Probing ultrafast excited state dynamics and nonlinear absorption properties of three star-shaped conjugated oligomers with 1,3,5-triazine core

Abstract

The effects of different bridging fluorene units on the two-photon absorption (TPA) properties and ultrafast response of organic conjugated oligomers were investigated for three star-shaped oligomers named TFT-1, TFT-2 and TFT-3. These three oligomers are composed of the same central core 1,3,5-triazine, with three arms that consist of specific numbers of bridging fluorene groups and with triphenylamine as electron-donating group at the terminal end of each arm. The studies on these oligomers were carried out by using two-photon excited fluorescence (TPF), degenerate pump–probe techniques, transient absorption spectroscopy and time-resolved photoluminescence (TRPL) methods. The TPA cross-sections were determined to be 1509 GM, 1260 GM and 789 GM for TFT-1, TFT-2 and TFT-3, respectively, decreasing with the increase of bridging fluorene number. Ultrafast dynamics results show that there is a fast intramolecular charge transfer (ICT) formation time of about several ps and a relatively long decay process of the ICT state. The formation time of ICT was found to increase from TFT-1 (1.9 ps), to TFT-2 (3.0 ps) and TFT-3 (6.3 ps), with the increased number of the fluorene bridge, which may explain the action of fluorene bridge on the electron transmission properties, the ICT properties and the TPA behavior.

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1 Introduction

The two-photon absorption (TPA) process is a kind of nonlinear optical (NLO) process that requires two photons to interact simultaneously with a molecule. There are several unique advantages of the TPA process in comparison with the one-photon case. First, the quadratic dependence of absorption on the incident light intensity permits a highly confined spatial excitation and high three-dimensional (3D) resolution. Second, due to the fact that the light scattering efficiency depends inversely on the excitation wavelength, the TPA process will provide improved penetration in scattering and absorbing media. Third, the application of a longer wavelength excitation light will not only afford deeper penetration of the excitation beam, but also reduced photobleaching and cellular autofluorescence from proteins and other intrinsic fluorophores which typically absorb UV or visible light. Due to these advantages, including nonlinear response, excellent 3D processing capabilities and high spatial resolution, the TPA process shows huge applicability in biology, medicine, physics, chemistry, and many other fields, holding potential applications in two-photon fluorescence (TPF) microscopy,¹ optical limitting,² three-dimensional microfabrication,³ optical data storage,⁴ photodynamic therapy,⁵ two-photon up-conversion lasing,⁶ and so on. Since the observation of TPA in CaF₂:Eu²⁺ crystals, a wide range of materials have been studied for their TPA properties.⁷ In 1963, Peticolas and Rieckhoff observed the TPA process in an organic dilute solution.⁸ In the middle of the last century, the organic conjugated systems with TPA properties attracted lots of attention as soon as the TPA dyes with large TPA parameters were first synthesized. Thereafter, design and synthesis strategies have been developed, and a series of organic conjugated materials with relatively superior TPA properties have been synthesized.

At present, there are mainly two kinds of materials being developed: organic small molecules are prone to crystallization, and typically high temperature vacuum deposition has to be used to fabricate the devices. Although polymers can be processed simply with methods such as spin-coating and inkjet printing, these materials are difficult to purify, and thus the stability and reproducibility remain big problems. However, monodispersed oligomers or dendrimers can combine the advantages of both small structure and conjugated polymers together. They have good solution processability and well-defined molecular structures, with high purity. In addition, star-shaped D-π-A (D is electron donor and A is electron acceptor) conjugated molecules are known to afford good nonlinear optical and TPA properties compared with their monomer counterparts due to the intramolecular charge transfer and cooperative enhancement effects.^9–11 Therefore, a useful design strategy for the synthesis of efficient TPA materials would be to incorporate a D-π-A or D-π-D type of TPA material into the multibranched structure.

