Switching of the Triplet Excited State of Rhodamine/Naphthaleneimide Dyads: An Experimental and Theoretical Study

Rhodamine–bromonaphthaleneimide (<b>RB–NI</b>) and rhodamine–bromonaphthalenediimide (<b>RB–NDI</b>) dyads were prepared for switching of the <i>triplet</i> excited states. Bromo-NI or bromo-NDI parts in the dyads are the spin converters, i.e., the triplet state producing modules, whereas the RB unit is the acid-activatable electron donor/energy acceptor. NI and NDI absorb at 359 and 541 nm, and the T₁ state energy levels are 2.25 and 1.64 eV, respectively. RB undertakes the reversible spirolactam (RB-c) ↔ opened amide (RB-o) transformation. RB-c shows no visible light absorption, and the triplet-state energy level is <i>E</i>_T1 = 3.36 eV. Conversely RB-o shows strong absorption at 557 nm, and <i>E</i>_T1 is 1.73 eV. Thus, the acid-activated fluorescence-resonance-energy-transfer (FRET) competes with the ISC of NI or NDI. No triplet state was observed for the dyads with nanosecond time-resolved transient absorption spectroscopy. Upon addition of acid, strong fluorescence and long-living triplet excited states were observed. Thus, the producing of triplet state is acid-activatable. The triplet state of <b>RB–NI</b> is <i>localized</i> on RB-o part, whereas in <b>RB–NDI</b> the triplet state is <i>delocalized</i> on both the NDI and RB-o units. The ISC of spin converter was not outcompeted by RET. These studies are useful for switching of triplet excited state