Excited-State Symmetry Breaking of a ... - ACS Publications

Article Cite This: J. Phys. Chem. C 2019, 123, 14564−14572

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Excited-State Symmetry Breaking of a Symmetrical Donor−π−Donor Quadrupolar Molecule at a Polymer/Glass Interface Masaaki Mitsui,*,† Yasushi Takakura,† Yoshiki Niihori,† Masashi Nakamoto,‡ Yutaka Fujiwara,‡ and Kenji Kobayashi*,‡ †

Department of Chemistry, College of Science, Rikkyo University, 3-34-1, Nishiikebukuro, Toshima-ku, Tokyo 171-8501, Japan Department of Chemistry, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan

‡

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S Supporting Information *

ABSTRACT: Symmetric multipolar (e.g., quadrupolar and octupolar) chromophores that undergo excited-state symmetry breaking in polar solvents have been drawing considerable attention for a long time as promising two-photon absorption and luminescent materials. However, this process has been generally restricted to liquid media. Herein, we report the excited-state symmetry breaking of a centrosymmetric D−π− D (donor−π−donor) quadrupolar molecule, DB2, occurring at a polymer/glass interface. In polar liquids, DB2 exhibits pronounced emission solvatochromism originating from symmetry-breaking intramolecular charge transfer in the S1 state. Single-molecule fluorescence spectroscopy shows that the fluorescence characteristics of DB2 at the interface are strikingly different from those of DB2 embedded in a polymer film, that is, the dramatic lengthening of fluorescence lifetimes and pronounced sharpening of fluorescence spectra are observed. With the aid of theoretical calculations, it is revealed that these characteristic behaviors are due to the strong electronic localization of the emitting excited state via asymmetric in-plane relaxation of the π-conjugated framework through dipolar interactions with the glass surface.

1. INTRODUCTION Multipolar chromophores consisting of several electrondonating (D) and electron-accepting (A) groups connected through a conjugated π-linker are known to have photoinduced intramolecular charge-transfer (ICT) properties, which render them promising two-photon absorption (TPA)1−4 and/or luminescent materials.5−7 In particular, centrosymmetric quadrupolar chromophores, in which two donors or acceptors are linked through an elongated π-conjugated unit (e.g., D− A−D and A−D−A), are attracting a great deal of attention because of their superb TPA abilities as compared to their dipolar counterparts (D−A).1−3 Interestingly, despite having negligible permanent dipole moment in the ground state (S0), centrosymmetric quadrupolar chromophores often show strong fluorescence solvatochromism.4,8−10 This intriguing phenomenon has been experimentally and theoretically interpreted by introducing the concept of symmetry-breaking ICT in the excited state,10−13 which refers to a breakup of the structural symmetry from a quadrupolar (symmetry-preserved) to a dipolar (symmetry-broken) excited state. Indeed, real-time observation of this process in polar solvents has recently been accomplished using ultrafast spectroscopic techniques.14−16 Most of the previous studies in this area have dealt with the excited-state symmetry breaking in polar liquid media, where conformational and orientational relaxations of solute and solvent molecules can occur almost freely and thus the ICT state is efficiently stabilized. Under rigid environments, in contrast, the energetic stabilization of the ICT state is expected © 2019 American Chemical Society

to be very small due to the lack of orientational polarization corresponding to solvent reorganization in polar liquids. Herein, we report the excited-state symmetry breaking of a centrosymmetric quadrupolar molecule at a polymer/glass interface (i.e., a solid/solid interface), where conformational and orientational relaxations are strongly restricted. In this work, single-molecule fluorescence spectroscopy (SMFS) was utilized to obtain statistical information on the photophysical parameters of the newly synthesized centrosymmetric D−π−D molecule DB2, in which a bis(phenylethynyl)benzene (π) unit is terminated by a p-dioctylaminostyryl (D) group at both ends (Chart 1) and then embedded in a polystyrene (PS) film or immobilized at a PS/glass surface. The density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were also performed to gain theoretical insight into the Chart 1. Schematic Molecular Structure of DB2

Received: April 17, 2019 Revised: May 23, 2019 Published: May 24, 2019 14564

DOI: 10.1021/acs.jpcc.9b03612 J. Phys. Chem. C 2019, 123, 14564−14572

Article

The Journal of Physical Chemistry C

used to excite DB2 at 440 nm. The repetition frequency was set to 10 or 20 MHz. The collimated excitation light was passed through a Glan−Thompson polarizer and subsequently through a λ/4 plate. Then, the circularly polarized beam was reflected using a dichroic mirror (DM, FF452-Di01-25 × 36, Semrock) and focused on a diffraction-limited spot size (ca. 210 nm fwhm) by an oil immersion objective lens (100×, NA 1.4, Olympus). Average excitation intensities were 200−400 W/cm2, which correspond to the excitation rate (kex) of 1.5 × 105 to 2.2 × 105 s−1 for DB2. Fluorescence photons from the excited molecule were collected through the same objective lens and then passed through a DM, a notch filter (NF01442U-25, Semrock), and a long-pass filter (LP02-442RS-25, Semrock) to reduce the scattered laser light. Then, the fluorescence was split by a 50:50 unpolarized beam splitter. Half of the detected fluorescence signal was sent to a polychromator (SpectraPro 2300i) coupled with a liquid nitrogen-cooled charge-coupled device camera (Spec10:100B/LN, Roper Scientific). The other half was focused through a pinhole (75 μm diameter) to reject the out-of-focus background. To confirm that the observed signals originated from the molecule, we recorded the fluorescence spectral traces (3 s integration). The APD signals were sent to a TCSPC card (TimeHarp 200, PicoQuant) operated in the time-tagged and time-resolved mode. Data acquisition and fluorescence decay analyses were performed using the SymPhoTime v5.2.4 (PicoQuant) software. To determine the fluorescence lifetime, monoexponentials were fitted to the fluorescence curves by maximum likelihood estimation. Each spectrum was fitted with the appropriate number of Gaussians to determine the wavelength of the fluorescence maximum. A fluorescence image of the sample was acquired by raster scanning of the laser focal spot. All the measurements were performed under high-vacuum conditions (∼0.1 Pa) to suppress photobleaching due to a photochemical reaction with oxygen. 2.3. Quantum Chemical Calculations. The program Gaussian 16 (ES64L-G16 RevB.01)19 was used for all quantum chemical calculations presented here. For the sake of simplicity, all octyl groups in DB1 and DB2 were replaced with methyl groups. These simplified molecules are referred to as DB1′ and DB2′. The ground-state (S0) geometries of DB1′ and DB2′ were optimized using the DFT with a polarizable continuum model (PCM). In this study, PCM/DFT calculations were carried out with three different functionals, including long-range correlated hybrid functionals (ωB97XD20), a global hybrid meta-generalized gradient approximation (GGA) functional (M06-2X21), and a hybrid GGA functional (MPW1K22). The selected functionals have previously been shown to provide accurate results in terms of reproducing photophysical properties of D−π−A dyes.17,23,24 The optimized structures (Figure S7) and frontier orbitals (Figure S8) were depicted using Winmostar (ver. 7.028) and GaussView 6.0.16 softwares, respectively. TD-DFT calculations, including solvent effects, by the PCM/TD-CAM-B3LYP/6-31+G(d,p) method were used for geometry optimizations in the lowest singlet excited state (S1). Harmonic vibrational frequencies of the optimized geometries in S0 and S1 were confirmed to correspond to potential minima by the absence of imaginary frequency. The simulated infrared (IR) spectra of all the optimized structures in S0 and S1 are presented in Figures S7 and S9, respectively. Vertical electronic transition energies in the S0 → S1 (absorption) and S1 → S0 (emission) transitions

