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Solvent Effect and Two-Photon Optical Properties of Triphenylamine-Based Donor-Acceptor Fluorophores Yilin Zhang, Meijuan Jiang, Guangchao Han, Ke Zhao, Ben Zhong Tang, and Kam Sing Wong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06762 • Publication Date (Web): 09 Nov 2015 Downloaded from http://pubs.acs.org on November 10, 2015

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The Journal of Physical Chemistry

Solvent Effect and Two-Photon Optical Properties of Triphenylamine-Based Donor-Acceptor Fluorophores Yilin Zhang1, Meijuan Jiang2, Guang-Chao Han3, Ke Zhao4, Ben Zhong Tang, 2, and Kam Sing Wong1,∗ 1

Department of Physics, the Hong Kong University of Science and Technology, Clear Water

Bay, Hong Kong, China 2

Department of Chemistry, the Hong Kong University of Science and Technology, Clear Water

Bay, Hong Kong, China 3

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids,

Institute of Chemistry, Chinese Academy of Sciences, Beijing, China, 100190 4

College of Physics and Electronics, Shandong Normal University, Jinan, China, 250014

∗[email protected] (K.S.W.)

Abstract:

ACS Paragon Plus Environment

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In this work we present a systematic investigation on the optical properties of two triphenylamine(TPA)-based donor-acceptor fluorophores: TPA-PA(phenyl-aldehyde) and TPABMO((Z)-4-benzylidene-2-methyloxazol-5(4H)-one). The two compounds are dissolved in nine different organic solvents as dilute solutions, in order to analyze the effect of solvent on their linear and nonlinear optical properties. For each compound under one-photon excitation, its fluorescence emission spectrum red-shifts more than 160 nm as the solvent polarity increases from hexane to MeCN, while the fluorescence quantum efficiency and lifetime reach maximum magnitudes in solvents with medium polarity. The quantum efficiency reaches as high as 0.72 in dioxane for TPA-PA, and 0.69 in Et2O for TPA-BMO respectively. These TPA-PA and TPABMO solutions are also strongly emissive upon appropriate two photon excitation, with fluorescence emission spectra identical to those under corresponding one-photon excitation. The maximum two-photon absorption cross-sections are ~160 GM (Goeppert-Mayer units) and 250 GM for TPA-PA and TPA-BMO, respectively, regardless of the solvent identity. Particularly, for TPA-BMO solutions in strongly polar solvents, dual fluorescence peaks are observed in steady state, and distinct relaxation dynamics are detected in fluorescence decays for the two emission peaks. These dual fluorescence emission spectra and dynamics could be interpreted as signs of charge-transfer state formation. Keyword: solvism, nonlinear optical materials, two-photon excited fluorescence, charge transfer state.

1.

Introduction


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Two-photon fluorescence microscopy is gaining popularity, as its selectivity of excitation within the focus leads to enhancement in the excitation penetration depth, reduction of the out-offocus noise and bleaching, and improvement in 3D resolution.1,2 Together with other applications such as two-photon lithography,3 and two-photon photodynamic therapy,4 these two-photon applications generate the need for novel chromophores with strong two-photon absorption (2PA). Several strategies have been proposed to design new organic molecules with large 2PA crosssections.3,5,6 Among the general strategy to enhance the charge transfer upon excitation, the donor (D)-π-acceptor (A) type of structure facilitates the prediction of the optimized two-photon excitation wavelength from ground-state absorption spectrum, since its non-centrosymmetric motif relaxes the parity forbidden rule of the 2PA process.7 With the D-π-A structure motif, TPA (triphenylamine)-PA (phenyl aldehyde) and TPA-BMO ((Z)-4-benzylidene-2-methyloxazol-5(4H)-one) (Fig. 1) are designed and synthesized, in which the TPA moiety serves as an electron-donating group, whereas PA and BMO are acceptors. To investigate their optical properties in dilute solutions, the two compounds are dissolved in nine organic solvents: hexane, toluene, diethyl ether (Et2O), 1,4-dioxane (dioxane), tetrahydrofuran (THF), ethyl acetate (EA), acetone, acetonitrile (MeCN) and dimethyl sulfoxide (DMSO). To gain a comprehensive picture of the optical properties of these two compounds, in this article, firstly the one-photon optical properties (absorption and fluorescence emission) of twocompounds in different solutions are analyzed in steady state and time-resolved measurements. Then, the 2PA properties are investigated by two-photon excited fluorescence (TPEF) experiment and quantum chemical calculation. The experiments and calculation details can be found in supplementary information.


