The Ultrafast Dynamics Of Liquid Two-photon Organic Oligomers

In recent years, the excited relaxation processes of two-photon organic materials has aroused great interest. With the advent of femtosecond laser, it becomes possible to study the ultrafast dynamics of liquid two-photon materials at the femtosecond time scale. From the pump-probe, fluorescence up-conversion and optical Kerr gate techniques, this paper chose one-color pump-probe technique, combined with our laboratory situation. Sample selection is two-photon organic oligomers based on phenothiazine.After liquid two-photon material molecules were excited to the electronic excited state by laser, two kinds of photophysical and photochemical processes occurred. One is intramolecular process, that is, bonds broken, bonding, isomerization, charge transfer (CT), the internal conversion between electronic states (ICR), the internal vibrational energy redistribution between different vibrational modes (IVR) and vibrational relaxation (VR), etc.; the other is intermolecular process, that is, the rapid intermolecular energy transfer, electron transfer (ET), hydrogen bonding, dipole-dipole interaction and the solvent effect of excited state, etc.. Deactivation of excited molecules return to ground state through the processes of radiation transitions, radiation-free transitions, energy transfer, electron transfer and chemical reactions. For the sample selected in this paper, the main processes are the intramolecular conversion of nonradiative transition and the fluorescence emission of radiative transition. 800nm femtosecond laser was used in one-color pump-probe experiment, the basic idea of pump-probe is to get the time delay between beams by space delaying, the specific optical path is shown in Figure 2.Pulse from the laser were splitted into two beams by the beam splitter, the stronger beam as pump beam, and the weaker one as probe beam, the ratio between the light intensity is about 10:1.Stimulate the sample with the stronger pump beam to establish a non-equilibrium state in sample, and probe the changes of transmittance caused by pump beam with the weaker probe beam. If the absorption changes caused by non-equilibrium state are more significant than the ground state, we can get the transmitted light intensity as a function of delay time. In general, relative to the pump beam, the intensity of probe beam is much weaker, or probe beam can also cause the nature change of the sample being measured, because the influence of probe beam on the physical properties of materials is much smaller than the changes induced by pump beam, therefore the transmittance of probe beam changes with the optical path difference can be seen as the evolution processes of particle distribution on non-equilibrium state and the materials physical characteristics changed with time, and then we can get the ultrafast dynamics information of non-equilibrium state back to the equilibrium state. If pump beam arrives to the sample earlier, excites the particles in the ground state to the excited state, so excited state was basically filled (because the energy of ultrashort pulses is very high), until the probe beam arrives, particles in the ground state is no longer excited to excited state, the sample almost does not absorb the probe beam, so this time the probe beam into detector is the most powerful; as the particles on the excited state gradually relaxed to the ground state, the sample also began to absorb the probe beam, so the transmitted light intensity is gradually reduced, until all the particles excited by the pump beam are relaxed back to ground state, the signal collected by the detector is also the smallest. The changes of probe light transmission intensity can reflect the relaxation process of excited particles. Analysis and processing on probe beam signals in different positon of the stepper motor can achieve the relaxation properties of particles on excited state. It can be calculated that the optical delay of one micron is equivalent to three microseconds of time delay, the stepper motor is very easy to reach this standard, so the pump-probe transient absorption time resolution can reach the magnitude of femtosecond.Samples selected are two-photon organic oligomers based on phenothiazine, using fluorene as a bridge to form a structure of donor-largeπbond-donor, or "D-π-D" structure. In which phenothiazine as the electron donor, fluorene neither give electronics, nor receive electronics, its role is to increase the conjugation length of oligomers and increase the solubility of oligomers. The structures are shown in Figure 3 and Figure 4. Array them in different order, two different oligomers are shown in Figure 5 and Figure 6. The main difference of two samples