P31

22BG044, 23BG033

BL-13

Ultrafast charge transfer through biphenyl and fluorene
monolayers studied by core-hole clock spectroscopy
Akinobu Niozua, Kakuto Yoshiokaa, Shogo Tendoa, Shohei Asakurab,
Yoshikazu Hanakib, Jumpei Koizumib, Yuri Ohurab, Junna Yamadab,and Shin-ichi Wadaa, ca

Graduate School of Science and Engineering, Hiroshima University,
Higashi-Hiroshima 739-8526, Japanb
Faculty of Science, Hiroshima University, Higashi-Hiroshima 739-8526, JapancHiroshima Synchrotron Radiation Center, Hiroshima University,Higashi-Hiroshima 739-0046, JapanKeywords: Self-assembled monolayers, charge transfer, core-hole clock spectroscopy

Nanoscale charge transfer dynamics are crucial for fundamental physical chemistry and applications,such as nanoelectronics and photovoltaics. For these applications, it is crucial to understand the correlation
between the molecular structure and charge transport properties. Although molecular conductance has been
extensively studied using single-molecule conductance measurements [1], a notable disparity exists between
these fundamental investigations of single-molecule junctions and their direct application in functional
devices. The use of self-assembled monolayers (SAMs) is a potential solution to bridge this gap given theircapacity to form functional devices, thus connecting the insights gained from fundamental studies to practicalapplications.
TABLE 1.

Molecular structures and torsion angles of the molecules in SAMs.

NameStructure

Torsion angle / °MFL

0MBP0 [3]

MmBP

52

MdmBP

90

In this study, we investigated ultrafast charge transfer through self-assembled monolayers of biphenyland fluorene molecules via resonant Auger electron spectroscopy (RAES) using the core-hole clock (CHC)
technique [2]. In this approach, molecules are resonantly core-excited with soft X-rays, and the subsequent
electron transfer is probed by monitoring Auger decay spectra. The thiolated derivatives of biphenyl andfluorene used in this study are listed in Table 1. The addition of methyl groups induces torsion between thetwo phenyl rings within the SAMs, resulting in a reduction in π-conjugation. MBP molecules are anticipatedto adopt an almost planar conformation [3], similar to that observed in bulk solids. Conversely, the methylsubstituted thiolates (MmBP and MdmBP) are expected to exhibit twisted conformations owing to sterichindrance from the methyl groups. Density functional theory (DFT) calculations of the relaxed moleculargeometries yielded torsion angles of 52° and 90° for MmBP and MdmBP molecules, respectively. Thesemolecules have a methyl ester (COOCH3) moiety as the tail group, which serves as the site resonantly excitedwith soft X-rays in the CHC experiments.
The methylester-terminated biphenyl and fluorene derivatives were custom-synthesized by and purchasedfrom Tokyo Chemical Industry Co., Ltd. (for MFL, MmBP, and MdmBP) and NARD Institute Ltd. (forMBP). The SAMs were formed by immersing the Au substrates in 1 mM ethanol (for MBP, MmBP, andMdmBP) and toluene (for MFL) solutions of the precursors at 30 ◦C for 24 h. After immersion, the SAM
samples were rinsed with solvent and dried under a flow of N2 gas. The SAM samples were characterizedusing near-edge X-ray fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). NEXAFS,XPS, and RAES experiments were conducted at HiSOR BL13. NEXAFS spectra were collected at the C andO K-edges using the total electron yield mode. The XPS and RAES spectra were acquired at an X-ray
incidence angle of 45° in the normal emission geometry using Omicron EA125 electron energy analyzer.
The C K-edge NEXAFS spectra showed several peaks corresponding to C 1s→ 𝜋 ∗ and C 1s→ 𝜎 ∗excitations. The polarization dependence of the 𝜋 ∗ intensities indicated the orientational order of themolecules within the SAMs. The S 2p XPS spectra of the target SAMs confirmed sulfur species chemisorbed
on Au. The NEXAFS and XPS spectra suggest the formation of ordered SAMs on the Au substrates.

FIGURE 1. RAES spectra of MFL, MBP, MmBP, and MdmBP SAMs. The spectra were compared with the pure resonant
spectrum recorded using the MHDA SAM and the normal Auger spectrum of the MBP SAM.

The RAES spectra at the O 1s→ 𝜋 ∗(C=O) excitation are shown in Fig. 1. The spectra were comparedwith the pure resonant spectrum recorded with an MHDA (HS-(CH2)15-COOCH3) SAM consisting of a long
aliphatic chain, which was assumed to show no charge transfer contribution. The spectra for the MFL, MBP,and MmBP SAMs showed an increase in intensity corresponding to normal Auger decay, which indicatesultrafast charge transfer from the molecule to the Au substrate. For a quantitative analysis of the chargetransfer dynamics, the RAES spectra were fitted with linear combinations of the normal Auger spectrum andthe pure resonant Auger spectrum recorded with the MHDA SAM. The fitting analysis yielded chargetransfer times of 16, 16, ~47, and ≳50 fs for the MFL, MBP, MmBP, and MdmBP SAMs, respectively. TheCHC results demonstrate the conformational control of ultrafast charge transfer and suggest a negligible
influence of the methylene bridge on the charge transfer dynamics.

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