Bridged EDA Molecules

In the present section, we first describe the details of the intramolecular charge-transfer (ICT) state formation in several directly connected (i.e., = 0) EDA molecules from our recent studies, and then review the reported features of the bridged EDA molecules. 

In the bridged EDA molecule I (Figure 7) where the donor dimethylaniline and the acceptor anthracene are bridged by a saturated hydrocarbon chain -(CH2),-, the features of the electron-transfer reaction are known to be strongly dependent on the chain length n. Mataga and co-workers demonstrated that in the condensed phase bridged EDA molecules with = 0, 1 and 2 formed the CT state only in polar solvents. In the case of = 3, the CT state was formed even in non-polar solvents [86]. The rise time of the CT emission in 2-propanol was 54 and 93 ps for n = 1 and 2, respectively. In -hexane solvent, a rise time of 2.6 ns was reported for the = 3 molecule. 

Zewail and co-workers first observed the behavior of the CT formation for the = 3 bridged EDA molecule in isolated jet-cooled conditions [87, 88]. They found that the rate of electron transfer depends on the excess energy given to the EDA molecule and not on the vibrational mode to be excited. This implies that the IVR in the LE state precedes the CT reaction. They confirmed this mechanism by demonstrating that an RRKM calculation could reproduce the energy dependence of the CT rate. The IVR accelerates the structural change, i.e., folding, of the EDA molecule in the LE state so that the donor part most favorably overlaps the acceptor part. 

In summary, for bridged EDA molecules in isolated jet-cooled conditions, folding is essential for the electron transfer to intensify the overlap between the donor and acceptor. In the condensed phase, a polar solvent assists the electron transfer even for molecules which cannot form a folded structure. The 1 1 bianthryl-H20... 

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