Abstract
Molecular engineering via functionalization has been a great tool to tune noncovalent intermolecular interactions. Herein, we demonstrate three-dimensional highly crystalline nanostructured D(C7CO)-BTBT films via carbonyl-functionalization of a fused thienoacene π-system, and strong Raman signal enhancements in Surface-Enhanced Raman Spectroscopy (SERS) are realized. The small molecule could be prepared on the gram scale with a facile synthesis-purification. In the engineered films, polar functionalization induces favorable out-of-plane crystal growth via zigzag motif of dipolar C = O···C = O interactions and hydrogen bonds, and strengthens π-interactions. A unique two-stage film growth behavior is identified with an edge-on-to-face-on molecular orientation transition driven by hydrophobicity. The analysis of the electronic structures and the ratio of the anti-Stokes/Stokes SERS signals suggests that the π-extended/stabilized LUMOs with varied crystalline face-on orientations provide the key properties in the chemical enhancement mechanism. A molecule-specific Raman signal enhancement is also demonstrated on a high-LUMO organic platform. Our results demonstrate a promising guidance towards realizing low-cost SERS-active semiconducting materials, increasing structural versatility of organic-SERS platforms, and advancing molecule-specific sensing via molecular engineering.
Highlights
Molecular engineering via functionalization has been a great tool to tune noncovalent intermolecular interactions
The analysis of the electronic structures and the ratio of the anti-Stokes to Stokes SERS, suggests that the π-extended and stabilized LUMOs with crystalline face-on orientations in varied directions, all of which are the direct results of carbonyl functionalization, are key to the realization of strong chemical enhancement mechanism
Methanol/dichloromethane), and the photo of the gram-scale solid obtained after drying. c Chemical structure of C8-BTBT, experimental HOMO/LUMO energy levels of C8-BTBT and D(C7CO)-BTBT determined via electrochemical methods combined with the optical band gaps (Eg’s)[26], and the density functional theory (DFT)(B3LYP/ 6-31 G**)-calculated topographical frontier orbital representations
Summary
Molecular engineering via functionalization has been a great tool to tune noncovalent intermolecular interactions. Our initial findings have convincingly demonstrated that low-lowest unoccupied molecular orbital (LUMO) oligothiophene semiconductors in their nanostructured films could enhance Raman signals via a chemical enhancement mechanism In this perspective, perfluoro-alkyl/-aryl substituents attached directly to the quaterthiophene π-core do lower the LUMO energy of the π-system (−3.2 to −3.4 eV) for effective charge-transfer (CT) interactions with analyte molecules and enable a high favorable crystal growth dynamics during the PVD processes. Carbonyl functionalities were observed to induce strong dipolar and π-interactions between the BTBTs, leading to enhanced cohesive energetics relative to non-functionalized analogs[26,27] These results have revealed the great potential of functionalized low-LUMO BTBTs in n-type OFETs and complementary circuits, and made this thienoacene molecular family, as pure organic films, quite attractive for nanostructured SERS-active platforms. The analysis of the electronic structures and the ratio of the anti-Stokes to Stokes SERS, suggests that the π-extended and stabilized LUMOs with crystalline face-on orientations in varied directions, all of which are the direct results of carbonyl functionalization, are key to the realization of strong chemical enhancement mechanism
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