Chapter 1 Introduction
1.2 Motivation
In order to estimate and improve azobenzene derivatives in optical storage and switching performance, it is essential to find out how molecular structure and the molecular environment affect the photoisomerization reaction. Recently, most studies have been carried out in solution, and relatively little researches in solid state. It should be emphasized, however, that most applications require performing in solid state. Therefore, understanding how molecular environment affects the isomerization process will be much useful for the synthesis of the desired materials. With tunable broadband sub-10 fs visible pulses, we can observe the relaxation dynamics and real-time frequencies of molecular vibrations in the ground state and in the excited state. In brief, the motivation of this study is to distinguish the ultrafast dynamics between solution and solid samples by investigating the real-time spectroscopy of molecular system.
Chapter 2
Preparation and characterization of samples
2.1 Preparation of samples
Disperse red 19 (DR19; Dye content 95%), bought from Sigma-Aldrich Co., was served as the azobenzene dye and used after re-crystallization. Trimethylolpropane triglycidyl ether (TMTE; technical grade) and 1, 2-Diaminopropane (DA; 99%) served as monomer and harden agent, respectively, to form epoxy resin, were obtained from Sigma-Aldrich and used as received. All the chemicals were stored in a dry box at room temperature. Figure 2-1 shows the chemical structures of all compounds.
Figure 2-1 Chemical structures of all compounds used in this study.
trimethylolpropane triglycidyl ether (TMTE)
4’-[(N,N-Dihydroxyethyl)amino]
-nitroazobenzene (DR19)
1,2-Diaminopropane (DA)
In order to study the characteristics of azobenzene dye in different environment, liquid state samples and solid polymer film samples were prepared in this research. For liquid state samples, DR19 molecules with concentration of 0.1 wt% were dissolved in TMTE completely through ultrasonic bath at 60℃ for 2 hours. The solution was then percolated through a syringe filter with a hole size of 0.2 μm. After these steps, the percolated solution was ready for the following measurements.
The solid polymer film samples were prepared by the doctor-blade method. First, DR19 molecules were added into DA. The solution was treated with ultrasonic bath for 10 minutes until the DR19 dissolved. Then, TMTE was added to the solution with mole ratio 1:1 between TMTE and DA, and the weight percentages of DR19 was kept at 0.1%. The solution was stirred at 60℃ for 50 minutes until it became homogeneously viscous solution. Further, the doctor-blade approach was used to cast the polymer solution on a flat glass. The samples were then polymerized in a vacuum oven at 45℃ for 24 hours to form the solid polymer film. The average thickness of the solid dry films were measured to be 0.5 mm (for pump-probe measurements) and 0.3 mm (for PL and Raman spectrum measurements), respectively. All liquid and solid samples were stored in sealed vials and dry box, respectively, in the dark state before use.
2.2 Optical properties of DR19 in solution and polymer films
2.2.1 UV-vis absorption spectra
UV-vis absorption spectra were performed on a HITACHI U-3310 spectrometer using tungsten iodide and deuterium lamp as light sources. The absorbance measurements of solution samples carried out in a quartz cell with the size of 1 cm. The absorption spectra of DR19 solutions and polymer films at room temperature are shown in Figure 2-2 (the contribution from the solvent and quartz cell was subtracted), and the spectra is dramatically different from azobenzene. Due to the charge transfer character of the transition, e.g.
intramolecular and intermolecular electron transfer reaction, the dipole moment increases.
Hence, the electronic charges redistribute in the excited state and results in the donor/acceptor substitution of azobenzene with shifting the π-π* band to longer wavelengths [13]. The absorption peaks in spectrum are at 494 nm and 500 nm for the solution and films, respectively, which are assigned to the strongly allowed π-π* transition. The weak n-π*
absorption, forbidden in the strictly planar molecules by symmetry selection rules and insensitive to donor/acceptor substitution, is not explicitly observed in the spectra because it is buried in the strong π-π* band. In order to obtain large overlap between the laser spectrum and π-π* absorption band tail of the samples, the higher concentration samples were used for the pump-probe experiments. On the other hand, the lower concentration samples were used for the measurements of photoluminescence spectra and Raman spectra.
