A new charge-transfer complex in UHV co-deposited tetramethoxypyrene and tetracyanoquinodimethane

TL;DR

This study investigates a newly formed charge-transfer complex between tetramethoxypyrene and tetracyanoquinodimethane deposited under ultra-high vacuum, revealing its structural, vibrational, and electronic properties through multiple spectroscopic techniques and theoretical calculations.

Contribution

It provides the first detailed characterization of a charge-transfer complex formed from TMP and TCNQ on gold, combining experimental and theoretical insights.

Findings

Formation of a new CT phase confirmed by XRD reflexes.

The CT complex exhibits a HOMO-LUMO gap of about 1.25 eV.

Electronic level shifts are consistent with DFT predictions.

Abstract

UHV-deposited films of the mixed phase of tetramethoxypyrene and tetracyanoquinodimethane (TMP1-TCNQ1) on gold have been studied using ultraviolet photoelectron spectroscopy (UPS), X-ray-diffraction (XRD), infrared (IR) spectroscopy and scanning tunnelling spectroscopy (STS). The formation of an intermolecular charge-transfer (CT) compound is evident from the appearance of new reflexes in XRD (d1= 0.894 nm, d2= 0.677 nm). A softening of the CN stretching vibration (red-shift by 7 cm-1) of TCNQ is visible in the IR spectra, being indicative of a CT of the order of 0.3e from TMP to TCNQ in the complex. Characteristic shifts of the electronic level positions occur in UPS and STS that are in reasonable agreement with the prediction of from DFT calculations (Gaussian03 with hybrid functional B3LYP). STS reveals a HOMO-LUMO gap of the CT complex of about 1.25 eV being much smaller than the gaps (>3.0 eV) of the pure moieties. The electron-injection and hole-injection barriers are 0.3 eV and 0.5 eV, respectively. Systematic differences in the positions of the HOMOs determined by UPS and STS are discussed in terms of the different information content of the two methods.