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2.

2.1.

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Result and discussion One-photon optical properties One-photon absorption The ground state absorption spectra are shown in Fig. 2 and relevant properties are listed in Table 1. Details of the physical properties of the solvents such as polarity are listed in Table S1. The order of solvents is based on the fluorescence emission peak wavelength of the fluorophores inside. The absorption peak around 370 nm for TPA-PA and 410 nm for TPA-BMO are assigned to the π →π* transition band.8 Small shoulder features appear to change the slope of the absorption spectra at the longer-wavelength band edges of TPA-BMO in MeCN and DMSO, which indicates a noticeable amount of internal charge transfer (CT) band formation.9 The absorption spectra in toluene and DMSO are slightly red-shifted compared to those in other solvents of these two fluorophores, which may be due to the specific interactions between the fluorophores’ π-electrons with surrounding toluene and DMSO molecules.9-11 The strength and distribution of one-photon absorption (1PA) spectrum of S0 to S1 transition can be described by the equation ε(ν) = A|μ μ10|2g1(ν),7 where ε(ν) is the 1PA (extinction) spectrum, A is a constant, ν stands for the frequency of the light, μ10 is the electric transition dipole moment between ground S0 and the first-excited S1 (the 1La state in Fig. 6) states, and g1(ν) represents the line shape function of the 1PA on frequency, which is normalized according to

∫ g (ν )dν 1

= 1.

The relatively larger one-photon absorptivity for TPA-BMO (~ 4 × 104 M-1 cm-1 at peak


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wavelength ) than for TPA-PA (~ 3×104 M-1 cm-1) is consistent with the larger transition dipole moment μ10 for TPA-BMO indicated from the quantum chemical calculation results in Table 2. •

One-photon fluorescence emission and solvatochroism The one-photon excited fluorescence emission shows significant dependence on solvents, as shown in Fig. 3c, Fig. S1 and listed in Table 1. There are many aspects of solvent-solute interactions. In general, if the solvent environment is considered as a polarizable continuous media of uniform dielectric constant (Table S1), for each compound, its fluorescence emission spectra would red-shift as the solvent polarity increases (Fig. 3, Table 1 and Fig. S1). For example, from non-polar solvent hexane to strongly polar solvent MeCN, the emission peak wavelength shifts from 406 nm to 562 nm for TPA-PA, while it shifts from 457 nm to 646 nm (under 420nm excitation) for TPA-BMO. Similar solvatochromism has been observed in asymmetric fluorophores with D-π-A structure in other literature,9 which could be generally explained by the Lippert-Mataga equation derived from Onsager’s reation field theory9,12,13

as

follows (here the equation is in SI units; there will be no 40 in the denominator of this equation

2∆µSS 2 ∆f + constant , where ∆ν if expressed in Gaussian units) ∆ν = 4πε 0 hca3

is the Stokes shift

between the spectra peak frequency of emission and absorption , ∆ µ S S refers to the effective molecular dipole moment difference between the ground- and solvent-stabilized excited- state, a is the Onsager cavity radius determined by the molecular volume calculated with Gaussian (Table 2 2), while ∆ f = ε − 1 − n 2− 1 is a quantity to describe the solvent polarity in which ε is the 2ε + 1 2 n + 1

static dielectric constant, and n is the refractive index of the solvents (see details of the solvent parameters in Table S1). Hence the slope of the Lippert-Mataga plot (Fig. 4) represents the


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2∆µSS 2 ∂∆ν = change in Stokes shift with the increase of solvent polarity , which has ∂∆f 4πε 0 hca3 quadratic dependence on the ∆ µ S S and inversely proportion to the Onsager volume a3. As the structures of TPA-PA and TPA-BMO are asymmetric, ∆ µ S S is nonzero due to the large charge separation (Table 2); hence the increase in solvent polarity ∆f would render a rise in Stokes shift

