Substituent-Controlled Photophysical Responses in Dihydropyridine Derivatives and Their Application in the Detection of Volatile Organic Contaminants

In the ever-expanding realm of organic fluorophores,
structurally
simple and synthetically straightforward molecules with unique photophysical
properties have received special attention. Among these, 1,4-dihydropyridine
(DHP) is an important scaffold that permits fine-tuning
of their photophysical properties through substituents on the periphery.
Herein, we describe a series of solid-emissive N-substituted 2,6-dimethyl-4-methylene-1,4-dihydropyridine
derivatives appended with electron-withdrawing substituents (dicyanomethylene
or 2-dicyanomethylene-3-cyano-2,5-dihydrofuran) at the C-4 position
and alkyl or alkylaryl groups on the DHP nitrogen. Electronic
and steric tuning exerted by these substituents resulted in interesting
photophysical properties such as negative solvatochromism, solidstate,
and aggregation-induced emission (AIE). Theoretical calculations were
carried out to explain the solvatochromic properties. Insight into
the AIE properties was obtained through variable-temperature nuclear
magnetic resonance and viscosity- and temperature-dependent emission
studies. The variations in molecular packing in the crystal lattice
with changes in the N-substituents contributed to the tuning of solid
state emission properties. Detection of aromatic volatile organic
compounds (VOCs) was achieved using the aggregates of the DHP derivatives. Among the VOCs, p-xylene elicited
a significant enhancement in emission, allowing its detection at submicromolar
levels.