J. Phys. Chem. B 2010, 114, 11737–11745
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Gigantic Two-Photon Absorption Cross Sections and Strong Two-Photon Excited Fluorescence in Pyrene Core Dendrimers with Fluorene/Carbazole as Dendrons and Acetylene as Linkages Yan Wan,† Linyin Yan,† Zujin Zhao,‡ Xiaonan Ma,† Qianjin Guo,† Mingli Jia,† Ping Lu,*,‡ Gabriel Ramos-Ortiz,*,§ Jose´ Luis Maldonado,§ Mario Rodrı´guez,§ and Andong Xia*,† The State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China, Department of Chemistry, Zhejiang UniVersity, ´ ptica A.P. 1-948, Hangzhou 310027, P. R. China, and Centro de InVestigaciones en O CP 37000 Leo´n, Guanajuato, Me´xico ReceiVed: May 27, 2010; ReVised Manuscript ReceiVed: July 10, 2010
We report a series of stiff dendrimers (referred to as T1, T2, T3, and T4) that have both gigantic two-photon absorption (TPA) cross sections up to 25 000 GM and strong two-photon excited fluorescence (TPEF) with fluorescence quantum yield of ∼0.5. The large TPA cross sections and high quantum yields of these dendrimers are directly related to their geometrical structures, where the polycyclic aromatic pyrene is chosen as the chromophoric core because of its planar and highly π-conjugated structure, fluorene moieties as dendrons extend the conjugation length through the planar structure, and carbazole moieties are modified at three-, six-, and nine-positions as electron donor. All of these groups are linked with acetylene linkage for effective π-electron delocalization, leading to large TPA cross section and high fluorescence quantum yield. The spectral properties of all dendrimers are investigated by one- and two-photon excitations. Furthermore, steady-state fluorescence excitation anisotropy and quantum chemical calculation are also employed to determine the structure-related mechanism of these dendrimers with gigantic TPA cross sections and high TPEF efficiency. We then show that the improvement of branched chains in the T-series dendrimers enhances the light-harvesting ability. The core emission spectra, fluorescence quantum yield, and fluorescence lifetime are almost invariable by directly exciting the dendrons. These results will provide a guideline for the design of useful two-photon materials with structural motifs that can enhance the TPA cross-section and fluorescence quantum yield of a molecule without causing a red shift of the one- and two-photon excitation wavelengths for specific applications. 1. Introduction In the last several decades, the nonlinear optical process of two-photon absorption (TPA) has become an area of intensive research, owing to its potential applications for optical limiting, 3-D microfabrication, photodynamic therapy, high-density optical storage, holographic data storage, frequency-upconverted lasing, and bioimaging.1-5 Much effort has been made to design and synthesize molecules with high TPA efficiency. In previous research, dipolar, quadrupolar, and multibranched molecules are the most frequently employed structural motifs for TPA materials.2,6 It is known that 2-D arrays, macrocycles, and dendrimers offer new and enhanced TPA effects for this field.7 For 2-D arrays and macrocycles, large TPA cross sections (over 105 GM, 1 GM ) 10-50 cm4 s/photon) have been achieved, but the fluorescence quantum yields are not valuable, mostly lower than 0.05.7,8 This limits their application in some fields that require strong two-photon excited fluorescence (TPEF), such as upconverted lasing and bioimaging.9-23 Dendrimers are synthetic, highly branched, monodisperse macromolecules that consist of a central core, one or more dendrons, and terminal groups.24 Their 3-D architectures mimicking that of natural photosynthetic centers give them * To whom correspondence should be addressed. E-mail: andong@ iccas.ac.cn (A.X.);
[email protected] (P.L.);
[email protected] (G.R.-O.); † Chinese Academy of Sciences. ‡ Zhejiang University. § ´ ptica. Centro de Investigaciones en O
advantages for light harvesting.25 Two features are unique to the dendritic structure. One is the “antenna effect”. The number of donor chromophores (periphery) can far exceed the number of acceptor chromophores (core moieties) and is a strict function of generation. The light harvesting ability (for both one- and two-photon absorptions) of donors can therefore be increased predictably, and the massive energy could transfer to the only acceptor by some approaches,26,27 resulting in a great amount of emission. The other is the “shell effect”. By providing a dense shell around the incorporated chromophores, dendrimers effectively prevent core molecules from aggregating and self-quenching, leading to highly efficient emission. Thereby, the dendritic structure is one of the best structures to design TPA materials with highly efficient TPEF and large TPA cross section.28,29 Anyway, the design of these materials calls for detailed understanding of underlying structure-property relations. In this Article, we report a series of stiff dendrimers (hereafter referred to as T1, T2, T3, and T4), which have gigantic TPA cross sections (as high as 25 000 GM) and strong TPEF (fluorescence quantum yield ΦF ≈ 0.5). The great TPA performance of these dendrimers is directly related to their geometrical structures, where their structural formulas are shown in Scheme 1. The dendrimers consist of a pyrene core, fluorene/ carbazole dendrons, and acetylene linkages. Polycyclic aromatic pyrene is chosen as the chromophoric core because of its excellent photoluminescence efficiency and its planar and highly π-conjugated structure.3,30 Fluorene has been extensively em-
10.1021/jp104868j 2010 American Chemical Society Published on Web 08/24/2010
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SCHEME 1: Molecular Structures of Dendrimersa
a
(a) T1, (b) T2, (c) T3, (d) T4, (e) the branch in second generation of T3, and (f) the branch in second generation of T4.
