Flavin/TCNQ 유기 LB 막에서 전자전달 효율 비교

조 경상, 최 정우, 이 원홍, 송 남웅*, 김 동호*

서강대학교 화학공학과, 한국 표준과학연구원 분광그룹*

Charge Separation Rate in Flavin/TCNQ Hetero LB Film

K.S. Cho, J.-W. Choi, W.H. Lee, N.W. Song*, D. Kim*

Department of Chemical Engineering, Sogang University

Spectroscopy Laboratory, Korea Research Institute of Standards and Science*

INTRODUCTION

Various artificial molecular devices have been fabricated by mimicking the electron transport function of biological photosynthesis [1]. One of the characteristics of photosynthesis applicable to the artificial molecular electronic device is the photoswitching effect induced by exciting electrons under light illumination, which initiates electron transfer and enables photodiode effect by transferring electrons unidirectionally from electron donors to acceptors due to their redox potential differences. To investigate the photoinduced electron transfer in hetero LB film, the measurement of time-resolved fluorescence decay of LB films has been done [2]. But, in the hetero LB film, the effects of distance of functional groups and redox potential difference between donor and acceptor on the photoinduced electron transfer rate have not been investigated using a time-resolved fluorescence measurement.

In this work, three different types of flavin-TCNQ hetero LB films such as the mixed layer, head (hydrophilic functional group)-to-head interfaced structure and tail (hydrophilic alkyl chain)-to-tail interfaced structure were fabricated. Flavin and TCNQ were used as electron donor (D) and acceptor (A), respectively. Fluorescence decay profiles of various structures of flavin-TCNQ hetero LB films were measured using a time correlated single photon counting (TCSPC) technique with a femtosecond Ti:Sapphire laser. Based on the fluorescence decay curves, the effects of distance of functional groups and redox potential difference between donor and acceptor on the photoinduced electron transfer rate were investigated.

EXPERIMENTAL DETAILS

1. Materials

Arachidic acid was purchased from Sigma Chemical Co. (St. Louis, USA). Three kinds of functional materials were used. 7,8-dimethyl-10-dodecyl isoalloxazine (flavin), N-docosilquinolinium TCNQ and N-Allyl-N'-[3-propylamido-N",N"-di(n-octadecyl)]-4, 4'-bipyridium dibromide (viologen) were used as donor and acceptors, respectively.

2. LB film deposition

The deposition of LB films was carried out using a circular Langmuir trough (Type 2022, Nima Tech., England). Three different structures of donor/acceptor hetero LB films (mixed layer, head/head interface, tail/tail interface) were fabricated. In the fabrication of mixed layers, the concentration of TCNQ was changed with the molar ratio of flavin to TCNQ such as 1:0.1, 1:0.2, 1:0.5 and 1:1.

3. Time-resolved fluorescence measurement

Fluorescence decay profiles were recorded by using a TCSPC technique with a femtosecond Ti:Sapphire laser pulse excitation. The laser pulse width is 150 fs, and the average power was 500 mW at 82 MHz operation. To excite the flavin of hetero LB films, the laser pulses were frequency doubled by using a □-BBO (□-barium borated) crystal. All the standard electronics used for the TCSPC system were purchased from EG & G Ortec. (TN, USA) The observed instrumental response function (IRF) for the excitation pulse was about 52 ps (FWHM) at 420 nm. This method allowed us to measure the fluorescence decay with a time resolution of ca. 10 ps after deconvolution.

RESULTS AND DISCUSSION

1. Effect of number of acceptor layer

The fluorescence decay profiles of flavin homo LB films (11-layer) and flavin-TCNQ hetero LB films with the various number of TCNQ layers are shown in Fig. 1. In flavin-TCNQ hetero LB films, the fluorescence decay of excited flavin was faster than that in flavin homo LB film. In the hetero LB films, the charge separation from flavin to TCNQ occurs by the irradiation of light. ks is the fluorescence decay rate of excited flavin in homo LB film and ka is the electron transport rate from excited flavin to TCNQ in hetero LB film. The decay rate (kt) of excited flavin in hetero LB films can be written as

kt =ks + ka                                                (1)

The relationship between electron transport rate and fluorescence lifetime (□t) is given by

□t =1/(ks + ka)                                              (2)

where □t is the fluorescence lifetime of flavin in hetero LB films. Thus the charge separation rate ka can be described in terms of the following relation.

ka =/ □t - ks = 1/ □t - 1/□s                                      (3)

where □s is the fluorescence lifetime of flavin in homo LB film.

