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Excited-state relaxation processes of three newly synthesized multi-branched alkyl-triphenylamine end-capped triazines
Chemical Physics Letters 736 (2019) 136800
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Excited-state relaxation processes of three newly synthesized multibranched alkyl-triphenylamine end-capped triazines
T
Xiang Lia, Zhiquan Wanga, Yuting Gaob, Zhengjun Shanga, Minghao Nia, Jianli Huab, Bo Lia, , Ye Chena ⁎
a b
Key Laboratory of Polar Materials and Devices (MOE), Department of Electronics, East China Normal University, Shanghai 200241, China Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, Shanghai 200237, China
HIGHLIGHTS
state and TICT state affect the relaxation rate of multi-branched polymers. • ICT effect influences the relaxation process of multi-branched polymers. • Solvent • Donor branch numbers cause the change of evolution process. ARTICLE INFO
ABSTRACT
Keywords: Intramolecular charge transfer (ICT) Fluorescence dynamic Multi-branched structure Solvent effect
The excited-state relaxation processes of three newly synthesized multi-branched alkyl-triphenylamine end-capped triazines ATT-(1–3) are characterized in different solvents by steady-state and time-resolved spectroscopy. In toluene, a weakly polar solvent, the emission originates from the intramolecular charge transfer (ICT) state; in tetrahydrofuran (THF), a strongly polar solvent, the existence of a nonradiative channel from ICT to twisted intramolecular charge transfer (TICT) accelerates the relaxation rate of the ICT state. The rate of the evolution process of ATT-(1–3) increases with increasing number of donor branches, which could ascribed to enhancements in the electron donor and acceptor abilities of the triazines.
1. Introduction Over the last two decades, organic materials have shown potential applications in many optoelectronic devices, such as field-effect transistors [1–4], photodetectors [5–7], and light-emitting diodes [8–12]. Multi-branched conjugated organic molecules with a geometry similar to that of donor-π-acceptor (D-π-A) compounds have recently attracted great research attention. Indeed, many multi-branched triazines have been synthesized for their excellent electron-donating and -transport capabilities and potential optical applications [13,14]. The notable optical properties of these triazines are distinctly related to their intramolecular charge transfer (ICT) state [15,16]. ICT occurs between electron acceptor and donor units. The polarity of a material changes through the ICT process, which can be influenced by solvent polarity. Therefore, the fluorescence dynamics of a material can be influenced by solvent polarity. Research on the solvent effect can help improve the understanding of the mechanism of the ICT state.
⁎
Considerable research on the fluorescence emission of multi-branched materials has been accomplished. Oberle and co-workers reported that pump-probe and time-resolved coherent anti-Stokes Raman spectroscopy could be used to determine the formation of the ICT state in polar solvents, which is associated with structural changes in the solute molecules [17]. Chi and co-workers used time-resolved methods to reveal that increasing the number of electron acceptors accelerates the fluorescence relaxation of D-π-A materials [18]. Steady-state and time-resolved photoluminescence (PL) spectroscopy are powerful tools for characterizing optical properties. In this study, the excited-state properties and fluorescence dynamics of three newly synthesized triazine dyes dissolved in tetrahydrofuran (THF) and toluene (Tol) are examined. Solvent polarity can influence the emission spectra of the samples. Moreover, solvent polarity and branch number of the samples affect their fluorescence dynamics. Fluorescence lifetimes decrease with increasing solvent polarity, while fluorescence evolution processes become faster with increasing branch number.
Corresponding author. E-mail addresses: [email protected] (B. Li), [email protected] (Y. Chen).
https://doi.org/10.1016/j.cplett.2019.136800 Received 20 June 2019; Received in revised form 8 September 2019; Accepted 26 September 2019 Available online 27 September 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 736 (2019) 136800
X. Li, et al.
2. Materials and methods
employed to study the excited-state relaxation process and dynamics. Here, the sample was pumped with 400 nm pulses and probed with 800 nm pulses.