∆ν . The deviations of the experimental data from good linear fitting in the Lippert-Mataga plot (Fig. 4) are probably due to the complicated nature of solvent-solute interactions. In the LippertMataga model the solvent environment is regarded as a polarizable continuum, and only the solvent polarity is taken into account. However in reality, the solvent can interact with solute in aspects such as thermal (e.g. thermal effect), mechanical (e.g. viscosity and rigidity), chemical (e.g. hydrogen-bonding) and others (e.g. intramolecular charge transfer), either as a whole medium or as individual molecules.12 Fluorescence quantum efficiency, lifetime and time-resolved emission spectra Details of the fluorescence quantum efficiency and lifetime of the two compounds in different solutions are listed in Table 1. The changes in fluorescence quantum efficiency φ with solvent polarity show segmented behavior: they remain quite high in low polarity solvents, such that the highest value is 0.72 for TPA-PA in dioxane and 0.69 for TPA-BMO in Et2O, and then decrease quickly as the solvent polarity becomes stronger (Table 1). The fluorescence decays which are used to estimate the fluorescence lifetimes of corresponding solutions are plotted in Fig. S2. Almost all the decays of TPA-PA solutions can be well-fitted with a single exponential decay function at their fluorescence emission peak


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wavelength, while for TPA-BMO in strongly polar solvents, a fast time component is observed resulting in a bi-exponential decay at wavelengths longer than their emission peak, which could be attribute to the existence of a lower energy excited state that undergoes either a faster radiative or non-radiative relaxation process (Table 1 and Fig. S2). Dual fluorescence for TPA-BMO in strongly polar solvents It should be noted that the emission spectra of TPA-BMO in acetone, MeCN and DMSO vary with different excitation wavelengths, with dual fluorescence emission peaks as shown in Fig. 5 and Fig. S1(c-d). Among the dual fluorescence peaks in Fig. S1(c-d), the peak at shorter wavelength (~550 – 580 nm) is assigned to the locally excited (LE) state, while the peak at longer wavelength (~660 – 700 nm) is probably ascribed to the twisted intramolecular charge-transfer (TICT) state.14 Details about this TICT state will be discussed in the following text. Among the intramolecular charge transfer (ICT) processes that describe the changes in overall charge distribution in a molecule, the TICT is a specific process in the excited state related to the twist in molecular conformation that lower the energy of the excited state, and result in a red-shifted emission band.15 The method for obtaining the LE/TICT spectra is illustrated in supplementary information. To investigate this faster decay process, a series of steady state and time-resolved fluorescence measurements are performed. For TPA-BMO in strongly polar solvents, the steady-state onephoton fluorescence emission spectra vary with the change of excitation wavelength from 410, 420 to 440 nm (Fig. S1c). When observed with the naked eye, the fluorescence emission is bright yellow under 410 nm excitation, while dim orange under 420 and 440 nm excitation. Two kinds of emission bands can be extracted by comparing the emission spectra under these different


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excitation wavelengths (Fig. S1d), in which one is peaked at around 550-580 nm, while the other peak is around 660-700 nm. According to this phenomenon, several assumptions such as excimer, dimer or TICT state formation

16

could be the possible reason for the dual fluorescence. Further

studies show that the ratio of intensities between the two fluorescence peaks is independent of fluorophore concentration (here concentration of 40 μM and 400 μM are studied), while the ∆μ10 from the calculations in Table 2 indicates that the largest charge separation occurs when the TPABMO molecules are being excited in strongly polar solvents. According to these additional information together with the donor-acceptor molecular structure, it becomes very likely that it is the addition of TICT state which is responsible for the dual fluorescence.14,16 In this case, according to fluorescence decays of TPA-BMO in strongly polar solvents, the fluorescence bands at the short wavelength with decay lifetimes similar to the corresponding TPA-PA ones in the same solutions are mainly from the LE states, while the fluorescence bands at long wavelengths are mainly from the TICT states. In many systems, the TICT state emission is forbidden in nature, with rapid non-radiative decay due to the intramolecular fluorescence quenching. However, in our TPA-BMO system, the number of π-electrons is large enough to relax the forbidden nature of the TICT emission, making the TICT state emissive with a relatively longer lifetime.16 To know whether there is TICT emission in other solutions, the time-resolved emission spectra are captured as shown in Fig. 5. According to Fig. 5(a and d) for TPA-BMO in MeCN, under the time-resolution determined by the instruments, the fluorescence emission peak first appears at around 520 nm, and then another peak becomes dominant at around 680 nm, and gradually shifts to around 580 nm as the fluorescence gradually decays. The fluorescence intensity around 580 nm decays much more slowly than the one around 680 nm, indicating that distinct excited states are involved in these two fluorescence bands. In Fig. 5 (b,e,c and f) for TPA-BMO in THF and