is that the end unit of sampleA (PFPFP) is phenothiazine, the end unit of sample B (FPFPF) is fluorene. And there are three phenothiazine units in sample A, which has one more phenothiazine unit than sample B. Because the different structural characteristics of two samples and the strong electron giving ability of phenothiazine, we can get that the electron giving ability of sample A is stronger than sample B, facilitate to analyze the influence of electron giving ability on the electronic excited relaxation process of two-photon organic oligomers. Excited two samples with 800nm femtosecond laser, their fluorescence spectra are shown in Figure 7 and Figure 8. It can be seen from the figure that the fluorescence emission peak of PFPFP is about 525nm, the fluorescence emission peak of FPFPF is about 428nm, while the femtosecond laser that induced the fluorescence emission is 800nm, which can verify the selected samples PFPFP and FPFPF were both two-photon materials.Mix the two samples into 5×10-5mol/L solutions respectively, toluene is selected as the solvent. Using one-color pump-probe technique, we get the transient absorption spectra of two samples, the two transient absorption spectrum are shown in Figure 9 and Figure 10. The relaxation time of FPFPF is about 200ps, which can be divided into an apparent fast process an an apparent slow process, the fast process is about 50ps, the slow process is about 150ps, the relaxation time of PFPFP is about 150ps, the distinction between its fast and slow process is not obvious. Comparing the two figures, we can see that the decay index of relaxation curve of PFPFP is similar to the fast process in the relaxation curve of FPFPF, while it is larger than the slow process in the relaxation curve of FPFPF. Fast process corresponds to the excited states nonradiative transition process, radiationless transitions occur in the same energy vibration-rotational levels of different electronic states, namely coupling between high vibrational levels of low-level electronic states and low vibrational levels of high-level electronic states. During the transition process, the electron excitation energy changes into the vibrational energy of lower electronic states, because the system's total energy does not change, so it does not emit photon. Two samples are both pentamers with D-π-D structure, and their conjugation length and two-photon absorption cross sections are similar, so the two non-radiative transition process, that is, the process of fast relaxation time in the figure is similar. Relaxation time difference between two samples is about 115ps, mainly in the slow process, which is the process of radiation, corresponding to the process of fluorescence emission of the sample. The electron giving ability of PFPFP is stronger than FPFPF, when affects the fluorescence yield, it also affects the excited relaxation processes, the relaxation time of radiative transition process of the PFPFP, whose electron giving ability is stronger,is less than that of FPFPF, whose electron giving ability is weaker.Then we selected FPFPF with different concentrations, l×l0-5mol/L, 3×10-5mol/Land 5×10-5 mol/L, respectively, the solvent is toluene, too. Using one-color pump-probe technique, we get the transient absorption spectra of FPFPF with different concentrations, as shown in Figure 11,Figure 12 and Figure 13. Fig 11 The transient absorption spectra of (FPFPF)of 1mol/L Fig 12 The transient absorption spectra of (FPFPF)of 3mol/L It can be seen from the figure that when the concentration becomes smaller, the relaxation time of the solution becomes smaller, the excited relaxation time of FPFPF of l×10-5mol/L is 160ps, the excited relaxation time of FPFPF of 3×10-5mol/L is 180ps, the excited relaxation time of FPFPF of 5×10 -5mol/L is 200ps. From this, we can see that the concentration has a significant effect on the excited relaxation of two-photon organic oligomers. Figure 14 shows the concentration is proportional to the relaxation time, when the concentration is larger, the relaxation time is longer, when the concentration is smaller, the relaxation time is shorter.In conclusion, the strength of electron giving ability mainly affects the relaxation time of fluorescence emission process, and the relaxation time of oligomers with stronger electron giving ability is shorter. The influence of concentration on the relaxation time is also obvious, the smaller the concentration of the faster relaxation time, the relationship between the two is proportional. By test and analysis on the influence of electron giving ability and concentration to the two-photon excited relaxation process, we got conclusions, and can provide a theoretical basis to the fabrication of two-photon organic oligomers.