White Light Emission and Second Harmonic Generation from Secondary Group Participation (SGP) in a Coordination Network

We describe a white emitting coordination network solid
that can
be conveniently applied as a thin film onto a commercial UV-LED lamp
for practical white lighting applications. The solid state material
was discovered in an exercise of exploring molecular building blocks
equipped with secondary groups for fine-tuning the structures and
properties of coordination nets. Specifically, CH3SCH2CH2S- and (S)-CH3(OH)­CHCH2S- (2-hydroxylpropyl) were each attached as secondary groups
to the 2,5- positions of 1,4-benzenedicarboxylic acid (bdc), and the
resultant molecules (L1 and L2, respectively)
were crystallized with Pb­(II) into the topologically similar 3D nets
of PbL1 and PbL2, both consisting of interlinked
Pb-carboxyl chains. While the CH3S- groups in PbL1 are not bonded to the Pb­(II) centers, the hydroxy groups in PbL2 participate in coordinating to Pb­(II) and thus modify the
bonding features around the Pb­(II), but only to a slight and subtle
degree (e.g., Pb–O distances 2.941–3.116 Å). Interestingly,
the subtle change in structure significantly impacts the properties,
i.e., while the photoluminescence of PbL1 is yellowish
green, PbL2 features bright white emission. Also, the
homochiral side group in PbL2 imparts significant second
harmonic generation, in spite of its seemingly weak association with
the main framework (the NLO-phore). In a broad perspective, this work
showcases the idea of secondary group participation (SGP) in the construction
of coordination networks, an idea that parallels that of hemilabile
ligands in organometallics and points to an effective strategy in
developing advanced functions in solid state framework materials.