  Quantitative phase imaging (QPI) quantifies the sample-specific
optical-phase-delay enabling objective studies of optically-transparent
specimens such as biological samples, but lacks chemical sensitivity limiting
its application to morphology-based diagnosis. We present wide-field
molecular-vibrational microscopy realized in the framework of QPI utilizing
mid-infrared photothermal effect. Our technique provides mid-infrared
spectroscopic performance comparable to that of a conventional infrared
spectrometer in the molecular fingerprint region of 1,450 - 1,600 cm-1 and
realizes wide-field molecular imaging of silica-polystyrene beads mixture over
100 {\mu}m x 100 {\mu}m area at 1 frame per second with the spatial resolution
of 430 nm and 2 - 3 orders of magnitude lower fluence of ~10 pJ/{\mu}m2
compared to other high-speed label-free molecular imaging methods, reducing
photodamages to the sample. With a high-energy mid-infrared pulse source, our
technique could enable high-speed, label-free, simultaneous and in-situ
acquisition of quantitative morphology and molecular-vibrational contrast,
providing new insights for studies of optically-transparent complex dynamics.
