localization in multibranched push-pull chromophores.

Solvent-dependent intramolecular charge transfer delocalization/localization in multibranched push-pull chromophores Yang Li, Meng Zhou, Yingli Niu, Qianjin Guo, and Andong Xia Citation: The Journal of Chemical Physics 143, 034309 (2015); doi: 10.1063/1.4926998 View online: http://dx.doi.org/10.1063/1.4926998 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/143/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The effect of structural changes on charge transfer states in a light-harvesting carotenoid-diaryl-porphyrinC60 molecular triad J. Chem. Phys. 140, 204309 (2014); 10.1063/1.4876075 Effect of intramolecular charge transfer on the two-photon absorption behavior of multibranched triphenylamine derivations J. Appl. Phys. 111, 053516 (2012); 10.1063/1.3692076 On the influence of nonlocal molecular vibrations and charge transfer on the spectra of aggregated push–pull chromophores J. Chem. Phys. 134, 154512 (2011); 10.1063/1.3580516 Intramolecular charge transfer of 4-(dimethylamino)benzonitrile probed by time-resolved fluorescence and transient absorption: No evidence for two ICT states and a π σ ∗ reaction intermediate J. Chem. Phys. 131, 224313 (2009); 10.1063/1.3270165 Femtosecond time-resolved absorption anisotropy spectroscopy on 9 , 9 ′ -bianthryl: Detection of partial intramolecular charge transfer in polar and nonpolar solvents J. Chem. Phys. 130, 014501 (2009); 10.1063/1.3043368

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THE JOURNAL OF CHEMICAL PHYSICS 143, 034309 (2015)

Solvent-dependent intramolecular charge transfer delocalization/localization in multibranched push-pull chromophores Yang Li, Meng Zhou, Yingli Niu, Qianjin Guo, and Andong Xiaa) Beijing National Laboratory for Molecular Sciences (BNLMS) and Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

(Received 26 April 2015; accepted 6 July 2015; published online 21 July 2015) The effect of the solvent polarity on excitation delocalization/localization in multibranched push-pull chromophores has been thoroughly explored by combining steady state absorption and fluorescence, as well as femtosecond transient spectral measurements. We found that the excited-state relaxations of the push-pull chromophores are highly dependent on both solvent polarity and the polar degree of the excited intramolecular charge transfer states. The symmetry of multibranched chromophores is preserved in less polar solvents, leading to excitation delocalization over all of the branches because of the negligible solvent reaction field. In contrast, symmetry is broken for multibranched chromophores in more polar solvents because of intense solvent reaction field, and the excitation is consequently localized on one of the dipolar molecular branches. The results provide a fundamental understanding of solvent-dependent excitation delocalization/localization properties of the multibranched chromophores for the potential applications in nonlinear optics and energy-harvesting applications. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4926998]

I. INTRODUCTION

Organic molecular materials with intramolecular charge transfer (ICT) properties are of potential applications in making organic light emitting diodes (OLEDs), fluorescence probe for bio-imaging, and artificial energy-harvesting applications.1–4 It is well known that local environments, especially solvents, have profound effects on the emission behavior of ICT type compounds,5–11 where the surrounding solvent molecules respond by slightly modifying their translational and rotational motion and then eventually dissipate the excitation energy of excited ICT chromophore.12–14 The degree of intramolecular charge transfer in multibranched chromophores can be estimated from the dynamics of solvent-coupled excited state relaxation.7,15,16 The simplest ICT chromophore is represented by so-called electron push-pull chromophore, which is commonly composed of single electron-donating (D) and electron-accepting (A) groups through π-conjugated linker (with dipole character). Moreover, increasing the dimensionality of donor-π-acceptor molecules gives rise to some special electronic properties compared to the linear counterparts due to extended one-dimensional backbones with delocalized mobile π-electron systems.17–20 This may result in fast spectral response and efficient movement of charge carriers and exciton diffusion through the backbone.20–24 Numerous multibranched chromophores have been synthesized with varying donor-πacceptor configurations, as well as different π-bridging units, and different donor-acceptor strengths to achieve different structure-function relationships.25–31 In particular, specific investigations aimed at understanding the inter-branches’ coherence or the effects of disorder within multibranched a)Author to whom correspondence should be addressed. Electronic mail:

[email protected]. 0021-9606/2015/143(3)/034309/12/$30.00

systems indicate that depending on the coupling intensity among the different branches, either localized or delocalized ICT states can be expected, with important consequences for the spectral properties and behaviors.32 While recent theoretical and experimental studies have contributed greatly to the description of the structure-function relationships, the exact environmental effect on excitation delocalization/localization in multibranched push-pull chromophores is still not well understood because of the intrinsic complexity of the ICT chromophores; considerable efforts are currently being devoted towards understanding the solvent effects and excited state dynamics of the ICT states with different donor-π-acceptor conjugated structures.33–37 To understand the effects of the environmental polarity on the nature of ICT states (localization/delocalization) of multibranched chromophores, in this article, we present the results of the spectroscopic properties and excited-state dynamics of three newly synthesized push-pull chromophores (named monomer, dimer, and trimer) in different solvents studied by steady state absorption and fluorescence, as well as femtosecond time-resolved transient absorption measurements. The molecular structures of these novel multibranched push-pull chromophores are shown in Fig. 1, in which a strong electron donor triphenylmethane is in the center, with 2,1,3benzothiadiazole as acceptor in the end of each branch, linked by conjugated ethylene bond. Supposing that excitation in monomer is always localized due to its single branch structure, thus comparing the relaxation rate of solvent-coupled ICT states in dimer and trimer with that of monomer in different solvents allows us to extract information about solventdependent excitation delocalization/localization in multibranched chromophores. Femtosecond time-resolved transient measurements have provided the solvent-dependent dynamics of excited state deactivation of all the chromophores.15,16,38–41

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FIG. 1. The molecular structures for monomer, dimer, and trimer.