The synthesis of the three new dyes was carried out as described in our previous work [13]. The structure of these dyes, as presented in Fig. 1, includes a triazine core acting as an electron acceptor and alkyltriphenylamine acting as an electron donor. In the present research, the samples featured different numbers of branches and were dissolved in two solvents with different polarities, namely, THF and Tol. Steadystate absorption spectra were measured by a UV–visible spectrophotometer (TU-1901, Purkinje General), while fluorescence spectra were recorded by a spectrophotometer (iHR550, HORIBA). The laser source was a mode-locked Ti:sapphire femtosecond laser system (Coherent, 80 fs pulse duration, 250 kHz repetition rate, centered at 800 nm). Frequency-doubled femtosecond pulses were used to excite the samples. The PL evolution curves of the three samples in THF and Tol were measured by time-correlated single-photon counting with an avalanche photodiode. The time constant of the instrument response function was about 40 ps. The two-color pump-probe technique was
3. Results and discussion The impacts of solvent polarity and branch number on the steadystate spectra and excited state dynamics of the dyes are investigated. Here, Δf is selected as the measure of polarity and calculated as follows [19]:
f=
1 2 +1
n2 1 2n2 + 1
where represents the static dielectric constant and n represents the optical refractive index of the solvent. The Δf of Tol and THF are 0.013 and 0.207, respectively.
Fig. 1. Structure of ATT-(1–3).
Fig. 2. (a) The absorption spectrum and fluorescence spectrum of three samples in Tol. (b) The absorption and fluorescence spectrum of ATT-3 in Tol and THF. 2
Chemical Physics Letters 736 (2019) 136800
X. Li, et al.
The evolution curves of ATT-(1–3) in Tol show an initial growth with a rise time of about 100 ps and then monoexponential decay (approximately 3 ns). The evolution curves of ATT-(1–3) in THF are much faster than those in Tol, thereby demonstrating dual-exponential decay.
Table 1 Steady-state absorption and fluorescence peak positions in solvents with different polarity (Δf). Solvent Sample
ATT-1 ATT-2 ATT-3
Tol(Δf = 0.013)
THF(Δf = 0.207)
λab (nm)
λpl (nm)
λab (nm)
λpl (nm)
443 449 455
601 603 604
439 447 452
685 688 692
3.2.1. Dynamics in Tol The evolution process of ATT-(1–3) in Tol consists of two processes. The rise time τr is related to the accumulating population of emission states, which, in turn, are attributed to the formation of a stabilized ICT state. The rise time τr is approximately 100 ps; the fluorescence observed in this case could be inferred to mainly arise from the ICT state. The formation time of the LE state is less than 10 ps, which is much shorter than the rise time of the three samples in Tol. After excitation of the solute molecules, the electrons transfer from donor branch to acceptor unit, and the dipole moment of the solute molecules changes. Surrounding solvent molecules reorganize to stabilize the excited solute molecules. This process forms a stable ICT state. τ3 refers to the relaxation time from the ICT state to the ground state and decreases from around 3 ns to 2.7 ns with increasing number of branches.
3.1. Steady absorption and fluorescence spectrum The steady-state absorption and fluorescence spectra of ATT-(1–3) in Tol at room temperature are shown in Fig. 2(a). The absorption and emission curves of all three samples in THF display the same trend; thus, only the results of ATT-3 are selected to describe this trend. Specific data on the steady-state spectra obtained are listed in Table 1. Fig. 2(a) shows that the absorption and fluorescence spectra of the three samples in Tol are comparable. Red-shifting of the absorption and fluorescence spectra increases as the number of branches increases from ATT-1 to ATT-3. An apparent Stokes shift (red-shifting) in the fluorescence spectra compared with the steady-state absorption spectra in Tol could also be observed. The absorption spectra of ATT-(1–3) in Tol and THF (around 450 nm) are nearly identical, indicating small differences in dipole moment between the ground and excited states. However, an obvious red-shift in emission spectra could be observed. Specifically, the emission peak of ATT-(1–3) shifted from 601, 603, and 604 nm in Tol to 685, 688, and 692 nm in THF, respectively. Differences in the characteristics of the absorption and emission spectra could be attributed to differences in the dipole moments of the ground and excited states. The large red-shift in the emission spectra indicates the formation of a new state, which is called the ICT state. Since the ICT state has a larger dipole moment than the locally excited (LE) state, the former is more stable in solvents with stronger polarity, leading to a larger red-shift in fluorescence spectra.
