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Ultrafast relaxation processes of multi-branched compounds based on 1,3,5-triazine: An investigation of the causes of a high fluorescence quantum yield after modification with perfluoroalkyl chains
Journal of Luminescence 190 (2017) 89–93
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Ultrafast relaxation processes of multi-branched compounds based on 1,3,5triazine: An investigation of the causes of a high fluorescence quantum yield after modification with perfluoroalkyl chains ⁎
MARK
⁎
Yaochuan Wanga, , Yihua Jiangb, Dajun Liua, , Yizhuo Wanga, Guiqiu Wanga, Jianli Huab a b
Department of Physics, Dalian Maritime University, Dalian 116026, People's Republic of China Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, People's Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: Ultrafast response Two-photon fluorescence Multi-branched Perfluoroalkyl chain modification
The effect of modifying a 1,3,5-triazine-based multi-branched compound (T03-a) with perfluoroalkyl chains (T03-d) on the fluorescence and ultrafast dynamics was investigated by two-photon fluorescence and femtosecond transient absorption experiments. After modification, a significantly increased fluorescence quantum yield was observed. The fluorescence quantum yield of the perfluoroalkyl chain modified compound T03-d (0.86) was approximately 3.2 times greater than that of T03-a (0.27). The ultrafast dynamics experiments revealed the effect of the perfluoroalkyl chain modification on the intramolecular charge transfer (ICT) property. The ultrafast dynamics results agree with the changes in the fluorescent property. Our results indicate that the modification of compounds with perfluoroalkyl chains based on the 1,3,5-triazine tri-branched compound would be a very useful strategy for enhancement of the fluorescence, and the 1,3,5-triazine based multi-branched derivatives have the potential to be employed as molecular probes for biological fluorescence imaging.
1. Introduction Two-photon fluorescent materials are essential for two-photon fluorescence microscopy [1]. For imaging, the materials must have both a large two-photon absorption (TPA) cross-section and a high fluorescence quantum yield [2,3]. Materials science has focused on the exploration of new TPA materials with improved properties [2–5]. Although some strategies effectively enhance the TPA cross-section, most strategies result in materials with relatively low fluorescence quantum yields, such as some dipole compounds and octupolar multibranched molecules [6,7]. The excited state decay of a material contains many photophysical processes that can provide significant information about the excited state and the interaction between light and matter [8–10]. We have investigated the ultrafast dynamics of TPA compounds including dipolar, quadrupolar, multi-branched, and polymeric compounds [11–14]. These results have provided information that has assisted in the explanation of TPA. In this study, the nonlinear optical (NLO) properties and ultrafast dynamics of the multi-branched compounds T03-a and T03-d were investigated by two-photon fluorescence and femtosecond (fs) pumpprobe experiments to study the effects of introducing perfluoroalkyl chains to the end donor of a multi-branched compound. After modifica-
⁎
Corresponding authors. E-mail addresses: [email protected] (Y. Wang), [email protected] (D. Liu).
http://dx.doi.org/10.1016/j.jlumin.2017.05.037 Received 23 March 2017; Received in revised form 10 May 2017; Accepted 13 May 2017 Available online 17 May 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
tion with perfluoroalkyl chains, the fluorescence quantum yield increased substantially. The ultrafast dynamics experiments revealed the effects of perfluoroalkyl chain modification on the fluorescent property and the intramolecular charge transfer (ICT) properties of the compounds. The fast process and subsequent long decay process were due to the formation of the ICT state and the relaxation of the ICT state, respectively. The 1,3,5-triazine based multi-branched derivatives have the potential to be molecular probes for biological fluorescence imaging in the life sciences. 2. Materials and experiments The synthesis of the multi-branched compound T03-a and the perfluoroalkyl chain modified compound T03-d have been previously reported [15]. For this experiment, the compounds were dissolved in chloroform (CHCl3), which was used without further distillation. The tri-branched compounds investigated are shown in Fig. 1. In compound T03-a, the electron acceptor (A) group 1,3,5-triazine serves as the central core, and three electron donor (D) triphenylamine groups serve as the terminal ends, which forms a tri-branched structural configuration containing three D-π -A subunits. T03-d can be described with the electron-withdrawing perfluoroalkyl chains as side groups that
Journal of Luminescence 190 (2017) 89–93
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Fig. 1. The structures of the 1,3,5-triazine-based multi-branched compounds (T03-a and T03-d).
