Changes in molecular film metallicity with minor modifications of the constitutive quinonoid zwitterions

Abstract

Molecular films of quinonoid zwitterions, of the general formula C 6 H 2 (/O) 2 (/NHR) 2 , have been shown to display electronic properties highly dependent on the nature of the N-substituent R when deposited on gold substrates. The different spacing and organization of the molecules can lead to molecular films with semi-metal or dielectric behavior. With the long term goal to establish how packing effects in the solid state correlate with properties as thin films, we first attempted to identify by X-ray diffraction analysis candidate molecules showing suitable packing arrangements in the crystalline state. To this end, we have prepared a series of new functionalized, enantiopure or sterically-hindered quinonoid zwitterions and established the crystal structure of those with R ¼ CH 2 -CH 2 -Ph (6), CH 2 -CH 2 -CH 2 -Ph (7), CH 2 -CH 2 -C 6 H 4 ) (15), CH 2 -((4-NH 2 )-C 6 H 4 ) (19) and CH 2 -CH 2 -((3,4-(OCH 3 ) 2 )-C 6 H 3 ) (24). An analysis of the crystal packing of three molecules, 5, 13 and 15, selected as illustrative examples for comparisons, was carried out and it was unexpectedly found that these chemically very similar molecules gave rise to different packing in the bulk, with resulting thin films showing different electronic properties. Various methods have been used for the characterization of the films, such as synchrotron radiation-based FTIR spatial spectra-microscopy, which provided an anchoring map of zwitterion 15 on a patterned substrate (Au/SiO 2 ) showing its selective anchoring on gold. This is one of the best examples of preferential anchoring of a zwitterion and the sole example of spatial localization for a quinonoid zwitterion thin film.We have also used combined photoemission and inverse photoemission spectra and the data were compared to occupied and unoccupied DFT density state calculations.

Pancakes under Pressure: A Case Study on Isostructural Dithia- and Diselenadiazolyl Radical Dimers

“…For example, while enhancement of S-, Se-, and Te-based polyacenes has been observed, formation of a metallic state by closure of the valence-conduction band gap E g usually requires pressures above 20 GPa, approaching the region where covalent bonds begin to rupture . Various molecular design strategies to reduce Δ E and hence E g have been pursued, the most notable involving the use of internal salts and charge-separated zwitterions, , where the resulting small Δ E can be fine-tuned by substituent and heavy atom effects. With favorable packing motifs, semiconductors capable of metallization at pressures well below 10 GPa have been made …”

Small Panchromatic and NIR Absorbers from Quinoid Zwitterions

“…In parallel, the convenient functionalization of BQIs combined with their large intrinsic dipole (ca. 10 D) made them candidates of choice for the elaboration of molecular films following adsorption or polymerization on metallic surfaces along with interfaces and, more recently, as interlayers in graphene transistors …”

The Quinonoid Zwitterion Interlayer for the Improvement of Charge Carrier Mobility in Organic Field-Effect Transistors

“…10 D) is deposited between the silicon wafer and the OSC layer. We decided to investigate zwitterionic benzoquinonemonoimine (BQMI) dyes 1–4 ( Scheme 1 ), which have recently been proposed for the creation of molecular films following adsorption [ 20 , 21 , 22 , 23 , 24 ], the polymerization of metallic surfaces [ 25 , 26 , 27 ], for use in interfaces [ 28 , 29 ], and, more recently, as interlayers in graphene transistors [ 30 ]. Lin et al reported a zwitterion-based cathode interlayer in organic photovoltaics, showing that the introduction of such interlayer significantly suppresses trap-assisted recombination in photovoltaics.…”