3.2.2. Dynamics in THF The evolution of fluorescence dynamics in THF differs from that in Tol. Here, τ1 refers to the formation time of the solvent-stabilized ICT state, while τ2 refers to the relaxation time from the ICT state to the ground state. When the molecules are excited by laser pulses, the ICT state demonstrates a planar structure, which is a relatively unstable state. Thus, the acceptor and donor units tend to twist vertically to form a twisted intramolecular charge transfer (TICT) state [20]. The TICT state has nonradiative characteristics, and no fluorescence emission from the TICT state is observed. Since the TICT state has very high polarity, it is easily generated in strongly polar solvents. In Tol, fluorescence decays from the ICT state to the ground state; by comparison, in THF, the decay of the ICT state occurs through two channels–from ICT to TICT and to the ground state. Thus, the relaxation rate in THF is faster than that in Tol, which presents only one channel for relaxation. Monoexponential decay in Tol is getting faster with increasing number of branches from 3 ns to 2.7 ns. In THF, the same tendency is observed. Thus, Tol was selected as the solvent in further studies on the influence of donor branch number on relaxation dynamics because its solvent effects can be ignored. The decay process is becoming faster with increasing number of branches (from ATT-1 to ATT-3), and τ3 decreases from 2989 ps to 2747 ps. The ability of a material to donate and accept electrons is enhanced by increases in number of donor branches, leading to a large dipole moment in the ICT state. Moreover, increases in number of donor
3.2. Excited-state dynamics The mechanism of ICT was investigated via time-resolved fluorescence spectroscopy and transient absorption studies. The evolution curves in Fig. 3 and fitting results in Table 2 indicate that the evolution processes of ATT-(1–3) in Tol are clearly different from those in THF.
Fig 3. PL evolution curves for center wavelength of ATT-(1–3) in Tol and THF. 3
Chemical Physics Letters 736 (2019) 136800
X. Li, et al.
Table 2 Fitting parameters of the PL evolution curves at center wavelength in different solvents. The relative weight in decay component are given in brackets. Solvent Sample
Detection (nm)
THF
Detection (nm)
τr (ps) ATT-1 ATT-2 ATT-3
685 690 691
τ1 (ps)
τ2 (ps)
144.36 (0.92) 114.82 (0.58) 109 (0.59)
490.36 (0.08) 261.27 (0.42) 258.09 (0.41)
τ3 (ps)
τr (ps) 592 600 604
64 108 68
Tol τ1(ps)
τ2(ps)
τ3(ps) 2989.73 2840.12 2747.70
Fig. 4. Results of ultrafast two-color pump-probe. The solid line is the fitted curve. (a) the decay dynamics of ATT-(1–3) in THF. (b) the decay dynamics of ATT-(1–3) in Tol.
4. Conclusion
Table 3 The fitting parameters of ultrafast dynamics of ATT-(1–3) in different solvent.
In summary, the optical properties of three newly synthesized multibranched triazines were characterized in Tol and THF by using steadystate spectroscopy, time-resolved fluorescence dynamics, and two-color pump-probe techniques. The steady-state spectra revealed differences in the peak positions of the three samples in Tol and THF. In THF, the curves observed are slightly blue-shifted in the absorption spectrum and remarkably red-shifted in the emission spectrum. This obvious red-shift indicates the formation of the ICT state. Triazines ATT-(1–3) showed a slight red-shift in absorption and emission spectra in both solvents. The results of excited-state dynamics in Tol reveal that the evolution process includes an initial increase (of approximately 100 ps) followed by subsequent decay (of about 3 ns), which could be attributed to the formation and depletion of the ICT state. These two processes occur much faster in THF than in Tol because of the solvent effect. Both in excited-state dynamics and transient absorption. The evolution process occurred faster with increasing number of donor branches, likely to enhancements in the electron donating and accepting abilities of the dyes.
