Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C 2019, 123, 13047−13056
Highly Efficient Orange-Red/Red Excimer Fluorescence from Dimeric π−π Stacking of Perylene and Its Nanoparticle Applications Yue Shen,†,∥ Zhe Zhang,†,∥ Haichao Liu,†,∥ Yan Yan,‡ Shitong Zhang,† Bing Yang,*,† and Yuguang Ma§ State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, and ‡State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China § State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, People’s Republic of China Downloaded via UNIV OF SOUTHERN INDIANA on July 23, 2019 at 08:43:39 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
S Supporting Information *
ABSTRACT: To achieve high-efficiency red excimer fluorescence, two novel perylene (PE) derivatives (mTPA-3PE and 2SF-3PE) are designed and synthesized with different monosubstituent triphenylamine (TPA) and spirofluorene (SF), respectively. The photophysical investigations reveal that mTPA-3PE and 2SF-3PE exhibit red/orange-red excimer fluorescence (637 and 610 nm) with long lifetimes (37.86 and 72.41 ns) in crystals but significantly different photoluminescence efficiencies (24 and 77%). Both crystal structure and excited-state property emphasize that the discreteness of the PE dimer with π−π stacking is essentially responsible for the high-efficiency excimer emission in the crystal. Using PE as a π−π emissive core in this work, highly efficient red excimer fluorescence is harvested in the crystal for the first time. Moreover, their nanoparticles with the same excimer fluorescence exhibit the advantage of easy processing and the promising application in cell imaging. Once again, our results validate the mechanism of excimerinduced enhanced emission (EIEE) by the discrete dimeric π−π stacking of PE in the solid state, which opens a new way to develop high-efficiency narrow band gap (e.g., nearinfrared) light-emitting materials using the EIEE mechanism in the future. matter of fact, solid-state fluorescence of aromatic molecules is often encountered with the aggregation-caused quenching (ACQ), which can be ascribed to the formation of low-lying and non-emissive “dark” excited-state species upon the aggregations through the strong π−π interactions, such as excimer.15−18 Previously, this kind of stacking form is strictly prevented by bulky-substitution and host−guest doping in many organic aromatic systems for the efficient monomer fluorescence.19−21 As we know, excimer (i.e., excited dimer) is a typical bimolecular excited species, which is the smallest and simplest model to better understand the structure−property relationship of supramolecular luminous aggregates.22−26 To be different from the previous excimer quenching fluorescence, the strong excimer emission [their photoluminescence (PL) efficiencies are around 80%] of several anthracene (AN) derivatives in solids have been reported recently, which are all featured with a discrete pairwise π−π stacking of AN units in crystals.27−31 Green excimer fluorescence was commonly obtained from AN π−π stacking, but the red excimer materials are still rare so far.32−39 For the further red-shifted excimer emission, our strategy is to enlarge the π-conjugation degree
1. INTRODUCTION High-efficiency organic red light-emitting materials are always highly desired owing to the important applications in many fields, such as flat-panel displays, solid-state lighting, photodynamic therapy, organic solar cells and chemo-sensing, and so forth.1−8 Generally, the achievement of red fluorescent materials mainly depends on two approaches: covalent molecular synthesis and noncovalent intermolecular aggregation.3,9−13 As is known to all, the modification and tailoring of organic structures endow the diversity of molecular designs for single-molecule red fluorescent materials, such as extending πconjugation of aromatic systems and selecting strong donor− acceptor framework to decrease the emission band gap between the highest occupied molecular orbital (HOMO) of the donor and lowest unoccupied molecular orbital (LUMO) of the acceptor.3,9 Beyond that, the intermolecular aggregation actually induces the orbital energy level splitting and band gap narrowing, and sometimes this is a more favorable way to realize red emission relative to the single molecule (which may be subject to complicated synthetic routes and rigorous reaction conditions) by the strong supramolecular interactions in solid states, such as π−π stacking, hydrogen and halogenbonding, and so forth.10−13 However, one of the biggest challenges to achieve high-performance red fluorescent materials is still the sharp conflict between high efficiency and narrow band gap according to the energy gap law.14 As a © 2019 American Chemical Society
Received: March 14, 2019 Revised: April 27, 2019 Published: May 2, 2019 13047
DOI: 10.1021/acs.jpcc.9b02447 J. Phys. Chem. C 2019, 123, 13047−13056
Article
The Journal of Physical Chemistry C