In addition, upon optical excitation in multibranched pushpull chromophores (dimer and trimer), the solvent-dependent intramolecular excitation transfer among the disorder-induced localized ICT states has further been explored by using the steady-state fluorescence excitation anisotropy, which is helpful for us to identify the localized and delocalized ICT states in multibranched push-pull chromophores in solvents with different polarities.42,43

where I|| and I⊥ are the polarized fluorescence intensities parallel and perpendicular to excitation polarization, respectively; G = I⊥/I|| is the geometrical factor of fluorescence spectrophotometer when the excitation is vertically polarized. To avoid fast rotation of molecule during fluorescence lifetime, the compounds were immobilized in Zeonex and pTHF matrixes during fluorescence excitation anisotropy spectra measurements. C. Femtosecond transient absorption measurements

II. MATERIALS AND METHODS A. Materials

The compounds of monomer, dimer, and trimer were synthesized by palladium-catalyzed Heck reaction from triphenylamine and benzothiadiazole. Details on the synthesis and characterization had been reported elsewhere.44 Their chemical structures and purities were identified by NMR, Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectroscopy (MALDI-TOF-MS), and elemental analysis. All solvents including n-hexane, toluene, diethyl ether, tetrahydrofuran (THF), and acetone used in this work were of analytical grade or higher and purchased from the Beijing Chemical Plant. For anisotropy spectral measurements, all multibranched chromophores were immobilized in different polymer matrices of Zeonex E48R (Zeon, Japan) and poly(tetrahydrofuran) (pTHF) (Aladdin). B. Steady state spectroscopy

Optical absorption measurements were carried out on UVvis spectrophotometer (Model U-3010, Hitachi, Japan). Steady state fluorescence measurements were carried out on a fluorescence spectrometer (F-4600, Hitachi, Japan). The comparative method was applied to determine the fluorescence quantum yield of multibranched chromophores in different solvents with fluorescein as standard (0.90). The concentrations of the samples for fluorescence quantum yield measurements were adjusted to have the optical density below 0.1 at the excitation wavelength in a 1 cm cuvette in order to minimize the selfabsorption effect. Fluorescence excitation anisotropy spectra were measured by changing the detection polarization on fluorescence routes parallel or perpendicular to the polarization of excitation light. The anisotropy (r) was calculated with r=

I|| − GI⊥ , I|| + 2GI⊥

(1)

The excited state deactivation and solvation of these chromophores in the toluene and THF have been determined with time resolution about 90 fs by using the homemade transient absorption setup described previously in detail.45–47 In brief, a regeneratively amplified Ti:sapphire laser (Coherent Legend Elite) produces 40 fs, 1 mJ pulses at a 500 Hz repetition rate at 800 nm with a bandwidth of about 30 nm. The output of laser beam was split to generate pump and probe pulses with a beam splitter (90% and 10%). The pump beam was doubled with a 0.5 mm thick β-barium borate (BBO) (type I) crystal to provide the pump pulse at 400 nm (∼80 nJ/pulse), which was focused into the sample with 120 µm spot to generate the FranckCondon state. The probe beam was delayed with a computer controlled optical delay and then focused into a 2 mm thick water cell to generate a white light continuum which was split into two beams using a broadband 50/50 beam splitter as the signal and reference beams. The signal beam was focused into a flow cell with 1 mm path length and spatially and temporally overlapped with the pump beam at 400 nm in the liquid sample, while the reference beam passed through the unexcited volume of the sample. Both reference and signal beams after the sample were focused into optical fibers of a dual-channel spectrometer (Avantes AvaSpec-2048-2-USB2) triggered from the same synchronized optical chopper driver at 500 Hz. A synchronized optical chopper (New Focus Model 3501) with a frequency of 250 Hz was inserted into the pump beam path in order to record probe spectra that were classified as pumped and not-pumped spectra, thereby reducing background effects. A wavelengthdependent time-zero correction was performed to account for the group velocity dispersion of the probe beam. To measure isotropic signals, the mutual polarizations of pump and probe beams were set to the magic angle (54.7◦) using a half-wave plate. For the pump-probe measurements, the concentration of multibranched chromophores in different solvents was adjusted to an absorbance around 0.3 OD at 400 nm in a 1 mm path length quartz cuvette. No photodegradation was observed after femtosecond transient absorption measurements.