nature of the excited-state symmetry breaking of this chromophore.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. A detailed description of the synthesis of 1-((E)-4-(dioctylamino)styryl)-4-(phenylethynyl)benzene (DB1) and 1,4-bis(4-((E)-4-(dioctylamino)styryl)phenylethynyl)benzene (DB2) is presented in the Supporting Information (Scheme S1 and Figures S1−S4). PS (average molecular weight Mw = 5.0 × 104, Sigma-Aldrich), polyvinylidene difluoride (PVDF, average molecular weight Mw = 5.3 × 105, Sigma-Aldrich), n-hexane (HPLC grade, SigmaAldrich), toluene (JIS special grade, Sigma-Aldrich), chloroform (spectrochemical analysis grade, Wako), tetrahydrofuran (THF, ACS reagent, Sigma-Aldrich), acetone (JIS special grade, Sigma-Aldrich), N,N-dimethylformamide (DMF, JIS special grade, Sigma-Aldrich), and acetonitrile (JIS special grade, Sigma-Aldrich) were used as received. The absorption spectra of DB1 and DB2 in solution were recorded on a spectrometer (Lamda 650, PerkinElmer). The fluorescence spectra and fluorescence lifetimes in solution were measured using a RF-5300PC fluorospectrometer (Shimadzu) and an avalanche photodiode (APD, PerkinElmer) equipped with a time-correlated single-photon counting (TCSPC) card (TimeHarp 200, PicoQuant), respectively. The absolute fluorescence quantum yields were measured on a Hamamatsu Photonics Quantaurus-QY calibrated integrating sphere system. Borosilicate glass coverslips (Matsunami) were washed ultrasonically in acetone, ethanol, NaOH aqueous solution (1 M), and ultrapure water for 15 min at each step and dried under a flow of argon gas. The cleaned coverslips have high wettability because of the removal of organic adsorbates and partial functionalization of the glass surface with hydroxy groups. Two samples were prepared for the SMFS: dyes embedded in PS thin films or bound to a PS/glass interface. The former sample was prepared by spin-coating (2000 rpm) one drop of a toluene solution containing DB2 (∼10−10 M) and PS (10 mg/mL) onto a cleaned glass coverslip, and the latter sample was prepared by spin-coating (2000 rpm) one drop of a toluene solution of DB2 (∼10−9 M) onto a glass coverslip, followed by spin-coating PS (in toluene, 10 mg/mL) to suppress dye photobleaching. Because the spin-coating without PS resulted in much lower sticking probability than that of the spin-coating with PS, a higher initial concentration was employed in the preparation of the latter sample. In either case, only the areas, where the DB2 molecules were very sparsely dispersed with approximately 0.2 molecules/μm2 (Figure S5), were measured to exclude influences of aggregation. The thickness of the DB2-doped PS film, measured using atomic force microscopy (AFM, SPM-9700, Shimadzu), was approximately 170 nm (Figure S6). We have also tried to sparsely disperse the molecules into a polar polymer film such as PVDF (εs = 6−13 and n = 1.43; Δf = 0.179−0.239);17 however, this turned out to be extremely difficult and dye segregation readily occurred because of the low solubility of DB2 in polar media. 2.2. Single-Molecule Fluorescence Spectroscopy. SMFS measurements were performed using a laser-scanning optical microscope built in-house (this has been thoroughly described elsewhere18). Picosecond pulsed diode lasers (PiL044X, Advanced Laser Diode System) with a pulse width of 50 ps full width at half-maximum (fwhm) were 14565


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DB2 in different solvents. As can be seen, the absorption spectra show less dependence on the type of solvent than the fluorescence spectra, which exhibit a pronounced red shift and a broadening of the bands accompanying the increase of solvent polarity. This solvatochromic behavior is a typical signature of the excited-state symmetry breaking of multipolar molecules.4,8−10 As shown in Figure 1b, the Stokes shifts (Δνst) increase linearly with increasing orientational polarizability (Δf), indicating the stabilization of the polar emitting excited state in polar solvents. A relatively large deviation is found for n-hexane, which suggests that the excited-state symmetry breaking does not occur in apolar media. We can determine the dipole moment change between the excited state (μe) and ground state (μg) dipole moments by using the Lippert−Mataga relationship 125,26

were computed with the linear-response and the state-specific PCM methods, respectively, using the CAM-B3LYP functional in combination with the 6-311++G(d,p) basis set. The Franck−Condon (FC) spectrum simulation was also performed using the CAM-B3LYP functional. The FC stick spectrum (Figure S10) was convoluted with a Gaussian profile to account for inhomogeneous broadening with a FWHM value of 450 cm−1, as employed in Gaussian 16.

