Dendrimers exciplex formation

In acetonitrile-dichloromethane 1 1 v/v solution, their absorption spectra are dominated by naphthalene absorption bands and they exhibit three types of emission bands, assigned to naphthyl localized excited states (/Wx = 337 nm), naphthyl excimers (Amax ca. 390 nm), and naphthyl-amine exciplexes (/lmax = 480 nm) (solid lines in Fig. 3). The tetraamine cyclam core undergoes only two protonation reactions, which not only prevent exciplex formation for electronic reasons but also cause strong nuclear rearrangements in the cyclam structure which affect excimer formation between the peripheral naphthyl units of the dendrimers. 

Extensive investigation has been performed on the interaction of dendrimers 1 and 2 with metal ions [17a, e-h]. Complexation with Zn2+ engages the nitrogen lone pairs and thereby prevents exciplex formation, with a resulting intense naphthyl fluorescence (dashed lines in Fig. 3). This strong fluorescent signal is... 

The non-linear spectral changes of the naphthyl-localised exciplex and exci-mer bands occur after addition of the first two equivalents of trifluoroacetic acid. It is thus unnecessary to protonate all of the nitrogen atoms in order to suppress exciplex formation since - expressed figuratively - the nitrogen atoms share the protons. Moreover, protonation not only provides protection against exciplex formation but also leads to conformational changes in the cyclam unit itself, which in turn affects excimer formation between peripheral naphthyl units of the dendrimer. 

Fig. 4. Schematic (a) representation of excimer and exciplex formation in a dendrimer and (b) energy level diagram showing the three types of emissions that can result.

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