MW ≈ 1700, styrene content = 68%) and solvent tetrahydrofuran (THF, anhydrous, 99.9%) were purchased from Sigma-Aldrich (Shanghai, China). CCDC 1893816, 1893817 contains the supplementary crystallographic data for 2SF-3PE and mTPA-3PE. UV−vis spectra were recorded using a Shimadzu UV-3100 Spectrophotometer. Steady-state fluorescence spectra and fluorescence lifetimes were measured with a FLS980 spectrometer. Single-crystal diffraction measurements were performed on a Bruker APEXII CCD system. The crystal structures were solved with direct methods and refined with a full-matrix least-squares technique using the SHELXS programs. All angles and distances in crystals were measured using Mercury 1.4.1 software. The particle sizes of nanoparticles in bulk solution were characterized by dynamic light scattering (DLS, Malvern Zetasizer NanoS). Transmission electron microscopy (TEM) measurements were recorded on a JEOL JEM-2100F using ultrathin carbon films as grids. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were used to describe the ground-state and excited-state properties. For all of the calculations in single molecule, optimized ground- and excited-state geometries were obtained at the levels of M06-2X/6-31G(d,p). For all of the dimers, optimized excited-state geometries were obtained by using the M06-2X hybrid functional at 6-31G(d,p) level. The natural transition orbitals (NTOs) were evaluated with the dominant “particle”−“hole” pair contributions and the associated transition weights. All of the calculations were carried out using Gaussian 09 package on a Power Leader cluster. Furthermore, the Multiwfn software was used to calculate the wave function of electron−hole pair from the transition density matrix (TDM) by TD-DFT and was plotted in a two-dimensional (2D) color-filled map for easily distinguishing the dimer ground- and excited-state character.
(the number of fused rings) of π-moiety monomer, aiming at decreasing the energy of excimer emissive state by two contributions: the enhanced π−π interaction between two πmoieties, as well as the stabilized monomeric locally excited (LE) state. In the meantime, this enhanced π−π interaction will preferentially induce the molecular aggregation in a π−π stacking mode between two monomers, which surely facilitates the formation of dimeric π−π stacking in solid states, especially in the more practical amorphous films or nanoparticles. Perylene (PE) is an aromatic molecule with a larger πconjugation plane than naphthalene (NA) and AN, whose derivatives have been widely applied in the biological probes and sensors because of the excellent optical and electronic properties.40−42 AN as linear molecule cannot form excimer even in high-concentration solution and packs in a sort of herringbone mode in solid state, but some modifications of AN unit can achieve the formation of AN excimer.43,44 In sharp contrast, PE as planar molecule can form sandwich-type dimers, which after excitation can relax to an excimer, both in solution and solid state.45−47 The reason is that PE has the larger π-conjugation and stronger π−π interaction than AN. Compared with the AN, PE shows not only the more redshifted emissions in monomers but also the more likely π−π stacking to form excimer in solid state. Therefore, it is the most favorable π-moiety to achieve the high-efficiency red excimer fluorescence by using the same strategy of discrete dimeric π−π stacking in solid state, as mentioned in our previous works.27,28,48 On the one hand, the universality of this strategy will be further verified to better understand the excimerinduced enhanced emission (EIEE) mechanism according to the various excimer species with different emission colors. On the other hand, the discrete dimeric π−π stacking of PE is expected to be formed more easily in amorphous film or nanoparticle for the development of practical applications than those of AN, as a result of the greatly enhanced π−π interaction between two PEs. Herein, we designed and synthesized two PE derivatives. First, triphenylamine (TPA) was chosen to produce a new compound 3-(perylen-3-yl)-N,N-diphenylaniline (mTPA3PE). As a result, mTPA-3PE was found to display the largely red-shifted emission (λmax = 637 nm) in the crystal relative to that in its monodisperse film (λmax = 496 nm), which can be ascribed to the PE excimer emission from antiparallel dimeric PE π−π stacking. Consequently, its PL efficiency is 24% due to the contact interactions between luminophors−PE dimers. To improve PL efficiency, a larger side group spirofluorene (SF) was introduced to obtain the other compound 2-(perylen-3yl)-9,9′-spirobi[fluorene] (2SF-3PE). Not only is highefficiency orange-red PE excimer emission (77%) observed in crystal but also this excimer fluorescence can be harvested in nanoparticles, which further enables the cell-imaging application with the excellent dyeing effect. This work suggests that the EIEE mechanism is an effective and practical way to achieve high-efficiency and narrow band-gap light-emitting materials.