ployed in TPA material research because of its capability of extending the conjugation length through the planar structure.3 Carbazole is easily modified at its three-, six-, and ninepositions31 and acts as an electron donor.32 Acetylene linkage facilitates effective π-electron delocalization and ensures the planarity of the molecule.33,34 The spectral properties of all dendrimers are investigated by one-photon absorption and fluorescence spectroscopy as well as by following their TPA spectra and comparing their TPA cross sections at spectral
maximum positions to extract the corresponding two-photon properties. Furthermore, steady-state fluorescence excitation anisotropy and quantum chemical calculation are also employed to determine the structure-related mechanism of these dendrimers with gigantic TPA cross sections and high TPEF efficiency. 2. Materials and Methods 2.1. Materials. The synthesis of dendrimers (T1-T4) was described in ref 35, where the structures of the dendrimers were
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verified by 1H and 13C NMR spectroscopies, elemental analysis, and MALDI-TOF MS measurement. Rhodamine B, quinine sulfate, and polystyrene were purchased from Sigma-Aldrich. Toluene, methanol, tetrahydrofuran (THF), and sulfuric acid used in this work were AR grade or higher. Water from a Milli-Q Millipore system was used for experiments. 2.2. Methods. One-photon absorption spectrum and fluorescence spectrum were measured with a UV-vis spectrophotometer (U3010, HITACHI) and a fluorescence spectrophotometer (F4600, HITACHI), respectively. Fluorescence quantum yields (ΦF) of the dendrimers were measured by using quinine sulfate (ΦF ) 0.546, in 0.5 M H2SO4) as a standard.36 The energy transfer efficiencies (φET) for dendrimers in toluene solution were estimated from the ratio of fluorescence excitation spectrum and absorption spectrum in the absorption maximum of donor (branched chain) after normalizing the acceptor’s (pyrene) absorption maximum.37 Fluorescence excitation anisotropy spectra were measured by a fluorescence spectrophotometer (F4600, HITACHI) with two polarizers in excitation and detection light routes, respectively. The anisotropy (r) was calculated with
r)
I| - GI⊥ I| + 2GI⊥
(1)
where I| and I⊥ are the polarized fluorescence intensities parallel and perpendicular to excitation polarization, respectively; G (G ) I⊥/I|) is the geometrical factor of fluorescence spectrophotometer when the excitation is vertically polarized.38 To avoid fast rotation of molecule in solution, dendrimers were dissolved in toluene solutions with saturated polystyrene during fluorescence excitation anisotropy spectra measurement. Fluorescence lifetimes (τF) were measured by time-correlated single-photon counting (TCSPC). The measurement setup was previously described.39 Our laser system for TPEF measurements comprised a Ti/ sapphire femtosecond oscillator (Mira-seed, Coherent) pumped by a continuous wave 532 nm laser (Verdi-5, Coherent) and a 1 kHz repetition rate Ti/sapphire regenerative amplifier (Legend, Coherent), which produced a 50 fs pulses at 2.5 mJ energy per pulse. The pulses from the amplifier were parametrically converted with an optical parametric amplifier (TOPAS, Coherent), which yielded 50 fs pulses in the range of 470-2700 nm. The pulse width of the femtosecond laser from TOPAS was controlled by a standard grating pair. The TPEF was collected with 90° geometry and recorded with a highly sensitive LNCCD detector (CCD-3000 V, Jobin Yvon) attached to a highquality emission spectrophotometer (TRIAX 320, Jobin Yvon). The TPA cross sections (δ) of samples (in the wavelengths of 610-870 nm) were obtained following the TPEF method described in refs 40 and 41. Rhodamine B in methanol was used as a reference. During the TPEF measurement, the 1 kHz repetition-rate pulses were used to avoid the excited-state absorption, which sometimes occurred with high repetition rate (such as 76 MHz) or nanosecond pulses leading to higher effective cross sections after the direct TPA process.42,43 After that, and to know the actual effect of the pulse repetition rate on the determination of δ, the TPEF measurement was repeated, but now with 100 fs pulses at 76 MHz repetition rate (delivered from a Ti/sapphire laser, Tsunami Spectra Physics). In this case, however, because of the intrinsic limitations of tunability in femtosecond oscillators, the TPEF experiments were performed only in the 740-810 nm wavelength range. We calculated the