are linked to the terminal ends of the electron donor triphenylamines. The UV–visible absorption spectra of the compounds in CHCl3 were recorded on a Hitachi spectrophotometer with 2 nm spectral resolution, and the steady-state fluorescence spectra were recorded with dilute solutions (10−5 M) using an Edinburgh FLS 920 spectrometer with 1 nm spectral resolution. The NLO fluorescence spectra were recorded on a TRISTAN light spectrometer. The fluorescence quantum yields of the compounds were measured with Rhodamine B in basic ethanol as a standard. The laser beam (370 nm, 1.4 mW) was slightly focused on the CHCl3 solutions (10−5 M concentration). The spectra were measured and corrected. The detailed experimental conditions can be found in our previous publication [15]. The ultrafast responses of these compounds were investigated by femtosecond (fs) pump-probe experiments. The fs pulses for the NLO absorption and ultrafast transient absorption measurements were generated by the fs laser system (Spitfire, Spectra-Physics). The average output power of the Spitfire was approximately 300 mW. The pulse duration was 140 fs, the central wavelength was 800 nm, and the repetition rate was 1 kHz. The fs pump-probe experimental setup was described previously [16]. The fs pulses from the fs laser system (800 nm), were divided into two parts by a beam splitter. To efficiently pump the compounds into the excited state, a portion of the beam was frequency-doubled to 400 nm by a 0.5 mm thick β -barium metaborate (BBO) crystal. The other portion of the beam was focused on a 5 mm thick cell with flowing water to generate a supercontinuum in order to sample the photoinduced excited state at various wavelengths. A monochromator after the sample cell selected the probe wavelength. To detected dynamics without diffraction effects or coherent artifacts, the polarization of the pump beam was set perpendicular to that of the probe beam [11,17]. All of the measurements were performed at room temperature.
Fig. 2. The one-photon absorption spectra and normalized fluorescence spectra of T03-a and T03-d dissolved in CHCl3. Table 1 A summary of the data for compounds T03-a and T03-d. Compounds
T03-a T03-d
Solvents
CHCl3 CHCl3
1PA-related optical properties
TPA-related optical properties
a λ max (abs)/nm
b λ max (fl)/nm
ϕc
d (TPEF)/ λ max nm
σ e /GM
421 395
535 490
0.27 0.86
545 510
447 603
a λ max is the wavelength for the maximum of the one-photon absorption spectra. b λ max is the wavelength for the maximum of the one-photon emission spectra. c ϕ is the fluorescence quantum yield, which was determined by using Rhodamine B in ethanol as a standard. d λ max is the wavelength for the maximum of the two-photon fluorescence spectra. e σ is the TPA cross-section.
3. Results and discussion 3.1. Linear absorption and fluorescence emission of T03-a and T03-d
band gap of the compound. Usually, in a conjugated system with delocalized π electrons, stronger ICT effects will cause a redshift in the absorption spectra. Thus, modification with perfluoroalkyl chains linked to the end of terminal triphenylamine groups may not lead to a stronger ICT effect. In the following investigations, the effect will be further discussed.
The absorption and normalized fluorescence spectra of the tribranched compound T03-a and perfluoroalkyl chain modified compound T03-d dissolved in CHCl3 in dilute concentrations (10−5 M) are shown in Fig. 2. The spectral results are shown in Table 1. Two major absorption bands are observed. The band located at approximately 311 nm is attributed to a localized multi-triazine aromatic π -π * transition, and the band located at approximately 421 nm is attributed to the charge transfer state [18,19]. The charge transfer absorption maximum of T03-d is located at 395 nm, which has a 26 nm blueshift from the T03-a maximum due to the addition of the electron-withdrawing perfluoroalkyl chains as side groups to the end triphenylamine donors. These 1,3,5-triazine-based tri-branched compounds exhibit adequate fluorescence emission. The fluorescence peaks of T03-a and T03-d are located at 535 nm and 490 nm, respectively. These results indicate that modification with perfluoroalkyl chains may increase the
3.2. Fluorescence properties Most two-photon imaging applications employ fs pulses at a wavelength of 800 nm. The two-photon fluorescence properties of the two compounds were investigated at this wavelength. Both T03-a and T03-d emitted intense fluorescence emission under irradiation of unfocused fs pulses with a pulse energy of several microjoules. Since no obvious linear absorption was observed at wavelengths longer than 750 nm, any absorption in this wavelength range would be attributed to 90
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Fig. 3. a, b: The TPF intensities of T03-a and T03-d dissolved in CHCl3. c: the TPF intensity versus the square of the excitation power density.