Solvent Sample
ATT-1 ATT-2 ATT-3
THF
Tol
τ (ps)
τ1 (ps)
τ2 (ps)
1.30 0.97 0.74
98 (0.2) 64 (0.27) 28 (0.47)
2870 (0.8) 2046 (0.73) 1875 (0.53)
branches cause red-shifting of the steady-state spectrum in Tol to approximately 5 nm. This shift indicates that the energy of the molecules is decreased, resulting in acceleration of fluorescence dynamics [21]. 3.2.3. Transient absorption The excited-state relaxation dynamics of ATT-(1–3) are investigated using two-color pump-probe experiments. The results are shown in Fig. 4, and the fitting parameters are listed in Table 3. Mono- and twoexponential decay functions are used to fit the results of the samples in THF and in Tol, respectively. The two-color pump-probe results show a tendency similar to that observed in the fluorescence dynamics experiments. The value of τ in THF solvent is 1 ps, which can be ascribed to vibrational relaxation to the LE state. The absence of fluorescence dynamics in this process may be due to the different techniques adopted. In Tol, τ1, which refers to the formation time of the ICT state, is similar to the rise time τr during fluorescence evolution. In addition, τ2, which describes the relaxation time from the ICT state to the ground state, is similar to τ3 in fluorescence dynamics. In THF, the relaxation time to the LE state is accelerated from 1.30 ps to 0.74 ps; in Tol, τ1 and τ2 accelerate from 98 and 2870 ps to 28 and 1875 ps, respectively. The transient absorption results agree well with the observed fluorescence evolution.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Key Research and Development Program of China (Grand No. 2016YFB0501604), and the National Natural Science Foundation of China (Grand Nos. 11575062, 61227902). 4
Chemical Physics Letters 736 (2019) 136800
X. Li, et al.
References
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Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Excited-state relaxation processes of three newly synthesized multibranched alkyl-triphenylamine end-capped triazines
T
Xiang Lia, Zhiquan Wanga, Yuting Gaob, Zhengjun Shanga, Minghao Nia, Jianli Huab, Bo Lia, , Ye Chena ⁎
a b
Key Laboratory of Polar Materials and Devices (MOE), Department of Electronics, East China Normal University, Shanghai 200241, China Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, Shanghai 200237, China
HIGHLIGHTS
state and TICT state affect the relaxation rate of multi-branched polymers. • ICT effect influences the relaxation process of multi-branched polymers. • Solvent • Donor branch numbers cause the change of evolution process. ARTICLE INFO
ABSTRACT
Keywords: Intramolecular charge transfer (ICT) Fluorescence dynamic Multi-branched structure Solvent effect
The excited-state relaxation processes of three newly synthesized multi-branched alkyl-triphenylamine end-capped triazines ATT-(1–3) are characterized in different solvents by steady-state and time-resolved spectroscopy. In toluene, a weakly polar solvent, the emission originates from the intramolecular charge transfer (ICT) state; in tetrahydrofuran (THF), a strongly polar solvent, the existence of a nonradiative channel from ICT to twisted intramolecular charge transfer (TICT) accelerates the relaxation rate of the ICT state. The rate of the evolution process of ATT-(1–3) increases with increasing number of donor branches, which could ascribed to enhancements in the electron donor and acceptor abilities of the triazines.
1. Introduction Over the last two decades, organic materials have shown potential applications in many optoelectronic devices, such as field-effect transistors [1–4], photodetectors [5–7], and light-emitting diodes [8–12]. Multi-branched conjugated organic molecules with a geometry similar to that of donor-π-acceptor (D-π-A) compounds have recently attracted great research attention. Indeed, many multi-branched triazines have been synthesized for their excellent electron-donating and -transport capabilities and potential optical applications [13,14]. The notable optical properties of these triazines are distinctly related to their intramolecular charge transfer (ICT) state [15,16]. ICT occurs between electron acceptor and donor units. The polarity of a material changes through the ICT process, which can be influenced by solvent polarity. Therefore, the fluorescence dynamics of a material can be influenced by solvent polarity. Research on the solvent effect can help improve the understanding of the mechanism of the ICT state.
