External Electric Field Effects on Excited-State Intramolecular Proton

Article pubs.acs.org/JPCA

External Electric Field Effects on Excited-State Intramolecular Proton Transfer in 4′‑N,N‑Dimethylamino-3-hydroxyflavone in Poly(methyl methacrylate) Films Kazuki Furukawa,† Kazuyuki Hino,‡ Norifumi Yamamoto,§ Kamlesh Awasthi,∥ Takakazu Nakabayashi,⊥ Nobuhiro Ohta,∥ and Hiroshi Sekiya*,† †

Department of Chemistry, Faculty of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan Department of Chemistry, Faculty of Education, Aichi University of Education, 1 Hirosawa, Igaya, Kariya, Aichi 448-8542, Japan § Department of Life and Environmental Sciences, Faculty of Engineering, Chiba Institute of Technology, Tsudanuma 2-17-1, Narashino, Chiba 275-0016, Japan ∥ Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University 1001, Ta-Hsueh Road, Hsinchu 30010, Taiwan ⊥ Graduate School of Pharmaceutical Sciences, Tohoku University, Aoba-Ku, Sendai 980-8578, Japan ‡

S Supporting Information *

ABSTRACT: The external electric field effects on the steady-state electronic spectra and excited-state dynamics were investigated for 4′-N,N-(dimethylamino)3-hydroxyflavone (DMHF) in a poly(methyl methacrylate) (PMMA) film. In the steady-state spectrum, dual emission was observed from the excited states of the normal (N*) and tautomer (T*) forms. Application of an external electric field of 1.0 MV·cm−1 enhanced the N* emission and reduced the T* emission, indicating that the external electric field suppressed the excited-state intramolecular proton transfer (ESIPT). The fluorescence decay profiles were measured for the N* and T* forms. The change in the emission intensity ratio N*/T* induced by the external electric field is dominated by ESIPT from the Franck−Condon excited state of the N* form and vibrational cooling in potential wells of the N* and T* forms occurring within tens of picoseconds. Three manifolds of fluorescent states were identified for both the N* and T* forms. The excited-state dynamics of DMHF in PMMA films has been found to be very different from that in solution due to intermolecular interactions in a rigid environment.

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INTRODUCTION Excited-state intramolecular proton transfer (ESIPT) is one of the most basic reactions in chemistry and biology. The molecule 3-hydroxyflavone is a model for the ESIPT reaction,1,2 in which substitution of a dialkylamino group (alkyl = CH3, C2H5, etc.) remarkably changes the excited-state dynamics. ESIPTs of 4′-N,N-(dialkylamino)-3-hydroxyflavones (Figure 1), including 4′-N,N-(dimethylamino)-3-hydroxyflavone (DMHF), and 4′-N,N-(diethylamino)-3-hydroxyflavone (DEHF) have been extensively studied in various environments, such as solutions,3−19 cyclodextrin,16,20 micelles,21,22 ionic liquids,23−26 and polymers.27−29 Steady-state and timeresolved studies of DMHF and DEHF have shown the occurrence of proton-coupled charge transfer,9,12,14−19 which forms the basis of many important chemical processes, including energy conversion in living cells. Theoretical calculations of DMHF and DEHF have shown that potential minimum corresponding to the locally excited (LE) state due to a large electric dipole moment (μe) in the lowest excited state of the normal (N) form is absent.15,30−32 © 2015 American Chemical Society

Figure 1. Structures of the normal and tautomeric forms of 4′-N,N(dialkylamino)-3-hydroxyflavone.

The photoexcited normal (N*) form undergoes ESIPT to generate the excited tautomer (T*) form. Observation of the N* and T* emissions has provided information on the excitedstate dynamics.3−20,23−26 The red shift of the N* emission and Received: April 17, 2015 Revised: August 24, 2015 Published: August 24, 2015 9599

DOI: 10.1021/acs.jpca.5b03672 J. Phys. Chem. A 2015, 119, 9599−9608

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The Journal of Physical Chemistry A