The application of TPA materials will greatly benefit from large TPA cross-section or strong TPF emission, or both. Hence, producing materials with large TPA cross-section and strong TPF has become one of the hottest research topics. Unfortunately, most known organic molecules have relatively small TPA cross-section. Therefore, it becomes critical to understand the structure–property relationships to design and synthesize highly efficient two-photon absorbing chromophores. In that case, a proper understanding of the excited state dynamics of the two-photon excitation is of great importance. In past years we investigated the NLO properties and ultrafast dynamics of several organic conjugated systems by using ultrafast spectroscopy,^12–16 and many other scientists did a lot of quantum-chemical studies of structure and spectral properties of organic conjugated compounds.^17–19 All of these results provide important contributions to better understanding of the structure–property relationship and in guiding the synthesis and optimization of new compounds.

1,3,5-Triazine containing materials have been widely used in industry owing to their high thermal stability derived from the structural symmetry of 1,3,5-triazine units,^20,21 and recently quite a bit of attention has been paid to 1,3,5-triazine containing π–conjugated systems because of the unique properties such as high electron deficiency and spatial coplanarity.^22–26 Fluorene is frequently used as an efficient building block for the construction of chromophores with large TPA properties, and excellent thermal and photochemical stabilities. However, there have only been limited reports on the ultrafast response of 1,3,5-triazine based oligomers. In this paper, a series of highly soluble 1,3,5-triazine based D-π-A star-shaped oligomers, TFT-1, TFT-2, and TFT-3, were prepared through a convergent synthetic strategy. All these three oligomers are composed of the same center core 1,3,5-triazine with three arms that consist of specific numbers of bridging fluorene units and with triphenylamine as electron-donating group at the terminal end of each arm. The TPA properties and ultrafast response were fully studied, which showed distinct correlations with the structures.

2 Materials and experimental section

The synthesis methods of TFT-1, TFT-2 and TFT-3 were reported elsewhere in detail.²⁷ As shown in Fig. 1, these star-shaped oligomers employ electron-withdrawing 1,3,5-triazine group as the central core, with different numbers of dialkylfluorene aromatic fluorene units between 1,3,5-triazine core and strong electron-donating group triphenylamine at the terminal end of each arm. It was found that the addition of moderately long alkyl pendants to the fluorene bridging group improved solubility effectively. Tetrahydrofuran (THF) has been used without further distillation.

Fig. 1 Molecular structures of TFT-1, TFT-2, and TFT-3.

The UV-visible absorption spectra in THF were recorded on a Hitachi spectrophotometer with 2 nm spectral resolution and the steady-state one-photon induced fluorescence spectra were recorded in dilute solutions (10⁻⁵ M) by using an Edinburgh FLS 920 spectrometer with 1 nm spectral resolution.

The TPA spectra of three oligomers were measured by using laser pulses of 650–810 nm generated by a mode-locked Ti:sapphire laser system (OPA, Spectra-Physics), with 150 fs pulse duration, 1 kHz repetition rate. All the results of TPA cross-section were measured by using two-photon excited fluorescence (TPF) method.²⁸ The TPF spectra were recorded by a TRISTAN light spectrometer. The ultrafast responses of these oligomers were investigated by fs pump–probe and time-resolved photoluminescence (TRPL) experiments. The setups of which have been described in ref. 29. The fs pulses employed in ultrafast dynamics measurements were generated by an amplification stage of the used fs laser system (Spitfire, Spectra-Physics). The average output power from the Spitfire was about 300 mW. The pulse duration was 140 fs, the wavelength was 800 nm and the repetition rate was 1 kHz. All the experiments were carried out at room temperature.

3 Results and discussions

3.1 Linear absorption and fluorescence spectra

The linear absorption and steady state fluorescence spectra of the three oligomers in THF are shown in Fig. 2, and the optical properties are summarized in Table 1. No obvious absorption is observed in the wavelength range longer than 475 nm. Interestingly, the absorption spectra as well as the fluorescence spectra peaks show a gradual blue-shift with increasing fluorene number in each arm, from TFT-1, to TFT-2 and TFT-3. For example, the absorption peaks of TFT-2 and TFT-3 are located at 390 nm and 380 nm, respectively, showing a 2 nm and 12 nm blue-shift in comparison with TFT-1 (392 nm). The fluorescence peaks of TFT-2 and TFT-3 are located at 508 nm and 456 nm, respectively, showing 12 nm and 64 nm blue-shifts in comparison to TFT-1 (520 nm). These results indicate that the expansion of the fluorene bridges could increase the band gap. Usually, in a larger conjugated system with delocalization of π electrons, stronger interaction among chromophores and stronger ICT effect will cause the red-shift of the absorption spectra. Thus, the increase of fluorene bridge number in each arm may not lead to a stronger ICT effect. In the following parts, we will further approve the result.