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Figure 2-2 Absorption spectra of DR19 dispersed in solution and a polymer film. The spectrum of NOPA (noncollinear optical parametric amplifier) output (black curve).
2.2.2 Photoluminescence spectra
Photoluminescence spectra were performed on the home-built micro-PL system using He-Cd laser. The 325 nm UV line was used as the excitation light source with the power of 5 mW. The fluorescence emission spectra of DR19 in solution and polymer films at room temperature are shown in Figure 2-3, and the corresponding absorption spectrum (dash line) in each sample is also included to illustrate the Stoke’s shift. The position of the maximum emission wavelength of DR19 in solution and polymer films is at 586 nm and 548 nm, respectively. It is worth to mention that the intensive fluorescence peak around 754 nm in solution case is ascribed to the fluorescence emission of the solvent (see Figure 2-4).
Additionally, the spectrum in the short wavelength range is dominated by the solvent effects.
The main fluorescence band of DR19 in a polymer film shows a good mirror image of the absorption band, and the major component is closely related to π-π* fluorescence since the n-π* fluorescence is extremely insensitive in push-pull derivatives.
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Figure 2-3 Photoluminescence spectrum of DR19 in (a) solution and (b) a polymer film.
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PL intensity (a.u.)
Frequency (cm-1)
PL spectrum
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Wavelength (nm)
Abs. spectrum
Figure 2-4 Photoluminescence spectrum of TMTE.
2.2.3 Raman spectra
Raman spectra were carried out with a micro-Raman spectrometer (Horiba Jobin Yvon, Labram HR 800) with 514 nm excitation. Data was accumulated for 30 seconds in each measurement. Figure 2-5 shows the typical Raman spectra of DR19 in solution and a polymer film. Table 2-1 presents the experimental Raman frequencies of the relevant vibrational modes with the corresponding assignments. Since the molecular structure of DR19 is similar to 4-nitro-4’-dimethylaminobenzene (DA), these vibrational modes are extracted from the literature with Raman spectra of DA and other relative azo compounds [32- 34].
Mode ων /cm-1(in solution / polymer film) Approximate assignmentsa
ν1 1107 / 1103 ν (C-C) + δ (C-C) + ν (C-N)NO
ν2 1140 / 1139 ν (C-N) + δ (C-C) + ν (C-C)
ν3 1200 / 1195 ν (C-C) + ν (C-N) + δ (C-C)
ν4 1343 / 1338 s. ν (NO2) + ν (C-N)NO
ν5 1391 / 1390 ν (N=N) + ν (C-N) + ν (C-C)
ν6 1426 / 1422 ν (C-C) + ν (N-O) + δ (C-N)
ν7 1448 / 1447 ν (C-C) + δ (C-N)
ν8 1593 / 1592 ν (C-C) + δ (C-C)
Table 2-1 Vibrational assignments of DR19 in solution and a polymer film.
a ν, stretch; δ, in-plane bend; s., symmetric
Resonance Raman spectroscopy has been used to study the solvent effects on molecular structure of DA [35]. The results indicate that enhanced charge transfer character in various solvents may cause more structure distortions following photoexcitation to the charge transfer state. In this work, the vibrational frequency shifts between the solution and polymer film samples is attributable to the different components of intra- and inter-molecular charge transfer reactions which depends on the surrounding environment. In both samples, mode ν4
assigned to the NO2 symmetric stretching is the most intense Raman peak, followed by mode ν5 and ν6 corresponding to N=N stretching and C-C stretching. A few more peaks with weak intensity at 819, 860, 928, and 1008 cm-1 observed in the solution case are assigned to C-C in-plane bending, C-H out-of-plane bending, C-C stretching, and C-C in-plane bending, respectively, while they are not clearly observed in the polymer film samples.
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Chapter 3
Experimental method and setup
3.1 The principle of pump-probe technique
Figure 3-1 Scheme of a pump-probe experiment.