two-photon absorption spectra by normalizing the TPEF excitation spectra to the TPA cross sections in the 610-870 nm range, where the pulse duration and energy were almost kept the same at different wavelengths during the TPEF measurements. In the TPA cross-section per molecular weight (δmax/MW) calculation, the MW was calculated by assuming that alkyl ) methyl, because the long chain alkyl groups will not contribute to δmax/ MW.3 To test the possible aggregation effects in spectral properties, the TPEF characterization of nanoparticles obtained from the T-series dendrimers was also performed. To prepare these nanoparticles, the reprecipitation method was employed: the materials were dissolved in THF at the concentration of 1 × 10-4 M. Then, a small volume of these solutions (typically between 0.25 and 0.5 mL) was injected quickly in 8 mL of poor solvent (aqueous solution of cetyl trimethyl ammonium bromide (CTAB), 0.08 mM) while the mixture was sonicated. The THF was then removed from the aqueous solution by partial evaporation under vacuum, followed by filtration through 200 nm PTFE membrane filters. The amount of each dendrimer contained in the aqueous solution of nanoparticles before filtration was determined directly from the volume and concentration of THF solution injected in water. The absorbance of the aqueous solution was then measured, and the molar absorption coefficient was calculated. After filtration, the measurement of the absorbance allowed us to calculate the final molar concentration. With this procedure, it was observed that >80% of the dendrimers used in the reprecipitation method were retained in the solution of nanoparticles. We optimized the ground-state geometries of all molecules studied by using the AM1 method. The electronic excited states had also been computed by ZINDO method with the corresponding ground-state optimal conformations. The ZINDO calculations were used to construct the charge difference densities (CDDs) according to refs 44 and 45, where the CDD means the difference in electron densities upon electronic transition between the ground and excited states. All calculations were carried out using the Gaussian 03 package.46 3. Results and Discussion 3.1. Two-Photon Excitation Properties of T-Series Dendrimers. In T-series dendrimers, the chromophore of the polycyclic aromatic pyrene core has high photoluminescence efficiency (ΦF ) 0.60 in cyclohexane).47,48 Fluorene has been reported extensively with large TPA cross section because of its capability of extending the conjugation length through the planar structure.1-4,6 Carbazole is the junction between different branches through three-, six-, and nine-positions31 and acts as an electron donor in dendrimers.32 Acetylene (phenylethynyl) linkages facilitate effective π-electron delocalization and ensure the planarity of the molecule.33,34 Therefore, the specific dendritic structure with pyrene as π-core, fluorenes and ethynylenes (phenylethynyls) as π-conjugated bridges, and sequentially carbazoles as terminal π-electron donors is expected to be one of the most remarkable structural motifs for the T-series dendrimers in this Article, simultaneously with large TPA cross section and high TPEF quantum yield.1-3,6 Figures 1 and 2 show the two-photon absorption spectra and TPE fluorescence of T-series dendrimers, where the corresponding one-photon excitation (OPE) emission spectra are also shown for comparison. The power-law dependence of TPEF spectra of T-series dendrimers is checked with the slope in the range of 1.94 to 1.97, indicating the typical two-photon excitation process without photodegradation or saturation. As shown in
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Figure 1. Two-photon absorption spectra of dendrimers in toluene. Incident power was 5 mW. Inset: Dependence of the two-photon excited fluorescence intensity on incident power at 660 nm. The lines represent a linear fit of the logarithmic data.
Figure 2. Normalized two-photon excited fluorescence spectra (red, λex ) 740 nm) of dendrimers in toluene (1 × 10-5 M). One-photon excited fluorescence spectra are also shown for comparison (blue, λex ) 370 nm).
