Diamond-Based Molecular Platform for Photoelectrochemistry Yu Lin Zhong, † Anupam Midya, † Zhaoyue Ng, † Zhi-Kuan Chen, ‡ Michael Daenen, § Milos Nesladek, §,| and Kian Ping Loh* ,† Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543, Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, IMOMEC, Hasselt UniVersity, Wetenschapspark 1, B 3590 Diepenbeek, Belgium, and Institute of Physics, Czech Academy of Sciences, 18200 Prague, The Czech Republic Received July 30, 2008; E-mail: chmlohkp@nus.edu.sg Photoelectrochemistry is one of the major field of studies for molecular-based platforms due to the ease of tailoring highly efficient photodevices via organic chemistry. 1 There are numerous reports on architectured donor-acceptor molecules or polymers on indium tin oxide (ITO) targeting photovoltaic applications, 2 pho- toswitching applications, and electrogenerated chemiluminescence (ECL). 4 In terms of the performance criteria of these molecular- based platforms, generic issues such as efficient charge transfer between the electrode and organic interface as well as photostability of the platforms are fundamental to all applications. Interest in boron-doped diamond (BDD) thin films as a signal transduction platform for sensing applications 5 stems from its excellent properties such as chemical robustness, wide electro- chemical potential window, biocompatibility, and optical transpar- ency. Due to its intrinsic high carrier mobilities and wide band gap, diamond electronic devices can potentially deliver outstanding performance in power diodes and high-frequency field effect transistors. In the past few years, there has been tremendous progress in the chemical vapor deposition (CVD) of nanocrystalline dia- monds (NCDs) prepared on large area glass 6 and polymer substrate, 7 which opens up many new technological possibilities. NCDs can be boron-doped to metallic levels, and specific resistivity as low as 0.005 Ω cm can be achieved. The B-doped NCD layers deposited on quartz and glass possess very high optical transparency in the 300 to 1000 nm spectral range. 6 In addition, the electron affinity of diamond can be tuned by controlling its surface termination 8 and organic molecules can be attached 5 via chemically stable C-C bonding, which are advantageous for efficient charge transfer and robust applications. Previous work on the photoconductive proper- ties of BDD focused mainly on the charge generation mechanism from the bare substrate. 9,10 Here we report the spectrophotochemical characteristics of a light harvesting molecular platform constructed on BDDs using a donor-acceptor molecular wire. An optically transparent diamond electrode was fabricated by the CVD of a thin layer of BDD film on glass substrate (160 nm thick, boron concentration 7 × 10 20 cm -3 ). The construction of the diamond-based molecular platform begins with the functionalization of the BDD thin film with arylboronic ester or arylhalide which acts as a synthon for subsequent Suzuki coupling to target light harvesting molecules. We have successfully coupled two donor- acceptor molecules: bithiophene-C 60 (2TC 60 ) and bithiophene- dicyano (2T(CN) 2 ) as summarized in Figure 1. Photoelectrochem- ical measurements were performed in 0.1 M Na 2 SO 4 solution containing 5 mM methyl viologen (MV 2+ ) as the electron carrier, using a three electrode system: the diamond-based molecular platform as working electrode, a platinum mesh counter electrode, and a Ag/AgCl reference electrode, under 1 sun irradiation (100 mW/cm 2 , AM 1.5G). From the action spectra in Figure 1a, the arylboronic ester functionalized BDD shows a small photocurrent peak at 310 nm due to the arylboronic ester groups; in contrast the H-terminated BDD shows minimal photocurrent. After coupling to 2TC 60 , a large photocurrent peak centered at 360 nm is observed. The photocurrent peak is shifted bathochromically to 440 nm for 2T(CN) 2 owing to the extended conjugation length of the dicyano group which also acts as the electron acceptor. This photocurrent peak correlates well with the UV-vis absorption spectrum of 2(dicyanovinyl)-5-iodobithiophene (I-2T(CN) 2 ) as shown in Figure S2. From Figure 1b, the photocurrent of 2TC 60 and 2T(CN) 2 on BDD under board solar spectrum (AM 1.5G) illumination yields similar values despite a dissimilar action spectrum. On application of a negative bias (-0.2 V vs Ag/AgCl), an ∼80% increase in photocurrent could be produced compared to zero potential, although there was a slight accompanied increase in the dark current due to the reduction of MV 2+ (redox peak at -0.6 V vs Ag/AgCl). † National University of Singapore. ‡ Institute of Materials Research and Engineering. § Hasselt University. | Czech Academy of Sciences. Figure 1. Surface functionalization and Suzuki coupling scheme of: bithiophene-C 60 (2TC 60 ) and bithiophene-dicyano (2T(CN) 2 ) on boron- doped diamond (BDD) thin film. (a) Action spectrum of (i) H-terminated BDD, (ii) arylboronic ester functionalized BDD, (iii) 2T(CN) 2 , and (iv) 2TC 60 molecular platform on BDD at 0 V (vs Ag/AgCl). (b) Photocurrent response at 0 V (vs Ag/AgCl) of 2TC 60 (red) and 2T(CN) 2 (blue, at -0.2V: orange) and 4T(CN) 2 (blue) under 1 sun (100 mW/cm 2 , AM 1.5G) illumination. Published on Web 12/02/2008 10.1021/ja805977f CCC: $40.75  2008 American Chemical Society 17218 9 J. AM. CHEM. SOC. 2008, 130, 17218–17219