Spiro-conjugated indenodiarylethenes: enabling steric-induced electronic tuning of photochromic and photoluminescent properties by spiro-conjugation

Light-operated excited-state photochemical reactions and the resulting photochromic materials have gained significant attention for their promising technological applications in molecular electronic and photonic devices. To obtain photochromic materials (e.g., diarylethenes) with desired properties, the way to tune energy processes of the excited state is essentially important, from both fundamental and practical viewpoints. The modified diarylethenes mostly focused on structural improvements to achieve efficient photochromism. We report here two new spiro-conjugated indenodiarylethenes (named, SFI-Th and SFI-Sul) with an orthogonally arranged spiro π-scaffold. The spiro fluorene sterically constrains the free volume of photoactive side aromatic rings, while the spiro-conjugation points toward modulation of the decaying channels of excited-state energy, thus allowing for tuning photochromic performances. These spirodiarylethenes SFI-Th and SFI-Sul are readily accessible with low cost and show distinct photochromic and photoluminescent properties. On one hand, the SFI-Th bearing bisthienyl side rings, is an excellent photochromic switch showing high thermal stability and high fatigue resistance in both solution and amorphous state, likely benefited from steric-induced excited-state energy transfer into reversible photochemical reactions, owing to higher π-orbital spiro-conjugation between the two molecular halves. On the other hand, the SFI-Sul with bissulfonyl side rings, most notably, displays quantitative photocyclization to yield SFI-Sulc upon excitation. However, the ring-closed SFI-Sulc has an extremely slow photocycloreversion, and meanwhile immediately switches on emissive state with a strong fluorescence (Φf=0.57 in amorphous powder state). We study the modulated roles of spiro π-scaffold in both cases computationally and experimentally, and attribute the facilitated photochemical reaction to the positively electronic effects of spiro-conjugation, and the photo-responsive radiative transition to the negatively electronic effects of spiro-conjugation, respectively. In all, this work demonstrates that the spiro-conjugation is a new and efficient strategy to deliver excitation energy transfer (EET) for featured photochromic properties, due to its advantage in steric-induced electronic modulation.