Effect of the ethynylene linker on the properties and carrier mobility of naphthalene derivatives with hexylbithienyl arms

Abstract

A series of new naphthalene derivatives with (5′-hexyl-2,2′-bithiophene-5-yl)ethynyl side arms attached to the central naphthalene core at positions 1 and 4 (H2TA14N), 1 and 5 (H2TA15N), and 2 and 6 (H2TA26N) were synthesized by a sequencing of alkynylations performed according to a Negishi cross-coupling protocol. This synthetic approach provides higher yields compared to Sonogashira-based alkynylations, particularly when aryl bromides are used as starting materials. Compared to the analogous derivatives with hexylbithienyl arms directly attached to the naphthalene core (H2T14N, H2T15N, and H2T26N), an ethynylene linker ensures the planarity of the molecules, causing a change in the order of derivatives by intramolecular conjugation from H2T15N<H2T14N<H2T26N to H2TA15N≈H2TA26N<H2TA14N, as can be seen from the solution, and thin film optical and electrochemical properties as well. However, the order of derivatives by the hole mobility in a field effect transistor remains unaltered by an ethynylene linker (H2TA14N<H2TA15N<H2TA26N). The tendency in charge mobility can be explained by different packing motifs affected by molecular symmetry and was revealed by single crystal X-ray analysis and 2D X-ray GIWAXS and AFM characterization of the thin films.