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D. Data analysis

Spectral chirp corrections for the obtained transient absorption spectra were performed for group velocity dispersion of the probe beam before global fitting. The differential absorbance ∆A(t, λ) was analyzed using the population dynamics modeling graphical interface program Glotaran and TIMP.48,49 The TA spectra are globally fitted with the sequential model; the associated spectra are called evolution associated difference spectra (EADS) and can be described as ∆A (t, λ) =

n 

ciEADS (t) E ADSi (λ) ,

(2)

i=1

  with ciEADS (t) = ij=1 b j i exp (−k jt) ⊕ i (t) and bi j = im=1 i k m/ n=1, n, j k n − k j , where i (t) is instrument response function (IRF), k j is the decay rate of component j, and the amplitudes b j i of the exponential decays are defined for j ≤ i assuming b11 = 1.The EADS represent the spectral evolution with successively increasing lifetime.50 E. Quantum chemical calculations

We performed the density functional theory (DFT) methods as implemented in the Gaussian 03 software package. Solvation effect was neglected during calculation. The ground state geometries of the three molecules are fully optimized under B3LYP/6-311G (d, p) level. The excited-state properties of the three molecules were characterized and investigated with the three-dimensional cube representation of the charge difference density (CDD), which shows the distribution of net change in electron and hole densities as a result of the electronic transitions and the orientation of the possible ICT states. The details of these calculations can be found in the Appendix.51 III. RESULTS AND DISCUSSION A. Steady-state spectra

Fig. 2 shows the normalized steady-state absorption and emission spectra of monomer, dimer, and trimer dissolved in toluene and THF, respectively. It is found that there is slight red-shift for the absorption spectra from monomer, dimer, and trimer either in toluene or in THF, respectively. All the compounds show two intense absorption bands in the near-UV around 320 nm and visible spectral regions from 400 to 500 nm, and the low-energy absorption around 450 nm for all the chromophores is ascribed to the charge-transfer transition, while the high energy absorption band arises from π − π ∗ transition.7,8,15 Typically, the slightly red-shift ICT state absorption of trimer compared to that of the monomer compound indicates that the three branches in trimer compound are mutually conjugated though the common core, giving rise to a delocalized electron system in two dimensions. Furthermore, as shown in Fig. 2, with increasing solvent polarity, the absorption spectra of all of the compounds have no obvious difference on solvent polarity, except a slight blue-shift. The absorption behaviors are easily understood on the basis of

FIG. 2. ((a) and (b)) Normalized absorption and fluorescence spectra of multibranched chromophores in toluene and THF. The absorptions of these samples are normalized to the peak of ICT states.

symmetry reasons: the multibranched chromophores have a nonpolar ground states, so that the ground state absorption is expected to be weakly dependent on the solvent polarity. In addition, the solute molecules in higher polar solvents could experience a finite solvent reaction field, which will readjust their geometry, giving rise to molecular structure with lower conjugated degree and showing the slight blue-shift of ICT state absorption relative to that in lower polar solvents, for all the chromophores, as shown in Table I. Similar solvent dependence on optical absorption features was also observed for multibranched chromophores.7,8,33 In contrast to absorption spectra, all the chromophores show an obvious positive emission solvatochromism. That is, increasing solvent polarity leads to a pronounced bathochromic shift of emission bands (see in Fig. 2). The emission peaks in toluene are around 560 nm, and those in THF are around 625 nm. The fact that the Stokes shifts significantly increase with increasing solvent polarity, indicates that significant reorganization takes place after excitation prior to emission, which is related to the electronic redistribution occurring upon photo excitation. The observed bathochromic shift is consistent with CDD calculations that the pronounced electronic density shifts from triphenylamine moiety to the benzothiadiazole moiety (see Tables V–VII in the Appendix). In addition, the relationship between Stokes shift and solvent polarity was usually given by Lippert-Mataga equation,52,53 TABLE I. Photophysical data of monomer, dimer, and trimer. λabs (nm) Solvent

∆f

n-hexane Toluene Diethyl ether THF Acetone

0.00 0.02 0.16 0.21 0.29

λem (nm)

Monomer Dimer Trimer Monomer Dimer Trimer 435 442 433 434 431

446 452 445 447 443

453 461 452 454 451

509 554 568 610 647

504 552 567 613 655

504 555 569 619 667


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∆ν = νabs − νem = 2∆µ2∆ f / hca3 + const, ∆ f = (ε − 1) / (2ε + 1) − n2 − 1 / 2n2 + 1 ,

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(3)

TABLE II. Stokes shift and fluorescence quantum yields of monomer, dimer, and trimer.

(4)

where ∆ν is the Stokes shift, the difference in energy between the absorption and emission maxima; νabs and νem are the wavenumbers of absorption and emission peaks, respectively, h is Planck’s constant, c is the speed of light, a is the Onsager cavity radius in which the chromophore resides, and ∆µ = µe − µg is the difference between the excited- and ground-state dipole moments as presented by Equation (3); ∆ f is the orientational polarizability of solvent which could be deduced from the dielectric constant ε and the refractive index n of the solvent represented by Equation (4). Accordingly, if ∆µ was independent of the solvent polarity, a straight line would be obtained for ∆ν versus ∆ f . The Lippert relationship between the Stokes shifts and solvent polarity of multibranched chromophores is thus shown in Fig. 3 after careful measurements in a series of different polar solvents. Stokes shift of multibranched chromophores in toluene deviated from linearity results from large quadrupolar moment of toluene molecule. A larger slope of Lippert-Mataga plot of trimer indicates that the effective dipole moment change (∆µ) is larger compared to that of monomer. However, a closer look at the absorption and emission maxima (Table I) indicate that the Stokes shift of the monomer is marginally higher than that of the dimer and trimer in low polar solvents, which suggests that significant reorganization occurred before emission for the monomer compared to the dimer and trimer, leading to stronger excited state charge-transfer character of monomer than that of dimer and trimer. While in high polar solvents, a similar Stokes shift of all the multibranched chromophores is observed, indicating that the effective dipole moment change (∆µ) is almost equivalent for all the chromophores. In addition, as shown in Table II, the photoluminescence characteristics are found to depend on the dimensionality of molecules and solvent polarity environment, where trimer exhibits higher fluorescence quantum yields than monomer in low polar solvents while an opposite behavior is observed in high polar solvents.