3. RESULTS AND DISCUSSION 3.1. Emission Solvatochromism in Solutions. Figure 1a shows the normalized absorption and fluorescence spectra of

Δνst = [2μe (μe − μg )/hca0 3]Δf

(1)

where Δf can be expressed according to eq 2 as follows: Δf = (εs − 1)/(2εs + 1) − (n2 − 1)/(2n2 + 1)

(2)

In these equations, a0 is the Onsager sphere radius, εs is the solvent dielectric, and n is the solvent refractive index. We performed DFT calculations on an analog of DB2 (hereafter referred to as DB2′), in which the four octyl groups were replaced by methyl groups. According to the results, the Onsager sphere radius and ground-state dipole moment were established to be a0 = 8.9 Å and μg = 0.47 D, respectively (Figure S7). Thus, the excited-state dipole moment was estimated to be 27.1 D. Note that this value of the Onsager radius was adopted for DB2 under the presumption of the excited-state localization on the half moiety of the symmetrical D−π−D structure; namely, the value of a0 was set to approximately one-fourth of the overall π-conjugated framework of DB2.27 The validity of this assumption is partly supported by the fact that this value (μe = 27.1 D) is comparable to that of DB1 (μe = 27.7 D, see Figure S11). Moreover, at least two-fold increments in radiative lifetime (τr) were observed in highly polar solvents, such as acetone and DMF, compared to other solvents (Table 1). Noticeably, the radiative lifetimes of DB2 in acetone and DMF show good agreement with those obtained for DB1 in polar solvents (Table S1). Because the radiative rate constant, kr = τr−1, is proportional to the square of emission transition dipole

Figure 1. (a) Solvent dependence of the absorption (dashed lines) and fluorescence (solid lines) spectra of DB2 and (b) Lippert− Mataga plot, where a data point for DB2 in a PS film is shown. The slope of the linear fit for the Lippert−Mataga plot is also presented. Note that the data point for n-hexane was excluded in the analysis because of a large deviation.

Table 1. Photophysical Parameters of DB2 in Various Environments dye

environment

λabs/nm, εmax/M−1 cm−1

2nd λ1st em, λem /nm

Δνst/cm−1

Φf

τf/ns

τr/nsd

DB2

n-hexane toluene furan chloroform THF acetone DMF PS film PS/glass interface

403 411, 1.18 × 105 410 411 415 415 420 414 408a

439, 467 463, 490 475 498 517 543 554 458b, 488 448b, 477

2035 2733 3338 4251 4754 5680 5759 2313 2188

0.79 0.74 0.71 0.78 0.89 0.39 0.13 ∼1 ∼1

0.70 0.81 0.88 1.07 1.39 1.19 0.58 1.16 (0.46)c 4.52 (0.81) 5.59 (0.34)c

0.89 1.09 1.24 1.37 1.56 3.05 4.64

a Wavelength of absorption maximum obtained by ensemble absorption measurements for DB2 adsorbed on quartz (Figure S12). bλ1st em estimated c 1st 7 2nd −1 using the average values of λ2nd em (1/λem = 10 /λem + 1350 cm ), obtained by the Gaussian fits to the histograms in Figure 3d. Average lifetimes obtained by the Gaussian fits to the histograms in Figure 3a, and the values in parentheses are their fwhms. Two Gaussians were used in the fit of the histogram at the PS/glass interface. dRadiative lifetimes were calculated by using fluorescence quantum yields and fluorescence lifetimes; τr = τf/Φf.

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The differences between the photophysical parameters obtained for the PS film and the PS/glass interface are more clearly shown in the histograms of τf, λ2nd em , and W and the corresponding correlation plots of τf versus λ2nd em and τf versus W, as depicted in Figure 3. As seen in Figure 3a, the single-

moment (i.e., oscillator strength), these results indicate a considerable reduction of the oscillator strength of DB2 in highly polar solvents and hence electronic localization in the emitting excited state. 3.2. Emission Behaviors in Polymer Films and at the Polymer/Glass Interface. Panels a−c and d−f in Figure 2

Figure 2. Typical examples of single-molecule data obtained for (a− c) DB2 embedded in a PS film and (d−f) DB2 bound at a PS/glass interface. (a,d) Fluorescence intensity time traces, (b,e) fluorescence spectra, and (c,f) fluorescence decay curves. Note that below 460 nm (shaded region), the fluorescence spectra in panels (b,e) were interrupted by optical filters and hence the 0−0 peak is not observed in panel (e). The solid lines in panels (c,f) correspond to singleexponential fits.

Figure 3. (a) Fluorescence lifetime distributions of DB2 single molecules: 202 molecules in a PS film (blue) and 202 molecules bound at a PS/glass interface (red). Dashed lines are Gaussian fits of each distribution. Filled and open arrows indicate the radiative lifetimes (τr) of DB2 in toluene and in DMF, respectively (Table 1). Correlation plots of (b) fluorescence lifetime (τf) vs wavelength of the second peak (λ2nd em ) and (c) τf versus spectral width of the second peak (W). Histograms of (d) λ2nd em and (e) W.

show representative examples of fluorescence intensity time traces (a, d) along with the corresponding spectra (b, e) and lifetimes (c, f) for DB2 molecules embedded in the PS film (thickness: ca. 170 nm, Figure S6) and bound at the PS/glass interface, respectively. The intensity time traces show irreversible single-step photobreaching from signals to background levels, which indicates the occurrence of singlemolecule emission. In panels b and e, a vibronic feature of approximately 1350 cm−1, which is theoretically assignable to the vibrational excitation of several in-plane C−C, CC, and CC stretching modes (Figure S10), is prominently observed for both environments. Remarkably, the fluorescence spectrum for the PS/glass interface exhibits a considerably sharper vibrational feature (fwhm of the second peak; W = 655 cm−1) than that for the PS film (W = 1393 cm−1). Furthermore, the 0−1 vibronic peak at λ2nd em = 477 nm for the interface is marginally blue-shifted compared to that for the PS film, λ2nd em = 497 nm. All the fluorescence decay curves (e.g., panels c and f) could be fitted well with single-exponential functions; however, the fluorescence lifetimes (τf) obtained for the film and the interface were remarkably different.