a multi-photon process [20]. The fluorescence spectra of compounds T03-a and T03-d dissolved in CHCl3 (approximately 10−3 M) under different excitation powers are shown in Fig. 3a and b, respectively. Fig. 3c show the linear dependence of the fluorescence intensity on the square of the excitation intensity, which confirms that the fluorescence emission of the compounds excited by the 800 nm fs pulses is generated by a TPA process. The emission peak wavelengths of T03-a and T03-d are located at 545 nm and 510 nm, respectively, which are redshifted by 10 nm and 20 nm from the fluorescence spectra produced with onephoton. The red-shifts are attributed to reabsorption [21], because we used much higher solution concentrations for the two-photon fluorescence measurements. To quantify the differences in the two-photon fluorescence ability of the perfluoroalkyl chain modified T03-d compared to the T03-a, the fluorescence quantum yield was measured. The addition of perfluoroalkyl chains may not increase the ICT property, which is very important for the enhancement of TPA compounds. The steady-state absorption maximum of the perfluoroalkyl chain modified T03-d is blue-shifted from 421 nm (T03-a) to 395 nm, which is much closer to 400 nm, which creates a stronger two-photon near-resonance effect for the T03-d at 800 nm (the absorption of one photon at a wavelength of 400 nm is equal to the simultaneous absorption of two photons at a wavelength of 800 nm). However, the fluorescence quantum yield of T03-d, which was determined by using Rhodamine B in ethanol as a standard [22], increased by approximately 3.2 times after the modification with perfluoroalkyl chains (0.86) compared to the quantum yield of T03-a (0.27). This result indicates that the perfluoroalkyl chains do play an important role in the fluorescence quantum yield of the 1,3,5-triazine-based tri-branched compounds. In the following ultrafast dynamics section, the enhancement mechanism will be further discussed.
3.3. Ultrafast relaxation processes The increase in the band gap of T03-d compared to T03-a indicates that the perfluoroalkyl chains modification on the 1,3,5-triazine-based tri-branched compound did not cause a stronger ICT effect. Fs transient absorption measurements were performed to continue the study of the effects of the perfluoroalkyl chain modification on the ultrafast relaxation processes and ICT property of the tri-branched compound. The transient absorption spectra obtained from the compounds in CHCl3 solutions are shown in Fig. 4. At the initial time of 0 ps in Fig. 4a, the tri-branched compound T03-a produces a broad photoabsorption region with a transient photobleaching region from 450 to 510 nm. The photoabsorption region is attributed to the excited state absorption of the ICT state, and the photobleaching region is ascribed to the stimulated emission of the fluorescence emission state [3]. As described by Fuks-Janczarek, Sahraoui et al. [23], some essential local effects may contribute to the NLO susceptibility of materials that are under action of the ultrashort laser pulses, and these effects may also play a competition: intra-molecular electronic, ionic and nucleus reorientation including translations, rotations, and vibrations. The contribution of the electronic cloud deformation would be in the femtosecond scale, while the processes related to the nucleus, such as vibration (somewhere, it is called as phonon) and reorientation are mainly in several picosecond scale due to the electron-phonon interaction (also called vibronic relaxation). Hence, at relative long delay time such as 100 ps, the contribution from the electronic cloud deformation and the nucleus vibration can be neglected. Fig. 5a shows the ultrafast dynamic traces of T03-a in CHCl3 probed at various representative probe wavelengths with the calculated time constants of the excited states summarized in Table 2. For a probe wavelength of 425 nm, the main dynamic curve shows two different decay processes: a fast photoabsorption process of approximately 4 ps, and a long photoabsorption process of approxi91
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Fig. 4. The femtosecond transient absorption spectra of compounds T03-a and T03-d in CHCl3 solutions with a delay time of 0–20 ps.