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TPA-PA MeCN, though the bluer side of the emission spectra rise slightly earlier than the redderside, there is no obvious spectral shift after the fluorescence intensity reaches maximum, indicating that the general solvent relaxation takes place in a time scale comparable to or even faster than the temporal resolution of the system. Comparing the time-resolved emission spectra of the three different solutions shown in Fig. 5, it can be observed that for the same solute TPABMO, the TICT emission is much stronger in MeCN than in THF; while for the same solvent MeCN, the TICT emission is much stronger for solute TPA-BMO than with TPA-PA. Based on the relative strength of the TICT emission among the three solutions, indications could be made that the strength of the TICT emission will generally increase with increasing solvent polarity, and also increase by substituting the PA with BMO moiety. These trends above could possibly be explained according to the Jablonski diagram as proposed in Fig. 6, in which the states labeled 1La and 1Lb correspond to the two lowest singlet excited states using Platt’s notation with weak transition dipoles whose polarizations are parallel and perpendicular to the long axis of the solute molecule respectively,17 and the state labeled 1B is the 1Lb – type state in the charge-transferred (CT) geometry.16 In ground state geometry, the energies of 1La and 1Lb states are close and nearly degenerate,18 so that the fluorophores can be excited to both of 1La and 1Lb states when the excitation wavelength is around the longest absorption peak wavelength. It should be noted that the lowest-energy Franck Condon excited states are 1Lb states for TPA-PA, and 1La for TPA-BMO.18 The distribution of the excited populations between the 1La and 1Lb states depends on the excitation wavelength and thermal equilibrium, so the excited populations are larger on the excited state with relatively lower energy (i.e. 1Lb states for TPA-PA, and 1La for TPA-BMO).


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The solvent-solute interaction includes a series of processes such as solvent relaxation, intramolecular rotation, charge transfer and reorientation of the solvent molecules. Without intramolecular rotation, the conformations of the fluorophores remain in the ground state geometry, and the energy of both the 1La and 1Lb states will be lowered by solvent-solute interaction: the decrease in energy is generally more significant in solvent with stronger polarity. On the other hand, after intramolecular rotation, the fluorophore conformation is altered to the CT geometry.16 During this conformational change, the energy of 1Lb state (i.e. the 1B band) rises to a higher energy than that of the ground state geometry, while the energy of 1La state (denoted as TICT states) changes differently in various solvents: it may slightly increases in solvent with weak polarity, while decreases in solvent with medium to high polarity. Therefore, the 1La bands in solvent with medium to high polarity are more stabilized in CT geometry than in ground-state geometry. As the system tends to relax to lower energy states during solvent-solute interaction, in solvent with weak polarity, the excited populations for both of TPA-PA and TPA-BMO tend to relaxed to the LE state after solvent-solute interaction; whereas in solvent with medium to high polarity, the excited populations have divergent behaviors for TPA-PA and TPA-BMO. For TPAPA in strongly-polar solvent, only the minor excited populations on the 1La state can relax to the TICT state; while for TPA-BMO in strongly-polar solvent, the major excited populations on the 1

La state will relax to the more stablized TICT state. Considering the fluorescence quantum

efficiency of the TICT band is relative lower than those of the LE band, the TICT emission remains negligible for TPA-PA in strongly-polar solvents, whereas becomes discernible for TPABMO in strongly-polar solvents. Therefore, the fluorescence emissions for all TPA-PA solutions studied and TPA-BMO solutions in solvent of polarity lower than that of THF are mainly assigned to the LE bands after general solvent effect, while those of TPA-BMO in strongly-polar solvent are mainly from the stabilized TICT band.


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The internal rotation of the fluorophores from ground state geometry to CT geometry during solvent-solute interaction upon excitation is a dynamic relaxation process. Hence radiation from ground state and CT geometry can be resolved in time-resolved spectra, while both are captured as a time-averaged value in steady state fluorescence. According to Fig. 5, taking TPA-BMO in MeCN as an example, its general solvent relaxation takes place much faster than the time resolution of the streak camera system ~40ps (peak around 580 nm), while the TICT process of the internal rotation together with solvent reorganization (peak around 680 nm) occurs after a time scale of ~91ps. Radiative and non-radiative decay rates The radiative decay rate kr and non-radiative decay rates knr of these fluorophores are estimated from their ϕ and fluorescence lifetime τ according to the equations = ⁄ and = (1 − )⁄ . The general decrease of radiative decay rate with increasing solvent polarity (Table 1) is also observed in other literature,9,12,14,19,20 which could be explained by the reduction in transition probability for the new π-electron distribution of the excited states after solvent relaxation.16,17 The increase of the non-radiative decay rate in polar solvent is related with processes like quenching, energy transfer, and solvent-solute interaction.12 Particularly, for TPAPA it can be explained by a rapid depopulation pathway to the ground state12 such as selfabsorption. while for TPA-BMO in strongly-polar solvents, the extra large non-radiative decay rates are probably attributed to the formation of an TICT state.12

2.2.