molecule fluorescence lifetimes of DB2 in the PS film mostly lie within the range of 0.9−2.0 ns. Intriguingly, longer lifetimes (>3.5 ns) are observed when DB2 resides at the PS/glass interface, and their distribution is much broader than that for the PS film. This striking difference reveals the dissimilarity in the nature of the emitting excited state between the inside of the polymer film and the polymer/glass interface. Note that a trace amount of the molecules dispersed in the PS film exhibits long lifetimes (3.5−5.0 ns), which are well matched with the lifetimes of DB2 at the interface. Hence, they can be attributed to molecules trapped at the PS/glass interface during the spincoating process. Conversely, a few molecules having short lifetimes (∼1.2 ns) were also observed when dispersed on the glass surface. They were most probably dissolved into the PS layer during the spin-coating of the PS/toluene solution. In the PS film (εs = 2.6 and n = 1.593; Δf = 0.005), the average lifetime is 1.16 ns (Table 1), which agrees with the radiative lifetime τr of 1.09 ns in toluene (Δf = 0.013). Because PS and toluene have a comparable orientational polarizability, 14567


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Figure 4. (a,b) Second-order cross-correlation functions, g(2)(t), obtained for DB2 single-molecules in a PS film and at a PS/glass interface, respectively. Solid lines correspond to single-exponential fits. The ISC parameters obtained by the three-state model are also shown. (c) Correlation plot between triplet-state lifetime (τT) and S1−T1 ISC yield (ΦISC): 196 molecules in a PS film (blue) and 196 molecules bound at a PS/glass interface (red). The dashed line represents the linear fit of the correlation plot (R = −0.90).

this agreement suggests that the fluorescence quantum yield of DB2 in the PS film is almost unity. In addition, the average 2nd value of λ2nd em for DB2 in the PS film, ⟨λem ⟩ = 488 nm (Figure 2nd 3b,d, Table 1), is well matched with λem = 490 nm in toluene (Table 1). However, the spectral shape is apparently different from that in toluene and similar to that in n-hexane, where a vibrational feature is observed prominently. In Figure 1a, an apparent spectral broadening can be observed when going from n-hexane to toluene, suggesting that the excited-state symmetry breaking is slightly induced in toluene. Hence, we consider that the excited-state symmetry breaking of DB2 does not take place in the PS film and that the fluorescence emission occurs from the symmetry-preserved (quadrupolar) excited state. Meanwhile, the long fluorescence lifetimes (3.5−7.0 ns) of DB2 at the PS/glass interface are several times longer than the radiative lifetimes of DB2 in nonpolar solvents (τr ≈ 1 ns) and are comparable to (or longer than) those in highly polar solvents, such as DMF (τr = 4.64 ns, Table 1) and those of DB1 in polar solvents (Table S1). When fluorescence lifetime accords with radiative lifetime, nonradiative processes are negligible. Therefore, these results suggest that the excitedstate symmetry breaking occurs at the PS/glass interface with Φf ≈ 1. As is evident in Figure 3c,e, the average values of the spectral widths of the second peak, ⟨W⟩, are significantly reduced in the PS/glass interface (⟨W⟩ = 640 cm−1) compared with the PS film (⟨W⟩ = 1342 cm−1). The spectral line width is dominated by (1) the coupling between electronic excitation and lowfrequency intra/intermolecular vibrational modes and (2) environmental inhomogeneity, that is, inhomogeneous broadening. As shown in Figure S10, the observed vibrational feature is mainly composed of the contribution of relatively highfrequency intramolecular modes (>1000 cm−1), and thus the peak width (W) is likely to be caused mostly by the inhomogeneous broadening and the excitation of lowfrequency intermolecular vibrational modes between a doped molecule and surrounding polymer chains. Noticeably, the parameters λ2nd em and W at the interface are rather narrowly distributed as compared to those in the PS film (Figure 3b−e), indicating that the inhomogeneous broadening at the polymer/ glass interface is much smaller than that in the polymer film. In addition, conformational changes are expected to be more

strongly restricted at the PS/glass interface than inside the PS film because of the reduction of the free volume in the polymer film via the adhesive interaction between the glass and polymer.28 Therefore, the sharp spectral feature reflects a relatively uniform and conformationally restricted physisorption of DB2 onto the glass surface. Such an interfacial confinement effect also has a substantial impact on the intersystem crossing (ISC) kinetics. Panels a and b of Figure 4 show representative examples of fluorescence intensity autocorrelation curves [second-order cross-correlation functions, g(2)(t)] of a DB2 single-molecule embedded in the PS film and trapped at the PS/glass interface, respectively. The second-order cross-correlation functions obtained for individual molecules can be fitted well with single-exponential functions. The single-exponential decay of the cross-correlation is usually a signature of ISC kinetics.17,18,29−35 Then, the threestate model of S0, S1, and T1 was employed in the g(2)(t) analysis to determine the ISC parameters, that is, triplet-state lifetime (τT) and ISC quantum yield (ΦISC).29−38 The τT value can be determined directly from the average off-time (τoff), whereas the ΦISC value is calculated using the average on-time (τon) through the relation of 1/τon = kexΦISC, where kex represents the excitation rate. According to the three-state model, g(2)(t) is given by g(2)(t) = 1 + C·e−(1/τoff+1/τon)t, where C is a molecular constant.32 If Ion ≫ Ioff, the molecular constant is represented by C = τoff/τon. Using these expressions, for example, we can derive τoff (=τT) of 16 μs and τon = 0.14 ms (ΦISC = 4.76 × 10−2) for a PS film (Figure 4a) and 425 μs and τon = 2.97 ms (ΦISC = 2.05 × 10−3) for a PS/glass interface (Figure 4b). The correlation plot between τT and ΦISC is shown in Figure 4c. The triplet lifetimes for the PS/glass interface are mostly distributed at 1−2 ms, whereas those for the PS film are much shorter than 2 ms and widely distributed from 1 μs to 1 ms. Moreover, a strong negative correlation (correlation coefficient R = −0.90) was found between τT and ΦISC. This result indicates that both the S1−T1 and T1−S0 ISC processes hardly occur at the PS/glass interface, in contrast to the inside of the PS film. One possible explanation for this result is that molecules deeply embedded within the PS film, especially molecules at the PS/glass interface, have less access to molecular oxygen as compared to molecules close to the PS/gas interface.37 However, our SMFS measurements were 14568