mately 200 ps. The fast process is the formation of the ICT state, and the long decay process reflects the evolution of the ICT state. When the probe wavelength was tuned to 500 nm, a fast photobleaching process emerges. The dynamic curve shows a fast decay process of several ps and a long decay process of approximately 200 ps, and both decay processes show photoabsorption property. When the probe wavelength was tuned to wavelengths longer than 650 nm, the dynamic curves exhibit a fast photoabsorption process of approximately 4 ps and a long photoabsorption process of approximately 200 ps. The transient absorption measurement was also performed on the perfluoroalkyl chain modified compound T03-d, and different ultrafast relaxation processes were observed. In Fig. 4b, a transient photobleaching region of 450–500 nm was also observed for T03-d. However, the time duration of the photobleaching region is considerably longer than that of T03-a, indicating that some of the photobleaching processes have longer lifetimes and stronger relative intensities. The dynamic traces of the perfluoroalkyl chain modified T03-d probed at several representative wavelengths are shown in Fig. 5b, and the fitting time constants are also summarized in Table 2. At a probe wavelength of 400 nm, the dynamic trace of T03-d is similar to the trace from T03-a. When the probe wavelength was tuned to 450 nm, a fast photobleaching process was also observed, which is similar to T03-a. These fast photobleaching processes are explained as the competition between electron-phonon interactions and the formation of the fluorescence emission state. Additionally, the stimulated emission (of photobleaching property) of fluorescence at the probe wavelength affects the signal intensity and the photoabsorption/photobleaching during the long decay process. When the probe wavelength was tuned to 475 nm, both the fast and long decay proceses show photobleaching property at 100 ps, which correlates with the very high fluorescence quantum yield
Table 2 A summary of the time constants for compounds T03-a and T03-d in CHCl3 solutions. Compound
T03-a T03-d
λPump/probe/nm
400/725 400/725
Decay /ps τ1 /ps
τ2 /ps
3.9 8.5
220 > 1000
(0.86). The wavelength of 725 nm is distant from the linear absorption and fluorescence regions of the two compounds and was chosen as a special probe wavelength to compare the time constants of the two tribranched compounds. When the probe wavelength was tuned to 725 nm, the perfluoroalkyl chain modified compound T03-d displayed a fast process reflecting the formation of the ICT state and a long decay process reflecting the evolution of the ICT state. In D-π -A structural tribranched compounds with double bonds as the conjugation bridge, some additional relaxation processes would be included after the evolution of the ICT state: the rotation of the double bonds and the rotation of the electron donor/acceptor form a twisted ICT (TICT) state, which weakly emits [24]. These processes would decrease the fluorescence quantum yield of the compound. However, the time constants of T03-d are different from T03-a, which is illustrated in the dynamic traces and the fitting results. The fast process of the perfluoroalkyl chain modified compound T03-d probed at 725 nm is 8.5 ps, which is substantially longer than that of T03-a, indicating that modification with perfluoroalkyl chains did not increase the ICT property [25]. However, the subsequent long process (reflecting the evolution of the ICT state, also considered as the formation of the TICT state) of T03-d is longer than 1000 ps, which is much longer than that of T03-a (220 ps).
Fig. 5. The dynamic decay traces for compounds a) T03-a and b) T03-d at various probe wavelengths.
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These results support the high fluorescence quantum yield of the perfluoroalkyl chain modified compound T03-d. In principle, the evolution of the ICT state correlates with the fluorescence emission, the longer lifetime of the ICT state, and the larger fluorescence quantum yield. Due to the terminal perfluoroalkyl chains on the modified compound T03-d, the rotation of excited state electron donor/acceptor and the formation of TICT state require additional time, which increases the lifetime of the ICT state. The transient absorption results indicate that modification with perfluoroalkyl chains suppresses the nonradiative decay pathways which increases the fluorescence emission efficiently. Our results indicate that perfluoroalkyl chain modification based on a 1,3,5-triazine tri-branched compound is a suitable strategy for great enhancing fluorescence emission.
