Optical Properties of Chiral Azo-Schiff Base Mn(II) and Zn(II) Complexes with Silver Nanoparticles

Herein we have originally designed chiral azo-salen Mn(II) and Zn(II) complexes for interacting silver nanoparticles (AgNPs) exhibiting localized surface plasmon resonance (LSPR). Understanding excited state and reaction intermediate during light irradiation to return to ground state may be impor-tant for such composite systems. Therefore, we investigated such optical properties for systems using time-resolved luminescence and transient absorption measurements. DMSO solutions of the four newly prepared and characterized complexes (MMn, MZn, CMn, and CZn) and ethanol solutions of the composite materials of each complex with AgNPs were served for optical measurements. The time-correlated single photon counting (TCSPC), the streak camera which is much shorter period of time than TCSPC and transient absorption measurement, was performed for the eight samples. The fluorescence lifetime of the sole complexes and the composite materials with AgNPs was derived from curve-fitting analysis of luminescence decay curves of TCSPC. Lifetime of the composite systems with AgNPs was longer than that of the corresponding sole metal complexes for three cases. It was re-vealed that composite systems may go through three reaction intermediates during relaxation from excited state to ground state.


Introduction
In recent years, we have carried out photochemical studies on chiral azo-Schiff base metal complexes as hybrid materials, for example, focusing on (with titanium oxide) cis-trans photoisomerization of azobenzene moiety [1], non-linear optical effect of chiral molecules [2], (with gold nanoparticles) induced circular dichroism (CD) as supramolecular chiroptical measurements of chiral molecules [3] [4] and polarized light control of molecular orientation on the surface resulted from induced CD due to Weigert effect of azobenzene moiety [5]. In this way, we attempted new mechanisms to improve the performance of DSSC in future. In this study, herein, we have prepared hybrid materials of new chiral azo-Schiff base Mn(II) and Zn(II) complexes and AgNPs (with or without titanium oxide), and investigated their photophysical and chiroptical properties.
Especially, time-resolving optical measurements were employed in order to understand excited state and reaction intermediates during light irradiation to return to the ground state. Some complexes with AgNPs exhibited longer fluorescence lifetime than that sole complexes without AgNPs.

Synthesis of Metal Complexes
Preparation schemes of the four complexes were shown in Figure 1.

Synthesis of MZn
Pure MZn (as potassium salt) was obtained in the similar procedure to MMn using zinc(II) acetate dihydrate instead of manganese(II) chloride tetrahydrate.

Physical Measurements
Elemental analyses (C, H and N) were performed using a Perkin-Elmer 2400 II CHNS/O analyzer at the Tokyo University of Science and using an Elementer vario MICRO cube at TORAY Research Center. Infrared (IR) spectra were recorded as KBr pellets on a JASCO FT-IR 4200 plus spectrophotometer in the range 4000 -400 cm −1 at 298 K. Electronic (UV-vis) spectra were obtained on a JASCO V-650 UV-vis-NIR spectrophotometer in the range 800 -200 nm at 298 K. Circular dichroism (CD) spectra were obtained on a JASCO J-820 spectropolarimeter in the range 800 -250 nm at 298 K. 1 H-NMR spectra were recorded on a JEOL JMN-300 spectrometer (300 MHz). Electrochemical (cyclic voltammetry, CV) measurement were carried out on a BAS SEC2000-UV/VIS and ALS2323 system with Ag/AgCl electrodes range of −2.0 -1.5 V vs. Ag/Ag + . Raman spectroscopy was recorded using JASCO NRS-3200. I -V measurement was obtained Yamashita Denso YSS-80 at 298 K. XPS measurement was carried out using Mg Kα source (10 kV, 25 mA) on a SHIMADZU ESCA3400. Powder X-ray diffraction patterns were collected at 298 K with Pohang Light Source II 2D Supramolecular Crystallography Beamline (PLSII-2D-SMC). Nanosecond time-resolved TA measurements were carried out with a Ti: sapphire laser (LibraHE, Coherent). The second harmonic (400 nm) of the laser was used for excitation. The intensity of the laser pulse (70 mW) was measured with an air-cooled thermopile sensor (PM2, Coherent). A xenon lamp (C4251, Hamamatsu) was used as the probe light source. The light transmitted through the sample was passed through monochromator (CM110, Spectral Products) and detected with a Si photodiode (DET10A/M, Thorlabs). The photocurrent from the detector was amplified with an amplifier (DHPVA-201, Femto), and signals were processed with a digital oscilloscope (WaveRunner8254, Teledyne LeCroy) and analyzed with a computer. The DC offset of the photocurrent from the detector was subtracted by using the AC-coupled mode of the amplifier. TCSPC measurements were carried out using FluoroCube (Horiba). For the excitation light source, a pulsed laser diode (Horiba, NanoLED; wavelength of 410 nm, pulse width of 200 ps, and repetition rate of 1 MHz) was used. For streak camera measurements, the second harmonic (400 nm) light of the Ti: sapphire laser (Solstice, Spectra Physics; pulse width of