Stokes shift cm−1

Quantum yield

Solvent

∆ f Monomer Dimer Trimer Monomer Dimer Trimer

n-hexane Toluene Diethyl ether THF Acetone

0.00 0.02 0.16 0.21 0.29

3358 4587 5495 6573 7718

2580 4015 4841 5866 7296

2234 3663 4549 5720 7207

0.6 0.42 0.46 0.23 0.07

0.68 0.47 0.46 0.20 0.007

0.8 0.47 0.45 0.16 0.005

multibranched chromophores, we performed steady state fluorescence excitation anisotropy spectral measurement in matrices with different polarities. To avoid the rotation of chromophores during excited state relaxation, all the chromophores were embedded in polymer matrices. By choosing appropriate polymers with different polarity conditions, we provide a more comprehensive explanation on the polarity-dependent excited state character of multibranched chromophores. Zeonex and pTHF are similar to the polarity of methylcyclohexane and THF, respectively.54 Fig. 4 shows the fluorescence excitation spectra and corresponding fluorescence excitation anisotropy spectra of monomer, dimer, and trimer in Zeonex and pTHF. As shown in Fig. 4(a), the anisotropy value of monomer has a roughly constant value between 0.30 and 0.40 in matrices

B. Fluorescence excitation anisotropy measurement

To obtain a deeper insight into the influence of the polarity of environment on the nature of the ICT states of

FIG. 3. Stokes shifts versus the solvent polarity (∆ f ) for monomer, dimer, and trimer.

FIG. 4. ((a)–(c)) Fluorescence excitation anisotropy spectra of monomer, dimer, and trimer in nonpolar matrix Zeonex (black) and polar matrix pTHF (red). Normalized fluorescence excitation spectra (dashed lines) are also shown for comparison. The fluorescence excitation anisotropy spectra in Zeonex and pTHF were measured by monitoring the fluorescence at 550 and 620 nm, respectively.


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with different polarities due to the orientation of transition dipole moments between absorption and emission being close to collinear. The anisotropy values of monomer are close to the theoretical limiting 0.40 for random distribution in both matrices, indicating no depolarization of the emission through rotational diffusion. For dimer, as shown in Fig. 4(b), the anisotropy value about 0.20 is observed at the high-energy ICT side around 400–420 nm, and the anisotropy value gradually increases as the excitation energy decreases and then reaches a value larger than 0.35 around 500 nm in the low energy ICT region in both matrices. Significantly, the fluorescence excitation anisotropy spectra of trimer in different matrixes have a strikingly different behavior as shown in Fig. 4(c). In Zeonex, an apolar matrix, the anisotropy value stays almost constant around the value of 0.10 throughout the ICT excitation band, with slight increase in the far red tail of the excitation band while the fluorescence of trimer was monitored at 550 nm. As the fluorescence of trimer was monitored in 620 nm from the red-shift fluorescence maximum in polar matrix pTHF, it is found that excitation anisotropy increases inside the ICT excitation band from a value lower than 0.10 on the blue side to a value exceeding 0.35 on the red side of the band, which is dependent on the excitation wavelength. To interpret the depolarization for trimer in non-polar matrix, Frenkel exciton model is employed, which has been successfully applied to multibranched chromophores built from dipolar moieties.9,55,56 In the case of trimer with C3symmetry, from the interbranch coupling between three single branches, the original excited state |e⟩ of the single branch is then split into three states, where two degenerated lower excited states and one higher excited state are obtained (see Fig. 12 in the Appendix). Specially, the two degenerate excited states are one-photo allowed, with transition dipole moments pointing along perpendicular (x and y) directions but with the same magnitude, while the higher energy state is dark because it is an almost vanishing transition dipole moment from the ground state (see Tables V–VII in the Appendix). Once the trimer has been excited along one particular polarization, the excitation is redistributed between the two degenerate excited states, so that emission from the twofold degenerate excited states has equal probability, with parallel or perpendicular polarization with respect to excitation polarization. In the absence of extrinsic depolarization effects, the law of additivity of polarization gives the limiting anisotropy value as r 0 = 0.1. With the Frenkel exciton model, the low fluorescence

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excitation anisotropy value of trimer provides evidence for symmetry of ICT states preserved in low polar Zeonex matrix. In contrast, entirely different excitation anisotropy spectrum is observed for trimer in pTHF with high polarity. The anisotropy value gradually increases as the excitation energy decreases and then reaches an anisotropy larger than 0.35 around 500 nm in the low energy ICT region. This indicates that in pTHF, depolarization in blue edge of ICT band occurs because of intramolecular excitation energy transfer, which means significant redistribution of the excitation energy among the split high-energy and low-energy ICT states, prior to emission. The twofold degenerate excited states splitting in trimer depends on the reduction of symmetry.42 This is due to the fact that the high polar matrix interacts with trimer, which leads to the torsional disorder of one of the dipolar molecular branches, and hence reduces the global symmetry of trimer.37,57 All these observations suggest that polar and nonpolar matrices have very different effect on the nature of the multibranched chromophores. In low polar matrix, the global symmetry of trimer is preserved with a twofold degenerated first excited states caused by interbranch coupling, so that the value of fluorescence anisotropy is close to 0.10 throughout the ICT absorption band. While in high polar matrix, anisotropy varies from 0.10 to around 0.40 with excitation energy ranging from blue to red side of the absorption band. This suggests that excitation energy transfer from high-energy ICT to lowenergy ICT states will lead to substantial depolarization, which means the degeneracy between the first two excited states has been removed due to reduction of the symmetry of trimer. In addition, it is worth noting that interbranch coupling between two single branches in dimer leads to the original excited state |e⟩ of the single branch split into two states (see Fig. 12 in the Appendix), and hence significant redistribution of the excitation energy among the split high-energy and lowenergy ICT states in dimer. Therefore, excitation energy is also redistributed no matter whether the symmetry is preserved or broken, leading to no distinguishable difference of anisotropy spectra of dimer in Zeonex and pTHF.39,43 To determine the influence of environment polarity on excited states character of multibranched chromophores, especially for dimer, femtosecond transient absorption measurements can provide the solvent-dependent dynamical information, which further illustrate the solvent effect on excitation delocalization/localization of multibranched push-pull chromophores.