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4. Conclusions The fluorescence property, as well as and ultrafast response of perfluoroalkyl chain modified tri-branched compounds based on 1,3,5triazine in CHCl3 solutions were investigated to study the effects of perfluoroalkyl chain modification on the fluorescence and excited state relaxation properties. After modification with perfluoroalkyl chains, the fluorescence emission was enhanced. The ultrafast relaxation processes revealed the effect of the perfluoroalkyl chains modification on the ICT property. Therefore, these 1,3,5-triazine based multi-branched derivatives could be employed as molecular probes for biological fluorescence imaging in the life sciences. These results can also assist in the design and optimization of new molecular probes with both large TPA crosssections and high fluorescence quantum yields. Acknowledgments We sincerely thank the following organizations for the financial support for this project: the National Natural Science Foundation of China (11404048, 11604038, 11375034), the Liaoning Provincial Natural Science Foundation of China (201602061, 201602062), the Program for Liaoning Educational Committee (L2015071), and the Fundamental Research Funds for the Central Universities (3132017060,
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Ultrafast relaxation processes of multi-branched compounds based on 1,3,5triazine: An investigation of the causes of a high fluorescence quantum yield after modification with perfluoroalkyl chains ⁎
MARK
⁎
Yaochuan Wanga, , Yihua Jiangb, Dajun Liua, , Yizhuo Wanga, Guiqiu Wanga, Jianli Huab a b
Department of Physics, Dalian Maritime University, Dalian 116026, People's Republic of China Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, People's Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: Ultrafast response Two-photon fluorescence Multi-branched Perfluoroalkyl chain modification
The effect of modifying a 1,3,5-triazine-based multi-branched compound (T03-a) with perfluoroalkyl chains (T03-d) on the fluorescence and ultrafast dynamics was investigated by two-photon fluorescence and femtosecond transient absorption experiments. After modification, a significantly increased fluorescence quantum yield was observed. The fluorescence quantum yield of the perfluoroalkyl chain modified compound T03-d (0.86) was approximately 3.2 times greater than that of T03-a (0.27). The ultrafast dynamics experiments revealed the effect of the perfluoroalkyl chain modification on the intramolecular charge transfer (ICT) property. The ultrafast dynamics results agree with the changes in the fluorescent property. Our results indicate that the modification of compounds with perfluoroalkyl chains based on the 1,3,5-triazine tri-branched compound would be a very useful strategy for enhancement of the fluorescence, and the 1,3,5-triazine based multi-branched derivatives have the potential to be employed as molecular probes for biological fluorescence imaging.
1. Introduction Two-photon fluorescent materials are essential for two-photon fluorescence microscopy [1]. For imaging, the materials must have both a large two-photon absorption (TPA) cross-section and a high fluorescence quantum yield [2,3]. Materials science has focused on the exploration of new TPA materials with improved properties [2–5]. Although some strategies effectively enhance the TPA cross-section, most strategies result in materials with relatively low fluorescence quantum yields, such as some dipole compounds and octupolar multibranched molecules [6,7]. The excited state decay of a material contains many photophysical processes that can provide significant information about the excited state and the interaction between light and matter [8–10]. We have investigated the ultrafast dynamics of TPA compounds including dipolar, quadrupolar, multi-branched, and polymeric compounds [11–14]. These results have provided information that has assisted in the explanation of TPA. In this study, the nonlinear optical (NLO) properties and ultrafast dynamics of the multi-branched compounds T03-a and T03-d were investigated by two-photon fluorescence and femtosecond (fs) pumpprobe experiments to study the effects of introducing perfluoroalkyl chains to the end donor of a multi-branched compound. After modifica-
⁎
Corresponding authors. E-mail addresses: [email protected] (Y. Wang), [email protected] (D. Liu).
http://dx.doi.org/10.1016/j.jlumin.2017.05.037 Received 23 March 2017; Received in revised form 10 May 2017; Accepted 13 May 2017 Available online 17 May 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
tion with perfluoroalkyl chains, the fluorescence quantum yield increased substantially. The ultrafast dynamics experiments revealed the effects of perfluoroalkyl chain modification on the fluorescent property and the intramolecular charge transfer (ICT) properties of the compounds. The fast process and subsequent long decay process were due to the formation of the ICT state and the relaxation of the ICT state, respectively. The 1,3,5-triazine based multi-branched derivatives have the potential to be molecular probes for biological fluorescence imaging in the life sciences. 2. Materials and experiments The synthesis of the multi-branched compound T03-a and the perfluoroalkyl chain modified compound T03-d have been previously reported [15]. For this experiment, the compounds were dissolved in chloroform (CHCl3), which was used without further distillation. The tri-branched compounds investigated are shown in Fig. 1. In compound T03-a, the electron acceptor (A) group 1,3,5-triazine serves as the central core, and three electron donor (D) triphenylamine groups serve as the terminal ends, which forms a tri-branched structural configuration containing three D-π -A subunits. T03-d can be described with the electron-withdrawing perfluoroalkyl chains as side groups that
Journal of Luminescence 190 (2017) 89–93
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Fig. 1. The structures of the 1,3,5-triazine-based multi-branched compounds (T03-a and T03-d).