Pump-probe measurements of transient transmittance and reflectivity are the most straightforward methods for exploring ultrafast processes on a time scale of femtosecond or picosecond. The typical scheme of a pump-probe experiment is shown in Figure 3-1, both pump and probe beams are synchronized with a period longer than the lifetime of an ultrafast process. The probe pulses are typically attenuated by a factor of 10 with respect to the pump pulses, and then we can assume that it will not perturb the sample. One of the pulse replicas is sent to a fine delay line with a computer controlled stage. After pump pulses perturb the sample, the probe pulses will be affected by the pump-induced changes at delay time τ. The delay time τ is adjusted by the delay line. While the probe pulse interacts with the perturbed
sample, the characteristics of the probe pulse will be modified by the perturbed sample such as the transmittance or reflectivity changes as a function of delay time τ between pump and probe pulses as shown in Figure 3-2.
Figure 3-2 Illustration for the principle of pump-probe technique.
For recording the small changes in transmittance or reflectivity, the pump-probe experiments are commonly carried out with a lock-in amplifier (LIA). Usually the pump beam is modulated by a chopper, and the intensity of probe pulses is detected by the photodiode and then the signal of photodiode is fed to the LIA. The signal in a LIA is mixed with the frequency and phase of a chopper. Then, the output signals of LIA were changed by the ultrafast event n(τ) induced by pump pulses, i.e. a AC signal (ΔI(τ)) is added to a DC signal (I0). By varying the delay time τ between pump and probe pulses, ΔI(τ) will change as a function of delay time. Finally, the AC signal should be divided by the DC component for discriminating fluctuations of the laser. According to the relation,
where and . One can obtain the relative changes in
transmittance ΔT/T with the order of 10-6 by directly measuring the ΔI/I0, and ΔT/T is independent of the intensity of incident light Ii.
In a transmission pump-probe experiment, the data are presented in form of transient transmittance changes ΔT/T, and there are three different types of signals may be detected: (1) ground state photobleaching (PB), (2) stimulated emission from the excited state (SE), and (3) photoinduced absorption from the excited state (PA) [36]. Figure 3-3 shows a scheme of energy levels in a molecular system, indicating the possible signals observed in the pump-probe experiments. The pump pulses decrease the population of electrons in the ground state, then the transmittance increase (ΔT/T > 0) due to the decrease of absorption for probe pulses at photon energy = the difference between the ground state (S0) and the first excited state (S1), i.e. the ground state photobleaching (PB). When a probe photon with the suitable energy passes by the excited molecule system, it can stimulate the excited molecule to emit a photon and return to the ground state; what is called stimulated emission (SE) occurs at probe photon energy equal or lower than the energy of ground state absorption, also giving rise to a transmittance increase (ΔT/T > 0). Moreover, the so-called photoinduced absorption (PA) results in a transmittance decrease (ΔT/T < 0), which is possibly to occurre at any probe photon energy. It depends on the higher energy levels (Sn, n > 1) in the molecular system.
Figure 3-3 Scheme of energy levels in a molecular system with the possible absorption and emission processes.
3.2 Experimental setup
For the pump-probe measurements, a noncollinear optical parametric amplifier (NOPA) [37-39] was used as a light source as shown in Figure 3-4. The pump source of a NOPA system was a regenerative amplifier (Coherent, Legend L-USP-1K-HE) with the central wavelength of 800 nm, the pulse duration of 30 fs, the repetition rate of 5 kHz, and the average output power of 2.3 W. The output light of regenerative amplifier was split into two beams by a beam splitter. The fraction of output light was doubled in frequency by a BBO crystal and used as the pump for NOPA. On the other hand, the remainder light was focused into a sapphire plate to generate white-light continuum as the seed for NOPA. The output pulses from the NOPA were compressed with an optical pulse compressor composed of a diffraction grating telescopic dispersion line, specially designed multiplayer dielectric chirped mirrors, and a computer-controlled flexible mirror. After the compressor, the laser pulses were separated into two beams, one was used as a pump beam, the other was used as a probe beam after passing through an optical delay line. The setup was optimized to obtain pump and probe energies of 8 and 0.8 nJ, respectively. Both of pump and probe pulses were shorter than 10 fs
Figure 3-4 Schematic diagram of the visible sub-10 fs NOPA system and pump-probe experimental setup. PD: photodiode; BS: beam splitter; SP: sapphire plate; SM: spherical mirror; CM: ultrabroad-band chirped mirror; GR: 300-lines/mm ruled diffraction grating; QB:
10-cm quartz block; PS: periscope; VND: variable neutral-density filter.
in duration and covered the spectral range of 514-758 nm. Within the pulses, the spectral phase is almost constant resulting in nearly the Fourier-tansform limited pulses. The pump beam was mechanically chopped at 2.5 kHz in synchronization with the laser pulses at 5 KHz.