spectra were obtained by plotting the changes in the absorption intensity or fluorescence intensity, respectively, as a function of wavenumber. Applied field strength was evaluated by dividing the applied voltage by the thickness. Measurements of the field-induced changes in the fluorescence decay profiles were conducted using a singlephoton counting system combined with a pulse generator supplying a bipolar square wave.50 The second harmonic of the output from a femtosecond mode-locked Ti:sapphire laser (Spectra Physics, Tsunami) was used for excitation, whereas the repetition rate of the laser pulse was reduced with a pulse picker (model 350−160, Conoptics) from the original ∼81 MHz to ∼6 MHz. Fluorescence passed through a colored glass filter and monochromator (Nikon, G-250) was detected with a microchannel-plate photomultiplier (Hamamatsu, R3809U-52). The fluorescence signal was amplified, discriminated, and then sent to a time-to-amplitude converter system. Fluorescence decays were measured by a multichannel pulse height analyzer (SEIKO EG&G, model 7700). The instrumental response function (IRF) had a pulse width of ∼60 ps (fwhm). The observed fluorescence decay was fitted by the convolution of the IRF with a multi-exponential function. The minimum lifetime that could be distinguished in the present analysis was ∼10 ps.50 The applied voltage was a repetition of rectangular waves of positive, zero, negative, and zero bias. The time duration of each bias was 30 ms, including an initial 2 ms dead time to avoid overshooting of the applied field. Four different decays were collected that corresponded to positive, zero, negative, and zero sample bias. These decays were stored in each of the different memory segments of the multichannel pulse height analyzer. All the measurements were conducted at room temperature. Quantum chemical calculations were performed to examine the electronic structures of DMHF using the Gaussian 09 revision D.01 program package51 in the framework of the density functional theory (DFT) method with the mPW1PW91 functional and the 6-311+G(d,p) basis set. The electronic excited states of DMHF were examined using the timedependent DFT (TD-DFT) method. The equilibrium and transition-state geometries of DMHF were determined at the mPW1PW91/6-311+G(d,p) level for the normal and tautomer forms in the ground and excited states.

intensity ratio of the N* emission to the T* emission are very sensitive to environmental conditions. These features originate from the fact that the N* form has a much larger μe than that for the T* form. Therefore, in polar environments, stabilization of the energy of the N* form of DMHF and DEHF is much larger than that of the T* form, leading to significant changes in the shape of the excited-state potential energy surface (PES) on which CT and ESIPT reactions occur. Thus, DMHF and DEHF are suitable for the investigation of intermolecular interaction effects on excited-state dynamics, as well as applications that probe various environments.33−40 The electric field effects on ESIPT are of considerable interest in probing biological environments. Strong electric fields exist in the local environments of biomolecular systems41−44 and play important roles in the functions of biological systems. Klymchenko et al. investigated the internal electric field effect on ESIPT in DEHF.45 A remarkable change in the emission intensity ratio N*/T* was observed. The electric field was shown to stabilize or destabilize the energy of the N* form depending on the direction of the internal electric field. The pressure effect on ESIPT in DMHF in a poly(methyl methacrylate) (PMMA) film was studied by Zhu et al.27 They also observed a dramatic change in the N*/T*; however, to the best of our knowledge, no study has reported on external electric field effects on the ESIPT reaction. DMHF has a flexible dimethylamino group, whose torsional motion is significantly influenced by intermolecular interactions with surrounding media. Therefore, the excited-state dynamics of DMHF in PMMA will be very different from that in solution. We report the first observation of the external electric field effects on the steady-state electronic spectrum and fluorescence decay profiles of DMHF in PMMA films. It has been found that the application of external electric field suppresses the ESIPT reaction. Herein, we discuss characteristic features of the excited-state dynamics of DMHF in PMMA film.

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EXPERIMENTAL SECTION DMHF was synthesized as described in a previous report.46 PMMA was obtained from Tokyo Chemical Industry and was purified several times through precipitation in a mixture of benzene and methanol. PMMA films containing DMHF were prepared by a spin-coating technique on an indium−tin oxide (ITO) coated quartz substrate. A semitransparent Al film was deposited on the sample polymer films in a vacuum. The Al and ITO films were used as electrodes. The thicknesses of the polymer films (typically 0.3 μm) were measured with an interferometer microscope (Nano Spec/AFT 010−0180, Nanometrics). The concentration of DMHF relative to the monomer unit of PMMA was 1 mol %. Steady-state absorption and fluorescence spectra of DMHF in PMMA films were measured with a spectrophotometer (Hitachi U-3500) and a fluorescence spectrometer (JASCO FP777), respectively. Electric field-induced changes in the absorption and photoluminescence spectra were measured using electric field modulation spectroscopy, as previously described.47−49 A sinusoidal AC voltage with a modulation frequency of 40 Hz was applied between the electrodes, and the field-induced change of fluorescence intensity was detected with a lock-in amplifier at the second harmonic of the modulation frequency. The DC component of the transmitted light intensity or emission intensity was observed simultaneously. The electroabsorption (E-A) and electrophotoluminescence (E-PL)

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RESULTS Steady-State Electronic Spectra. Figure 2a shows the absorption and fluorescence spectra of DMHF at 1 mol % in a PMMA film. Two fluorescence spectra were measured by excitations at 366 and 424 nm. An absorption peak detected at 400 nm was assigned to the S1−S0 (ππ*) transition of the N form. The spectra show dual fluorescence similar to that observed in solution3−19 and PMMA film.27 The weak peak at ∼470 nm and strong peak at ∼560 nm were assigned to the normal fluorescence and tautomer fluorescence, respectively. The N*/T* depends on the excitation wavelength. The position of the T* emission peak varies with the excitation wavelength. Figure 2b displays the absorption spectrum and fluorescence excitation (FE) spectra detected at 470, 500, and 580 nm. The peak position of the FE spectrum depends on the detected wavelength of fluorescence. The observation of an absorption peak around 400 nm and the absence of an absorption peak due to the T form indicate that the excited state of the T* form is generated solely from the N* form via ESIPT. 9600