FIG. 5. Evolution of femtosecond transient absorption spectra of monomer (a) and trimer (b) in toluene at different delay times (pump at 400 nm).


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FIG. 6. ((a) and (b)) Contributions of GSB, SE, and ESA to a transient spectrum for dissolved monomer and trimer in toluene at 100 ps. ((c) and (d)) Time-dependent contributions to transient absorption spectrum due to GSB and SE.

C. Femtosecond transient absorption measurements

To unravel the intramolecular charge transfer (de)localization in the ICT state of multibranched chromophores and elucidate the solvent dependent excitation delocalization/ localization, broadband femtosecond transient absorption measurements were taken using a 400 nm excitation in toluene and THF. Fig. 5(a) shows the transient absorption spectra at different time delays of monomer in toluene. Steady-state absorption spectra in toluene and THF suggest that photoexcitation using 400 nm pulse directly excites the molecules to the higher vibrational levels of the ICT state. At a time delay of 200 fs, the transient spectrum is composed of broad excited state absorption (ESA) around 720 nm overlapping with ground state bleach (GSB) and stimulated emission (SE). A closer look at the initial transient absorption spectra found that a SE signal around 540 nm increases, simultaneously with a broad ESA signal that rises promptly in the 700-750 nm spectral region during the first picosecond, indicating the growth of a new transient state. The initial fast spectral evolution is attributed to a fast vibrational relaxation within the excited state.7,58 At progressively increasing time delays, the peak of SE red-shifts from 540 to 570 nm, which typically results from the solvent-coupled excited state relaxed because solvation gradually lowers the excited-state potential energy surface.13 Fig. 5(b) shows the transient absorption spectrum of trimer

in toluene. The lack of obvious red-shift of the SE is likely due to the negligible reorganization energy following excitation, which indicates that the ICT state of trimer experiences a smaller solvent reorganization compared to that of monomer. As shown in Figs. 5(a) and 5(b), evolution of the spectra mainly consists of a red shift of the SE band and rise of ESA around 500 nm. In order to assess the precise dynamic processes that underlie the excited state multibranched chromophores deactivation, the dynamic Stokes shift must be estimated through the stimulated emission peak in every transient spectrum as described previously in detail.12,59,60 In brief, we begin with the stationary absorption spectrum which is considered to be normalized at the peak of the ICT band. Then, from the stationary fluorescence, the band for stimulated emission at late time (symbolized by t = ∞) is derived. We now have a trial spectrum as σAbsSE (∞) = σabs + σSE (∞). When ασAbsSE (∞) is subtracted from a transient absorption spectrum measured at late time, the contribution σESA (∞) should remain. The contribution due to GSB is ∆ODGSB = ασAbs. The bleach amplitude α was estimated, for example, by the condition that ∆ODESA (∞) ≥ 0 everywhere. At this

TABLE III. Summary of time-dependent shift of SE band of monomer, dimer, and trimer in toluene and THF from transient absorption spectrum. Spectral parameters ν SE (t) were fitted by ν SE (t) = a 0 + a exp(−t /τ). Toluene a Monomer Dimer Trimer

(cm−1)

1602.6 1184 564.7

THF τ (ps) 2.92 5.26 5.08

a

(cm−1)

1816.0 1793.1 1800.1

τ (ps) 1.88 1.82 1.71

FIG. 7. Relaxation of SE peak for monomer, dimer, and trimer dissolved in toluene.


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FIG. 8. Evolution of femtosecond transient absorption spectra of monomer (a) and trimer (b) in THF at different delay times (pump at 400 nm).

point, we assume that the shape of the ESA band is unaffected over time. In addition, for multibranched chromophores, the ESA contribution is relatively weak in emission region. Under such conditions, subtraction of ∆ODESA (∞) from a measured transient absorption spectrum should give the GSB band and instantaneous stimulated emission band ∆ODSE (t, ν), as shown in Figs. 6(c) and 6(d). The spectral position ν(t) is determined from a quadratic fit of the SE band around its minimum. The subsequent time-resolved dynamic shift of stimulated emission used to monitor the relaxation of the excited ICT state free energy due to solvent motion only requires that no excess vibrational energy is deposited in excited ICT state, or that intramolecular vibrational relaxation is significantly faster than solvation. Due to significant vibrational relaxation of multibranched chromophores in toluene within few hundred femtoseconds, the dynamic of SE peak shift is fitted after 500 fs. The fitted parameters are listed in Table III. As shown in Fig. 7, compared to that of monomer, the trimer in toluene exhibits less solvent reorganization, identified by the slower SE peak shift as well as smaller amplitude of the SE peak red-shift during solvent coupled structure relaxed process. Since no specific solute-solvent interactions such as hydrogen

