Transformation of Nonporous Adaptive Pillar[4]arene[1]quinone Crystals into Fluorescent Crystals via Multi-Step Solid–Vapor Postsynthetic Modification for Fluorescence Turn-on Sensing of Ethylenediamine

Organic solid-state
fluorescent crystals have received extensive
attention owing to their remarkable and promising optoelectronic applications
in many fields. Current methods to obtain organic fluorescent crystals
usually involve two steps: (1) solution phase organic synthesis and
(2) crystallization of target fluorescent compounds. Direct transformation
from nonfluorescent organic crystals to fluorescent organic crystals
by postsynthetic modification (PSM) might be a potential alternative
to the traditional methods. Although it is common to implement PSM
for porous frameworks, it remains a huge challenge for nonporous organic
crystals. Herein, we report a novel method of multistep solid–vapor
PSM in nonporous adaptive crystals (NACs) of a pillar[4]­arene[1]­quinone
(M1) to prepare organic solid-state fluorescent crystals.
Fluorescent organic crystals can be simply generated when guest-free M1 crystals were exposed to ethylenediamine (EDA) vapor. However,
only nonemissive crystals of a thermodynamically metastable intermediate M2 are obtained through solid–vapor single-crystal-to-single-crystal
transformation of CH3CN-loaded M1 crystals.
Solution-phase reaction of M1 with EDA affords three
distinct compounds with different fluorescent properties, which are
demonstrated to be the main components of the fluorescent organic
crystals that are generated by the solid–vapor PSM. Mechanistic
studies show that the pillararene skeleton not only induces the solid–vapor
PSM by physical adsorption of EDA but also facilitates the fluorescent
emission in the solid state by restricting intermolecular π–π
interactions to avoid aggregation-caused quenching (ACQ). Furthermore,
this interesting phenomenon is applied for facile fluorescence turn-on
sensing of EDA vapor to distinguish EDA from other aliphatic amines.