Using an aromatic linker to optimize charge-resonance states, photodimerization and reversibility in covalent...

The intramolecular [4+4] photodimerization of anthracene chromophores in covalent assemblies can be harnessed to create negative photochromic systems. This paper reports the characterization of the photophysical and photochemical properties of a new class of asymmetric phenyl-linked bis(anthracene) photochromes and compares their behavior with that of a previously studied symmetric ethylene-linked analog. Steady-state and femtosecond time-resolved spectroscopic experiments show that both types of bis(anthracenes) support a neutral bright state along with a lower-energy charge resonance state. After photoexcitation, both states relax on sub-10 ps timescales, but with significantly different photodimerization quantum yields: 0.83 using 532 nm excitation of the charge-resonance state versus 0.46 for 400 nm excitation of the neutral bright state. The phenyl-linked bis(anthracene) derivatives exhibit superior thermal stability and reversibility due to a lower activation energy for dimer dissociation (94 kJ mol^-1versus 110 kJ mol^-1). Quantum chemical calculations reveal the structure of the neutral and charge-resonance excited states and can rationalize the higher photodimerization quantum yield of the latter. The phenyl linker enforces close alignment of the anthracene moieties, shifting the charge resonance state to lower energy and extending the wavelength range of the photochrome while also raising the energy of the photodimer ground state to enhance the backward reaction rate. The phenyl-linked bis(anthracenes) provide a promising system to harness the [4+4] photodimerization reaction with high quantum yield, room temperature reversibility, and cyclability.