⁎
Considerable research on the fluorescence emission of multi-branched materials has been accomplished. Oberle and co-workers reported that pump-probe and time-resolved coherent anti-Stokes Raman spectroscopy could be used to determine the formation of the ICT state in polar solvents, which is associated with structural changes in the solute molecules [17]. Chi and co-workers used time-resolved methods to reveal that increasing the number of electron acceptors accelerates the fluorescence relaxation of D-π-A materials [18]. Steady-state and time-resolved photoluminescence (PL) spectroscopy are powerful tools for characterizing optical properties. In this study, the excited-state properties and fluorescence dynamics of three newly synthesized triazine dyes dissolved in tetrahydrofuran (THF) and toluene (Tol) are examined. Solvent polarity can influence the emission spectra of the samples. Moreover, solvent polarity and branch number of the samples affect their fluorescence dynamics. Fluorescence lifetimes decrease with increasing solvent polarity, while fluorescence evolution processes become faster with increasing branch number.
Corresponding author. E-mail addresses: [email protected] (B. Li), [email protected] (Y. Chen).
https://doi.org/10.1016/j.cplett.2019.136800 Received 20 June 2019; Received in revised form 8 September 2019; Accepted 26 September 2019 Available online 27 September 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 736 (2019) 136800
X. Li, et al.
2. Materials and methods
employed to study the excited-state relaxation process and dynamics. Here, the sample was pumped with 400 nm pulses and probed with 800 nm pulses.
The synthesis of the three new dyes was carried out as described in our previous work [13]. The structure of these dyes, as presented in Fig. 1, includes a triazine core acting as an electron acceptor and alkyltriphenylamine acting as an electron donor. In the present research, the samples featured different numbers of branches and were dissolved in two solvents with different polarities, namely, THF and Tol. Steadystate absorption spectra were measured by a UV–visible spectrophotometer (TU-1901, Purkinje General), while fluorescence spectra were recorded by a spectrophotometer (iHR550, HORIBA). The laser source was a mode-locked Ti:sapphire femtosecond laser system (Coherent, 80 fs pulse duration, 250 kHz repetition rate, centered at 800 nm). Frequency-doubled femtosecond pulses were used to excite the samples. The PL evolution curves of the three samples in THF and Tol were measured by time-correlated single-photon counting with an avalanche photodiode. The time constant of the instrument response function was about 40 ps. The two-color pump-probe technique was
3. Results and discussion The impacts of solvent polarity and branch number on the steadystate spectra and excited state dynamics of the dyes are investigated. Here, Δf is selected as the measure of polarity and calculated as follows [19]:
f=
1 2 +1
n2 1 2n2 + 1
where represents the static dielectric constant and n represents the optical refractive index of the solvent. The Δf of Tol and THF are 0.013 and 0.207, respectively.
Fig. 1. Structure of ATT-(1–3).
Fig. 2. (a) The absorption spectrum and fluorescence spectrum of three samples in Tol. (b) The absorption and fluorescence spectrum of ATT-3 in Tol and THF. 2
Chemical Physics Letters 736 (2019) 136800
X. Li, et al.
The evolution curves of ATT-(1–3) in Tol show an initial growth with a rise time of about 100 ps and then monoexponential decay (approximately 3 ns). The evolution curves of ATT-(1–3) in THF are much faster than those in Tol, thereby demonstrating dual-exponential decay.