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Table 2. Configuration Interaction (CI) Expansion Coefficients (P), Absorption Peaks λabs (nm), and Oscillator Strengths f of the S0 → S1 Transition of DB1' and DB2' in Toluenea S0 geometryb

μg/D

dye

method

DB1'

TD-CAM-B3LYP//ωB97X-D TD-CAM-B3LYP//M06-2X TD-CAM-B3LYP//MPW1K TD-CAM-B3LYP//ωB97X-D

twist twist planar twist (C2)

4.11 4.11 6.44 1.45

TD-CAM-B3LYP//M06-2X

slightly twist (C2)

1.19

TD-CAM-B3LYP//MPW1K

planar (C2)

0.47

DB2'

dominant configuration

CI expansion coefficientc

H→L H→L H→L H-2 → L H-1 → L + 1 H→L H-2 → L H-1 → L + 1 H→L H-2 → L H-1 → L + 1 H→L

0.65076 (85%) 0.64893 (84%) 0.65974 (87%) 0.22189 (10%) −0.31172 (19%) 0.55495 (62%) 0.20989 (9%) −0.31548 (20%) 0.55880 (62%) 0.19083 (7%) −0.29708 (18%) 0.58179 (68%)

Abs. peak λabs/nmd 370.33 368.86 398.71 392.73

f

(3.348) (3.361) (3.110) (3.157)

2.131 2.117 2.229 4.575

397.43 (3.120)

4.613

437.52 (2.834)

4.758

exp. λabs/nme 392 (3.16)

412 (3.01)

a

6-31+G(d,p) and 6-311++G(d,p) basis sets were used in the geometrical optimization and the TD-DFT calculations, respectively. The linearresponse formalism within the PCM/TD-DFT method was used to include solvent effects. bSee Figure S7. cThe primary configurations with larger CI expansion coefficient for the S1 state. The value in parentheses represents the percentage contribution (P = 2|CI|2) for the corresponding oneelectron transition to S1. dThe value in parentheses is in eV. eThis work.

Figure 5. (a) Bar plots of bond length differences between the corresponding bonds in the right-half and left-half parts of the S1-optimized geometries of DB2′ in vacuo and solvents with different polarities. The frontier orbitals of the S1 geometries in vacuo and DMF are also shown. (b) Corresponding excited state dipole moments and (c) vertical emission energies computed at the state-specific PCM/TD-CAM-B3LYP method with the 6-311++G(d,p) basis set.

processes become slower. Because of the polymer−glass adhesive interaction, the free volume in the polymer film decreases when approaching the polymer/glass interface.28 Thus, the conformational restriction becomes stronger as molecules are more deeply embedded within the PS film, and the strongest conformational restriction is attained at the interface. Indeed, molecules residing at the PS/glass interface distinctly exhibit much lower ISC yields (≤10−3) and longer triplet-state lifetimes (≥1 ms) than those within the PS film. Because of the highly anisotropic shape of DB2, the flatlying orientation, in which the molecular plane is parallel to the glass surface,42 is most likely favored (Figure S13). In this adsorption mode, out-of-plane vibrational motions, such as torsional motions, are expected to be hindered at the

implemented under a high-vacuum condition (∼0.1 Pa), where the oxygen concentration was estimated to be approximately 10 nM. The rate constants for the triplet quenching of aromatic molecules (kTq ) are generally of the order of 109 to 1010 M−1 s−1,39 and thus, the virtual quenching rate kTq [O2] should be 10−100 s−1 or less, which is one (or two) order(s) of magnitude smaller than the slowest triplet decay rate observed for DB2 (kT = τT−1 ≈ 1000 s−1). Hence, it seems reasonable to conclude that the influence of triplet quenching by oxygen is negligible for the present system. Another plausible origin is the restriction of conformational relaxation. In stilbene and its derivatives, the torsion around the double bond plays a pivotal role in the ISC process.40,41 Therefore, if torsional motions are environmentally hindered, the ISC 14569


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The Journal of Physical Chemistry C interface.43 The symmetry-breaking dynamics of quadrupolar molecules in the liquid phase are often accompanied by torsional relaxations that stabilize the nuclear configurations in the excited-state potential energy surface.16,44,45 Therefore, the suppression of torsional motions results in a reduction of the energetic stabilization of the symmetry-breaking ICT state.43,46 Indeed, the Stokes shift of DB2 at the PS/glass interface (2188 cm−1) is much smaller than those in highly polar solvents (∼5700 cm−1) and comparable to that obtained in nonpolar nhexane (2035 cm−1), in which the excited-state symmetry breaking does not occur. This result brings to light an interesting question: why does the excited-state symmetry breaking of DB2 take place rather efficiently at the PS/glass interface? 3.3. Theoretical Insight into Excited-State Symmetry Breaking. To address the issue just mentioned above, theoretical calculations were conducted for DB1′ and DB2′. DB2′ in S0 exhibits a centrosymmetric, almost a planar structure with a small, out-of-plane dipole moment (Figures S7 and S8a). As summarized in Table 2, the vertical S0 → S1 transition is mainly described by the highest occupied molecular orbital → lowest unoccupied molecular orbital transition of (π, π*) nature and the vertical transition energies reproduce well the corresponding experimental values. As can be seen in Figure S8b, which depicts the difference of full electron density between S0 and S1, the absorption is accompanied by a symmetrical charge displacement from the lateral electron rich p-dioctylaminostyryl donor moiety toward the central bis(phenylethynyl)benzene portion, indicating a symmetric ICT character in the vertical absorption process. Subsequently, structural relaxation, that is, symmetry breaking, from this FC state to the S1 equilibrium state is likely to proceed in highly polar solvents. To verify this, geometrical optimizations for the S1 state were further performed. All the relaxed S1 geometries in vacuo and nonpolar and polar solvents were found to be almost planar (Figure 5 and S7), as in the case of the ground state. As can be seen in Figure 5a, however, structural and electronic asymmetries in the S1 geometry are enhanced with increasing solvent polarity. Note that the completely centrosymmetric structure was obtained only in vacuo. Another particularly notable point is that the excited-state symmetry breaking of DB2′ occurs with the asymmetric in-plane relaxation of the π-conjugated framework through bond length changes, which are especially pronounced around the stilbene moiety (bond labels 6−12 in Figure 5a). Such in-plane symmetry breaking can be expected to facilitate the excited-state symmetry breaking at the flat-lying geometry on the glass. As the solvent polarity increases, the excited-state dipole moment gradually increases (Figure 5b) and becomes closer to the value of μe = 27.1 D that was estimated from the Lippert−Mataga analysis, assuming that the excited state localizes on half the structure of DB2. Overall, the calculated vertical emission energies reproduce the experimental results (Figure 5c), but a large underestimation was obtained in the case of n-hexane. This discrepancy suggests that the excitedstate symmetry breaking does not occur in nonpolar solvents, as mentioned above. At the flat-lying geometry, the excitedstate dipole moment, depicted in Figure 5b, is generated in the surface-parallel direction, and adsorbed molecules can be effectively stabilized through dipole−dipole interaction with the surface charge of the borosilicate glass.47,48 In polar liquids, the excited-state symmetry breaking is driven by differences in the instantaneous orientation of the polar solvent molecules

