Multiple intramolecular motions consume the excited-state energy of luminogenic molecules upon photoexcitation and lower the emission efficiency. The low frequency rotational motion of aromatic rings can be facilely restricted by steric constraint in the condensed phase, but the high frequency bond stretching motion can hardly be suppressed by aggregation. In this work, three phosphorus-containing heterocycles, 1,2,3,4,5-pentaphenylphosphole-1-oxide (PPPO), 1,2,3-triphenylphosphindole-1-oxide (TPPIO), and 1,2,3-triphenylphosphindole (TPPI), were synthesized and characterized. Their optical properties, crystal-packing manners, electronic features, and fluorescence dynamics were systematically investigated, and theoretical calculations were performed to decipher structure-property relationships. The results reveal that these luminogens are weak emitters in solutions but show strong emission in aggregates, exhibiting obvious aggregation-induced emission (AIE) features. The aggregation-insensitive stretching motion, which is dominant in PPPO, is lowered in TPPIO, enabling TPPIO to fluoresce much more efficiently than PPPO in aggregates. The stretching motion is even more lowered in TPPI, but its relatively planar conformation suffers emission quenching due to strong π-π stacking interactions in aggregates. Therefore, a twisted molecular conformation consisting of a rigid stator and a rotatable periphery is demonstrated to be a rational design for more efficient AIE luminogens.

Aggregation-induced emission (AIE) is commonly observed for propeller-like luminogens with aromatic rotors and stators. Herein, we report that a coumarin derivative containing a seven-membered aliphatic ring (CD-7) but no rotors showed typical AIE characteristics, whereas its analogue with a five-membered aliphatic ring (CD-5) exhibited an opposite aggregation-caused quenching (ACQ) effect. Experimental and theoretical results revealed that a large aliphatic ring in CD-7 weakens structural rigidity and promotes out-of-plane twisting of the molecular backbone to drastically accelerate nonradiative excited-state decay, thus resulting in poor emission in solution. The restriction of twisting motion in aggregates blocks the nonradiative decay channels and enables CD-7 to fluoresce strongly. The results also show that AIE is a general phenomenon and not peculiar to propeller-like molecules. The AIE and ACQ effects can be switched readily by the modulation of molecular rigidity.

A series of BODIPY derivatives with tetraphenylethene (TPE) moieties were designed and synthesized. The effect of positions and numbers of substitution groups on the fluorescence of the BODIPYs was investigated. Theoretical calculation and single crystal structures proved that the TPE substitution groups on the 8-position of BODIPY contributed little to the conjugation, but benefited the aggregated state emission. On the other hand, the substitutions on the 3- or 5-position of BODIPY through vinyl bridges increased the conjugation length, and generated big coplanar π-conjugated structures with poor aggregated state emission. The compound with bright aggregated state emission has been further fabricated into biocompatible fluorescent nanoparticles and used as effective fluorescent contrast agents for intracellular imaging.