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polymer film, the field-induced change in absorption intensity (ΔA) can be expressed as the sum of the zeroth, first, and second derivatives of the absorption spectrum:52 ⎧ d[A(ν)/ν] ΔA(ν) = (f F)2 × ⎨A χ A(ν) + Bχ ν dν ⎩ + Cχ ν

d2[A(ν)/ν] ⎫ ⎬ dν 2 ⎭

(1)

where F is the externally applied electric field, ν is the frequency in wavenumbers, and f is the internal field factor. The coefficient Aχ depends on the transition dipole moment polarizability and hyperpolarizability, and Bχ and Cχ, which are given by Bχ =

Δα̅ /2 + (Δαm − Δα̅ )(3 cos2 χ − 1)/10 hc

(2)

{5 + (3 cos2 ξ − 1)(3 cos2 χ − 1)} 30h2c 2

(3)

and Cχ = (Δμ)2

where h is Planck’s constant and c is the speed of light. Here Δμ and Δα are the difference in electric dipole moment and molecular polarizability between the ground and excited states, respectively, with 1 Δμ = |Δμ| and Δα̅ = Tr(Δα) (4) 3

Figure 2. (a) Absorption and fluorescence spectra recorded by excitation of 1 mol % DMHF in a PMMA film at 366 and 424 nm. (b) Absorption and FE spectra recorded by fluorescence detection at 470, 500, and 580 nm.

To examine the origin of the spectral shifts in the FE and fluorescence spectra, we measured the absorption, FE, and fluorescence spectra of DMHF in a polystyrene (PS) film, where the PS film cannot act as a hydrogen bond acceptor (Figure S1, Supporting Information). The peak position of the T* emission depends on the excitation wavelength. Similarly, the peak position of the FE spectrum depends on the detected wavelength of fluorescene. We note that the spectral shifts are observed in the PS film where the intermolecular hydrogen bonds cannot be formed and the magnitude of the spectral shifts is larger in the PMMA film than that in the PS film. The observed wavelength-dependent spectral shifts in the FE and fluorescence spectra of DMHF in PMMA and PS films are ascribed to stabilization of the energy mainly in the S1 state due to van der Waals interaction between DMHF and surrounding molecules, where the DMHF molecules exist in an inhomogeneous environment. The larger spectral shifts for the FE and fluorescence spectra of DMHF in the PMMA film than those in the PS film are explained by the existence of a polar CO group in a PMMA moiety, whereas such a polar group is absent in a PS moiety. When an electric field is applied to a molecule, the magnitude of each energy level shift (Stark effect) depends on the electric dipole moment (μ) and molecular polarizability (α) of the concerned state. For an isotropic and immobilized sample, application of an external electric field will broaden the optical spectra due to a change in μ following optical transition. This change gives rise to a Stark effect line shape, which is given by the second derivative of the field-free absorption or emission spectrum. However, if the molecular polarizability varies following optical transition, the Stark effect line shape corresponds to the first derivative of the field-free spectrum. The theory of electric field effects on molecular spectra in condensed phase has been reported in detail.52−54 For isotropic distribution of samples in rigid matrices, such as PMMA

In these equations, Δαm denotes the diagonal component of Δα with respect to the direction of the transition dipole moment, χ is the angle between the direction of F and the electric field vector of the excitation light, and ξ is the angle between the direction of Δμ and the transition dipole moment. From eqs 1−4, the values of Δμ and Δα̅ can be obtained from analysis of the derivative parts of the E-A spectrum. Figure 3 shows the E-A spectrum of DMHF in a PMMA film together with the absorption spectrum and its first and second derivative spectra. The spectrum was measured under an applied field strength of 0.6 MV·cm−1 at room temperature and with χ = 54.7°. As shown in Figure 3, the E-A spectrum was similar in shape to the second derivative of the absorption spectrum, indicating that the field-induced change in absorption intensity essentially originates from Δμ following absorption. The E-A spectrum was fitted by the linear combination of the second derivative spectrum and a small contribution of the first derivative spectrum. The first and second derivative coefficients were found to be approximately 0.8 cm MV−2 and 2750 MV−2, respectively. From these coefficients, the magnitudes of the molecular polarizability change (Δα̅ ) and electric dipole moment change (Δμ) between the FC excited state and the ground state of the N form of DMHF were estimated to be 81 ± 12 Å3 and 13 ± 2 D (13 D = 43.4 × 10−30 C m), respectively, by assuming that the local field was identical to the applied field; that is, f = 1 in eq 1. The magnitude of Δμ in PMMA was similar to that in 1,4-dioxane (45.1 × 10−30 ± 0.95 × 10−30 C m), measured by Nemkovich et al.55−57 By using the electric dipole moment of DMHF in the electronic ground state in 1,4dioxane (14.1 × 10−30 C m),55−57 the electric dipole moment of DMHF in the FC state was estimated to be 17 D. Electric field-induced change in the fluorescence intensity ΔIF that is observed at the second harmonic of the modulation frequency in a polymer film is given by an equation similar to 9601


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Figure 3. (a) Absorption, (b) first derivative, (c) second derivative, and (d) E-A spectra of DMHF in a PMMA film. The simulated E-A spectrum is depicted by a broken line. The applied field strength was 0.6 MV·cm−1.