Two-photon optical properties


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The two-photon optical properties of these fluorophores are investigated with a relative twophoton excited fluorescence (TPEF) technique21 (see the details of this measurement in supplementary information). The intensity of TPEF shows quadratic dependence on the excitation power (Fig. S4), indicating that the process involved in absorption is a 2PA process. The TPEF emission spectra for these fluorophores are identical to those in the one-photon excitation process (Fig, S1 and Fig. 3c), while their TPEF excitation spectra are close to double the wavelength of their one-photon absorption (1PA) spectra (Fig. 7a). The similarity of the absorption and emission bands in the one- and two-photon processes indicates that the excited states involved in these processes are the same, since the parity selection rule 17 is relaxed as the molecular structures are asymmetric.7,22 The peak strength of 2PA cross-sections for the S1 state estimated from TPEF measurement are around 160 GM for TPA-PA and 250 GM for TPA-BMO in all nine solvents studied as depicted in Fig. 7b. According to Fig. 7b, the relative maximum strength of 2PA between TPAPA and TPA-BMO estimated by the TPEF is consistent with the relative strength indicated by quantum chemical calculations (Table 2), and the results from both TPEF and calculation of different solvents show that the maximum 2PA strengths are not sensitive to the change of solvents, with minor fluctuation within the 21% experimental error.22 If the two-photon excitation wavelength is longer than double of the longest absorption peak wavelegnth, the 2PA crosssections of TPA-BMO dramatically drop (Fig. 7) since the ~250 GM maximum 2PA crosssections for TPA-BMO are attributed from the weighted average of 2PA strength of the 1Lb and 1

La bands (Fig. 6), and the excitation of longer wavelengths with smaller photon energy can only

reach the 1La state with weaker 2PA strength (~600GM), rather than the 1Lb state which has larger 2PA cross-sections (~1200GM) estimtated from calculation. The smaller 2PA strength for


12

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the 1La state is probably due to its preference to take on configurations with large torsional angles between the donor-acceptor plane in polar solvents such as: acetone, MeCN and DMSO as the energy is lower in CT configuration. This conformational change reduces the planarity of the π – electron system, as a result jeopardizing the π – electron coherence of the wave-function and segmenting the π – electrons into shorter chains, which then leads to the decrease of 2PA strength.13,23-25 For TPA-PA, the 2PA strength does not show this dramatically drop, since the excited population on the 1La state with weaker 2PA strength is either minor or small at the twophoton excitation wavelength. When Δμ10 estimated from quantum chemical calculation (Table 2) and ΔμSS estimated by a Lippert Mataga plot (denoted as ΔμTPA-PA and ΔμTPA-BMO in Fig. 4) are compared, although they both refer to the difference between excited- and ground- state dipole moments, their distinct magnitudes reveal that they are intrinsically different. The Δμ10 influences the strength of the 2PA transition, while ΔμSS affects the degree of excited state energy shift after solvent relaxation. Hence, Δμ10 is related to the excited state right after an absorption process, while the ΔμSS corresponds to the excited state just before the fluorescence emission i.e. after solvent relaxation. There are relations Δμ μSS = μSS - μ00 and Δμ μ10 = μ11 - μ00 by definition, where μSS is the dipole moment vector for the solvent-stabilized excited state while μ11 is the one for the 1La-type Franck Condon excited state as labeled in Fig. 6. Under the assumption that the direction of dipole moments before and after excitation are nearly parallel for linear-structured D-π-A molecules, the vector relations above can be regarded as scalar relations. The relation that ΔμSS >Δμ10 indicates that the solvent relaxation processes finally results in a solvent-stabilized state (for TPA-PA when estimated from Fig. 4 and Table 2, μSS =μ00 + ΔμSS ~23.5D) with larger dipole moment than the 1La state (for TPA-PA when estimated from Fig. 4 and Table 2, μ11 =μ00 + Δμ10 ~14D).