Figure 1, these dendrimers have extremely high TPA cross sections in a very broad excitation wavelength range from 610 to 870 nm. Furthermore, the TPA absorption spectra of T-series compounds show large blue shifts with respect to one-photon absorption spectra (Figure S1 of the Supporting Information), which results from the different selection rules for one- and two-photon excitations, or alternatively, the vibronic transition in TPA borrows most of the energy from the pure electronic transition.6,49,50 Even so, the TPEF spectra are almost identical to the one-photon excited fluorescence and are independent of excitation wavelengths (610-870 nm), indicating that the emission results from the same excited state (S1) by either oneor two-photon excitations. It should be noted that the excellent two-photon performances of T-series compounds are strongly dependent on their unique molecular structure of dendrimers. For example, it is seen that in T1 (Figure S2 of the Supporting Information), pyrene, fluorene, and benzene comprise a large planar system through the rigid acetylene linking, which could help to increase the π-electron delocalization and simultaneously enhance the TPA cross section.51 The intramolecular interaction among the branched multichromophores leads to large TPA cross-section through the acetylene-linked pyrene core.52 We then obtained the TPA cross section of T1 of ∼8300 GM. To estimate the observed TPA properties of T-series compounds quantitatively, we take AF-50, an analogue of the single-branch counterpart of T1, as reference,32 where the TPA cross-section of AF-50 was reported to be 30 GM under femtosecond excitation. The observed TPA cross-section of T1 is much larger than the sum of four single-branch counterparts, suggesting that the cooperative enhancement of TPA among different branches in T1 is involved. The TPA cross section further increases to ∼10 800 GM in T2 when one more fluorene unit is added to each branch of T1 to increase the conjugation length. Furthermore, by
Wan et al. connecting new groups and branches to each branch of T1, the much large TPA cross sections are obtained up to ∼15 200 GM for T3 and 25 000 GM for T4, respectively. The obtained TPA cross sections of T-series dendrimers are listed in Table 1. It is worth mentioning that the TPA cross sections measured with amplified femtosecond pulses at 1 kHz repetition rate resulted practically identical to those measured with nonamplified femtosecond pulses at 76 MHz repetition rate; this was probed at least for the wavelength range 740-810 nm in which the femtosecond oscillator could be tuned. This means that twophoton processes due to excited-state absorption were absent or were negligible, even under pulse excitation at 76 MHz repetition rate. Materials, such as the T-series dendrimers, that exhibit intrinsic two-photon absorption and have large values of δ under pulsed excitation at high repetition rates are of major interest; this is because in TPA applications, a femtosecond oscillator is more accessible and practical than a femtosecond amplifier system. Furthermore, to know the usefulness of the T-series compounds for applications, it is necessary to estimate the ratio δmax/MW. The δmax/MW values of T1-T4 are all larger than 2.45, which are valuable because molecules with δmax/MW > 1.0 have been found to be useful for applications, such as optical limiting, 3-D microfabrication, and bioimaging.3 Therefore, it is concluded that the extending π-conjugation length in each branch plays an important role in the enhanced TPA cross sections because of the rigid T-series dendrimers. Following structure-dependent OPE properties of T-series dendrimers could help for further understanding the observed TPE properties. 3.2. Steady-State Absorption and Fluorescence Spectroscopy. Figures 3 and 4 show the one-photon absorption spectra, one-photon excited fluorescence spectra, and fluorescence excitation anisotropy spectra of the compounds. A summary of the spectral properties for all compounds are given in Table 1. Combining with the fluorescence excitation anisotropy spectra, three distinct absorption bands of T1-T4 are distinguished. The absorption band in the region of 300-350 nm is ascribed to the nonconjugated carbazole groups or the higher excited state of pyrene,37,53-55 the band in the region of 350-420 nm is mainly contributed by fluorene chromophores and/or conjugated fluorene/carbazole dendrons,56 and that in the region of 420-525 nm is assigned to the absorption of pyrene core (pyrene ring with a certain extension).57 Taking a closer look at the absorption spectra of all compounds, it is found that the absorption positions at 300-350 nm remain almost unchanged, suggesting that the terminal carbazole groups are not well conjugated with fluorene groups in the branches of these T-series dendrimers because of long arms and steric effect. The absorption of carbazole groups in the region 300-350 nm in T3 and T4 increases by almost three times that in T1, resulting from the addition of nonconjugated carbazole groups. Furthermore, the linear absorption at 350-420 nm shows a slight red shift from T1 to T2 or from T3 to T4, resulting from the increased conjugation length, in which one more fluorene unit was added to each branch. The molar absorption extinction coefficients of the visible band in the region of 420-525 nm from pyrene core remain constant because the peripheral fluorene/carbazole dendrons in the second generation do not efficiently conjugate to the pyrene core through intermediate 3,6,9-carbazole linkages in the first generation.58-62 On the contrary, the molar extinction coefficient of the UV band around 350-420 nm with two peaks (or shoulder) around 375 and 400 nm corresponding to the conjugated fluorene/carbazole dendrons increases with increasing generation, mainly resulting from the increasing number of
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TABLE 1: Summary of One- and Two-Photon Properties for the Compounds Studied compounds
λabs. (nm) (ε/105 M-1 cm-1)a
λem. (nm)b
τF (ns)c
ΦFd
φETe
δmax (GM)f
δmax/MWg
T1
392 (2.27) 494 (1.00) 398 (3.72) 496 (1.00) 377 (8.53) 494 (1.00) 383 (12.44) 494 (1.00)
522
1.5
0.51
0.98
3.71
523
1.4
0.52
0.99
522
1.4
0.50
0.97
414 522
1.4
0.49
0.95
8300 (620 nm) 10800 (660 nm) 15200 (620 nm) 25000 (660 nm)
T2 T3 T4
3.47 2.45 3.15
a Wavelengths of one-photon absorption spectra peaks. b Wavelengths of one-photon excited fluorescence spectra peaks. c Fluorescence lifetime, λex ) 370 nm. d Fluorescence quantum yield, λex ) 370 nm. e Energy transfer efficiency. f Maximum of two-photon absorption cross-section and corresponding wavelength. The error of measurement of δmax is ∼10%. g Two-photon absorption cross-section per molecular weight.
