Restriction of Molecular Twisting on a Gold Nanoparticle Surface

To understand the photophysical properties of intramolecular charge transfer (ICT) and twisted intramolecular charge transfer (TICT) states on a gold nanoparticle (Au NP) surface, we have designed and synthesized a new coumarin molecule (C3) that exists both as ICT and TICT states in its excited state in a polar environment. On a Au NP surface, an excited C3 molecule only exists as an ICT state owing to restricted molecular rotation of a diethylamino group; as a result, no conversion from the ICT to TICT state was observed. Selection of the preferential state of a molecule with dual emitting states can be helpful for selected biological applications. TICT off: The coumarin 3 (C3) molecule is found to have dual emitting intramolecular charge transfer (ICT) and twisted ICT (TICT) states. Rotation of the free 7‐amino substituent in the excited state generates the twisted structure, which gives the TICT state. Au nanoparticles (NPs) restrict the rotation of the 7‐amino substituent of C3; dual‐emitting C3 on the Au NP surface gives emission that is only due to the ICT state.