Table 1 Steady-state absorption and fluorescence peak positions in solvents with different polarity (Δf). Solvent Sample
ATT-1 ATT-2 ATT-3
Tol(Δf = 0.013)
THF(Δf = 0.207)
λab (nm)
λpl (nm)
λab (nm)
λpl (nm)
443 449 455
601 603 604
439 447 452
685 688 692
3.2.1. Dynamics in Tol The evolution process of ATT-(1–3) in Tol consists of two processes. The rise time τr is related to the accumulating population of emission states, which, in turn, are attributed to the formation of a stabilized ICT state. The rise time τr is approximately 100 ps; the fluorescence observed in this case could be inferred to mainly arise from the ICT state. The formation time of the LE state is less than 10 ps, which is much shorter than the rise time of the three samples in Tol. After excitation of the solute molecules, the electrons transfer from donor branch to acceptor unit, and the dipole moment of the solute molecules changes. Surrounding solvent molecules reorganize to stabilize the excited solute molecules. This process forms a stable ICT state. τ3 refers to the relaxation time from the ICT state to the ground state and decreases from around 3 ns to 2.7 ns with increasing number of branches.
3.1. Steady absorption and fluorescence spectrum The steady-state absorption and fluorescence spectra of ATT-(1–3) in Tol at room temperature are shown in Fig. 2(a). The absorption and emission curves of all three samples in THF display the same trend; thus, only the results of ATT-3 are selected to describe this trend. Specific data on the steady-state spectra obtained are listed in Table 1. Fig. 2(a) shows that the absorption and fluorescence spectra of the three samples in Tol are comparable. Red-shifting of the absorption and fluorescence spectra increases as the number of branches increases from ATT-1 to ATT-3. An apparent Stokes shift (red-shifting) in the fluorescence spectra compared with the steady-state absorption spectra in Tol could also be observed. The absorption spectra of ATT-(1–3) in Tol and THF (around 450 nm) are nearly identical, indicating small differences in dipole moment between the ground and excited states. However, an obvious red-shift in emission spectra could be observed. Specifically, the emission peak of ATT-(1–3) shifted from 601, 603, and 604 nm in Tol to 685, 688, and 692 nm in THF, respectively. Differences in the characteristics of the absorption and emission spectra could be attributed to differences in the dipole moments of the ground and excited states. The large red-shift in the emission spectra indicates the formation of a new state, which is called the ICT state. Since the ICT state has a larger dipole moment than the locally excited (LE) state, the former is more stable in solvents with stronger polarity, leading to a larger red-shift in fluorescence spectra.
3.2.2. Dynamics in THF The evolution of fluorescence dynamics in THF differs from that in Tol. Here, τ1 refers to the formation time of the solvent-stabilized ICT state, while τ2 refers to the relaxation time from the ICT state to the ground state. When the molecules are excited by laser pulses, the ICT state demonstrates a planar structure, which is a relatively unstable state. Thus, the acceptor and donor units tend to twist vertically to form a twisted intramolecular charge transfer (TICT) state [20]. The TICT state has nonradiative characteristics, and no fluorescence emission from the TICT state is observed. Since the TICT state has very high polarity, it is easily generated in strongly polar solvents. In Tol, fluorescence decays from the ICT state to the ground state; by comparison, in THF, the decay of the ICT state occurs through two channels–from ICT to TICT and to the ground state. Thus, the relaxation rate in THF is faster than that in Tol, which presents only one channel for relaxation. Monoexponential decay in Tol is getting faster with increasing number of branches from 3 ns to 2.7 ns. In THF, the same tendency is observed. Thus, Tol was selected as the solvent in further studies on the influence of donor branch number on relaxation dynamics because its solvent effects can be ignored. The decay process is becoming faster with increasing number of branches (from ATT-1 to ATT-3), and τ3 decreases from 2989 ps to 2747 ps. The ability of a material to donate and accept electrons is enhanced by increases in number of donor branches, leading to a large dipole moment in the ICT state. Moreover, increases in number of donor
3.2. Excited-state dynamics The mechanism of ICT was investigated via time-resolved fluorescence spectroscopy and transient absorption studies. The evolution curves in Fig. 3 and fitting results in Table 2 indicate that the evolution processes of ATT-(1–3) in Tol are clearly different from those in THF.