Fig. 2 The linear absorption spectra and steady-state fluorescence spectra of TFT-1, TFT-2 and TFT-3 dissolved in THF.

Table 1 Optical properties of TFT-1, TFT-2 and TFT-3 in THF

Compound	Number of fluorenes in each arm	λabs^a nm	λflu^b nm	Δs^c nm	Φf^d	λTPE^e nm	σMax^f GM
a The peak wavelength of absorption spectra.b The peak wavelength of one-photon fluorescence.c Stokes shifts are calculated from the absorption and emission spectra.d Fluorescence quantum yield.e The peak emission wavelength by two-photon excitation.f The maximum value of TPA cross-section measured by TPF method (1 GM = 10^−50 cm^4 s per photon).
TFT-1	1	392	520	128	0.47	528	1509
TFT-2	2	390	508	118	0.32	521	1260
TFT-3	3	380	456	76	0.16	460	789

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3.2 Two-photon absorption and two-photon fluorescence properties

All three oligomers have typical D-π-A type structure. The three oligomers TFT-1, TFT-2 and TFT-3 emitted intense up-conversion fluorescence under the excitation of fs pulses at a wavelength of 800 nm. The TPF can be easily observed by the naked eye even when excited by unfocused laser pulses with pulse energy of several microjoules, indicating the large TPA cross-section and high fluorescence quantum yield as well as resultant promising application in various fields such as TPF imaging and up-conversion lasing. The peak wavelengths of fluorescence emission located at 528 nm, 521 nm and 460 nm for TFT-1, TFT-2 and TFT-3, respectively, are 8 nm, 13 nm and 4 nm red-shifted in comparison with steady state fluorescence spectra. The red-shift could be attributed to the reabsorption effect,³⁰ as we used solutions with a much higher concentration (∼0.005 M) in the TPF measurement than in the one photon case (∼10⁻⁵ M); the fluorescence at shorter wavelength was reabsorbed by the solution with high concentration leading to red-shifts of the fluorescence spectra. Fig. 3 shows the TPF spectra of three oligomers excited at different intensities, and the inset shows the linear dependence of fluorescence intensity on the square of the excitation intensities, which confirms that the strong fluorescence emission excited by the 800 nm fs pulses is really generated by the TPA process.

Fig. 3 TPF intensities under different excitation power densities in THF. Insets show TPF intensity dependence on the square of the excitation power density.

In order to quantify the TPA/TPF ability of these oligomers, fluorescence quantum yield and TPA cross-section of the three oligomers were measured. The fluorescence quantum yields of THF solutions were determined to be 0.47, 0.32 and 0.16 for TFT-1, TFT-2 and TFT-3, respectively, using Rhodamine B (η = 0.64 in methanol, excited at 510 nm) as reference. The TPA spectra of these star-shaped conjugated oligomers with different number of fluorene bridges are measured in THF at a concentration of 0.001 M. The TPA cross-section values were calculated by the following equation.

σ_s = σ_r(S_s/S_r)(Φ_r/Φ_s)(φ_r/φ_s)(c_r/c_s) (1)