bonding in toluene, the dynamics of solvent reorganization around the excited solute molecule is mainly determined not only by solvent polarity but also by the charge transfer degree of excited solute molecules.8,61 Solvent-coupled excited state relaxation is considerable faster for monomer than trimer in toluene, indicating that the degree of charge transfer in ICT state of monomer is larger than that of trimer, that is, the overall changes of molecular dipole moments of the ICT states are considerably smaller in the trimer compared to the monomer. The prominent decrease of excited dipole moments of trimer compared to that of monomer indicates the delocalization of intramolecular charge transfer over all the three branches. Excited delocalization based on the global symmetry of trimer is preserved. This is in agreement of fluorescence anisotropy results that the first excited state of trimer is doubly degenerate in low polar matrix due to the nuclear geometry is undistorted. Furthermore, compared to the delocalization ICT state of trimer, the localization ICT state of monomer is more favorable for solvent-coupled ICT state relaxation, and then nonradiative transition will take a higher portion of the whole deactivation process of excited state, leading to the lower fluorescence quantum yield of monomer in low polar solvents, as shown in Table II.

FIG. 9. ((a) and (b)) Contributions of GSB, SE, and ESA to a transient spectrum for dissolved monomer and trimer in THF at 100 ps. ((c) and (d)) Time-dependent contributions to transient absorption spectrum due to GSB and SE.


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FIG. 10. Relaxation of SE peak for monomer, dimer, and trimer dissolved in THF.

The transient absorption spectra of monomer and trimer in the high polar solvent THF are illustrated in Fig. 8. The spectral evolution of multibranched chromophores in THF resembles that in toluene, which indicates similar dynamic process occurs in both solvents. Compared to low polar solvent toluene, stronger solvent-solute interaction is obviously observed in THF, which can be identified by the substantial solvent-induced red-shift of SE peak. Since the similar spectral evolution of multibranched chromophores is observed in both toluene and THF, we can also estimate the shift of the SE peak in THF (see in Fig. 9). The fitted parameters are also listed in Table III. As shown in Fig. 10, in contrary to the case in toluene, the rate of SE peak shift in THF slightly increases with increasing of branch number as shown in Table III, indicating slightly larger dipole moment of ICT state in trimer compared to that in monomer.16 We attributed the formation of a significant dipole moment in trimer to the localization of the ICT to a single branch, a localization driven by symmetry broken. The trimer experiencing an intense solvent reaction field responds by readjusting its geometry, which has been proved in steady state absorption measurements and fluorescence anisotropy measurements in pTHF, as shown in Figs. 2 and 4(c). In addition, the excitation energy is redistributed in dimer upon excitation no matter whether the symmetry of dimer is preserved or broken, leading to no distinguishable difference of anisotropy spectra of dimer. Thus, we evaluated the extent of charge transfer in the excited state of dimer by ultrafast pump-probe measurements in toluene and THF, which is influenced by electron redistribution upon photo excitation.

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The fitted parameters of the time dependence SE shifts are listed in Table III. It is worth noting that dimer has similar solvent-dependent dynamic behavior with trimer, that is, solvent-coupled excited state relaxation of dimer is considerably slower than that of monomer in toluene, whereas in THF it is almost same in dimer. Transient absorption measurements indicate that the excitation for dimer is delocalized in toluene and localized in THF, suggesting that the symmetry is conserved in low polar solvent and broken in high polar solvent, which cannot be distinguished in fluorescence anisotropy measurements. It can be seen from Table III that the time constants of solvation-coupled excited state relaxation for multibranched chromophores not only show a strong solvent dependence but also change with different branch numbers. In particular, the solvation-assisted relaxation of ICT state of monomer is significantly faster than that of dimer and trimer in toluene. However, an opposite trend is observed in THF. Fig. 11 summarized the solvent dependent ICT state relaxation model of multibranched chromophores in solvents with different polarities. As shown in Fig. 11(a), in low polar solvent, solvent coupling of trimer is weaker compared to that of monomer as a result of the less polar ICT state of trimer, corresponding to trimer with octupolar character (excitation delocalized). Photoinduced excitation delocalized (at least partially) over the three branches can be expected because symmetry is preserved which leads to slower solvent-coupled ICT state relaxation compared to that of monomer with dipolar character. While in high polar solvents such as THF, as a result of the interaction with polar solvent, symmetry is always broken in trimer, with consequent localization of the intramolecular charge transfer on one of the dipolar molecular branches. Excitation localization leads to formation of a significant dipole moment in ICT state of trimer, which contributes to the faster solventcoupled relaxation of ICT state for trimer compared to that of monomer. Finally, it is worth mentioning that the results from transient measurements of multibranched chromophores in toluene and THF are helpful for us to understand the excited state deactivation of chromophores. The excited state dynamics of multibranched chromophores can be described as follows: Upon photoexcitation of the chromophores, the molecule is vertically excited to its FC state, which is the high vibrational energy level of ICT state. Vibrational relaxation of the FC

FIG. 11. Solvent-coupled ICT state relaxation of trimer in toluene and THF, respectively.


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TABLE IV. Summary of solvent-dependent excited state lifetimes of monomer, dimer, and trimer obtained from global target analysis. Toluene τ 1 fs

τ 2 ps

Research Program of the Chinese Academy of Sciences (Grant No. XDB12020200).