Fig 3. PL evolution curves for center wavelength of ATT-(1–3) in Tol and THF. 3
Chemical Physics Letters 736 (2019) 136800
X. Li, et al.
Table 2 Fitting parameters of the PL evolution curves at center wavelength in different solvents. The relative weight in decay component are given in brackets. Solvent Sample
Detection (nm)
THF
Detection (nm)
τr (ps) ATT-1 ATT-2 ATT-3
685 690 691
τ1 (ps)
τ2 (ps)
144.36 (0.92) 114.82 (0.58) 109 (0.59)
490.36 (0.08) 261.27 (0.42) 258.09 (0.41)
τ3 (ps)
τr (ps) 592 600 604
64 108 68
Tol τ1(ps)
τ2(ps)
τ3(ps) 2989.73 2840.12 2747.70
Fig. 4. Results of ultrafast two-color pump-probe. The solid line is the fitted curve. (a) the decay dynamics of ATT-(1–3) in THF. (b) the decay dynamics of ATT-(1–3) in Tol.
4. Conclusion
Table 3 The fitting parameters of ultrafast dynamics of ATT-(1–3) in different solvent.
In summary, the optical properties of three newly synthesized multibranched triazines were characterized in Tol and THF by using steadystate spectroscopy, time-resolved fluorescence dynamics, and two-color pump-probe techniques. The steady-state spectra revealed differences in the peak positions of the three samples in Tol and THF. In THF, the curves observed are slightly blue-shifted in the absorption spectrum and remarkably red-shifted in the emission spectrum. This obvious red-shift indicates the formation of the ICT state. Triazines ATT-(1–3) showed a slight red-shift in absorption and emission spectra in both solvents. The results of excited-state dynamics in Tol reveal that the evolution process includes an initial increase (of approximately 100 ps) followed by subsequent decay (of about 3 ns), which could be attributed to the formation and depletion of the ICT state. These two processes occur much faster in THF than in Tol because of the solvent effect. Both in excited-state dynamics and transient absorption. The evolution process occurred faster with increasing number of donor branches, likely to enhancements in the electron donating and accepting abilities of the dyes.
Solvent Sample
ATT-1 ATT-2 ATT-3
THF
Tol
τ (ps)
τ1 (ps)
τ2 (ps)
1.30 0.97 0.74
98 (0.2) 64 (0.27) 28 (0.47)
2870 (0.8) 2046 (0.73) 1875 (0.53)
branches cause red-shifting of the steady-state spectrum in Tol to approximately 5 nm. This shift indicates that the energy of the molecules is decreased, resulting in acceleration of fluorescence dynamics [21]. 3.2.3. Transient absorption The excited-state relaxation dynamics of ATT-(1–3) are investigated using two-color pump-probe experiments. The results are shown in Fig. 4, and the fitting parameters are listed in Table 3. Mono- and twoexponential decay functions are used to fit the results of the samples in THF and in Tol, respectively. The two-color pump-probe results show a tendency similar to that observed in the fluorescence dynamics experiments. The value of τ in THF solvent is 1 ps, which can be ascribed to vibrational relaxation to the LE state. The absence of fluorescence dynamics in this process may be due to the different techniques adopted. In Tol, τ1, which refers to the formation time of the ICT state, is similar to the rise time τr during fluorescence evolution. In addition, τ2, which describes the relaxation time from the ICT state to the ground state, is similar to τ3 in fluorescence dynamics. In THF, the relaxation time to the LE state is accelerated from 1.30 ps to 0.74 ps; in Tol, τ1 and τ2 accelerate from 98 and 2870 ps to 28 and 1875 ps, respectively. The transient absorption results agree well with the observed fluorescence evolution.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Key Research and Development Program of China (Grand No. 2016YFB0501604), and the National Natural Science Foundation of China (Grand Nos. 11575062, 61227902). 4
Chemical Physics Letters 736 (2019) 136800
X. Li, et al.
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