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To analyze the structure-property relationship, the molecule TPA-BMO is compared with TPAPA. The difference in their one- and two-photon absorption transition probabilities is attributed to the fact that the BMO acceptor is physically larger in π-electron system than the PA acceptor, resulting in a greater transition dipole moment. In terms of the TICT state formation in TPABMO rather than TPA-PA, it is probably due to accessibility from the excited states due to symmetry. The underlying reason is that the slope of energies decrease vesus Hammett substituent constants26 are different for the lowest two excited states 1La and 1Lb, so that the energy of 1La state is higher than that of 1Lb for TPA-PA, and get reversed for TPA-BMO with stronger substituents.14 Therefore, the 1La state related with TICT emission is more accessible for TPA-BMO. In addition, the maximum 2PA cross-section in THF solution of TPA-PA (161.2 GM) is much larger than the one of DPA-TPE-PA (51 GM) in our previous study22 (in the publication the DPA-TPE-PA is named as DPA-TPE-CHO), although the molecular structure of TPA-PA and DPA-TPE-PA are similar to a certain extent. It has been found in the literature2,3,27-40 that many monomers containing the TPA moiety have 2PA cross-sections of hundreds to thousands of GM units. TPA (triphenylamine) seems to be a very promising backbone structure to compose molecules with large 2PA cross-sections. The plausible reason could be that the TPA group has a relatively greater number of π electrons3 and stronger electron-donating ability41 than many other organic structures. To summarize, in terms of fluorophore design for TPEF applications in general, using donor/acceptor groups with larger π-electron system could enlarge the two-photon absorptivity, and using substituents with larger Hammett constants could increase the possibilities to observe TICT emission. Conclusion


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In this work the one- and two-photon optical properties of TPA-PA and TPA-BMO in nine different solvents are investigated. General solvent effect, i.e. the solvent interaction with the fluorophore dipoles as a continuous medium, is the main cause of the basic bathochromic shift in fluorescence emission as the solvent polarity increases. The major excited state populations will relax to the TICT state rather than the LE state for TPA-BMO solutions in strongly polar solvents like acetone, MeCN and DMSO. The general solvent effect due to solvent polarity has minor effect on the 2PA strength, while the stablization of the TICT state correlates with the decrease in 2PA cross-section for TPA-BMO in strongly polar solvents. When the excited-state dipole moments indicated from 2PA are compared with those estimated from solvatochroism, a significant change in excited-state dipole moment is presented in solvent-solute interaction upon excitation. Acknowledgments We would like to express my appreciation to Prof Zhigang Shuai for the coordination of the molecular calculation, and to Prof Jiannong Wang, Ya Yi and Yiqun Xiao for the support in timeresolved fluorescence emission spectra measurement. This work is supported by Research Grants Council of Hong Kong (project HKUST2/CRF/10 and CUHK1/CRF/12G), the University Grants Committee of Hong Kong (project AoE/P-03/08 and AoE/P-02/12), and Shandong Provincial Natural Science Foundation, China (Grant No. ZR2014AM026).

Supporting Information Available


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Details of the solvent properties, UV/Vis spectroscopy, method to obtain LE/TICT state, fluorescence quantum efficiency, time-resolved spectroscopy, TPEF measurement, quantum chemical calculation, and material information such as synthesis, preparation and characterization details are available free of charge via the Internet at http://pubs.acs.org.

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Fig. 1 Molecular structures of TPA-based molecules studied in this work. The moieties colored blue are the electron-donating parts TPA, while those colored red are the electron-accepting groups PA and BMO.


20

-1 -1

Hexane Toluene Et2O Dioxane THF EA Acetone MeCN DMSO

TPA-PA

0.3

5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 -1 -1 Molar absorptivity (x 10 M cm ) Molar absorptivity (x 10 M cm )

Page 21 of 29

0.2

0.1

0.0

300

400 Wavelength (nm)

TPA-BMO

Hexane Toluene Dioxane THF EA Acetone MeCN DMSO

0.4

0.2

0.0

300

500

400 500 Wavelength (nm)

Fig. 2 Ground state absorption spectra of TPA-PA and TPA-BMO in various solvents at room temperature. Fluorophore concentration: 10 µM.


21


TPEF Intensity (a.u.)

TPA-PA


(a)

400


TPA-BMO (b) TPEF Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

700 Hexane Toluene Et2O Dioxane THF EA acetone MeCN DMSO



700

22

Page 23 of 29

Fig. 3 (a, b) TPEF emission spectra for TPA-PA and TPA-BMO. Fluorophore concentration: 40 µM. Excitation wavelength is 760 nm for TPA-PA and 840 nm for TPA-BMO. (c) Photographs of the TPA-PA and TPA-BMO in various solvents under UV irradiation.