3. RESULT AND DISCUSSION 3.1. Molecular Design. For high-efficiency red excimer fluorescence, two key points should be of particular concern in molecular design: red excimer emission core and its discrete dimeric π−π stacking. To construct red-shifted excimer emission core, our strategy is to gradually enlarge the πconjugation degree of π-monomer, for the purpose of the decreased energy of excimer emissive state, as shown in Figure 1a. Larger π-conjugation plane means the narrower HOMO− LUMO gap for monomer and stronger π−π interaction between two π-monomers can further induce the wider energy splitting between monomeric HOMO−HOMO and LUMO− LUMO for dimer. Consequently, with the increasing number of fused rings from NA, AN, to PE, both the narrowed HOMO−LUMO gap of monomer and the broadened energy splitting of dimer jointly give rise to the significantly redshifted emission of excimer according to a single one-electron excitation from HOMO to LUMO.49 From previous reports, the excimer emission of PE is right in the region of red light, while the excimers of NA and AN show ultraviolet and green emissions, respectively.45,46,49−52 As a result, the PE dimer with cofacial π−π stacking is confidently chosen as the red excimer emission core. Furthermore, for the high-efficiency red fluorescence of PE excimer, the discrete dimeric π−π stacking should be arranged for PE, according to a series of highly efficient AN excimers that have been reported in our group.27−31 To realize the discrete dimeric π−π stacking of PE, the target candidate is
2. EXPERIMENTAL SECTION The details of synthesis and structure characterization can be found in the Supporting Information. All of the reagents and solvents used for organic synthesis were purchased from Sigma-Aldrich and Acros companies and used without further purification. The amphiphilic functional polymer poly(styreneco-maleic anhydride) (PSMA, cumene-terminated, average 13048
DOI: 10.1021/acs.jpcc.9b02447 J. Phys. Chem. C 2019, 123, 13047−13056
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substituent play a very important role in the construction of spatially isolated dimeric π−π stacking of PE moieties for the purpose of the enhanced PE excimer emission. In this contribution, two PE derivatives (mTPA-3PE and 2SF-3PE) with one-sided substituents (TPA and SF) were designed and synthesized by Suzuki coupling reaction (see the Supporting Information and Scheme S1). Then, they were further investigated about the photophysical properties, together with the crystal packing structures, excited-state properties, and nanoparticle application in cell-imaging. 3.2. Photophysical Properties of mTPA-3PE. First, both absorption and emission of mTPA-3PE were measured in different solvents, corresponding to the photophysical properties of ground state and excited state, respectively. For UV−vis absorption spectra in Figure 2a, three main absorption bands can be assigned to the characteristic absorption of TPA (300 nm) and PE (250 nm, 350−475 nm with vibrational fine structure) (Figure S1), respectively.45 With the increasing solvent polarity, absorption spectra show the almost negligible changes. But for the PL spectra (Figure 2b), there is a new peak around 620 nm in high polar solvent such as acetonitrile, which is attributed to intramolecular charge-transfer (ICT) state (Figures S2 and S3). Besides, we observe that absorption and emission spectra exhibit good mirror symmetry, as a result of the planarity and rigidity of PE unit as an emissive core, as shown in Figure 2a,b. Furthermore, the photophysical properties of mTPA-3PE were carefully characterized in solid states (Figure 2c,d). For the 1 wt % doped film in polymethyl methacrylate (PMMA), mTPA-3PE exhibits the very bright bluish-green emission with the main peak at 496 nm. Although the obvious structured PL spectra remain, the 0−1 vibration dominates PL intensity in film while 0−0 vibration dominates PL intensity in solutions, probably arising from the formation of weakly coupled aggregate in film rather than in solutions.53
Figure 1. (a) Schematic diagram of electronic structures for redshifted excimer emission (NA: ground state of naphthalene; NA*: excited state of naphthalene; (NANA)*: excimer state of naphthalene; AN: ground state of anthracene; AN*: excited state of anthracene; (ANAN)*: excimer state of anthracene; PE: ground state of perylene; PE*: excited state of perylene; (PE−PE)*: excimer state of perylene). (b) Molecular design of discrete dimeric π−π stacking.