Figure 4. (a) Fluorescence spectrum of DMHF and deconvoluted N* and T* emission bands excited at 366 nm, (b) first derivative spectra of the N* and T* emission bands, (c) second derivative spectra of the N* and T* emission bands, and (d) E-PL spectrum. Simulations of the E-PL spectrum are depicted by broken lines. The applied field strength was 1.0 MV·cm−1.

eq 1, i.e., by a linear combination of the fluorescence spectrum and its first and second derivative spectra, as far as isotropic distribution of excited molecules is concerned. The first and second derivative components correspond to the spectral shift and spectral broadening resulting from Δα and Δμ, respectively, between the fluorescent state and the ground state. Figures 4 and 5 show the fluorescence and E-PL spectra excited at 366 and 424 nm, respectively, where the contribution of ΔA is negligible, with an applied field strength of 1.0 MV· cm−1. The 366 and 424 nm wavelengths were used because the change in ΔA was small. The two E-PL spectra suggest that the intensity of the N* emission increased, whereas that of the T* emission decreased, in the presence of F. To analyze the fieldinduced effects on the N* and T* emissions, the observed fluorescence spectra were fitted with a linear combination of four Gaussian functions. Two of these functions correspond to the N* emission and the other two functions to the T* emission. The reproduced fluorescence spectra of N* and T* are shown in Figures 4 and 5, together with their first and second derivative spectra. The zeroth components of the E-PL spectrum were reproduced by superposition of the first and second derivative spectra of the N* and T* emissions. Simulations of the zeroth components of the N* and T* emissions are shown in Figure S2 (Supporting Information). The zeroth-, first-, and second-derivative coefficients A, B, and C employed to simulate the E-PL spectra are summarized in Table 1. The first and second derivative coefficients for the N* emission do not show a clear difference upon changing the excitation wavelength. It is significant that the zeroth-derivative component of the N* emission is positive and that of the T* fluorescence is negative in both the E-PL spectra; that is, the external electric field induced enhancement of the N* emission and quenching of the T* emission. When the N form was

Figure 5. (a) Fluorescence spectrum of DMHF and deconvoluted N* and T* emission bands excited at 424 nm, (b) first derivative spectra of the N* and T* emission bands, (c) second derivative spectra of the N* and T* emission bands, and (d) E-PL spectrum. Simulations of the E-PL spectrum are depicted by broken lines. The applied field strength was 1.0 MV·cm−1.

excited at 366 nm with application of 1.0 MV cm−1, the relative intensity of the N* emission increased by 1.7%, whereas that of 9602


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The Journal of Physical Chemistry A Table 1. Fitted Parameters Used To Simulate E-PL Spectra of DMHF in PMMA Films excitation wavelength (nm)

A (10−2 cm2 MV−2)

B (cm MV−2)

C (102 MV−2)

N* band 366 424

1.7 1.5

25 20

30 20

366 424

−0.6 −0.7

0 2

9 2

T* band

the T* emission decreased by 0.6%. When the N form was excited at 424 nm, the relative intensity of the N* emission increased by 1.5%, whereas that of the T* emission decreased by 0.7%. The external electric field effect may induce a significant change in the potential energy of the N* form, but such an effect is negligible in the T* form as the electric dipole moment (μe) of the T* form is much smaller than that of the N* form. In PMMA films, the orientation of electric dipole moment vector μ is random; therefore, the interaction energy depends on the directions of μ and F. In the S1 state, the largest energy stabilization may occur when F is antiparallel to μ, whereas the largest destabilization occurs when F is parallel to μ. PMMA reduces the effective electric field through shielding, which is proportional to 1/ε,45 where ε (3.6) is the dielectric constant of PMMA.58 In our experiment the applied electric field F is not large (0.6−1.0 MV·cm−1). However, the increased intensity of the N* emission and the decreased intensity of the T* emission imply that the external electric field significantly stabilized the potential energy of the N* form, whereas the change in the potential energy of the T* form was small. Fluorescence Decay Profiles. The enhancement of the N* emission and the reduction of the T* emission in the steady-state fluorescence spectrum are induced by the external electric field. Thus, we measured the electric field effects on the fluorescence decay profiles to investigate the time evolution of the populations of N* and T* after excitation of the Franck− Condon (FC) state of the N form. Figure 6a displays a fluorescence decay profile of the N* form (I0(t)) in a PMMA film in the absence of an external electric field, in which case the excitation and detection wavelengths were 424 and 500 nm, respectively. The decay profile in the presence of 1.0 MV·cm−1 is denoted as IF(t), which was measured under the same excitation and detection wavelengths that were used for the measurement of I0(t). The difference in fluorescence intensity is given by ΔIF(t) = IF(t) − I0(t) and is displayed in Figure 6b, whereas the intensity ratio IF(t)/I0(t) is shown in Figure 6c. The decay profile of the N* emission in Figure 6a was fitted with three exponential functions. The obtained fluorescence lifetimes and the preexponential factors are listed in Table 2. It should be noted that ΔIF(t) is positive throughout the time range, indicating the increase in the fluorescence intensity due to the electric field, which is in agreement with the increase in the intensity of N* emission in the E-PL spectrum. If the initial population of the fluorescence state of DMHF is not affected by an external electric field, IF(t)/I0(t) should be 1.00 at t = 0. IF(t)/I0(t) for the N* emission is larger than 1.00 throughout the time range, indicating that the initial population of the N* form is increased by the external electric field.