Figure 3. UV-vis absorption spectra (left) of dendrimers in toluene (1.0 × 10-6 M) and normalized fluorescence spectra (right, fluorescence intensity relative to T4) in toluene (1.0 × 10-7 M) under one-photon excitation (λex ) 370 nm).
Figure 4. Absorption spectra (left) of T1-T4 (1 × 10-6 M) in toluene and fluorescence excitation anisotropy spectra (right, monitored at λem ) 520 nm) of T1-T4 (1 × 10-6 M) in saturated toluene solutions of polystyrene.
chromophores rather than extending conjugations. Furthermore, the intensity of the absorption peak around 375 nm in T3 (T4) shows an obvious increase with respect to that around 400 nm in T1 (T2), indicating that the branched chains in both the first and second generations are not well conjugated in both T3 and T4, leading to some short conjugated segments in the branches. As mentioned above, the absorption of the pyrene core remains almost constant, but the absolute fluorescence intensity of the pyrene core increases from T1 to T4 upon excitation at 370 nm in the absorption region of fluorene dendrons, as shown in Figure 3, indicating efficient energy transfer from branches to the core. No obvious emission from the branches in T1, T2, and T3 was observed, suggesting the efficient intramolecular fluorescenceresonance energy transfer (FRET) from branches to the core. By comparing the fluorescence excitation spectra and absorption spectra,37,63,64 the high-energy transfer efficiencies (φET) are obtained above 98% for both T1 and T2. (See Table 1.) Such high-energy transfer efficiency results from the nearly coplanar geometries of first generation branched chains and the core
chromophore within T1 and T2. The branched chains in the second generations of T3 and T4 are far away from the core; it is found that the energy transfer efficiencies slightly decrease to 97% in T3 and 95% in T4, respectively. The leakage emissions in the range of 400-470 nm from fluorene dendrons are seen in T4, suggesting that the conjugation between the fluorene dendrons and the pyrene core is slightly disrupted in high generations.31 Even so, the energy transfer efficiency was still as high as >95%. The high efficient energy transfer from dendrons to core within T3 and T4 mainly results from throughspace Fo¨rster energy transfer because of the large spectral overlap between the fluorescence of fluorene dendron and the absorption of pyrene core.38 The through-bond energy transfer between the branches in the second and the first generations may not be efficient in this case because the conjugation was disrupted by the intermediate carbazole linkages (Figures S4 and S5 in the Supporting Information). Such high efficient energy transfer from fluorene branches to the pyrene core in T-series compounds strongly relates to their specific molecular structures, where the structure-dependent spectral properties are further supported by the following results of molecular structure optimizations (Figures S2-S5 in the Supporting Information) and steady-state fluorescence excitation anisotropy spectra, as shown in Figure 4. The optimization of the ground-state geometries of T1 and T2 (Figure S2 in the Supporting Information) indicates that T1 and T2 adopt a “pancake-like” shaped structure. The basic units in T1 and T2, such as carbazole, benzene, fluorene, and pyrene moieties, are all planar moieties, linked to the rigid acetylene linkages, guaranteeing the planarity of the dendrimer molecules except for terminal carbazole moieties. The planar configuration is favorable for the conjugation among various chromophore moieties in T1 and T2,58 leading to large TPA cross sections. Such kind of conjugation occurred in T1 and T2 and leads to a large orientation angle between the absorption transition dipole moments of dendrons and emission transition dipole moments of pyrene core (Table 2 and Figure S2 in the Supporting Information). As shown in Figure 4, fluorescence excitation anisotropy spectra monitored at 520 nm were measured in saturated toluene solutions of polystyrene to avoid fast rotation of molecules. The anisotropy values of the four dendrimers in the region of 420-510 nm approach a value of 0.3, indicating that the emission of the core is highly oriented as terminal emitter. Interestingly, in the excitation band of 350-420 nm corresponding to the absorption of fluorene group in dendrimers, the anisotropy values are about -0.127 at 396 nm for T1 and about -0.111 at 406 nm for T2, respectively. Such large negative anisotropy values indicate that there is a large averaged orientation angle between the absorption transition dipole
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TABLE 2: Ground-State to Excited-State Transition Properties of T1: Comparison between Calculated Data and Experimental Data calculation data transitions
