Synthesis and Strong Solvatochromism of Push-Pull Thienylthiazole Boron Complexes

Unravelling photoisomerization dynamics in a metastable-state photoacid

Direct access to trans-cis photoisomerization in a metastable state photoacid (mPAH) remains challenging owing to the presence of competing excited-state relaxation pathways and multiple transient isomers with overlapping spectra. Here, we reveal the photoisomerization dynamics in an indazole mPAH using time-resolved fluorescence (TRF) spectroscopy by exploiting a unique property of this mPAH having fluorescence only from the trans isomer. The combination of these experimental results with time-dependent density function theory (TDDFT) calculations enables us to gain mechanistic insight into this key dynamical process.

Synthesis and Excited-State Properties of Donor-Acceptor Azahelical Coumarins

A set of three donor-acceptor azahelical coumarins (DA-AHCs), namely, H-AHC, Me-AHC, and Ph-AHC, were rationally designed and synthesized, and their excited-state properties were comprehensively investigated. All three DA-AHCs are shown to display very high fluorosolvatochromic shifts as a result of significant intramolecular charge transfer in their excited states. The para-quinoidal forms of the latter apparently contribute predominantly to large dipole moments in their excited states. By virtue of the fact that these helical systems structurally incorporate a highly fluorescent coumarin dye, they exhibit high quantum yields in both solution and solid states. Indeed, their emission behaviors in the crystalline media are shown to be remarkably correlated with their respective crystal packings. Incisive analyses demonstrate (i) strengthening of hydrogen bonding in the excited state promotes quenching (H-AHC), (ii) efficient crystal packing promotes high emission (Me-AHC) by precluding deactivations via vibrational motions, and (iii) loose crystal packing contributes to excited-state deactivation to account for low quantum yields of emission (Ph-AHC).