Figure 6. (a) Fluorescence decay curve of the N* form of DMHF in a PMMA film observed at zero field (indicated with a solid green line), where the excitation and detection wavelengths were 424 and 500 nm, respectively. The fitted curve and the instrumental response function are shown with broken red and solid black lines, respectively. (b) Difference (ΔIF(t), i.e., IF(t) − I0(t)) between the decay curves in the presence and absence of 1.0 MV·cm−1. (c) Ratio (IF(t)/I0(t)) between the decay curves in the presence and absence of 1.0 MV·cm−1.

Table 2. Lifetimes and Pre-exponential Factorsa of Fluorescence Decay Profiles of DMHF in PMMA Films λDet (nm) N* band

500

T* band

570

τ τ1: τ2: τ3: τ4: τ5: τ6:

100 ps (0.779) 1.29 ns (0.195) 3.83 ns (0.026) 0.88 ns (0.607) 2.61 ns (0.372) 3.02 ns (0.021)

a

Pre-exponential factor of each component is given in parentheses. The sum of the factor at zero fields is normalized.

Figure 7a shows a fluorescence decay profile of the T* form in the absence of F. The decay profile of the T* emission in Figure 7a was fitted with three exponential functions. The obtained fluorescence lifetimes and the pre-exponential factors are listed in Table 2. The fluorescence excitation spectrum depended on the detected wavelength (Figure 2b). This observation indicates that the observed three fluorescence lifetimes correspond to averaged lifetimes of three manifolds of electronic states. The ESIPT rate could not be determined with our instrumentation and we observed only the decays of the two forms in emission. The difference and the ratio between the decays with and without F are shown in Figure 7b,c, respectively, where the excitation and detection wavelengths were 424 and 570 nm. The applied field strength was 1.0 MV· cm−1. In contrast to the N* emission, ΔIF(t) of the T* emission is negative, as is evident from Figure 7b. This result is 9603


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Figure 8. Energy diagrams and electric dipole moments for the normal, tautomer, and transition-state (TS) forms of DMHF. The energies and electric dipole moments were calculated at the mPW1PW91/6-311+G(d,p) level.

Figure 7. (a) Fluorescence decay curve of the T* form of DMHF in a PMMA film observed at zero field (solid blue line), where the excitation and detection wavelengths were 424 and 570 nm, respectively. Decay curves obtained by simulation and instrumental response function are indicated with broken red and solid black lines, respectively. (b) Difference (ΔIF(t), i.e., IF(t) − I0(t)) between the decay curves in the presence and absence of 1.0 MV·cm−1. (c) Ratio (IF(t)/I0(t)) between the decay curves in the presence and absence of 1.0 MV·cm−1.

Figure 9. Molecular structure of the N form of DMHF, where the definitions of angles ϕ and θ are given.

C7−C1−C2, and θ, a dihedral angle for C16−N1−C4−C5. Panels a and c of Figure 10 show two-dimensional potential energy surfaces (2D-PESs) for the S0 and S1 states. Optimized geometries revealed that the normal form of DMHF has a

in agreement with the E-PL spectrum of the T* emission, where the T* emission intensity decreased. In Figure 7c, IF(t)/ I0(t) for the T* emission is less than 1.00 just after the photoexcitation, indicating that the initial population of the T* state is decreased by F. The field-induced change in the fluorescence decay profile was fitted without changing the fluorescence lifetime of each component in the presence of 1.0 MV·cm−1. Only the preexponential factors were affected by the application of 1.0 MV· cm−1. This result indicates the initial population of the N* and T* form is affected upon application of the external electric field. Potential Energies and Electric Dipole Moments. Figure 8 shows the relative energies and the electric dipole moments calculated at the mPW1PW91/6-311+G(d,p) level for the normal and tautomer forms of DMHF. Our quantum chemistry calculations indicate that the electric dipole moments of the N form are 5.8 D in the ground state and 14.5 D in the FC excited state, which are in agreement with the experimental value of the difference in electric dipole moment between the ground and excited states, Δμ, is 13 ± 2 D. For the T* form, the electric dipole moment is 5.8 D, which is much smaller than the value for the N* form. Figure 9 shows the optimized geometry for the N form of DMHF. We examined the changes in the ground and excited potential energies and electronic dipole moments of DMHF, along with two torsional angles, φ, a dihedral angle for C8−