energy gap (eV)/(nm)
osc.a
S1 r S 0 S2 r S0 S 3 r S0 S4 r S0 S5 r S0 S6 r S0
2.4991/496.12 3.0499/406.51 3.1186/397.57 3.1298/396.14 3.2871/377.19 3.3474/370.39
1.9475 1.3699 0.0006 4.9136 0.0045 1.2181
experimental data ori.b
λabs. (nm)c
ε (105 M-1 cm-1)d
ori.b
67°
494 392
1.00 2.27
70°
a Oscillation strength. b Orientation of the transition dipole moments of Sn r S0 (n ) 2-6) is presented relative to that of S1 f S0, where the calculated orientation angle is the weighted average from Sn r S0 (n ) 2-6) in T1. c Wavelengths of one-photon absorption spectra peaks. d Molar extinction coefficient at λabs..
moments of fluorene dendron and the emission transition dipole moments of pyrene core. We obtain experimentally the orientation angles about 70 and 67° for T1 and T2, respectively. Furthermore, as an example, the calculated excitation energies and corresponding oscillation strengths of T1 are consistent with the experimental results. For simplicity, we calculate the molecular orbits by taking T1 as a conjugated molecule. According to the calculated results listed in Table 2, the two absorption bands 420-525 and 350-420 nm are mainly ascribed to S1 r S0 transition from pyrene core and Sn r S0 (n ) 2-6) transition from fluorene dendrons, respectively. The CDD of transition is further calculated to visualize the difference in electron density upon electronic transition between ground and excited states,44 where the intense electron delocalization over the whole molecule backbone was observed in the CDDs of T1 (see Figure S3, SI), especially in Sn r S0 (n ) 2-6) transition. Because the efficient electron delocalization from conjugated fluorene dendrons in the branches usually enhances the TPA efficiency,51 the large TPA cross section of T1 is obtained, as shown in Table 1. As shown in Scheme 1, the 3,6,9-carbazole moieties in the terminal parts of the first generation branches (T1) were used to connect the branches of the second generation in T3. As an example, the structural optimization result shows that the two monodendron branches linked through 3,6-carbazole moiety (Scheme 1e and Figure S4 in the Supporting Information) in the second generation of T3 adopt a planar structure themselves, which is twisted by ∼45° with respect to the planar T1-like moiety connected through the nine-nitrogen of the carbazole to the second generation branches. The deviation from planarity slightly reduces the effective conjugation between the branches in the first and the second generations,58 leading to an increase in the absorption of T3 around 377 nm, which is from the analogues of short fluorene segments.31 Compared with the onephoton absorption spectrum of T1 (Figure 3), no red shift of absorption bands of T3 was observed, further indicating that the branches in the second generation are not well conjugated with the branches in the first generation, as mentioned above. The weak conjugation is favorable to retain the excellent photoluminescence performance of pyrene core without causing a red-shifted OPE and TPE fluorescence. As a result, the emission spectra, fluorescence quantum yield and fluorescence lifetime of T3 are almost invariable with respect to those of T1 (Table 1), but the absolute emission intensity of T3 is stronger than that of T1, as shown in Figure 3 under the same experimental conditions, because of the increase in molar extinction coefficient. Furthermore, as mentioned above, the two monodendron branches linked through 3,6-carbazole moiety (Scheme 1e) in the second generation increase the conjugation, therefore leading to large TPA cross section of T3 up to 15 200
GM. Even so, the TPA cross section does not increase linearly with increasing generations and increasing molecular weights from T1 to T3, which shows the decreases in ratio δmax/MW of the whole molecule (as shown in Table 1). On going from T3 to T4, the longer distance between branches in the second generation and the core in T4 compared with that in T3 also causes the decrease in the energy transfer efficiency from the second generation branches to the core. Furthermore, the optimal geometry (Figure S5 in the Supporting Information) of T4 is very similar to that of T3, where the elongated branches in the second generation of T4 (Scheme 1f) remain in the planar structure, which is twisted by ∼45° relative to the plane of T1-like moiety in the first generation. Compared with T3, the plane of a single branch in the first generation of T4 is twisted by ∼10° relative to pyrene core. Such deviation from planarity slightly affects the photoluminescence property of T4. As a result, the fluorescence quantum yield decreased from 0.50 in T3 to 0.49 in T4, and the energy transfer