(Invited) High-Yield and Long-Lived Triplet Excited States of Pentacene Alkanethiolate Monolayer Protected Gold Nanoparticles By Singlet Fission
Authors: Hasobe Taku , Sakai Hayato 
Published: February 2020

Photochemical conversion to high-yield and log-lived triplet excited states of organic molecules is an interesting research content. For example, three-dimensional self-assembled monolayers (SAMs) of organic dyes on metal nanoparticles (MNP) are highly promising as photofunctional nanomaterials. A major research topic is light energy conversion via photoinduced processes such as electron and energy transfer. The other application is optical molecular rulers (nanoruler) in the biological fields. In these cases, a serious problem of organic dye-modified SAMs on MNP is the significant deactivation pathway of the excited states of dyes caused mainly by the fast nanosurface energy transfer (NSET). Actually, more than 90% of fluorescence is quenched and formation of the excited triplet states is negligible. Therefore, the formation of high-yield and long-lived triplet excited states on MNP is favorable for various applications relaying on photoinduced processes.

Generation of efficient singlet fission (SF) on gold nanoparticles (GNP) is a promising method to overcome the above-mentioned problem. SF is described as a spin-allowed process in which one singlet exciton splits into two triplet excitons.1 In our study, TIPS-pentacene-alkanethiolate monolayer-protected gold nanoparticles with different chain lengths and particle sizes were successfully synthesized to efficiently generate excited triplet states by singlet fission (Fig. 1). Time-resolved spectroscopy revealed high-yield excited triplet states by suppressing ET to the gold surface.2

The mean diameters (R
CORE) of TP-C11-S-MPC and TP-C11-L-MPC are 1.65 ± 0.30 nm and 2.13 ± 0.33 nm, respectively. The core of TP-C11-S-MPC and TP-C11-L-MPC contains 158 and 338 Au atoms, respectively. Moreover, based on the values of elemental analysis, there are 51 and 75 TP molecules on one GNP. In addition, the coverage ratios of TP units (γ) to surface Au atoms in TP-C11-S-MPC and TP-C11-L-MPC are 63% and 51%, respectively.

Steady-state absorption and fluorescence spectra of TP-Cn-X-MPCs and TP-Ref were measured in toluene. In absorption spectra (Fig. 2A), the spectral shape of TP-C11-S-MPC (spectrum a) matches well with that of TP-Ref (spectrum b).The fluorescence of TP-C11-S-MPC was strongly quenched relative to TP-Ref. To compare the emission spectral shapes, the fluorescence spectra of TP-C11-S-MPC (spectrum a) and TP-Ref (spectrum b) were normalized in Fig. 2B. The fluorescence excitation spectrum was observed (spectrum c in Fig. 2A) to carefully check the fluorescence species of TP-C11-S-MPC. Then, Fig. 2C shows the femtosecond transient spectra of TP-C11-S-MPC. After laser pulse excitation, the triplet–triplet absorption of TP units at ca. 520 nm develops, whereas the singlet–singlet absorption at ca. 630 nm decays. The maximum Φ
Τ values attained 172 ± 26% and 157 ± 17% in TP-C11-S-MPC and TP-C7-S-MPC, respectively.

[1] Hasobe, T. et al, J. Phys. Chem. A
2016, 120, 1867.

[2] Hasobe, T. et al, Angew. Chem. Int. Ed.
2016, 55, 5230.

Figure 1