THF τ 3 ns

τ 1 fs

τ 2 ps

τ 3 ns

Monomer 640 ± 35 4.8 ± 0.3 3.6 ± 0.2 790 ± 50 3.3 ± 0.2 1.3 ± 0.1 Dimer 580 ± 30 6.6 ± 0.4 2.4 ± 0.1 660 ± 40 2.5 ± 0.1 1.4 ± 0.1 Trimer 550 ± 40 10.6 ± 0.7 2.5 ± 0.2 510 ± 30 2.2 ± 0.1 1.4 ± 0.1

state takes place very fast. The ICT state thus decays with a lifetime of about few picoseconds during solvation and structure relaxed to form the ICT state, which is dependent on solvent polarity and the degree of charge transfer in ICT state of multibranched chromophores.41 For comparison with the methods by extracting the dynamic of SE peak shift as mentioned above, we have also globally analyzed the time resolved data with a global fitting procedure combined with singular value decomposition (SVD), which was further employed to extract the time-dependent correlations and EADS from the transient data. The transient absorption spectra of multibranched chromophores in both solvents are fitted well using a sequential model with three components. The fitted parameters are listed in Table IV. It can be seen from Table IV that the similar influence of solvent polarity on excited relaxation of multibranched chromophores further indicates that intramolecular charge transfer delocalization in multibranched chromophores (dimer and trimer) in low polar solvents while localized to a single branch in high polar solvents. IV. CONCLUSIONS

In conclusion, we have demonstrated that excitation delocalization/localization in multibranched push-pull chromophores can be significantly influenced by solvent polarity using steady and femtosecond transient absorption experiments. The charge redistribution upon photoexcitation in multibranched push-pull chromophores leads to a delocalized ICT state in low polar solvents, while excitation localized on one of the branches in high polar solvents. That is, solvent reaction field is expected to be negligible in low polar solvent so that the symmetry of multibranched chromophores can be preserved, which leads to excitation delocalization (at least partially) over the three branches, whereas in high polar solvent, reduction of symmetry driven by solvent and solute reorganization can lead to excitation localized to a single branch, forming a significant dipole moment in ICT state. The results presented here provide insights into understanding the large influence of solvent polarity on excitation delocalization/localization in multibranched chromophores which is of great importance for the knowledge-guided development of multibranched chromophores for various applications. ACKNOWLEDGMENTS

A.X. thanks Professor Yongfang Li for providing samples. This work was supported by the 973 Program (Grant No. 2013CB834604), NSFCs (Grant Nos. 21173235, 91233107, 21127003, 21333012, and 21373232) and the Strategic Priority

APPENDIX: COMPLEMENTS OF FRENKEL EXCITON MODEL, ENERGY LEVEL, AND TRANSITION DIPOLE MOMENTS FOR MULTIBRANCHED CHROMOPHORES 1. Frenkel exciton model of multibranched chromophores

In this work, the Frenkel exciton model is used to analyze the transition dipole moments of multibranched chromophores (dimer and trimer) together with their single-branched counterpart chromophore (monomer).20 This approach is appreciable for multibranched chromophores systems with an electrostatic interaction between branches which is small compared to the typical transition energy. As shown in Fig. 12, dimer with C2 symmetry and trimer with C3 symmetry can be regarded as an assembly of two and three dipolarlike arms. With each individual arm (a, b) of the dimer and (a, b, c) of the trimer, one can associate a dominant low-lying ICT state, here after denoted (a∗, b∗) and (a∗, b∗, c∗). In addition, transition dipole moments can be characterized by (µa , µb ) of dimer and (µa , µb , µc ) of trimer, respectively. In the Frenkel exciton model, the energy level splitting of the excitonic states is caused by the dipole-dipole coupling of the transition dipole moments of the branches (see Fig. 12). Furthermore, the direction of transition dipole moments depends on the interbranch coupling. For the monomer with linear dipole moment geometry, the transition dipole moment µa of the first excited states |e⟩ is in the direction of the long axis of the molecule (shown in Fig. 12). The situation

FIG. 12. Schematic electronic-level diagram of the multibranched chromophores within the Frenkel excitonic model. TABLE V. Excited state charge difference density (CDD) and transition dipole moment of monomer. Transition dipole moment µ f g (a.u) Excited state |e⟩ = |a ∗⟩

Charge difference density

x

y

z

2.6568

µa −0.1295

−0.0213


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TABLE VI. Excited state charge difference density (CDD) and transition dipole moment of dimer. Transition dipole moment µ fg (a.u) Excited state

|e 1⟩ =

y

x


z

√

√

2 2 (µ a + µ b )

2 ∗ ∗ 2 (|a ⟩ + |b ⟩)

−3.5288

0.0611

0.0544

√

|e 2⟩ =

2 2 (µ a − µ b )

√ 2 ∗ ∗ 2 (|a ⟩ − |b ⟩)

0.1364

−1.3769

−0.0014

TABLE VII. Excited state charge difference density (CDD) and transition dipole moment of trimer. Transition dipole moment µ fg (a.u) Excited state

|e 1⟩ =

|e 2⟩ =

|e 3⟩ =


√1 (2|a ∗⟩ − |b ∗⟩ − |c ∗⟩) 6

√1 (|a ∗⟩ + |b ∗⟩ + |c ∗⟩) 3

FIG. 13. Energy levels for dimer in matrices of different polarities: (Left) Symmetric system at low polar matrix for Frenkel exciton model (delocalization) and (Right) symmetry lowed system for FRET model (localization) in high polar matrix.