efficiency decreased from 97% in T3 to 95% in T4, whereas a weak emission of T4 appeared around 390-470 nm (Figure 3), resulting from the short fluorene segments because of the slight distortion of structure that occurred in T4. Even so, as compared with T3, one more fluorene unit was added to each branch in the second generation of T4, which largely increased the conjugation, leading to the larger TPA cross section of T4 up to 25 000 GM. Furthermore, from Table 1, it is found that the improvement of branched chains in these four dendrimers only enhanced the light harvesting ability. The core emission spectra, fluorescence quantum yields, and fluorescence lifetimes are almost invariable, which is of advantage for their two-photon excitation application.65 Meanwhile, the fluorescence excitation anisotropy spectra (Figure 4) show that the anisotropy values of T3 and T4 approach nearly zero in the region of 300-420 nm, indicating the homogeneous distributions of dimensional orientation of conjugated fluorene groups in the branches of T3 and T4 for maximal light harvesting with both one- and two-photon excitations. Upon UV-vis excitation or two-photon excitation in the near-infrared region, the excitation is delocalized within each dendron branch and then efficiently transferred to the pyrene core through FRET, leading to strong fluorescence with high fluorescence quantum yield. 3.3. Nanoparticles of T-Series Dendrimers. Nanoparticles dispersed in water and obtained from the T-series compounds were also studied to test the possible aggregation effects in the fluorescence and TPA properties. The knowledge of these properties might be of interest for biophotonic applications, which demand efficient TPA compounds that can be dispersed and remain highly fluorescent in aqueous media. Stable water dispersions of nanoparticles with typical diameters ranging from
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Figure 5. Molar absorption coefficient and normalized OPE emission spectra of THF solutions (dashed lines) and nanoparticles (solid lines) for the T-series at the concentration of 1 × 10-6 M. The OPE emission spectra were obtained under excitation at 370 nm.
TABLE 3: Summary of One- and Two-Photon Properties for the Compounds in THF Solutions and Aqueous Solution of Nanoparticles solution compounds T1 T2 T3 T4
5
λabs. (nm) (ε/10 M
-1
393 (3.11) 472 (1.28) 500 (1.52) 397 (6.18) 472 (1.61) 500 (1.88) 373 (13.25) 472 (1.62) 500 (1.81) 381 (18.37) 472 (1.55) 498 (1.65)
-1 a
cm )
nanoparticles λem. (nm)
b
τF (ns)
522 558
1.5
521 558
1.5
519 558
1.5
519 558
1.5
c
5
λabs. (nm) (ε/10 M
-1
370 (2.65) 478 (1.08) 507 (1.10) 372 (4.99) 477 (1.29) 508 (1.31) 367 (10.38) 478 (1.24) 504 (1.20) 375 (16.23) 479 (1.37) 500 (1.35)
cm-1)a
λem. (nm)b
τF (ns)c
(Φ′F/ΦF)d
(δ′/δ)e
535 570
1.5 (76%) 4.6 (24%)
0.46
0.17
536 565
1.3 (83%) 4.4 (17%)
0.49
0.20
1.4 (79%) 4.4 (21%)
0.57
0.27
560
1.2 (83%) 4.6 (17%)
0.44
0.39
565
a Wavelengths of one-photon absorption spectra peaks. b Wavelengths of one-photon excited fluorescence spectra peaks, λex ) 370 nm. Fluorescence lifetimes are monitored at 520 nm for T-compounds in THF and at 570 nm in aqueous solution of nanoparticles excited at 436 nm. d Φ′F is the fluorescence quantum yield of nanoparticles in water for T-series. e δ′ is the TPA cross sections for nanoparticles measured with femtosecond pulses at 76 MHz repetition rate at the wavelength of 740 nm. c
30 to 100 nm (measured through transmission electron microscopy images) were obtained; the attainable diameters depended on the concentration of solutions and the volume injected into water during the reprecipitation method (in which lower concentrations produced smaller particles). The obtained nanoparticles dispersed in water remained stable for several months and did not show sign of light scattering during measurements. Figure 5 presents the absorption and OPE emission spectra obtained from these nanoparticles dispersed in water at the concentration of 1 × 10-6 M. It is observed that the molar absorption coefficient of nanoparticles decreases with respect to that observed in molecular solutions at the same concentration; note that this effect is not attributed to light scattering in nanoparticles because scattering in UV-vis spectroscopy is rather associated with apparent increments of absorbance due to a baseline superimposed to the actual absorption bands of the compounds. It is then concluded that scattering effects are negligible in these experiments. Figure 5 also shows that the maximum of absorption, attributed to fluorene chromophores and conjugated fluorene/carbazole dendrons (350-420 nm), is blue-shifted, whereas the bands assigned to the pyrene core (420-525 nm) are red-shifted. The peak of fluorescence at 522