designed into one-substituent PE, as shown in Figure 1b. That is to say, PE derivatives should consist of two moieties: a planar aromatic PE (π-moiety) and a one-sided substituent (side-moiety). First, the one-sided substitution is still favorable enough for PE π-moiety to form antiparallel dimeric cofacial π−π stacking with appropriate steric hindrance. Next, the onesided substituent can serve as the spacer to separate PE dimer from each other using its proper size and suitable orientation, aiming at the discrete dimeric π−π stacking of PE moiety in the solid state. Notably, the size and orientation of the one-side
Figure 2. (a) UV−vis absorption spectra of mTPA-3PE with increasing solvent polarity. (b) Fluorescence spectra of mTPA-3PE with increasing solvent polarity; inset is the chemical structure of mTPA-3PE. (c) Fluorescence spectra of mTPA-3PE in different solid states; photographs of mTPA-3PE in different solid states under the illumination of a 365 nm UV lamp. (d) Transient PL decay measurements of mTPA-3PE in different solid states. 13049
DOI: 10.1021/acs.jpcc.9b02447 J. Phys. Chem. C 2019, 123, 13047−13056
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Figure 3. (a) Fluorescence spectrum of 2SF-3PE of 1 wt % film in PMMA. (b) Transient PL decay measurement of 2SF-3PE of 1 wt % film in PMMA. (c) Fluorescence spectrum of 2SF-3PE crystal. (d) Transient PL decay measurement of 2SF-3PE crystal; inset is the chemical structure of 2SF-3PE.
emitting diodes and organic field-effect light-emitting transistors in the near future. 3.3. Photophysical Properties of 2SF-3PE. To promote PL efficiency of the PE excimer, the TPA substituent is replaced with an SF unit, and then, we designed and synthesized the other PE derivative (2SF-3PE). Like those in mTPA-3PE, the photophysical properties of 2SF-3PE were carefully characterized in solution, film, and crystal (it was prepared by sublimation). In different solvents (Figures S5 and S6), 2SF-3PE shows very similar absorption spectra compared with those of mTPA-3PE, except for the structured absorption band at 300 nm from the SF moiety. As for the PL spectra, no ICT characteristic can be observed even in a high polar solvent. For 1 wt % doped film in PMMA (Figure 3a,b), 2SF3PE demonstrates a slightly red-shifted maximum emission wavelength at 509 nm relative to 496 nm of mTPA-3PE, which is ascribed to the smaller twisting angle between PE and its substituent group in the excited state (Figures S12 and S17). The lifetimes at different emission wavelength (476 and 509 nm) were measured, and the average lifetimes were estimated to be 8.24 and 11.00 ns, respectively. More interestingly, the crystal of 2SF-3PE exhibits a very intense orange-red fluorescence with an emission maximum wavelength at 610 nm (Figure 3c), which is somewhat blue-shifted in contrast to 637 nm of the mTPA-3PE crystal but red-shifted in contrast to 578 nm of α-phase PE dimer crystal.45,47 It is noteworthy that the PL efficiency of 2SF-3PE crystal was measured to be 77% as we expected in molecular design, which is much higher than that of mTPA-3PE (ηPL = 24%) and α-phase PE dimer (ηPL = 36%) crystals. Furthermore, the transient PL spectrum (Figure 3d) revealed a monoexponential decay with a lifetime as long as 72.41 ns. In addition, the other 2SF-3PE crystal was obtained through a slow evaporation of solventTHF, which is named as crystal-G because of the green fluorescence emission (λmax = 528, 557 nm) under UV irradiation (Figure S8). The average lifetime was calculated to be 3.90 ns at λmax =