S is the integral area of the measured TPF spectrum, Φ is the fluorescence quantum yield, φ is the overall fluorescence collection efficiency, and C is the molar concentration of the compound. In the TPF measurement, the collection efficiency of samples and reference compound is the same. Rhodamine B was chosen as the reference compound. The measured TPA spectra of the three oligomers are shown in Fig. 4, and the data are also summarized in Table 1. At a wavelength of 810 nm, the TPA cross-sections were determined to be 480 GM (1 GM = 1 × 10⁻⁵⁰ cm⁴ s per photon), 415 GM and 378 GM for TFT-1, TFT-2 and TFT-3, respectively, decreasing with the number of fluorene bridge groups. When the excitation wavelength was changed to the shorter wavelength region, the value of the TPA cross-section increased gradually. The TPA cross-section values at a wavelength of 650 nm were determined to be 1509 GM, 1260 GM and 789 GM for TFT-1, TFT-2 and TFT-3, respectively. AF-455 and its derivatives (AF-450, AF-457) have similar structures to our compounds and are known to have large two-photon absorption cross-sections and are therefore promising optical materials for many applications.³¹ AF455 was also doped in PMMA polymers to study the photodegradation and recovery of two-photon fluorescence.³² These compounds were also widely studied by theoretical calculation.³³ The TPA cross-sections of our compounds are larger than those of some other triazine derivatives T3 (407 GM), AF-450 (137 GM), AF-455 (127 GM), AF-457 (112 GM).^12,31 It is well-known that higher strength of electron acceptor–donor group will cause a greater degree of charge transfer in the donor–acceptor system, resulting in a higher nonlinear optical response. The diphenylamine groups in our compounds TFT-1, TFT-2 and TFT-3 have a much stronger electron-donating ability compared with the phenothiazine group in AF-455. The enhancement of TPA property is due to the stronger electron-donor group of triphenylamine which cause a more effective ICT effect between terminal triphenylamine group and central 1,3,5-triazine core. Among the TFT series, TFT-1 has the strongest TPA ability. Due to the wavelength limitation of the used fs laser system, we cannot measure the TPA cross-section at wavelengths shorter than 650 nm, where a much larger value may be expected. The value of the maximum TPA cross-section decreases by a factor of 0.83 and 0.52 when going from TFT-1, to TFT-2 and TFT-3 (the number of fluorenes in a single arm: 1, 2, 3), in that order, and the fluorescence quantum yield value decreases by a factor of 0.68, and 0.34. This indicates that the TPA activity is mainly attributed to efficient ICT properties of the molecules. Although, TFT-2 and TFT-3 gradually gained in conjugated length as well as delocalization of π electrons, the electron transmission ability of the fluorene group is poorer than common π-conjugaged bridges, such as C–C double bond, C–C triple bond, benzene ring, thiophene. The results observed above indicate that fluorene is not an efficient π-conjugated spacer, the electron transmission ability of π-conjugated spacer is an important parameter for TPA activities of TPA dyes. Besides, electronic interactions between conjugated π-orbitals and atomic orbitals of the main group elements are maximized in the planar configuration, thus, the molecular planarity is also an important parameter for the enhancement of π-conjugation. Zheng, Prasad et al. introduced ladder-type oligo-p-phenylenes into a rigid and planar structure, and found that not only did this greatly facilitate the π-electron delocalization, but it also increases fluorescence efficiencies, because such a structure can efficiently restrict geometric relaxations from the excited states.³⁴ The more fluorenes in a single arm, the worse the ICT properties, probably due to the non-planar biphenyl configuration. In the following section, we will further discuss the relation between structure and the ICT properties.

Fig. 4 Two-photon fluorescence spectra of TFT-1, TFT-2 and TFT-3 dissolved in THF, under the excitation of laser pulses over 650–810 nm.

3.3 Ultrafast excited state dynamics

The band gap of TFT-1 was effectively lowered due to the strong ICT effect. The band gaps of TFT-2 and TFT-3 were increased gradually as the π-spacer became longer. To further study the effect of the fluorene number on the excited state relaxation dynamics and the ICT properties of these D-π-A star-shaped oligomers, femtosecond one/two-color pump–probe and time-resolved photoluminescence (TRPL) measurements were carried out. Shown in Fig. 5a are experimental results of degenerate pump–probe measurements for TFT-1 at a wavelength of 800 nm under parallel and perpendicular polarization configurations. All the curves show transient absorption properties, indicating that the absorption cross-section of excited state is larger than that of ground state. The relaxation curve under parallel polarization configuration shows an ultrafast process about 200 fs, followed by a fast process about 1.9 ps and a long process with lifetime longer than 100 ps. However, the ultrafast process disappeared under perpendicular polarization configuration. Sometimes, the ultrafast peak was ascribed to the possible coherent artifacts or special TPA process which simultaneously absorbs each photon from the pump and probe beam in the transition from the ground state to the two-photon excited state. The fast process of about several ps was ascribed to the formation of the ICT state (including vibration relaxation process from the TPA excited state to the lowest vibration state of the first excited state). The subsequent long process is attributed to the relaxation of the ICT state.¹⁵ Degenerate pump–probe results of TFT-1–3 under parallel polarization configuration are shown in Fig. 5b, and the lifetimes of dynamics are summarized in Table 2. It should be noticed that the formation time of ICT increases from TFT-1 (1.9 ps), to TFT-2 (3.0 ps), to TFT-3 (6.3 ps), indicating the action of the number of the fluorene bridge. This fact can explain the poor electron transmission properties of the fluorene bridge, the resultant poor ICT properties and decrease of TPA behavior.