around each D−π branch of the chromophore.14 At the interface, each D−π branch of DB2 with the flat-lying orientation (Figure S13) should also undergo different interactions with the polar glass surface; otherwise, the excited-state symmetry breaking is not driven at this orientation. In this respect, surface charge nonuniformity should present on the glass surface and affect the degree of symmetry breaking of the adsorbates. Therefore, we infer that such heterogeneity may be responsible for the broad fluorescence lifetime distribution in Figure 3a, irrespective of the relatively uniform adsorption on the glass. As can be seen in Table 1, the Φf value of DB2 remains almost constant from nonpolar to weakly polar solvents, whereas an abrupt decrease is observed in highly polar solvents. This might be attributed to the energy gap law,38,49 according to which the symmetry-breaking ICT state would be stabilized by solvent reorganization in polar solvents, thereby approximating its energy level to the potential energy surface of S0, which would lead to an increase of the FC overlap and the concomitant enhancement of nonradiative processes. In contrast, even if the excited-state symmetry breaking occurs at the polymer/glass interface, the energetic stabilization of the ICT state is small due to the lack of orientational polarization corresponding to solvent reorganization in polar solvents. Therefore, the energy gap law is not applicable to DB2 at the PS/glass interface and no enhancement of nonradiative processes occurs.

4. CONCLUSIONS In conclusion, we found that the excited-state symmetry breaking of the centrosymmetric D−π−D chromophore occurs not only in polar solvents but also at the polymer/glass interface, whereas the emission behaviors in these dielectric environments are strikingly different. In contrast to the case in polar solvents, the excited-state symmetry breaking at the interface is not accompanied by a large Stokes shift; however, an asymmetric in-plane symmetry breaking is efficiently induced by dipolar interaction with the glass surface. Such symmetry-broken adsorbates exhibit several times longer fluorescence lifetimes that coincide with the radiative lifetimes of the symmetry-broken dipolar states formed in highly polar solvents. Because of the lack of large surface reorganization in the glass, the energy gap law is not applicable, and strong conformational restriction at the solid/solid interface efficiently suppresses the nonradiative processes. This interfacial effect enables the symmetry-broken molecules to boost the fluorescence quantum yield up to Φf ≈ 1. Hence, we envision that our findings on the symmetry breaking at the interface provide new insight into multipolar chromophore-based luminescent materials for applications in organic light-emitting diodes6,7 because emitting chromophores are deposited on substrate surfaces, thereby promoting the excited-state symmetry breaking at the interfaces of such devices.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b03612. Synthesis methods and NMR data of DB1 and DB2; additional data of quantum-chemical calculations, including the S 0 and S 1 optimized geometries, corresponding IR spectra, frontier molecular orbitals, 14570