Computational Methods
Calculations of all complexes were performed using the Gaussian 09W software Revision D.02 (Gaussian, Inc.) [8]. Starting models (four-coordinated square planar) of structural optimization is based on preliminary results of Rietveld analysis and TG-DTA. The gas phase geometry optimizations were carried out by using TD-DFT with B3LYP functional. The vertical excitation energy was calculated with the Lanl2dz for each complexes, with the 6 -31 + G (d) basis set for H, C, N and O method based on the singlet ground state geometry.    Figure 4, fluorescence spectra of carboxyl complexes (CMn and CZn) exhibited more clear quenching than that of methoxy complexes (MMn and MZn).

Nanosecond Time-Resolved Spectroscopy of Fluorescence Decay
Time-correlated single photon counting (TCSPC) were carried out to investigate the photoexcited state and the reaction intermediate state when each complex and its composite system (8 samples in total) were illuminated [9]. The fluorescence decay plots ( Figure 5) were analyzed to obtain the fitting parameters (Table 1) to the exponential functions (1) and (2) using an ORIGIN software. It was found that this task could be achieved by compositing silver nanoparticles. The relaxation constants are faster in the order τ 1 > τ 2 > τ 3 . A 1 , A 2 , and A 3 are the component ratios of the respective relaxation components. The average fluorescence lifetime was calculated by the following equation.
Only for MZn + AgNP composite, τ avg was longer than sole complex (MZn), which indicated that the average fluorescence lifetime of other complexes is     shortened by the addition of AgNPs ascribed to effective energy transfer through carboxyl groups. Due to Mn(II) ion unsuitable for excitation, MMn + AgNP composite showed the shortest τ avg of 1.55 ns. In this way, the results observed were caused by the energy coupling between the plasmon resonance and the emission of the fluorescent molecules, which shortens the electronic recombination time from the excited state to the ground state and accelerates the radiation relaxation. A previous study [9] reported that the presence of metal nanoparticles in the vicinity of fluorescent molecules shortens the fluorescence lifetime of the fluorescent molecules. This suggests that AgNP could not exist in the vicinity of the composite due to aggregation of AgNPs in MZn + AgNP. While CZn has many carboxyl groups to adsorb on AgNPs. Therefore, when AgNP were combined with the complex, the carboxyl groups of the complex were able to bind to AgNPs and maintain a close proximity.

Application for DSSC Dyes
Discussion in 3.2 suggests that MZn + AgNP may be suitable for sensitizer of DSSC among the four complexes, and cells [1] assembled as Figure 6, in which adsorption of MZn to AgNP was monitored with Raman peaks at 1417 and 1042 cm −1 . The DFT calculation shows that the positional relationship of the orbitals before and after excitation is in an advantageous position with respect to the adsorption substituents. Redox potentials from CV suggested that the four complex may be appropriate for sensitizer electrochemically (LUMOs are higher than −0.540 V for TiO 2 , HOMOs are lower than 0.460 V for 3 I I − − ). Photoelectric conversion efficiency could be evaluated to be 0.433%, 0.0377%, and 0.342% for AgNP, MZn, and MZn + AgNP, respectively based on I -V curves. In contrast, employing MZn shorten t ave , the corresponding values are 0.433%, 0.0837%, and 0.655% for AgNP, CZn, and CZn + AgNP, respectively. The results indicated that other factors associated with cells may make it inefficient besides excited state of MZn + AgNP.

Conclusion
In this work, we evaluated the physical properties of chiral Schiff azo-salen Mn(II) and Zn(II) complexes and their composites with AgNPs in order to search useful dyes for DSSC. UV-vis spectra of the four complexes with AgNP exhibited longer wavelength shift than the complex solely. Finally, TCSPC mea-Journal of Materials Science and Chemical Engineering surements were carried out to compare the photo-excited states and reaction intermediate states of the complexes alone and the composite systems with AgNPs.
As a result, it was found that the average fluorescence lifetime became shorter by addition of AgNPs except for MZn. This may be due to the energy coupling between the plasmon resonance and the emission of the fluorescent molecules, which reduces the electronic recombination time from the excited state to the ground state and accelerates the radiation relaxation. As described above, the changes in photo-responsivity due to the differences in the substituents and central metal of the complexes were confirmed by several spectroscopic measurements.