Room-Temperature Phosphorescence of Nicotinic Acid and Isonicotinic Acid: Efficient Intermolecular Hydrogen-Bond Interaction in Molecular Array

Pure organic room-temperature phosphorescence
(RTP) has attracted
wide interest due to its unique advantages and promising applications.
However, it is still challenging to develop efficient RTP through
precise molecular design. In this work, RTP is observed from two simple
aromatic acids, nicotinic acid (NA) and isonicotinic acid (INA), in
the crystal state. Single crystal structure analysis indicates that
an intense hydrogen bond between the pyridine nitrogen atom and the
carboxyl group results in zigzag and linear molecular packing modes
in NA and INA crystal. From theoretical calculations, the hydrogen
bond can effectively promote the intersystem crossing process and
stabilize triplet exciton. The identical molecular orientations in
the molecular array contribute to the larger dipole moment of INA
as compared to that of NA, which should be responsible for the red-shifted
photoluminescence and RTP of INA. When the hydrogen bond is destructed
by grinding or deprotonation, the RTP decreases sharply, further confirming
the crucial role of the hydrogen bond on RTP.