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Electron-Transfer Kinetics of S1 Bianthryls. J. Phys. Chem. 1988, 92, 6800−6807. (12) Vauthey, E. Photoinduced Symmetry-Breaking Charge Separation. ChemPhysChem 2012, 13, 2001−2011. (13) Dereka, B.; Vauthey, E. Solute-Solvent Interactions and Excited-State Symmetry Breaking: Beyond the Dipole-Dipole and the Hydrogen-Bond Interactions. J. Phys. Chem. Lett. 2017, 8, 3927− 3932. (14) Dereka, B.; Rosspeintner, A.; Li, Z.; Liska, R.; Vauthey, E. Direct Visualization of Excited-State Symmetry Breaking Using Ultrafast Time-Resolved Infrared Spectroscopy. J. Am. Chem. Soc. 2016, 138, 4643−4649. (15) Beckwith, J. S.; Rosspeintner, A.; Licari, G.; Lunzer, M.; Holzer, B.; Fröhlich, J.; Vauthey, E. Specific Monitoring of Excited-State Symmetry Breaking by Femtosecond Broadband Fluorescence Upconversion Spectroscopy. J. Phys. Chem. Lett. 2017, 8, 5878−5883. (16) Kim, W.; Sung, J.; Grzybowski, M.; Gryko, D. T.; Kim, D. Modulation of Symmetry-Breaking Intramolecular Charge-Transfer Dynamics Assisted by Pendant Side Chains in π-Linkers in Quadrupolar Diketopyrrolopyrrole Derivatives. J. Phys. Chem. Lett. 2016, 7, 3060−3066. (17) Mizukami, T.; Inoue, S.; Ohsawa, H.; Niihori, Y.; Kobayashi, K.; Mitsui, M. Impacts of Environmental Rigidity on Photophysical Characteristics and Behaviors of a Quadrupolar Chromophore Revealed by Single-Molecule Fluorescence Spectroscopic Study. J. Phys. Chem. C 2018, 122, 21295−21307. (18) Mitsui, M.; Kawano, Y.; Takahashi, R.; Fukui, H. Photophysics and Photostability of 9,10-Bis(phenylethynyl)anthracene Revealed by Single-Molecule Spectroscopy. RSC Adv. 2012, 2, 9921−9931. (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; et al. Gaussian 16, Revision B.01; Gaussian, Inc.: Wallingford CT, 2016. (20) Chai, J.-D.; Head-Gordon, M. Long-range corrected hybrid density functionals with damped atom-atom dispersion corrections. Phys. Chem. Chem. Phys. 2008, 10, 6615−6620. (21) Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Functionals. Theor. Chem. Acc. 2008, 120, 215−241. (22) Lynch, B. J.; Fast, P. L.; Harris, M.; Truhlar, D. G. Adiabatic Connection for Kinetics. J. Phys. Chem. A 2000, 104, 4811−4815. (23) Bernini, C.; Zani, L.; Calamante, M.; Reginato, G.; Mordini, A.; Taddei, M.; Basosi, R.; Sinicropi, A. Excited State Geometries and Vertical Emission Energies of Solvated Dyes for DSSC: A PCM/TDDFT Benchmark Study. J. Chem. Theory Comput. 2014, 10, 3925− 3933. (24) Xu, B.; Li, Y.; Song, P.; Ma, F.; Sun, M. Photoactive Layer Based on T-Shaped Benzimidazole Dyes Used for Solar Cell: From Photoelectric Properties to Molecular Design. Sci. Rep. 2017, 7, 45688. (25) Lippert, E. Spectroscopic Determinations of the Dipole Moment of Aromatic Compounds in the First Excited Singlet State. Z. Elektrochem. 1957, 61, 962−975. (26) Mataga, N.; Kaifu, Y.; Koizumi, M. Solvent Effects upon Fluorescence Spectra and the Dipolemoments of Excited Molecules. Bull. Chem. Soc. Jpn. 1956, 29, 465−470. (27) Amthor, S.; Lambert, C.; Dümmler, S.; Fischer, I.; Schelter, J. Excited Mixed-Valence States of Symmetrical Donor−Acceptor− Donor π Systems. J. Phys. Chem. A 2006, 110, 5204−5214. (28) Xia, W.; Hsu, D. D.; Keten, S. Dependence of Polymer Thin Film Adhesion Energy on Cohesive Interactions between Chains. Macromolecules 2014, 47, 5286−5294. (29) Haase, M.; Hübner, C. G.; Reuther, E.; Herrmann, A.; Müllen, K.; Basché, T. Exponential and Power-Law Kinetics in SingleMolecule Fluorescence Intermittency. J. Phys. Chem. B 2004, 108, 10445−10450.

total electron difference density, and FC simulation; and additional experimental data including photophysical parameters, absorption and emission spectra, and Lippert−Mataga plot of DB1 in solutions, fluorescence images, AFM image of a PS film, and proposed adsorption geometry of DB2 on glass (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +81-3-3985-2364 (M.M.). *E-mail: [email protected]. Fax: +81-54-2384933 (K.K.). ORCID

Masaaki Mitsui: 0000-0001-5800-0415 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Theoretical calculations were performed using Research Center for Computational Science, Okazaki, Japan. The authors would like to thank Enago (www.enago.jp) for the English language review. This work is supported by Iwatani Science and Technology Research Grant and Grants-in-Aid for Scientific Research (C), nos. 24550018 and 15K05398.

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REFERENCES

(1) Albota, M.; Beljonne, D.; Brédas, J.-L.; Ehrlich, J. E.; Fu, J.-Y.; Heikal, A. A.; Hess, S. E.; Kogej, H. T.; Levin, M. D.; Marder, S. R.; et al. Design of Organic Molecules with Large Two-Photon Absorption Cross Sections. Science 1998, 281, 1653−1656. (2) He, G. S.; Tan, L.-S.; Zheng, Q.; Prasad, P. N. Multiphoton Absorbing Materials: Molecular Designs, Characterizations, and Applications. Chem. Rev. 2008, 108, 1245−1330. (3) Pawlicki, M.; Collins, H. A.; Denning, R. G.; Anderson, H. L. Two-Photon Absorption and the Design of Two-Photon Dyes. Angew. Chem., Int. Ed. 2009, 48, 3244−3266. (4) Ricci, F.; Elisei, F.; Foggi, P.; Marrocchi, A.; Spalletti, A.; Carlotti, B. Photobehavior and Nonlinear Optical Properties of PushPull, Symmetrical, and Highly Fluorescent Benzothiadiazole Derivatives. J. Phys. Chem. C 2016, 120, 23726−23739. (5) Ventelon, L.; Blanchard-Desce, M.; Moreaux, L.; Mertz, J. New Quadrupolar Fluorophores with High Two-photon Excited Fluorescence. Chem. Commun. 1999, 20, 2055−2056. (6) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic Light-Emitting Diodes from Delayed Fluorescence. Nature 2012, 492, 234−238. (7) Data, P.; Pander, P.; Okazaki, M.; Takeda, Y.; Minakata, S.; Monkman, A. P. Dibenzo[a,j]phenazine-Cored Donor-AcceptorDonor Compounds as Green-to-Red/NIR Thermally Activated Delayed Fluorescence Organic Light Emitters. Angew. Chem., Int. Ed. 2016, 55, 5739−5744. (8) Amthor, S.; Lambert, C.; Dümmler, S.; Fischer, I.; Schelter, J. Excited Mixed-Valence States of Symmetrical Donor−Acceptor− Donor π Systems. J. Phys. Chem. A 2006, 110, 5204−5214. (9) Raju, M. V. N.; Mohanty, M. E.; Bangal, P. R.; Vaidya, J. R. Synthesis and Ultrafast Dynamics of a Donor−Acceptor−Donor Molecule Having Optoelectronic Properties. J. Phys. Chem. C 2015, 119, 8563−8575. (10) Terenziani, F.; Painelli, A.; Katan, C.; Charlot, M.; BlanchardDesce, M. Charge Instability in Quadrupolar Chromophores: Symmetry Breaking and Solvatochromism. J. Am. Chem. Soc. 2006, 128, 15742−15755. (11) Kang, T. J.; Kahlow, M. A.; Giser, D.; Swallen, S.; Nagarajan, V.; Jarzeba, W.; Barbara, P. F. Dynamic Solvent Effects in the 14571