changes upon dimer formation, and the original excited states |e⟩ split into two new excited states, |e1⟩ and |e2⟩, respectively. The two exited states, allowed by linear absorption, have transition dipole moments polarized along perpendicular directions. In the case of trimer with C3 symmetry, from the

z

√1 (2µ a − µ b − µ c ) 6

−0.8343

3.1435

0.0000

√

√ 2 ∗ ∗ 2 (|b ⟩ − |c ⟩)

y

x

2 2 (µ b − µ c )

3.1435

−0.8343

0.0000

√1 (µ a + µ b + µ c ) 3

0.0000

0.0000

−0.2159

FIG. 14. Energy levels for trimer in exciton model description: (Left) Symmetric system at low polar matrix and (Right) symmetry lowed system for FRET model (localization) in high polar matrix.

interaction of between three single branches, the original excited state |e⟩ of the single branch is then split into three states, where two degenerated lower excited states and one higher excited state are obtained. The two degenerate lowenergy excited states, allowed by linear absorption, have transition dipole moments polarized along perpendicular directions.


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TABLE VIII. Calculated fluorescence excitation anisotropy spectra values for blue edge excitation and red edge excitation of dimer. f i : The fractional contribution of the i state to the total absorption at blue edge or ( ) red edge of ICT band. β i : The angle between the absorption and emission transitions. r i : r i =  r : r (λ) = f i r i (λ).

2 5

3cos2 β i −1 2

.

i

Low polar matrix

Blue edge excitation Red edge excitation

f1 ≈0 ≈1

Excitation transition Blue edge excitation Red edge excitation

|g ⟩ → |e 2⟩

|g ⟩ → |e 1⟩

Excitation transition

f1 ≈0 ≈1

β1 90◦ 0◦

r1 f2 −0.2 ≈1 0.4 ≈0 High polar matrix

|g ⟩ → |s 1⟩ β1 120◦ 0◦

r1 −0.05 0.4

f2 ≈1 ≈0

β2 0◦ 90◦

|g ⟩ → |s 2⟩ β2 0◦ 120◦

r2 0.4 −0.2

r −0.2 0.4

r2 0.4 −0.05

r −0.05 0.4

TABLE IX. Calculated fluorescence excitation anisotropy spectra values for blue edge excitation and red edge excitation of trimer. f i : The fractional contribution of i state to the total absorption at blue edge or ( ) 2 red edge of ICT band. β i : The angle between the absorption and emission transitions. r i : r i = 52 3cos 2β i −1 .  r : r (λ) = f i r i (λ). i

Low polar matrix |g ⟩ → |e 1⟩


Emission state: |e 1⟩ Emission state: |e 2⟩

f1 0.5 0.5

r1 0.4 −0.2

f2 0.5 0.5

|g ⟩ → |s 1⟩


Blue edge excitation Red edge excitation

β1 0◦ 90◦

|g ⟩ → |e 2⟩

f1 ≈0 ≈1

β1 0◦ 0◦

While the state |c⟩ has the same C3 symmetry with the ground state which leads to the transition dipole moment from ground state to the state |c⟩ is almost zero (shown in Tables V–VII in the Appendix). In addition, Tables V–VII show the CDD calculated by using TD-DFT for the electronic transitions from ground state to the lowest ICT states of multibranched chromophores, where green and red stand for the holes and electrons, respectively. µfg is the transition dipole moment from the ground state to the excited states.

2. Energy levels and transition dipole moments for multibranched chromophores in low polar matrix and high polar matrix

The left panel of Fig. 13 shows the exciton model for dimer chromophore in low polar matrix. In the case of dimer with C2-symmetry, the interbranched coupling between two single branches in dimer leads to the original excited state |e⟩ of the single branch split into two states, allowed by linear absorption, which have transition dipole moments polarized along perpendicular directions. As shown by right panel of Fig. 13, in the high polar matrix, the molecular symmetry is

β2 90◦ 0◦

r2 −0.2 0.4

r 0.1 0.1

|g ⟩ → |s 2⟩|g ⟩ → |s 3⟩ r1 0.4 0.4

f2 ≈1 ≈0

β2 90◦ 120◦

r2 −0.05 −0.05

r −0.05 0.4

lowed due to the presence of an intense reaction field, and hence the interbranched coupling decreases and gives new eigenstates that we label |s1⟩ and |s2⟩. Table VIII shows the calculated fluorescence excitation anisotropy spectra values for blue edge excitation and red edge excitation of dimer. The left panel of Fig. 14 shows the exciton model for trimer chromophore in low polar matrix Zeonex. As mentioned above, the interbranched coupling between three single branches in trimer leads to the original excited state |e⟩ of the single branch split into three states; the two degenerate low-energy ICT states allowed by linear absorption have transition dipole moments polarized along perpendicular directions. While, in high polar matrix, the molecular symmetry is lowed due to the presence of an intense reaction field, and hence, the interbranched coupling decreases and gives new eigenstates that we label as |s1⟩, |s2⟩, and |s3⟩. Calculated fluorescence excitation anisotropy spectra values for blue edge excitation and red edge excitation of trimer can be seen in Table IX.62 1M.

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Custom-made versus off-the-shelf multibranched endografts for endovascular repair of thoracoabdominal aortic aneurysms.

Visceral branch occlusion following aneurysm repair using multibranched thoracoabdominal stent-grafts.

Multiple exciton generation in quantum dots versus singlet fission in molecular chromophores for solar photon conversion.

H2S induced coma and cardiogenic shock in the rat: Effects of phenothiazinium chromophores.

Quantifying charge resonance and multiexciton character in coupled chromophores by charge and spin cumulant analysis.