are linked to the terminal ends of the electron donor triphenylamines. The UV–visible absorption spectra of the compounds in CHCl3 were recorded on a Hitachi spectrophotometer with 2 nm spectral resolution, and the steady-state fluorescence spectra were recorded with dilute solutions (10−5 M) using an Edinburgh FLS 920 spectrometer with 1 nm spectral resolution. The NLO fluorescence spectra were recorded on a TRISTAN light spectrometer. The fluorescence quantum yields of the compounds were measured with Rhodamine B in basic ethanol as a standard. The laser beam (370 nm, 1.4 mW) was slightly focused on the CHCl3 solutions (10−5 M concentration). The spectra were measured and corrected. The detailed experimental conditions can be found in our previous publication [15]. The ultrafast responses of these compounds were investigated by femtosecond (fs) pump-probe experiments. The fs pulses for the NLO absorption and ultrafast transient absorption measurements were generated by the fs laser system (Spitfire, Spectra-Physics). The average output power of the Spitfire was approximately 300 mW. The pulse duration was 140 fs, the central wavelength was 800 nm, and the repetition rate was 1 kHz. The fs pump-probe experimental setup was described previously [16]. The fs pulses from the fs laser system (800 nm), were divided into two parts by a beam splitter. To efficiently pump the compounds into the excited state, a portion of the beam was frequency-doubled to 400 nm by a 0.5 mm thick β -barium metaborate (BBO) crystal. The other portion of the beam was focused on a 5 mm thick cell with flowing water to generate a supercontinuum in order to sample the photoinduced excited state at various wavelengths. A monochromator after the sample cell selected the probe wavelength. To detected dynamics without diffraction effects or coherent artifacts, the polarization of the pump beam was set perpendicular to that of the probe beam [11,17]. All of the measurements were performed at room temperature.
Fig. 2. The one-photon absorption spectra and normalized fluorescence spectra of T03-a and T03-d dissolved in CHCl3. Table 1 A summary of the data for compounds T03-a and T03-d. Compounds
T03-a T03-d
Solvents
CHCl3 CHCl3
1PA-related optical properties
TPA-related optical properties
a λ max (abs)/nm
b λ max (fl)/nm
ϕc
d (TPEF)/ λ max nm
σ e /GM
421 395
535 490
0.27 0.86
545 510
447 603
a λ max is the wavelength for the maximum of the one-photon absorption spectra. b λ max is the wavelength for the maximum of the one-photon emission spectra. c ϕ is the fluorescence quantum yield, which was determined by using Rhodamine B in ethanol as a standard. d λ max is the wavelength for the maximum of the two-photon fluorescence spectra. e σ is the TPA cross-section.
3. Results and discussion 3.1. Linear absorption and fluorescence emission of T03-a and T03-d
band gap of the compound. Usually, in a conjugated system with delocalized π electrons, stronger ICT effects will cause a redshift in the absorption spectra. Thus, modification with perfluoroalkyl chains linked to the end of terminal triphenylamine groups may not lead to a stronger ICT effect. In the following investigations, the effect will be further discussed.
The absorption and normalized fluorescence spectra of the tribranched compound T03-a and perfluoroalkyl chain modified compound T03-d dissolved in CHCl3 in dilute concentrations (10−5 M) are shown in Fig. 2. The spectral results are shown in Table 1. Two major absorption bands are observed. The band located at approximately 311 nm is attributed to a localized multi-triazine aromatic π -π * transition, and the band located at approximately 421 nm is attributed to the charge transfer state [18,19]. The charge transfer absorption maximum of T03-d is located at 395 nm, which has a 26 nm blueshift from the T03-a maximum due to the addition of the electron-withdrawing perfluoroalkyl chains as side groups to the end triphenylamine donors. These 1,3,5-triazine-based tri-branched compounds exhibit adequate fluorescence emission. The fluorescence peaks of T03-a and T03-d are located at 535 nm and 490 nm, respectively. These results indicate that modification with perfluoroalkyl chains may increase the
3.2. Fluorescence properties Most two-photon imaging applications employ fs pulses at a wavelength of 800 nm. The two-photon fluorescence properties of the two compounds were investigated at this wavelength. Both T03-a and T03-d emitted intense fluorescence emission under irradiation of unfocused fs pulses with a pulse energy of several microjoules. Since no obvious linear absorption was observed at wavelengths longer than 750 nm, any absorption in this wavelength range would be attributed to 90
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Fig. 3. a, b: The TPF intensities of T03-a and T03-d dissolved in CHCl3. c: the TPF intensity versus the square of the excitation power density.