The transient PL spectra reveal the averaged lifetime is 12 ns in the monodisperse film.54 In the meantime, the crystal of mTPA-3PE was grown through a slow solvent evaporation process in THF. Interestingly, the crystal exhibits the significantly distinct photophysical properties relative to those of doped film and solution. Not only is the largely redshifted emission (637 nm) observed together with disappeared vibrational fine structure and obviously broadened full width at half maximum (fwhm = 135 nm) but also mTPA-3PE crystal shows a monoexponential decay of transient PL spectrum with a single lifetime as long as 37.86 ns, which is much longer than those in doped film and solution. Compared with the single molecule, the emission behaviors (large red shift, broadening, and long lifetime) of the crystal are in very good agreement with the spectral characteristics of PE excimer.45,47,55 Subsequently, PL efficiency of mTPA-3PE was evaluated by integrating-sphere experiment to be 97 and 24% for the doped film and crystal, respectively. From the monomer to dimer, the PL efficiency of mTPA-3PE shows obvious ACQ, which appears to conform the conventional view that excimer is usually of low-efficiency. Though the PL efficiency of mTPA3PE crystal is not as high as those of AN excimer we have reported previously, it is much better than some classic red dyes, such as 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran, rubrene and Nile red, and so forth.32 Besides, for the future practical application, the neat thin film of mTPA-3PE was also prepared by vacuum evaporation, and its emission is peaked at about 600 nm (Figure S4a). Although the emission of neat film is some blue-shifted relative to that of crystal, the transient PL spectrum confirms that there is long lifetime component (Figure S4b), indicating the formation of PE excimer to a certain degree. If the operating conditions were further optimized during the vacuum evaporation process, the fluorescence properties of neat films could be expected to be completely consistent with crystal, which makes the excimer film possible to be applicable to organic light13050
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mTPA-3PE and 2SF-3PE crystals. Specifically, the PE dimer in the 2SF-3PE crystal is well isolated by peripheral four SF moieties through multiple C−H···π interaction between PE moieties and fluorene moieties (Figure S10b), that is, discrete PE dimer stacking. However, in the mTPA-3PE crystal, the PE dimer is surrounded by the other four PE dimers through multiple C−H···π interactions between PE and PE moieties (Figure S10a), that is, nondiscrete PE dimer stacking. Obviously, the structure−property relationship can be better understood for the distinct dimer emission in two crystals of mTPA-3PE and 2SF-3PE. Namely, the discrete dimeric π−π stacking of PE is mainly responsible for high-efficiency excimer fluorescence in 2SF-3PE, whereas the nondiscrete dimeric stacking results in a relatively low efficiency of excimer emission in mTPA-3PE crystal, which is essentially related to the intensified nonradiation triggered by the exciton delocalization and diffusion due to the interactions between PE dimers (Table S3). At this point, α-phase crystal of PE further confirms that the nondiscrete dimeric π−π stacking causes low PL efficiency (ηPL = 36%) of excimer fluorescence, which has almost the same crystal packing of PE dimer as that in the mTPA-3PE crystal (Figure S11). Thus, the discrete dimeric π−π stacking is substantially verified to be important for high-efficiency excimer fluorescence, as a result of the enhanced localization of emissive exciton. 3.5. Excimer Excited-State Properties. Moreover, the excited-state properties were evaluated to gain a deep insight into the intrinsic photophysics of these π−π dimeric structures in crystal. First, the lowest excited-state (S1) geometries were obtained by geometry optimization at the level of TDDFT (M06-2X/6-31G) using monomer and dimer in mTPA-3PE and 2SF-3PE crystals as initial geometry, respectively.30 For the monomers at excited state, both mTPA-3PE and 2SF-3PE show the more planar PE moiety and the reduced twist angle between PE and TPA/SF relative to the ground state in vacuum conditions/crystals, indicating the enhanced πconjugation along the molecular backbone (Figures S12 and S17). For the dimers at excited state, the π−π interplanar distances between two PE moieties are obviously shortened by 0.155 and 0.331 Å relative to those in initial crystals of mTPA3PE and 2SF-3PE, respectively, together with the enlarged overlap ratio between two PE moieties from 60 to 88%/84% (Figures S16 and S21). Compared with the dimer structures in both crystals, the optimized geometries