Figure 10. Two-dimensional PES for the (a) N form (S0) and (b) N* form (S1). Two dimensional surfaces for the electric dipole moments of the (c) N form (S0) and (d) N* form (S1). The energies and electric dipole moments were calculated at the mPW1PW91/6311+G(d,p) level. The changes in magnitude of the energies and electric dipole moments are shown with bars on the right side of each figure. 9604


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The Journal of Physical Chemistry A nonplanar structure with φ = 0° and θ = −7° in the S0 state, and a planar structure with φ = θ = 0° in the S1 state. As shown in Figure 10a,c, the PESs show single minima at φ = 0° and θ = −7° for the S0 state, and φ = θ = 0° for the S1 state. Panels b and d of Figure 10 show changes in the electric dipole moment of DMHF for the S0 state along with changes in the torsional angles, φ and θ. The value of the dipole moment decreases with increasing the torsional angles in the S0 state; the value, however, increases with increasing the torsional angles in the S1 state.

coordinate is highly asymmetric and the effective potential energy barrier for ESIPT is high enough to localize the proton in a well of the N* form when the excess energy from the vibrationless state is small. As a result ESIPT from the states of the N* form with small vibrational energies will be very slow. Therefore, the major portion of the T* emission comes from ESIPT via the FC state of the N* form. Among the three time constants for N* emission, τ1 (100 ps), τ2 (1.29 ns), and τ3 (3.83 ns), the pre-exponential factor of the component τ3 is very small. Therefore, the dominant fluorescent states of the N* form have lifetimes τ1 (100 ps) and τ2 (1.29 ns). The former lifetime is much shorter than the latter, whereas the pre-exponential factor of 0.779 for τ1 is much larger than that of 0.195 for τ2. Conversely, three time constants, τ4 (0.88 ns), τ5 (2.61 ns), and τ6 (3.02 ns), were obtained from the fluorescence decay profile of the T* form. The major contribution to the decay profile of the T* form comes from the two components with τ4 and τ5, whose preexponential factors are 0.607 and 0.372, respectively, whereas the contribution of the component with τ6 is minor, as the preexponential factor for τ6 is very small (0.021). The three fluorescent states of the N* form may be due to conformational isomers having cyclic intramolecular hydrogen bonds. It is reasonable to consider that the three manifolds of fluorescent states of the T* form are produced by ESIPT from the three conformers of the N* form. In this study, no definite results have been obtained about the conformations of the N* and T* forms and correlations between the conformers as well. Comparison of Excited-State Dynamics in PMMA Films and Solution. PMMA film provides a weakly polar environment for DMHF. The absorption and fluorescence spectra in PMMA films are similar to those in nonpolar and weakly polar solutions. The fluorescence peak positions of the N* and T* emissions in PMMA were observed at approximately 470 and 560 nm, respectively. The observed peak positions of the N* and T* emissions in PMMA were similar to those in diethyl ether (466 and 567 nm) and chloroform (477 and 560 nm), indicating that the local polarity in the PMMA film is small.9 However, the fluorescence lifetimes of DMHF in PMMA films are very different from those in solution. Shynkar et al.10 and Roshal et al.9 studied time-resolved emission of DMHF in nonpolar and polar solvents such as cyclohexane, tetrahydrofuran, chloroform, ethyl acetate, dichloromethane, and acetonitrile at 293 K. In tetrahydrofuran solution, two time constants 16 and 1190 ps were obtained from the N* emission band, whereas a rising component (16 ps) and a decay component (308 ps) were observed for the T* emission band. The result showed that irreversible ESIPT occurred in tetrahydrofuran solution, and the time constant 308 ps was ascribed to a lifetime of zwitterionic type T* form. On the contrary, in chloroform solution, two decay components with time constants 79 and 773 ps were obtained. These time constants are in agreement with time constants for a rising component (79 ps) and a decay component (773 ps) of the T* emission bands. This observation is an evidence for the occurrence of reversible ESIPT. In general, the N* and T* forms of DMHF are not equilibrated in weakly polar solvents. In aprotic polar solvents, ESIPT in DMHF is a fast and reversible process; thus, the intensity ratio of the N* and T* emission bands is determined by the ESIPT equilibrium in the excited state. On the basis of the time-resolved study, criteria for reversibility were discussed in detail by Tomin et al.19 The magnitude of the three time