Fig. 5 (a) Degenerate pump-probe results of TFT-1 at 800 nm in two polarization configurations. (b) The relaxation dynamics results (800–800) of TFT-1, TFT-2 and TFT-3 in parallel polarization configuration.

Table 2 Summary of the time constants of TFT-1, TFT-2 and TFT-3 in THF. P is the shortened form of parallel configuration of pump–probe and Per is the shortened form of perpendicular configuration

Compounds	λPump/probe nm	Decay/ps (%)
		τ1/ps	τ2/ps	τ3/ps
TFT-1	800/800 (P)	0.2	1.9	>100
	400/450 (Per)	1.1 (76.5%)	1964 (23.5%)	—
	400/725 (Per)	2.5 (57.5%)	1818 (42.5%)	—
	400/750 (Per)	2.1 (64.5%)	1950 (35.5%)	—
	400/775 (Per)	1.9 (61.4%)	1900 (38.6%)	—
	TRPL	2.1 ns	11.1 ns	—
TFT-2	800/800 (P)	0.2	3.0	>100
	400/450 (Per)	2.6 (77.6%)	>2000 (22.4%)	—
	400/725 (Per)	3.1 (78.2%)	774 (21.8%)	—
	400/750 (Per)	3.0 (72.9%)	803 (27.1%)	—
	400/775 (Per)	3.1 (56.8%)	740 (43.2%)	—
	TRPL	0.89 ns	6.2 ns	—
TFT-3	800/800 (P)	0.2	6.3	>100
	400/450 (Per)	18.9 (77%)	>2000 (23%)	—
	400/725 (Per)	12.4 (42.3%)	171 (57.7%)	—
	400/750 (Per)	9.4 (36.1%)	158 (63.9%)	—
	400/775 (Per)	9.0 (31.3%)	175 (68.7%)	—
	TRPL	0.28 ns	6.5 ns	—

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We also did fs transient absorption measurements to further investigate the excited state relaxation and the nature of the ICT state. The fs pulses (800 nm) generated by the amplification stage were divided into two parts by a beam splitter. In order to pump the samples to the excited state via linear absorption efficiently, a portion was frequency-doubled to 400 nm by a 0.5 mm thick BBO crystal. Another portion was focused on a 5 mm thick cell with flowing water to generate a supercontinuum which served as the probe pulses. In order to avoid diffraction effects leading to coherent artifacts, the polarization of the pump beam was set to be perpendicular with respect to that of the probe beam.³⁵ Shown in Fig. 6a is the transient absorption spectrum of oligomer TFT-1 in THF. At the initial time of 0 ps, a transient photobleaching band in the region of 420–485 nm and photoabsorption signals are observed in the region longer than 485 nm. The photoabsorption region can be ascribed to the absorption of ICT state, and the photobleaching can be attributed to the stimulated radiation of fluorescence emission. Shown in Fig. 6b are excited state relaxation traces of TFT-1 at two representative probe wavelengths: 450 nm and 750 nm. The fitting time constants at several probe wavelengths are also summarized in Table 2.

Fig. 6 (a) Femtosecond transient absorption spectra of compound TFT-1 in THF in the delay time range of 100 ps. (b) Dynamics decay traces for compound TFT-1 at representative probe wavelengths.

