Suppression of the harpooning mechanism in donor-bridge-acceptor systems by aza substitution in the bridge

An aza substituent was introduced in the bridge of donor-bridge-acceptor systems known to undergo harpooning (long-range photoinduced charge separation followed by electrostatically driven folding). Although the substitution does not reduce the conformational flexibility of the bridge, it fully suppresses the harpooning mechanism. From a comparison with a reference compound, the solvatochromic shifts of charge transfer fluorescence and transient absorption results, it is concluded that suppression is due to the low ionization potential of the aza substituent, which leads to a strong reduction of the positive charge on the terminal donor in the excited state.