Article

The Journal of Physical Chemistry C (30) Ha, T.; Enderle, T.; Chemla, D. S.; Selvin, P. R.; Weiss, S. Quantum Jumps of Single Molecules at Room Temperature. Chem. Phys. Lett. 1997, 271, 1−5. (31) Yip, W.-T.; Hu, D.; Yu, J.; Vanden Bout, D. A.; Barbara, P. F. Classifying the Photophysical Dynamics of Single- and MultipleChromophoric Molecules by Single Molecule Spectroscopy. J. Phys. Chem. A 1998, 102, 7564−7575. (32) Weston, K. D.; Carson, P. J.; DeAro, J. A.; Buratto, S. K. SingleMolecule Detection Fluorescence of Surface-Bound Species in Vacuum. Chem. Phys. Lett. 1999, 308, 58−64. (33) Mitsui, M.; Higashi, K.; Takahashi, R.; Hirumi, Y.; Kobayashi, K. Enhanced Photostability of an Anthracene-Based Dye Due to Supramolecular Encapsulation: A New Type of Photostable Fluorophore for Single-Molecule Study. Photochem. Photobiol. Sci. 2014, 13, 1130−1136. (34) Mitsui, M.; Unno, A.; Azechi, S. Understanding Photoinduced Charge Transfer Dynamics of Single Perylenediimide Dyes in a Polymer Matrix by Bin-Time Dependence of their Fluorescence Blinking Statistics. J. Phys. Chem. C 2016, 120, 15070−15081. (35) Mitsui, M.; Unno, A.; Mori, K. Methodology for Discriminating between Competitive Photophysical Processes in Photoblinking: Application to the Fluorescence Blinking of Single Dye Molecules Adsorbed on TiO2. Chem. Lett. 2017, 46, 866−869. (36) Piwoński, H.; Hartschuh, A.; Urbańska, N.; Pietrazkiewicz, M.; Sepiol, J.; Meixner, A. J.; Waluk, J. Polarized Spectroscopy Studies of Single Molecules of Porphycenes: Tautomerism and Orientation. J. Phys. Chem. C 2009, 113, 11514−11519. (37) Würsch, D.; Hofmann, F. J.; Eder, T.; Aggarwal, A. V.; Idelson, A.; Höger, S.; Lupton, J. M.; Vogelsang, J. Molecular Water Lilies: Orienting Single Molecules in a Polymer Film by Solvent Vapor Annealing. J. Phys. Chem. Lett. 2016, 7, 4451−4457. (38) Adhikari, A. N.; Capurso, N. A.; Bingemann, D. Heterogeneous Dynamics and Dynamic Heterogeneities at the Glass Transition Probed with Single Molecule Spectroscopy. J. Chem. Phys. 2007, 127, 114508. (39) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Modern Molecular Photochemistry of Organic Molecules; University Science Books: Sausalito, CA, 2010. (40) Han, W.-G.; Lovell, T.; Liu, T.; Noodleman, L. Density Functional Studies of the Ground- and Excited-State Potential-Energy Curves of Stilbenecis-trans Isomerization. ChemPhysChem 2002, 3, 167−178. (41) Yang, J.-S.; Chiou, S.-Y.; Liau, K.-L. Fluorescence Enhancement oftrans-4-Aminostilbene byN-Phenyl Substitutions: The “Amino Conjugation Effect”. J. Am. Chem. Soc. 2002, 124, 2518−2527. (42) Yokoyama, D. Molecular Orientation in Small-Molecule Organic Light-Emitting Diodes. J. Mater. Chem. 2011, 21, 19187− 19202. (43) Thompson, D. W.; Fleming, C. N.; Myron, B. D.; Meyer, T. J. Rigid Medium Stabilization of Metal-to-Ligand Charge Transfer Excited States. J. Phys. Chem. B 2007, 111, 6930−6941. (44) Suhina, T.; Amirjalayer, S.; Mennucci, B.; Woutersen, S.; Hilbers, M.; Bonn, D.; Brouwer, A. M. Excited-State Decay Pathways of Molecular Rotors: Twisted Intermediate or Conical Intersection? J. Phys. Chem. Lett. 2016, 7, 4285−4290. (45) Carlotti, B.; Benassi, E.; Spalletti, A.; Fortuna, C. G.; Elisei, F.; Barone, V. Photoinduced Symmetry-Breaking Intramolecular Charge Transfer in a Quadrupolar Pyridinium Derivative. Phys. Chem. Chem. Phys. 2014, 16, 13984−13994. (46) Gao, B.-R.; Wang, H.-Y.; Hao, Y.-W.; Fu, L.-M.; Fang, H.-H.; Jiang, Y.; Wang, L.; Chen, Q.-D.; Xia, H.; Pan, L.-Y.; et al. TimeResolved Fluorescence Study of Aggregation-Induced Emission Enhancement by Restriction of Intramolecular Charge Transfer State. J. Phys. Chem. B 2010, 114, 128−134. (47) Kim, Y.; Lee, M.; Wang, H. S.; Song, K. The Effect of Surface Polarity of Glass on Liquid Crystal Alignment. Liq. Cryst. 2018, 45, 757−764.

(48) Mullen, M.; Euler, W. B. The Influence of Interfacial Effects on the Photophysics of Rhodamine 6G Thin Films on a Poly(vinylidene fluoride) Surface. Langmuir 2017, 33, 2194−2204. (49) Liu, C.; Tang, K.-C.; Zhang, H.; Pan, H.-A.; Hua, J.; Li, B.; Chou, P.-T. Studies of Excited-State Properties of Multibranched Triarylamine End-Capped Triazines. J. Phys. Chem. A 2012, 116, 12339−12348.

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