of the excited state tend to be an almost fully cofacial π−π stacking with a “compressed” π−π interplanar distance, revealing a great enhancement in the rigidity of excimer geometry. By the way, the emission energies were estimated based on the optimized geometry of excited state, which is in very good agreement with experimental results (Figure 5). Second, both NTO and electron−hole pair wave function were used to describe the excited-state character and transition composition. For the monomers of mTPA-3PE and 2SF-3PE, both “hole” and “electron” are mainly localized on the PE moiety for S1 → S0 transition (Figures S12 and S17), which is very well consistent with the LE state emission of PE in solution and doped films. For their dimers, the emission NTOs (S1 → S0) obviously show the stronger interaction between two monomers of PE than those of absorption (S0 → S1) (Figures S13 and S18), as a result of the almost completely overlapping and “compressed” π−π distance between two PE planes. At the dimer excited state, a “hole” is obtained from negative linear combination between two HOMO orbitals of monomers, while an
528 nm by the transient PL spectrum. According to both fluorescence emission and lifetime, it can be speculated that the crystal-G showed the monomer-like fluorescence properties, which are also similar to those of single molecule in all solutions. However, compared with the red-emitting crystal, the PL efficiency of crystal-G is obviously decreased to be only 14% probably because of fluorescence quenching from the intermolecular interaction between PE units. Therefore, the 2SF-3PE crystal not only successfully achieved the highefficiency red PE excimer fluorescence for the first time but also confirmed the EIEE mechanism once again. 3.4. Nondiscrete and Discrete Dimeric Stacking. To better understand the difference in photophysical properties of these two compounds, especially for the distinct PL efficiency in two crystals, the crystal structures of mTPA-3PE and 2SF3PE were determined by the single-crystal X-ray diffraction experiment, respectively. Their crystal structures are shown in Figure 4 for the purpose of comparison, and the detailed
Figure 4. Single-crystal structures of (a) mTPA-3PE and (b) 2SF3PE.
crystal data are listed in the Supporting Information (Tables S1 and S2). Similarly, both mTPA-3PE and 2SF-3PE crystals belong to monoclinic crystal system and centrosymmetric space groups of P21/c. As expected by the molecular design, both crystal structures show the antiparallel dimeric π−π stacking of PE moieties with the nearly same overlap ratio of 60% between two PE planes (top view). Also, the dimeric π−π stacking is all formed through two different kinds of conformations of a single molecule, which are exactly in Ci symmetry for this dimer. To be different, for the single molecule, the twisting angle of mTPA-3PE between PE and its substituent group is 54.20°, which is smaller than 69.31° of 2SF-3PE. For the dimeric π−π stacking of PE, the interplanar distance between two PE moieties is about 3.601 Å in the 2SF3PE crystal (side view), which is obviously larger than 3.456 Å of mTPA-3PE crystal within a typical π−π interaction distance. For the reason, it can be mainly ascribed to the fact that there is a larger twisting angle between the PE unit and substituent in 2SF-3PE crystal, and the nearly vertical benzene ring structure prevents the PE unit to be close to each other. Most importantly, the PE dimer stacking is totally different in 13051
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probability of |c(xi, yi)|2 of finding both electron and hole. At the dimer geometry of ground state from crystal, the brightest zone is localized primarily along the diagonal of 2D color-filled map (Figures S15 and S20), which indicates that both electron and hole are localized on a single PE unit, and thus, the electronic transition has a dominant LE character. In a different way, the brightest zone are distributed along both diagonal and off-diagonal as shown in Figure 5, corresponding to two kinds of electronic transitions with LE character and intermolecular charge-transfer (CT) character, respectively. Both LE and CT transition characters coexist in one excited state, which can also be regarded as a hybridized local and CT (HLCT) excited state.4,58−67 As a comparison, both mTPA3PE and 2SF-3PE have a very similar HLCT character and composition of the excited state; thus, the excited-state property is impossible to be key factor responsible for the significant difference of excimer fluorescence between mTPA3PE and 2SF-3PE. Therefore, the high-efficient excimer fluorescence of 2SF-3PE crystal can be attributed to two main reasons: HLCT-featured state induces “compressed” excimer geometry for the effective suppression of nonradiative vibrational quenching, as well as the discrete dimeric π−π stacking guarantees the purity and singleness of excimer state as a result of excition localization, preventing the occurrence of “dark” state from nonradiative energy transfer. In a word, the discreteness of π−π stacking PE dimer is of great importance to achieve high-efficiency excimer fluorescence in solid state. 3.6. Nanoparticle Cell-imaging Applications. To develop the practical applications of excimer materials, the primary requirement is that the discrete dimeric π−π stacking of PE should be easily formed in noncrystalline (amorphous) solids, such as film, nanoparticle, and so forth. At least, the dimeric π−π stacking should be formed with a sufficient proportion to satisfy the complete energy transfer from monomer to excimer. As mentioned above, the excimer emissions have been clearly observed in the vacuum-
Figure 5. (a) NTOs from S1 to S0 of mTPA-3PE (left) dimer and 2SF-3PE (right) dimer from respective optimized geometries. (b) TDM color-filled maps of the S1 state of mTPA-3PE (left) dimer and 2SF-3PE (right) dimer.
“electron” is corresponded to the positive linear combination of two LUMO orbitals of monomers. Only on the basis of the NTOs, the excited-state character and quantitative transition composition cannot be further analyzed because of a completely symmetrical distribution of wave functions. Finally, the wave function of the electron−hole pair was plotted into a two-dimensional color-filled map to disclose the nature of excited state according to TDM using Multiwfn software.56,57 For the 2D color-filled map, both the horizontal axis xi and the vertical axis yi run over all of the nonhydrogen atoms of dimer, as labeled in Figures S14 and S19. The brightness of each coordinate point (xi, yi) is directly proportional to the
Figure 6. Characterizations of 2SF-3PE nanoparticles and specific cell labelings. (a) Hydrodynamic diameter of the nanoparticles. The inset shows a representative TEM image. The scale bar represents 50 nm; (b) spectral character of the nanoparticles at the ratio at 80%; (c,d) are confocal images of Hela cell after incubation them with nanoparticles. 13052
DOI: 10.1021/acs.jpcc.9b02447 J. Phys. Chem. C 2019, 123, 13047−13056
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The Journal of Physical Chemistry C evaporated film for both mTPA-3PE and 2SF-3PE, but the film-forming conditions need to be further carefully optimized. Here, mTPA-3PE was attempted to make into nanoparticles to investigate its properties. The nanoparticles were prepared by the reprecipitation method, in which the emitter molecule was encapsulated by PSMA because of hydrophilic and hydrophobic interactions resulting in stable aqueous solutions. With the increase of doping concentration (mTPA-3PE/PSMA), absorption bands around 390 nm gradually appear and increase in contrast with those in dilute solutions (Figure S22), which is assigned to H-aggregation of PE unit. For the PL spectra, when the doping concentration is lower than 10%, only monomeric fluorescence can be observed. The excimer fluorescence gradually arises, as the doping concentration is more than 20%. Once the doping proportion reaches 60%, the excimer fluorescence peaked at 632 nm dominates the whole PL spectra, together with the entirely disappeared monomer emission, indicating the pure excimer emission from π−π stacking PE dimer. For the transient fluorescence decay spectra, the lifetimes are very short (