a multi-photon process [20]. The fluorescence spectra of compounds T03-a and T03-d dissolved in CHCl3 (approximately 10−3 M) under different excitation powers are shown in Fig. 3a and b, respectively. Fig. 3c show the linear dependence of the fluorescence intensity on the square of the excitation intensity, which confirms that the fluorescence emission of the compounds excited by the 800 nm fs pulses is generated by a TPA process. The emission peak wavelengths of T03-a and T03-d are located at 545 nm and 510 nm, respectively, which are redshifted by 10 nm and 20 nm from the fluorescence spectra produced with onephoton. The red-shifts are attributed to reabsorption [21], because we used much higher solution concentrations for the two-photon fluorescence measurements. To quantify the differences in the two-photon fluorescence ability of the perfluoroalkyl chain modified T03-d compared to the T03-a, the fluorescence quantum yield was measured. The addition of perfluoroalkyl chains may not increase the ICT property, which is very important for the enhancement of TPA compounds. The steady-state absorption maximum of the perfluoroalkyl chain modified T03-d is blue-shifted from 421 nm (T03-a) to 395 nm, which is much closer to 400 nm, which creates a stronger two-photon near-resonance effect for the T03-d at 800 nm (the absorption of one photon at a wavelength of 400 nm is equal to the simultaneous absorption of two photons at a wavelength of 800 nm). However, the fluorescence quantum yield of T03-d, which was determined by using Rhodamine B in ethanol as a standard [22], increased by approximately 3.2 times after the modification with perfluoroalkyl chains (0.86) compared to the quantum yield of T03-a (0.27). This result indicates that the perfluoroalkyl chains do play an important role in the fluorescence quantum yield of the 1,3,5-triazine-based tri-branched compounds. In the following ultrafast dynamics section, the enhancement mechanism will be further discussed.
3.3. Ultrafast relaxation processes The increase in the band gap of T03-d compared to T03-a indicates that the perfluoroalkyl chains modification on the 1,3,5-triazine-based tri-branched compound did not cause a stronger ICT effect. Fs transient absorption measurements were performed to continue the study of the effects of the perfluoroalkyl chain modification on the ultrafast relaxation processes and ICT property of the tri-branched compound. The transient absorption spectra obtained from the compounds in CHCl3 solutions are shown in Fig. 4. At the initial time of 0 ps in Fig. 4a, the tri-branched compound T03-a produces a broad photoabsorption region with a transient photobleaching region from 450 to 510 nm. The photoabsorption region is attributed to the excited state absorption of the ICT state, and the photobleaching region is ascribed to the stimulated emission of the fluorescence emission state [3]. As described by Fuks-Janczarek, Sahraoui et al. [23], some essential local effects may contribute to the NLO susceptibility of materials that are under action of the ultrashort laser pulses, and these effects may also play a competition: intra-molecular electronic, ionic and nucleus reorientation including translations, rotations, and vibrations. The contribution of the electronic cloud deformation would be in the femtosecond scale, while the processes related to the nucleus, such as vibration (somewhere, it is called as phonon) and reorientation are mainly in several picosecond scale due to the electron-phonon interaction (also called vibronic relaxation). Hence, at relative long delay time such as 100 ps, the contribution from the electronic cloud deformation and the nucleus vibration can be neglected. Fig. 5a shows the ultrafast dynamic traces of T03-a in CHCl3 probed at various representative probe wavelengths with the calculated time constants of the excited states summarized in Table 2. For a probe wavelength of 425 nm, the main dynamic curve shows two different decay processes: a fast photoabsorption process of approximately 4 ps, and a long photoabsorption process of approxi91
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Fig. 4. The femtosecond transient absorption spectra of compounds T03-a and T03-d in CHCl3 solutions with a delay time of 0–20 ps.
