Intramolecular Charge Transfer in Dual Fluorescent 4-(Dialkylamino)benzonitriles. Reaction Efficiency Enhancement by Increasing the Size of the Amino and Benzonitrile Subunits by Alkyl Substituents

Abstract

The intramolecular charge transfer (ICT) reaction from the locally excited state (LE) to the charge transfer state (CT) in the singlet excited state is investigated for the dual fluorescent 4-(dialkylamino)benzonitriles DRABN and 4-dialkylamino-2,6-dimethyl-benzonitriles RDB (R = methyl, ethyl, n-propyl) in toluene as a function of temperature by photostationary and time-resolved experiments. The efficiency of the ICT reaction, as expressed by the CT/LE fluorescence quantum yield ratio Φ‘(CT)/Φ(LE), is enhanced by the increase in the size of the dialkylamino group as well as by the presence of the two extra methyls in the phenyl ring. This increase is mainly brought about by slowing down the thermal back reaction (k d) due to a larger activation energy E d or a smaller preexponential factor k d o, respectively. The free enthalpy change ΔG becomes more negative (∼6 kJ/mol) for 4-(diethylamino)benzonitrile (DEABN) and 4-(di(n-propyl)amino)benzonitrile (DPrABN) as compared with 4-(dimethylamino)benzonitrile (DMABN) and also for the 4-dialkylamino-2,6-dimethylbenzonitriles EDB (ethyl) and PrDB (n-propyl) relative to MDB (methyl). The ΔG is about 2 kJ/mol more negative for RDB than for DRABN. In both series DRABN and RDB, the ICT stabilization enthalpy, −ΔH, increases from R = methyl to n-propyl. The increase in the ICT rate constant k a for the pairs DEABN/DMABN and EDB/MDB is primarily caused by a larger preexponential factor k a o counteracted but not reversed by a likewise larger activation energy E a. For the dependence of the energy E(CT) of the CT state on the difference between the redox potentials of the dialkylamino and benzonitrile moieties in DRABN and RDB a substantially smaller correlation coefficient (0.29) is found than the value of 1.0 expected for the TICT model, showing that the amino and benzonitrile groups in the CT state are not electronically decoupled. The energy δE rep of the Franck−Condon ground state reached upon CT emission decreases with increasing size of the dialkylamino group in DRABN and RDB.

Intramolecular Charge Transfer with N,N-Dialkyl-4-(Trifluoromethyl)anilines and 4-(Dimethylamino)benzonitrile in Polar Solvents. Investigation of the Excitation Wavelength Dependence of the Reaction Pathway

“…The ratio Φ (ICT)/Φ(LE) increases when the amino-alkyl chain becomes longer, in particular from DMCF3 to DECF3: 0.06 (DMCF3), 0.50 (DECF3), 0.54 (DPrCF3), 0.64 (DPeCF3) ( Table II). A similar increase in Φ (ICT)/Φ(LE) with increasing alkyl chain length has been found with DXABN and derivatives in toluene [27]. …”

“…For a series of DXABN D/A molecules, a correlation between the energy of the ICT state E(ICT) and the difference between the oxidation and reduction potentials of the D and A subgroups has been established, with a correlation coefficient f = 0.29, see Eq. 1 [27]:…”

Structure and spectal properties of cinnamoyl pyrones and their vinylogs

“…As it was earlier found for other organic substances [21][22][23], these transitions are of different nature, and they are accompanied by inter-fragmental charge transfer. The long-wavelength bands corresponding to these transitions have different shapes (see Figs.…”

Ultrafast Intramolecular Charge Transfer and Internal Conversion with Tetrafluoro‐aminobenzonitriles

“…[9] In the series of 4-aminobenzonitriles with a heterocyclic amino group PnC, where n represent the size of the ring, the ICT efficiency becomes larger with increasing ring size. Whereas for this series only 4-(aziridinyl)benzonitrile (P3C) and 4-(azetidinyl)benzonitrile (P4C, or AZABN in this paper) do not show ICT in DEE, ICT fluorescence is observed [10,11] with P4C in MeCN (but not with P3C).…”

“…[15] Although the absence of ICT in the case of MABN and ABN, in contrast to DMABN, is indirectly related to the weaker electron-donor strength (larger ionisation or oxidation potential) of the methyl-amino and amino as compared with the dimethylamino substituent, [16] or alternatively of the N-methylaniline or aniline relative to the N,N-dimethylaniline, [17] this phenomenon cannot be explained on the basis of the energies of the twisted intramolecular charge transfer (TICT) [2] states, for which an electronically decoupled amino group is assumed. Rather, the energy of the ICT state E(ICT), the crucial factor determining the possibility of an ICT reaction, depends with a correlation factor considerably smaller than unity (0.2-0.3) [9] on the difference between the redox potentials of the amino and benzonitrile subunits.…”