab initio CASPT2//CASSCF quantum-chemical calculations assign the TA signal to the 90 • twisted TICT state, and the ICT fluorescence to a partially twisted (∼54.4 • ) intramolecular charge transfer (pTICT) state. 12 The results and interpretations, summarized above, have recently been questioned by Zachariasse et al, 13 who assign the 700 nm transient of DMABN to the absorption spectrum of the LE state. As the main support, they provided the observation of ∼700 nm ππ* ← ππ* absorptions from 2,6-dimethyl-4X (X = F, Br, CF 3 )-N,N-dimethylanilines that do not contain a low-lying πσ* state.…”

Solvent-dependent intramolecular charge transfer delocalization/localization in multibranched push-pull chromophores

“…1-4 It is well known that local environments, especially solvents, have profound effects on the emission behavior of ICT type compounds, [5][6][7][8][9][10][11] where the surrounding solvent molecules respond by slightly modifying their translational and rotational motion and then eventually dissipate the excitation energy of excited ICT chromophore. [12][13][14] The degree of intramolecular charge transfer in multibranched chromophores can be estimated from the dynamics of solvent-coupled excited state relaxation.…”