The pump and probe beams were set collinear and focused on the samples. Then the probe beam intensity was synchronously detected by a Si photodiode attached to a monochromator (Prinston Instruments, SpectraPro 2300i). The intensity changes of transmitted probe beam induced by the pump were processed through a 128 channel lock-in amplifier referenced to the frequency and phase of chopper in pump beam.
3.3 Experimental detail
In this study, we performed both the shorter time region and the longer time region pump-probe experiments. The delay-time ranges were set between -300 fs and 1400 fs and between -0.9 ps and 14.5 ps, respectively. There are 500 data point in time region and 96 data points in the spectral range between 514 nm and 758 nm in each experimental data, and step width was 3.6 fs in the shorter time region experiment and 31 fs in the longer time region experiment. For these experiments, the full width at half maximum (FWHM) of the pulse was determined by second-harmonic generation frequency-resolved optical gating (SHG-FROG) measurements [40, 41] to be slightly shorter than 10 fs, within which it carried a nearly constant spectral phase. The pulse energies of the pump and probe were about 8 nJ and 0.8 nJ, respectively.
During the measurements, a liquid DR19 sample was taken in a 1-mm-thick quartz flow cell (Starna, Type : 48-Q-1) , and rapidly recirculated with a peristaltic pump through a 4 mm i.d. Tygon tube (R3603) to ensure the fresh sample after each laser shot. The configuration of circulation system was shown in Figure 3-5 (a). Large volumes of the liquid samples with 60 ml were chosen to avoid accumulation of photo-generated cis isomer. For the experiment of polymer films, the samples were placed on the holder which was connected with a vertical stage as shown in Figure 3-5 (b). The measured position of a film was varied after every measurement to avoid the damage on the surface caused by laser pulses. In case of the polymer films, the effect of accumulation may be considerable (The cis isomer thermally reforms the trans isomer with a time constant of several seconds or milliseconds) [42]. All the experiments were performed at room temperature.
Figure 3-5 The photograph of the measurements for (a) a liquid sample and (b) a polymer film.
Chapter 4
Results and discussion
4.1 Femtosecond absorption spectroscopy in the shorter time region (from -300 fs to 1400 fs)
4.1.1 2D absorbance changes
The experimental data were represented with a two-dimensional (2D) plot of the transmittance changes (ΔT/T) as a function of both the probe frequency and delay time.
According to the calculating formula , one can obtain the absorbance changes. Figure 4-1 shows the two-dimensional view of absorbance change spectrum, which has 500 data points in time region between -300 fs and 1400 fs with the interval of 3.6 fs and 96 data points in spectral region between 514 nm and 758 nm. The contour line of ΔA = 0 is shown as black line. The photo induced absorbance changes ΔA for solution samples is negative in a spectral range of 514 nm to 578 nm and positive beyond this spectral range.
Additionaly, the negative ΔA for polymer films is in a spectral range of 514 nm to 591 nm.
The negative ΔA is caused by the absorption bleaching, which means the population of electrons depleted when DR19 was photoexcited by pump pulses. The positive ΔA is a result of the induced absorption of the excited state. The intense signal of ΔA near zero-delay time is attributed to a coherent spike, and the interference due to the superimposition of pump pulses and probe pulses is various at different wavelengths. Figure 4-2 and 4-3 show the ΔA as a function of delay time of DR19 in solution and a polymer film, respectively, for probe wavelengths of 558 nm, 583 nm, 609 nm, 635 nm, 660 nm, 686 nm, and 712 nm. The
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Probe frequency (cm-1)
(b)
Figure 4-1 2D transient absorbance change spectrum of DR19 in (a) solution and (b) a polymer film.
