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

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 Δ G 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 Δ G is about 2 kJ/mol more negative for RDB than for DRABN. In both series DRABN and RDB, the ICT stabilization enthalpy, −Δ H, 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 δ E rep of the Franck−Condon ground state reached upon CT emission decreases with increasing size of the dialkylamino group in DRABN and RDB.