Molecular Cooperativity in the Intense Raman Scattering on the Surface of an Organic Molecular Microcrystal

Abstract

AbstractMetal free substrates as promising surface enhanced Raman spectroscopy (SERS) platforms have recently gained significant attention owing to their high spectral stability, ease of synthesis, environmental stability, biocompatibility, and target molecule specificity. In this context, molecular crystals of π‐conjugated small molecules can represent a class of prominent candidates, the SERS properties – especially analyte specificity – of which can be tuned based on molecular bonding and interaction. Herein it is suggested and demonstrated that microcrystals of a small organic molecule such as those of π‐conjugated terephthalic acid (TA) can be used as novel platforms for SERS. TA microcrystals show unprecedented Raman signal enhancements up to 1.50 × 106 for the analyte molecule rhodamine‐6G (R6G), 3.23 × 104 for rhodamine B (RhB), and 7.17 × 104 for the eosin yellowish (EY) at 785 nm laser stimulation. Experimental results also show that molecular cooperativity in the Raman signal enhancement or reduction of binary mixtures over TA microcrystals can be analyte‐specific. The density functional theory (DFT) based electronic structure of the crystals, photo‐induced charge transfer transitions, and the concept of hybrid orbitals through noncovalent interactions in the combined system provide insights into the cooperative, molecule‐specific intense SERS performance. These findings represent a substantial exploratory breakthrough in the relatively nascent domain of organic SERS.