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Figure 4-2 Transient absorbance changes of DR19 in solution at various probe wavelengths.
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-0.01
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-0.002
Figure 4-3 Transient absorbance changes of DR19 in a polymer film at various probe wavelengths.
negative photoinduced absorbance changes found at probe wavelength 558 nm are attributed to the bleaching of the ground state population. The absorbance changes is close to zero at 583 nm, where is neither bleaching nor photoinduced absorption. At the other probed wavelengths, the positive sign of ΔA is caused by the induced absorption of the excited state.
The photoinduced absorbance changes exist up to 1400 fs due to the lifetime longer than the measured time region. These signals are composed of a slow relaxation dynamics and a periodical oscillation. The former is assigned to the electronic relaxation, and the latter is attributed to molecular vibration. The oscillatory signals exist up to 1400 fs after excitation, and they seem to be composed of two dominate frequencies. The Fourier transform of the periodical oscillation will reveal information about the vibrational amplitude and phase.
4.1.2 Lifetime fitting
The time-dependent absorbance changes is analyzed by the double-exponential decay fitting. Figure 4-4 shows the lifetime at all probed wavelengths. In these results with short scanning range in delay time, the longer lifetime cannot be fitted well and it will be discussed in subsection 4.2.2. However, the fast decay with a time constant of ~100 fs was observed in both samples. It has been attributed to a large amplitude motion of the excited molecule out of the Franck-Condon region [43]. Additionally, this attribution of the rapid internal conversion from the S2 state to the S1 state is also proposed in previous works by other groups [44, 45].
These experimental results indicate that the fast dynamic behavior in solution samples is similar to that in the polymer films.
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t1~103 fs
(a)
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(b)
Figure 4-4 Lifetime of DR19 in (a) solution and (b) a polymer film.
4.1.3 Fast Fourier transform of the absorbance changes
With a multi-channel lock-in amplifier, the time-dependent absorbance changes were taken over the spectral range from 514 nm to 578 nm. The Fourier power spectra of oscillating components were obtained from the real-time trace by fast Fourier transform (integrated over the time range from 100 fs to 1400 fs since there are interference signals caused by the probe pulses and scattered pump pulses in the delay time earlier than 100 fs).
The two-dimension plots of FFT power as a function of the molecular vibrational frequencies and probe wavelengths are shown in Figure 4-5. Several vibrational modes appeare in FFT power spectrum, especially two prominent modes around 1105 cm-1 and 1320 cm-1. These modes may be expressed as the modes mixing due to the small difference in frequency between the adjacent modes, assigned to the stretching of C-C and symmetric stretching of NO2, respectively. Both signals are also observed in the stationary Raman spectrum, e.g.
Figure 2-5.
Figure 4-6 and 4-7 show the Fourier amplitude spectrum of real-time traces for probe wavelengths of 555 nm, 573 nm, 591 nm, 609 nm, 627 nm, 645 nm, 663 nm, and 681 nm. The intense peaks in the spectrum of a solution sample located around 470 cm-1, 1105 cm-1, 1180 cm-1, 1320 cm-1, and 1560 cm-1 which are also observed in a polymer film (shown in Fig. 4-7) except for the 470 cm-1 attributed to the solvent molecule of TMTE. In the FFT amplitude spectra, the dominant modes at 1105 cm-1 and 1320 cm-1 is strong probe wavelength dependence, and the FFT amplitude of peak in 1105 cm-1 is drastically reduced at 591 nm in
Figure 4-6 and 4-7 show the Fourier amplitude spectrum of real-time traces for probe wavelengths of 555 nm, 573 nm, 591 nm, 609 nm, 627 nm, 645 nm, 663 nm, and 681 nm. The intense peaks in the spectrum of a solution sample located around 470 cm-1, 1105 cm-1, 1180 cm-1, 1320 cm-1, and 1560 cm-1 which are also observed in a polymer film (shown in Fig. 4-7) except for the 470 cm-1 attributed to the solvent molecule of TMTE. In the FFT amplitude spectra, the dominant modes at 1105 cm-1 and 1320 cm-1 is strong probe wavelength dependence, and the FFT amplitude of peak in 1105 cm-1 is drastically reduced at 591 nm in