Photoluminescence Spectral Reliance on Aggregation Order of 1,1-Bis(2‘-thienyl)-2,3,4,5-tetraphenylsilole

1,1-Bis(2‘-thienyl)-2,3,4,5-tetraphenylsilole (1) was prepared and characterized crystallographically. Silole 1
exhibited aggregation-induced emission (AIE) behavior like other 2,3,4,5-tetraphenylsiloles. Unexpectedly,
aggregates formed in water/acetone (6:4 by volume) mixture emitted a blue light that peaked at 474 nm,
while aggregates formed in the mixtures with higher water fractions emitted green light that peaked at 500
nm. Transmission electron microscopy demonstrated that the aggregates formed in the mixture with water
fraction of 60% were single crystals, while aggregates that formed in the mixture with water fraction of 90%
were irregular and poorly ordered particles. The unusual PL spectral reliance on aggregation order was further
confirmed by PL emissions of macroscopic crystal powders and amorphous powders of the silole in the dry
state. PL spectral blue shifting was observed upon aging of the poorly ordered aggregates formed in mixtures
with water fractions of 70−90%, and they finally exhibited the same blue emission as the crystalline aggregates.
The as-deposited thin solid film was amorphous and it could be transformed to a transparent crystalline film
upon treatment in the vapor of an ethanol/water (1:1 by volume) mixture, along with PL spectral blue shifting
due to changing of aggregation order. It was also found that the crystalline film showed a blue-shifted absorption
spectrum relative to the amorphous film and the shift of the absorption edge of the spectra could match that
of corresponding PL spectra. The FT-IR spectrum of crystal powders of 1 displayed more vibration modes
compared with that of amorphous powders, suggesting the existence of different π-overlaps or different
molecular conformations. The crystals of 1-methyl-1,2,3,4,5-pentaphenylsilole and hexaphenylsilole also showed
blue-shifted PL emissions of their amorphous solids, with a comparable PL spectral shift of 1. Developing of
a silole solution on a TLC plate readily brought about an amorphous thin layer. Our results suggest that
crystalline films of AIE-active siloles are potential emissive layers for efficient blue OLEDs with stable color
and long lifetime.