Lewis Pairs of Pyridylated Benzothienobenzothiophene (BTBT) Derivatives and Tris(Pentafluorophenyl)Borane (TPFB): Molecular Arrangements, Solid-State Photophysical Properties and Single-Molecule Raman Spectroscopy

Abstract

A series of organic crystalline solids is prepared by Lewis pairing the pyridyl derivatives of benzothienobenzothiophene (BTBT) and tris(pentafluorophenyl)borane (TPFB). The Lewis pair series shows structural variations in the arrangements of the BTBT moiety: 1D π-stacked columns covered by TPFB, slipped columns, and cofacial dimers. Quantum chemical calculations are performed to quantify the intermolecular electronic coupling and bandgaps, which reveal that the 1D columns are electronically fragmented at the dimer or tetramer level. In response to the degree of electronic aggregations, the Lewis pairs exhibit red-shifted fluorescence, reaching 5810 cm⁻¹ from the emission maximum of the non-substituted BTBT (397 nm) to that of one crystal form (516 nm). Scanning tunneling microscopy reveals that one Lewis pair is loosely gathered on the Ag(111) surface without any ordered arrangement of the adsorbates, differing from that in the solid phase. Furthermore, vibrational fingerprinting of the Lewis pair is achieved using tip-enhanced Raman scattering techniques at the single-molecule level, including the B–N and C–S stretching characteristics. These results provide insight into modulating the molecular arrangements of small-molecule semiconducting units to alter the emission properties, as well as fabricating molecular devices adsorbed on surfaces via post-modification.