Based on the above equation, the charge separation rate can be obtained from the fluorescence lifetimes of flavin in homo and hetero LB films. Since the maximum wavelength of absorption of TCNQ does not overlap with the emission wavelength of flavin, Foster-energy transfer process can be neglected. Thus the interaction of between excited flavin and TCNQ could be considered to be a charge separation process. The fluorescence decay profiles of flavin were well described as the triple exponential components. From these results, it can be suggested that there exist various energy states due to different conformation and geometry of flavin monolayer LB films [2].

In flavin (2 layer) /TCNQ (3 layer) and flavin (2 layer) /TCNQ (7 layer) hetero LB films, the fluorescence decay of flavin becomes faster than that in flavin homo LB film. But the fluorescence lifetimes of the above two hetero LB films show little difference from each other. This result can be explained in view point of the interface structure. Because the charge separation occurs at the interface between donor and acceptor, the hetero LB films with similar interfaced structure exhibits the analogous fluorescence decay curves with little difference. The effect of number of acceptor layer on charge transfer rate could be investigated effectively by measuring the transient photocurrent.

2. Effect of the distance between functional groups on electron transport rate

Fig. 2 shows the fluorescence decay curves of flavin in flavin-TCNQ head/head interfaced and tail/tail interfaced hetero LB films. In comparison with tail/tail interfaced LB films, head/head interfaced hetero LB films exhibit faster fluorescence decay. This phenomenon is attributable to the distance variation between functional groups of donor and acceptor. The distance is about 47 □ in the tail/tail interfaced LB film and 6 □ in the head/head interfaced LB film, respectively. The results suggested that the distance of functional groups between donor and acceptor affects the photoinduced electron transfer rate.

3. Mixed layer of flavin/TCNQ LB film

The fluorescence decay profiles of mixed layer of flavin/TCNQ hetero LB films with respect to the various flavin/TCNQ molar ratio were shown in Fig. 3. In this case, the electron transport from donor to acceptor is not in the perpendicular direction of the adjacent layer but in the lateral direction. As the mole percentage of TCNQ is increased, the fluorescence decay becomes faster. This phenomena can be ascribed to the fact that the average distance between flavin and TCNQ becomes smaller and the charge separation rate increases as the TCNQ mol percent is increased. The mixed layer of flavin/arachidic acid (AA) hetero LB film with 1:1 molar ratio did not show any fluorescence quenching effect. This result suggested that the fluorescence quenching of the mixed layer of LB films is not by the difference in the LB film geometry or conformation but mainly due to the redox potential difference.

3.4. Effect of redox potential difference between donor and acceptor on the electron transport rate

Fig. 4 shows that the fluorescence decay profiles of flavin/viologen hetero LB films. In the case of flavin/viologen hetero LB films, the fluorescence decay of flavin was faster than that of flavin homo LB films but slower than that of flavin/TCNQ hetero LB films. This is due to the redox potential difference between flavin/viologen and flavin/TCNQ. In flavin/viologen hetero LB films, the redox potential difference between donor and acceptor is smaller than that in flavin/TCNQ hetero LB films. This leads to the slower electron transfer from flavin to viologen in flavin/viologen hetero LB films than that in flavin/TCNQ hetero LB films. These results suggested that the redox potential difference between donor and acceptor affects the photoinduced electron transfer rate.

REFERENCES

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[2]. K. Akiyama, S. Nishikawa, S. Uyeyama and S. Isoda, Jpn. J. Appl. Phys., 34, 3942(1995).