nm is red-shifted ∼15 nm for the nanoparticles of T1 and T2 with respect to those in organic solutions. In the case of T3 and T4, however, that peak vanishes, and only peaks at 560 and 565 nm remain, respectively. This effect, also observable in thin solid films made of these dendrimers, can be originated by excimer formation of pyrene rings to some extent.35 Fluorescence lifetime results, as shown in Table 3, further support the excimer formation of pyrene ring, where a long lifetime of T-compounds around 4.5 ns in T-series nanoaggregates is observed, in comparison to the lifetime of T-compounds around 1.5 ns in THF. One- and two-photon excitation experiments produce the same emission spectra for the compounds in THF solutions and nanoparticles in water. A summary of the spectral properties for these nanoparticles is given in Table 3. It must be mentioned that the formation of the nanoparticles quenched the photoluminescence of T-series to some extent. The ratio between the fluorescence quantum yield of T-series compounds in solution and nanoparticles in water is approximately Φ′F/ΦF ≈ 0.5, where the primed term corresponds to the nanoparticles. Because the method of the integrating sphere was employed to measure the quantum yields, possible
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light scattering effects in nanoparticles did not introduce significant error measurements. The aggregation of T-compounds to form the nanoparticles also resulted in a reduction of TPA. For instance, the TPA cross sections for nanoparticles of T1, T2, T3, and T4 were reduced by a factor of 0.17, 0.20, 0.27, and 0.39 with respect to the values measured in organic solutions, respectively (see Table 3). In this case, the comparison of TPA activity between nanoparticles and organic solutions was performed with femtosecond pulses at 76 MHz repetition rate at the wavelength of 740 nm. Similar reduction in TPA due to aggregation effects was reported for hydrophobic chromophores loaded into the core of micelles.66 Although the reduction in TPA can be attributed to the geometrical distortions of T-compounds that takes place in aggregation, the exact mechanism for that TPA reduction in organic nanoparticles is unknown, and the effect deserves further investigation. Therefore, although the TPA action of nanoparticles dispersed in water diminished with respect to that observed in organic solutions, it is clear that the maximum TPA cross section values in nanoparticles are still on the order of 103 GM. For instance, according to Tables 1 and 3, the maximum attainable values of TPA cross sections in nanoparticles are 1410, 2160, 4100, and 9750 GM for T1, T2, T3, and T4, respectively. These values are certainly much larger than those obtained in some compound that exhibits aggregation enhancement of photoluminescence and two-photon absorption.67 4. Conclusions In conclusion, we report a series of dendrimers T1-T4 that have gigantic TPA cross sections (as high as 25 000 GM) and strong TPEF (fluorescence quantum yield ≈ 0.5) in a very broad excitation wavelength range from 610 to 870 nm. To the best of our knowledge, this is one of the largest cross sections reported for stiff dendrimer molecules on the femtosecond time scale.29,49,65 The relationship between the molecular structure and one- and two-photon excitation properties has been investigated by one- and two-photon spectra to explain the experimental finding of a strong enhancement of the two-photon absorption cross section over the specific branch structure, where the improvement of branched chains in the T-series dendrimers only enhances the light-harvesting ability. With increasing generation, the photoluminescence intensity increases, whereas the core emission spectra, fluorescence quantum yield, and fluorescence lifetime are almost invariable by direct excitation of the dendrons, which is of advantage for the two-photon excitation application. Therefore, it is concluded that the specific dendritic structure for the T-series dendrimers in this Article with pyrene as π-core, fluorenes and ethynylenes (phenylethynyls) as π-conjugated bridges, and sequentially carbazoles as terminal π-electron donors is one of the most remarkable rigid macromolecules simultaneously with large TPA cross section and high TPEF quantum yield. Acknowledgment. This work was financially supported by NSFC, 973 Programs, Chinese Academy of Sciences and CONACyT (grant J49512F). Supporting Information Available: Computational results and one- and two-photon absorption spectra of T1, T2, T3, and T4. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) He, G. S.; Tan, L.-S.; Zheng, Q.; Prasad, P. N. Chem. ReV. 2008, 108, 1245.
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