10000

8000 Stoke Shift (/cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


slope = 1.71E4 ∆µTPA-PA = 18.0±1.5 D (a = 5.75 Å)

6000

slope = 1.56E4 ∆µTPA-BMO = 18.7±1.4 D (a = 6.07 Å)

4000

TPA-PA TPA-BMO

2000 0.0

0.1

∆f

0.2

0.3

Fig. 4 Lippert-Mataga plot of the relation between Stokes shift Δν with solvent polarity ∆f for TPA-PA and TPA-BMO, in which a refers to the Onsager radius. The fluorescence emission peaks for TPA-BMO in acetone, MeCN and DMSO are 622nm, 646nm and 646nm under 420nm excitation. Table 1 One-photon and two-photon excited fluorescence properties of TPA-PA and TPA-BMO in various solvents

TPA-PA Hexane Toluene Et2O Dioxane THF EA

λabs /nm

λem /nm

φ

τ /ns

kr /108 s-1

knr /108 s-1

σ TPEF /GM

σ 2PA /GM

364 374 365 365 366 371

406, 427

0.07 0.68 0.71 0.72 0.67 0.61

0.25 2.24 2.88 3.03 3.61 3.27

2.48 3.04 2.46 2.38 1.86 1.86

34.6 1.43 1.01 0.92 0.91 1.19

9.4 94.9 126.8 116.2 107.8 92.1

140.3 138.6 177.9 160.2 161.2 152.1

446 456 464 483 485


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Acetone MeCN DMSO TPA-BMO Hexane Toluene Et2O Dioxane THF EA Acetone

370 370 371

408 419 406 407 412 406

MeCN

408

DMSO

413

λabs and λem are

529 562 565

0.29 0.10 0.17

2.74 1.08 1.69

1.06 0.93 1.01

Page 24 of 29

2.59 8.33 4.91

0.46 1.51 3.05 3.58 509 0.68 2.23 3.03 1.46 526 0.69 3.13 2.2 0.99 530 0.62 2.88 2.14 1.33 576 0.51 3.74 1.36 1.31 581 0.28 3.2 0.88 2.25 b a b 550 0.09 2.33 0.39 3.9 660c 0.03 0.53c 0.51 18.4 b a b 580 0.03 0.83 0.36 11.7 700c 0.005 0.13c 0.39 76.5 b a b 555 0.04 2.00 0.20 4.8 685c 0.01 0.15c 0.66 65.6 the peak wavelength of 1PA and emission spectra, 1GM 457, 482

50 15.9 26

171.8 159.3 152.7

136.2 296 151.1 223.8 156.7 227.5 175.7 285.3 130.9 257.1 57.4 205 d 6.0 263d 4.8 176.3 d 1.2 239d 0.2 44 d 3.4 241d 0.4 40 −50 4 ≡10 cm s/photon.

The other symbols are explained in the text. a Measured with a 370nm SHG excitation. b The quantities are for LE state.

c

The quantities are for TICT state, determined using excitations

whose wavelength are so long, that the LE emissions in bright yellow regime are hardly observed for TPA-BMO in these strongly-polar solvents.

d

With 820nm two-photon excitation. Unless

otherwise noted, the excitation wavelengths are 380nm for TPA-PA and 420nm for TPA-BMO in the one-photon process, and double the corresponding excitation wavelengths in the two-photon process.


24

Page 25 of 29

TPA-BMO MeCN

(a) Intensity (a.u.)

0

Delay Time (ps)

400

84

300

168 252 336

200

420 486

100

TPA-BMO MeCN 680nm Delay Time (d) 0ps 29ps 57ps 91ps 139ps 186ps 288ps 400ps

520nm

580nm

0 500 600

TPA-BMO THF570nm (e)

200

Intensity (a.u.)

Delay Time (ps)

400

0

500 600 700 Wavelength(nm)


0 80 160 240 320 400 480

500 600 Wavelength(nm)

700

Intensity (a.u.)

400 200

0 TPA-PA MeCN 450


TPA-PA MeCN

0 80 160 240 320 400 480

600

Delay Time 0ps 24ps 71ps 165ps 371ps 580ps

(b)

TPA-BMO THF

Delay Time (ps)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Delay time 0 ps 23ps 47ps 78ps 109ps 300ps 627ps

560nm

(f)

(c)

500 550 600 Wavelength(nm)

650

500 600 Wavelength(nm)

Fig. 5 (a-c) Contour plots of time-resolved fluorescence spectra of TPA-BMO in MeCN, THF and TPA-PA in MeCN. The instrumental response function of the system is ~ 40 ps. (d-f) Time-


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

resolved fluorescence spectra (smoothed by average) for TPA-BMO in MeCN, THF and TPAPA in MeCN, fluorophore concentration: 400 µM. Table 2 Theoretical result of the typical electronic transition dipole moment and 2PA crosssection of our compounds from TD-DFT calculation solvent

ε α0/a.u.