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DISCUSSION Decay Pathways to Fluorescence States. The energy difference between the N* and T* forms in the gas phase was estimated to be 1290 cm−1 (Figure 8). Conversely, the excess energy of the absorption FC state of the N* form from its potential minimum was roughly estimated to be 1000 cm−1 from the positions of the red edge of the absorption spectrum at 450 nm and the excited wavelength at 424 nm. Upon the photoexcitation of the FC state (N*FC), the three manifolds of fluorescence states of the N* from are populated via energy relaxation competing with ESIPT. On the contrary, the three manifolds of fluorescence states of the T* form are mainly populated through ESIPT followed by vibrational relaxation. In addition to the energy relaxation in potential wells of the N* and T* forms, S 1 −S 0 internal conversion and S 1 −T 1 intersystem crossing will occur. An energy diagram including the triplet states as well as the singlet states for the N* and T* forms calculated at the mPW1PW91/6-311+G(d,p) level are shown in Figure S3 (Supporting Information). We observed the electric field effect on ESIPT by measuring the steady-state electronic spectra and fluorescence decay profiles. The fluorescence lifetime did not significantly change upon application of an electric field, indicating that the increase in the N* emission intensity and the decrease in the T* emission intensity were due to the change in the initial populations of the N* and T* forms. When the external field of 1.0 MV·cm−1 was applied, the E-PL spectra revealed that the intensity of the N* emission increased by ∼2%, whereas that of the T* emission decreased by ∼1%. This result was ascribed to the suppression of ESIPT. The rising component was not resolved in the fluorescence decay profile for the T* form in Figure 7, indicating that the ESIPT time is less than the instrument detection limit (∼10 ps). Therefore, the change in the emission intensity ratio N*/T* induced by the external electric field is dominated by ESIPT from the NFC * state and vibrational cooling to the fluorescence states of the N* and T* forms occurring within 10 ps. The ESIPT reaction from the N* form may also populate the T* form, followed by energy relaxation in a potential well of the fluorescence state of the T* form. The potential energy curve along the proton transfer coordinate is asymmetric, where the energy of the T* form is much lower than that of the N* form (Figure 8). The ESIPT rate may decrease with decreasing the excess energy of the N* form due to the existence of a potential barrier along the proton transfer reaction coordinate. The external electric field effects on the fluorescence decay profile suggest that the ESIPT rate from the NFC * state is much larger than those from the N* form with smaller excess energies. One may raise a question why the N* emission was observed in the steady-state fluorescence spectrum in Figure 2 although no onset of the T* emission was observed. We infer that the potential energy curve along the intramolecular proton transfer 9605


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Research on Innovative Areas (No. 26288010) and challenging Exploratory Research (No. 15K13678) from the Ministry of Education, Culture, Sports, Science.

constants for the N* emission band is considerably different from those for the T* emission band in PMMA films. Therefore, the rate for back ESIPT process in PMMA films is predicted to be slow. The observed lifetimes of the N* and T* emissions are widely spread, suggesting that the radiative and nonradiative rates are dependent on the environment of DMHF molecules, where the twisting of the dimethylamino and/or flavonol moieties may occur. The absence of reorganization induces anisotropic intermolecular interactions between the N* form and surrounding PMMA, which significantly stabilizes the energy depending on the direction of μe, leading to different PESs from those in solution. Application of a femtosecond pump−probe spectroscopy will provide further detailed information about the excited-state PES and dynamics of DMHF and its analogs in PMMA films.