mately 200 ps. The fast process is the formation of the ICT state, and the long decay process reflects the evolution of the ICT state. When the probe wavelength was tuned to 500 nm, a fast photobleaching process emerges. The dynamic curve shows a fast decay process of several ps and a long decay process of approximately 200 ps, and both decay processes show photoabsorption property. When the probe wavelength was tuned to wavelengths longer than 650 nm, the dynamic curves exhibit a fast photoabsorption process of approximately 4 ps and a long photoabsorption process of approximately 200 ps. The transient absorption measurement was also performed on the perfluoroalkyl chain modified compound T03-d, and different ultrafast relaxation processes were observed. In Fig. 4b, a transient photobleaching region of 450–500 nm was also observed for T03-d. However, the time duration of the photobleaching region is considerably longer than that of T03-a, indicating that some of the photobleaching processes have longer lifetimes and stronger relative intensities. The dynamic traces of the perfluoroalkyl chain modified T03-d probed at several representative wavelengths are shown in Fig. 5b, and the fitting time constants are also summarized in Table 2. At a probe wavelength of 400 nm, the dynamic trace of T03-d is similar to the trace from T03-a. When the probe wavelength was tuned to 450 nm, a fast photobleaching process was also observed, which is similar to T03-a. These fast photobleaching processes are explained as the competition between electron-phonon interactions and the formation of the fluorescence emission state. Additionally, the stimulated emission (of photobleaching property) of fluorescence at the probe wavelength affects the signal intensity and the photoabsorption/photobleaching during the long decay process. When the probe wavelength was tuned to 475 nm, both the fast and long decay proceses show photobleaching property at 100 ps, which correlates with the very high fluorescence quantum yield
Table 2 A summary of the time constants for compounds T03-a and T03-d in CHCl3 solutions. Compound
T03-a T03-d
λPump/probe/nm
400/725 400/725
Decay /ps τ1 /ps
τ2 /ps
3.9 8.5
220 > 1000
(0.86). The wavelength of 725 nm is distant from the linear absorption and fluorescence regions of the two compounds and was chosen as a special probe wavelength to compare the time constants of the two tribranched compounds. When the probe wavelength was tuned to 725 nm, the perfluoroalkyl chain modified compound T03-d displayed a fast process reflecting the formation of the ICT state and a long decay process reflecting the evolution of the ICT state. In D-π -A structural tribranched compounds with double bonds as the conjugation bridge, some additional relaxation processes would be included after the evolution of the ICT state: the rotation of the double bonds and the rotation of the electron donor/acceptor form a twisted ICT (TICT) state, which weakly emits [24]. These processes would decrease the fluorescence quantum yield of the compound. However, the time constants of T03-d are different from T03-a, which is illustrated in the dynamic traces and the fitting results. The fast process of the perfluoroalkyl chain modified compound T03-d probed at 725 nm is 8.5 ps, which is substantially longer than that of T03-a, indicating that modification with perfluoroalkyl chains did not increase the ICT property [25]. However, the subsequent long process (reflecting the evolution of the ICT state, also considered as the formation of the TICT state) of T03-d is longer than 1000 ps, which is much longer than that of T03-a (220 ps).
Fig. 5. The dynamic decay traces for compounds a) T03-a and b) T03-d at various probe wavelengths.
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These results support the high fluorescence quantum yield of the perfluoroalkyl chain modified compound T03-d. In principle, the evolution of the ICT state correlates with the fluorescence emission, the longer lifetime of the ICT state, and the larger fluorescence quantum yield. Due to the terminal perfluoroalkyl chains on the modified compound T03-d, the rotation of excited state electron donor/acceptor and the formation of TICT state require additional time, which increases the lifetime of the ICT state. The transient absorption results indicate that modification with perfluoroalkyl chains suppresses the nonradiative decay pathways which increases the fluorescence emission efficiently. Our results indicate that perfluoroalkyl chain modification based on a 1,3,5-triazine tri-branched compound is a suitable strategy for great enhancing fluorescence emission.
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4. Conclusions The fluorescence property, as well as and ultrafast response of perfluoroalkyl chain modified tri-branched compounds based on 1,3,5triazine in CHCl3 solutions were investigated to study the effects of perfluoroalkyl chain modification on the fluorescence and excited state relaxation properties. After modification with perfluoroalkyl chains, the fluorescence emission was enhanced. The ultrafast relaxation processes revealed the effect of the perfluoroalkyl chains modification on the ICT property. Therefore, these 1,3,5-triazine based multi-branched derivatives could be employed as molecular probes for biological fluorescence imaging in the life sciences. These results can also assist in the design and optimization of new molecular probes with both large TPA crosssections and high fluorescence quantum yields. Acknowledgments We sincerely thank the following organizations for the financial support for this project: the National Natural Science Foundation of China (11404048, 11604038, 11375034), the Liaoning Provincial Natural Science Foundation of China (201602061, 201602062), the Program for Liaoning Educational Committee (L2015071), and the Fundamental Research Funds for the Central Universities (3132017060,
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