Modulating Room-Temperature Phosphorescence through the Synergistic Effect of Heavy-Atom Effect and Halogen Bonding

Organic
room-temperature phosphorescence (RTP) materials have been
paid great attention for their promising applications in anticounterfeiting,
optical device, and bioimaging. However, owing to inefficient intersystem
crossing (ISC), it still remains a challenge to develop organic RTP
materials with both high quantum yields (Φp) and
long lifetime (τp). Herein, a reasonable strategy
is presented to modulate and balance the Φp and τp through the synergy effect of halogen bonding and heavy-atom
effect (HAE). Modulated RTP properties are successfully achieved by
the introduction of halogen atoms into 4-(9-H-carbazol-9-yl)
benzonitrile due to enhanced ISC. Especially, CzBzBr shows
the highest Φp of 23.50% and CzBzCl exhibits
the longest τp of 607.4 ms. The excessive HAE of
the bromine atom decreases the τp of CzBzBr, while moderate HAE of the chlorine atom endows CzBzCl with both high Φp and long τp.
In addition, the halogen bondings lead to specific halogen-mediated
molecular cluster packing, further suppressing nonradiative transition
for ultralong RTP emission. Through simple physical co-crystallization
with adjusting the mass ratio of CzBzCl/CzBzBr, co-crystals with modulated RTP properties and white-light emission
phenomena are obtained. Our study provides a rationale method to develop
modulated high-efficiency RTP materials, which will expand their practical
applications.