1.89 2.38 4.33 2.25 7.58 6.02 20.7 37.5 46.7

solvent

ε

399 413 444 409 465 457 487 493 495

α0/a.u.

µ10/D 8.51 8.67 8.56 8.58 8.66 8.61 8.63 8.62 8.71

µ10/D

µ00/D 5.05 5.18 5.43 5.14 5.58 5.53 5.72 5.76 5.77

µ00/D

TPA-PA µ11/D 13.7 14.0 13.9 13.8 14.1 14.0 14.1 14.1 14.4

∆µ10/D λ2pa /nm

σ2pacalc /GM

8.60 8.82 8.52 8.70 8.57 8.52 8.39 8.34 8.60

656 661 661 658 665 663 665 665 668

315 344 335 332 354 345 352 351 379

λ2pa /nm

σ2pacalc /GM

TPA-BMO µ11/D ∆µ10/D

Hexane 1.89 530 11.0 2.60 13.7 11.1 763 522 Toluene 2.38 549 11.1 2.69 13.9 11.3 770 564 4.33 588 11.0 2.87 14.3 11.4 770 569 Et2O Dioxane 2.25 543 11.0 2.66 13.9 11.2 768 545 THF 7.58 616 11.1 2.99 14.5 11.5 778 597 EA 6.02 606 11.0 2.95 14.4 11.5 775 583 Acetone 20.7 644 11.0 3.11 14.6 11.5 778 602 MeCN 37.5 652 11.0 3.14 14.6 11.5 778 603 DMSO 46.7 655 11.1 3.15 14.7 11.6 783 636 is the polarizability in the ground state. µ00 and µ11 are the dipole moments for the ground, and the 1La excited state (Fig. 6) respectively. µ10 refers to the transition dipole moment between the ground state and the 1La excited state, and ∆µ10 = µ11 - µ00 is the dipole moment difference between the 1La excited state and ground state. All the dipole moments listed here are in the unit


26

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of Debye (D). and correspond to the 1La state in all solvents. The 1PA properties are also calculated. The calculations show that the same excited states are involved in 2PA and 1PA. The Onsager radius is 5.75 angstrom for TPA-PA and 6.07 angstrom for TPA-BMO in solutions studied respectively. Details of the quantum chemical calculation involved is described in the supplementary information.

Fig. 6 Schematic presentation of the photophysical processes. In this diagram, thicker lines indicate relatively higher population in states and stronger transitions. Other details of the diagram are discussed in the text.


27


σ 2PA (GM)

0.6 -1

TPA-BMO TPA-PA

0.5

-1

TPA-BMO TPA-PA

250

500

1PA

2PA

0.4

200

0.3

150 100

0.2

(a)

0.1

50

THF solution 0

5

300

Molar absorptivity (x10 M cm )

1PA wavelength (nm) 400 450

350

0.0 800 900 2PA wavelength (nm)

300

1000

(b)

600

200 400

200

DMSO

MeCN

0 Acetone

EA

THF

Dioxane

Toluene

Hexane

0

Et2O

TPA-PA calc TPA-BMO calc TPA-Pa 760nm exc TPA-BMO 840nm exc TPA-BMO 820nm exc

100

σ 2PA (GM) from calculation

700

σ 2PA (GM) from TPEF

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 29

Fig. 7 (a) 2PA spectra (left axis) compared with 1PA spectra (right axis) of TPA-PA and TPABMO solutions in THF. For easy -comparison, 2PA wavelength (bottom) is plotted as double of the 1PA wavelength (top). (b) Maximum 2PA cross-sections in different solvents determined by TPEF experiment (scattered points, left axis) and by quantum chemical calculation (lines, right axis). Experimentally, for TPA-BMO solutions in weak polar solvents such as THF, the 2PA cross-sections reach maximum at around 840 nm, while for the TPA-BMO in polar solvents, the 2PA cross-sections peak at around 820 nm, and drop quickly as the two-photon excitation wavelength reaches 840 nm.


28

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TOC graphic


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Solvent Effect and Two-Photon Optical Properties ... - ACS Publications

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