The same transient absorption measurements were also done for oligomers TFT-2 and TFT-3. The transient absorption spectra as well as dynamic traces probed at 450 nm and 750 nm are shown in Fig. 7a and b and Fig. 8a and b, respectively. Similar photobleaching regions were observed in the wavelength ranges 425–485 nm and 425–475 nm for TFT-2 and TFT-3, respectively. The other wavelength ranges show photoabsorption behavior. The fitting time constants at some probe wavelengths are also summarized in Table 2. However, the time constants of TFT-2 and TFT-3 are absolutely different from TFT-1, which can be clearly seen from fitting results as well as dynamic traces. The results are in good accordance with degenerate pump–probe results at a wavelength of 800 nm. The different number of fluorenes affects the ICT process in these oligomers, leading to the decrease of the TPA behavior.

Fig. 7 (a) Femtosecond transient absorption spectra of compound TFT-2 in THF in the delay time range of 100 ps. (b) Dynamics decay traces for compound TFT-2 at representative probe wavelengths.

Fig. 8 (a) Femtosecond transient absorption spectra of compound TFT-3 in THF in the delay time range of 100 ps. (b) Dynamics decay traces for compound TFT-3 at representative probe wavelengths.

In some literature, a dynamic trace which is similar to that measured at 450 nm in this paper, was ascribed to a new transient, a solvent-stabilized and conformationally relaxed ICT state. However, after cautious consideration, we ascribe the traces to a competitive and composite result of pure excited state dynamics and stimulated radiation of fluorescence emission. It can be seen from Fig. 9 that the dynamic traces show regular transformation with the probe wavelength tuning in the range of 475–725 nm. Besides, the rise and decay dynamics of the fluorescence (photobleaching signal) show obvious wavelength dependence.³⁶ Thus, the detected dynamics traces could be ascribed to the competition between the pure excited state relaxation and stimulated emission. The observed fast component of about several ps is ascribed to the formation of the ICT state (competition of two components: electron–phonon interaction, elsewhere it is called vibronic relaxation, and the formation of the fluorescence emission state). The long decay component is attributed to the decay of the ICT state which is always accompanied with strong fluorescence emission. The stimulated emission of fluorescence at the probe wavelength may affect the signal intensity of the long process. In principle, the lifetime of the long process could reflect the evolution of the ICT state, which is also in good accordance with the fluorescence quantum yields of these oligomers. The lifetimes of fluorescence for the three oligomers were also measured by TRPL method and the fitting results are summarized in Table 2. The fluorescence lifetimes of these oligomers accord well with the fluorescence quantum yields (Fig. 10).

Fig. 9 The competition between pure excited state relaxation and stimulated emission, (a) for TFT-2, (b) for TFT-3.

Fig. 10 Time-resolved photoluminescence results of TFT-1, TFT-2 and TFT-3.

4 Conclusions

In conclusion, a series of 1,3,5-triazine based star-shaped conjugated oligomers with different number of dialkylfluorene bridging fluorene units and with electron-donating group triphenylamine at the terminal end of each arm, were systematically investigated. The photophysical properties as well as ultrafast responses of these oligomers showed distinct correlations with their specific structures. TPF measurement indicates that all of these oligomers showed intense frequency up-converted fluorescence emission. Among them, TFT-1 shows the largest TPA cross-section and fluorescence quantum yield due to the strongest ICT effect. The observed nonlinear optical results indicate that the electron transmission ability of π-conjugated spacer is an important parameter for TPA activities of TPA dyes. The more bridging fluorene units in a single arm, the worse the ICT properties, probably due to the non-planar biphenyl configuration, and poor electron transmission ability. In order to study the length of the oligomeric fluorene arms on the effect of ICT, fs transient absorption technique was used. The formation of ICT state takes a longer time from TFT-1 (1.9 ps), to TFT-2 (3.0 ps) and TFT-3 (6.3 ps), increasing with the number of the fluorene bridge, which can explain the poor electron transmission properties of the fluorene bridge and resultant decrease of ICT properties and TPA behavior.

Acknowledgements

We sincerely thank the financial support from National Natural Science Foundation of China (no.60978055, 11004042, 10705007, and 11005025.) and “The Fundamental Research Funds for the Central Universities 3132013104, 3132013106”.

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