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(1) Sengupta, P. K.; Kasha, M. Excited State Proton-Transfer Spectroscopy of 3-Hydroxyflavone and Quercetin. Chem. Phys. Lett. 1979, 68, 382−385. (2) Formosinho, S. J.; Arnaut, L. G. Excited-State Proton Transfer Reactions II. Intramolecular Reactions. J. Photochem. Photobiol., A 1993, 75, 21−48. (3) Itoh, M.; Fujiwara, Y.; Sumitani, M.; Yoshihara, K. Mechanism of Intramolecular Excited-State Proton Transfer and Relaxation Processes in the Ground and Excited States of 3-Hydroxyflavone and Related Compounds. J. Phys. Chem. 1986, 90, 5672−5678. (4) Chou, P. T.; Martinez, M. L.; Clements, J. H. Reversal of Excitation Behavior of Proton-Transfer vs. Charge-Transfer by Dielectric Perturbation of Electric Manifolds. J. Phys. Chem. 1993, 97, 2618−2622. (5) Swinney, T. C.; Kelley, D. F. Proton Transfer Dynamics in Substituted 3-Hydroxyflavones: Solvent Polarization Effects. J. Chem. Phys. 1993, 99, 211−221. (6) Ormson, S. M.; Brown, R. G.; Vollmer, F.; Rettig, W. Switching Between Charge- and Proton-Transfer Emission in the Excited State of a Substituted 3-Hydroxyflavone. J. Photochem. Photobiol., A 1994, 81, 65−72. (7) Klymchenko, A. S.; Pivovarenko, V. G.; Demchenko, A. P. Elimination of the Hydrogen Bonding Effect on the Solvatochromism of 3-Hydroxyflavone. J. Phys. Chem. A 2003, 107, 4211−4216. (8) Klymchenko, A. S.; Demchenko, A. P. Multiparametric Probing of Intermolecular Interactions with Fluorescent Dye Exhibiting Excited State Intramolecular Proton Transfer. Phys. Chem. Chem. Phys. 2003, 5, 461−468. (9) Roshal, A. D.; Organero, J. A.; Douhal, A. Tuning the Mechanism of Proton-Transfer in a Hydroxyflavone Derivative. Chem. Phys. Lett. 2003, 379, 53−59. (10) Shynkar, V. V.; Mély, Y.; Duportail, G.; Piémont, E.; Klymchenko, A. S.; Demchenko, A. P. Picosecond Time-Resolved Fluorescence Studies Are Consistent with Reversible Excited-State Intramolecular Proton Transfer in 4′-(Dialkylamino)-3-hydroxyflavones. J. Phys. Chem. A 2003, 107, 9522−9529. (11) Shynkar, V. V.; Klymchenko, A. S.; Piémont, E.; Demchenko, A. P.; Mély, Y. Dynamics of Intermolecular Hydrogen Bonds in the Excited States of 4′-Dialkylamino-3-hydroxyflavones. On the Pathway to an Ideal Fluorescent Hydrogen Bonding Sensor. J. Phys. Chem. A 2004, 108, 8151−8159. (12) Douhal, A.; Sanz, M.; Carranza, M. A.; Organero, J. A.; Santos, L. Femtosecond Observation of Intramolecular Charge- and ProtonTransfer Reactions in a Hydroxyflavone Derivative. Chem. Phys. Lett. 2004, 394, 54−60. (13) Ameer-Beg, S.; Ormson, S. M.; Poteau, X.; Brown, R. G.; Foggi, P.; Bussotti, L.; Neuwahl, F. V. R. Ultrafast Measurements of Charge and Excited-State Intramolecular Proton Transfer in Solutions of 4′(N,N-Dimethylamino) Derivatives of 3-Hydroxyflavone. J. Phys. Chem. A 2004, 108, 6938−6943. (14) Chou, P. T.; Huang, C. H.; Pu, S. C.; Cheng, Y. M.; Liu, Y. H.; Wang, Y.; Chen, C. T. Tuning Excited-State Charge/Proton Transfer Coupled Reaction via the Dipolar Functionality. J. Phys. Chem. A 2004, 108, 6452−6454. (15) Chou, P. T.; Pu, S. C.; Cheng, Y. M.; Yu, W. S.; Yu, Y. C.; Hung, F. T.; Hu, W. P. Femtosecond Dynamics on Excited-State Proton/ Charge-Transfer Reaction in 4′-N,N-Diethylamino-3-hydroxyflavone. The Role of Dipolar Vectors in Constructing a Rational Mechanism. J. Phys. Chem. A 2005, 109, 3777−3787. (16) Sanz, M.; Organero, J. A.; Douhal, A. Proton and Charge Transfer Reactions Dynamics of a Hydroxyflavone Derivative in a Polar Solvent and in a Cyclodextrin Nanocavity. Chem. Phys. 2007, 338, 135−142.

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CONCLUSIONS We investigated the external electric field effects on the E-A spectra, E-PL spectra, and fluorescence decay profiles of the N* and T* forms of DMHF in PMMA films. We measured Δμ to be 13 D ± 2 D, yielding 17 D for the FC state of the N* form. When an external field of 1.0 MV·cm−1 was applied, the E-PL spectra indicated that the N* emission intensity increased by ∼2%, whereas that of the T* emission decreased by ∼1%. This result was ascribed to the suppression of ESIPT. The measurement of the fluorescence decay profile also showed the suppression of ESIPT through application of an external electric field; however, the lifetimes did not change. The excited-state dynamics of DMHF in PMMA films is very different from that in solution. We calculated the 2D surfaces for the potential energies and electric dipole moments along the twisting angles of the dimethylamino and flavonol moieties for the S0 and S1 states of the N* form. These results indicated that twisting of the two angles significantly changed the potential energies and electric dipole moments. The results reflect that the conformation of DMHF may easily change in a rigid environment. Therefore, we have demonstrated that the investigation of the external field effect on ESIPT has the potential to reveal new features of excited-state dynamics of flexible molecules such as DMHF and DEHF.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b03672. Absorption, fluorescence, and fluorescence excitation spectra of DMHF in a polystyrene film, E-PL spectra and simulation of the zeroth components of the N* and T* emissions, energy diagram of DMHF (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*H. Sekiya. E-mail: [email protected]. Tel: +81-92642-2574. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by Grant-in-Aid for JSPS Fellows (No. 13J04747) and by Grand-in-Aid for Scientific 9606


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