Spectral Features and Excited-State Transformations of Hydroxy

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Spectral Features and Excited-State Transformations of Hydroxy Derivatives of 4'-N,N-Dimethylaminoflavone in PVA Films and on Plasmonic Platforms Illia E. Serdiuk, Anna Synak, Beata Grobelna, Ignacy Gryczynski, and Piotr Bojarski J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07386 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 24, 2016

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The Journal of Physical Chemistry

Spectral Features and Excited-State Transformations of Hydroxy Derivatives of 4'-N,N-Dimethylaminoflavone in PVA Films and on Plasmonic Platforms

†‡

†

‡

§

†

Illia E. Serdiuk, * Anna Synak, Beata Grobelna, Ignacy Gryczynski and Piotr Bojarski

†

Faculty of Mathematics, Physics and Informatics, University of Gdańsk, Wita Stwosza 57, 80-308

Gdańsk, Poland ‡

Faculty of Chemistry, University of Gdańsk, Wita Stwosza 63, 80-308 Gdańsk, Poland

§

Center for Fluorescence Technologies and Nanomedicine, University of North Texas, Health Science Center, 3500 Camp Bowie Boulevard, Fort Worth, Texas 76107, United States

*Corresponding author Phone: +48 58 523 51 13, Phone/fax: +48 58 523 50 12, e-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Compounds which undergo excited-state intramolecular proton-transfer (ESIPT) have been widely applied in biophysical investigations as fluorescence sensors enabling several analytical signals. Low fluorescence quantum yields in protic media seem to be, however, their main drawback. In this study, we investigated spectral features of 4'-N,N-dimethylaminoflavone derivatives containing hydroxyl groups at positions 3 and/or 7 in protic solid media – polyvinyl alcohol (PVA) films at various concentrations and under conditions of fluorescence intensity enhancement by plasmonic resonance. Our experimental findings indicate that due to ESIPT and susceptibility of the investigated compounds to polarity and hydrogen-bonding ability of medium various tautomeric species localized in different domains of PVA can be distinguished in the ground and electronically excited states what determines multiple spectral parameters of such materials. According to the evaluated rate constants, relaxation and aggregation processes as well as proton-transfer transformations occurring in the excited state in PVA films are relatively slow and proceed in the nanosecond time domain. Under the conditions of plasmonic resonance, the multiband fluorescence intensity of compounds increases up to 7 times, whereas rates of excited-state processes including ESIPT increase up to 15 times. Due to acceleration of proton-transfer plasmon resonance can be successfully applied for enhancement of dual fluorescence of ESIPT sensors.


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INTRODUCTION Excited state intramolecular proton transfer (ESIPT) represents one of the fundamental photochemical reactions. ESIPT takes place in compounds which contain proton donor in the vicinity of proton acceptor, which are connected by the hydrogen bond (HB).1 Generally, ESIPT has no or very low energetic barrier and is registered in the femto- and subpicosecond time domain,2 which results in single-band fluorescence with abnormally high Stokes shift of some organic compounds. From the point of view of possible applications, one of the most interesting features of the ESIPT fluorophores is their ability to exhibit dual fluorescence under certain conditions. Together with a specific sensibility of this group of compounds to intermolecular H-bonding and pH, this feature enables monitoring various changes in molecular environment with the help of not only absolute parameters, but relative ones, for example, relative maxima positions and intensities of bands, ratio of fluorescence decay components etc.3,4 From the point of view of creation of fluorescent materials, use of dual emission of the ESIPT fluorophores can afford generation of various emission color from blue to red and even white light.1,5 One of the most important factors which can reduce ESIPT rate and induce appearance of the unreacted species emission (dual fluorescence) are: (i) decrease of the proton donor acidity, which can be achieved by introduction of the electron-releasing substituents (internal factor)2,6 and (ii) disruption of the intramolecular HB, usually due to competitive H-bonding with other proton acceptors (external factor).7,8,9 In most of the cases, the fluorescence quantum yield of the ESIPT fluorophores does not, however, exceed 30–40%,1 whereas in protic media, where the mentioned above external factor acts, this value is much lower. Low quantum yields of fluorescence seem to be the main drawback of the ESIPT fluorophores and the reason of relatively rare examples of applications compared to the photoinert ones. From the point of view of the problems mentioned above, the plasmon resonance effect seems to be useful. The plasmon resonance phenomenon has numerous potential applications ranging from


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biological to chemical sensing.10,11 In particular, it offers a way to increase brightness and photostability at the same time.12,13 The ability to increase the emission intensity of fluorescence strongly depends on the size and shape of the nanoparticles and distance between the chromophores and the metallic structure.14 It is worth to add, that the enhancement is a result of a resonance interaction of localized surface plasmons in metallic nanoparticles with propagating surface plasmons polaritons induced in the metal film through the near-field interactions.15 In this study, we investigated for the first time the plasmon resonance effect on flavonols (3hydroxy-2-phenyl-4H-chromen-4-ones), which have been one of the most popular systems for investigations and applications of ESIPT for nearly half of century.5 Proton transfer in these compounds represents an example of a keto-enol tautomerization (Scheme 1). Flavonol derivatives containing dimethylamino group at position 4' (para position of the lateral benzene ring) exhibit especially interesting properties as in such compounds ESIPT is coupled with intramolecular charge transfer (ICT)16,17 and is reversible.18 Due to ICT, 4'-N,N-dimethylaminoflavones in general show high sensitivity of fluorescent features on polarity. Moreover, in protic environments, intermolecular H-bonding lead to formation of species without intramolecular hydrogen bond (Scheme 1) what can result in appearance of additional band in fluorescence spectra.7 We focused our attention on 4'-N,N-dimethylaminoflavone derivatives containing hydroxyl groups at positions 3 and/or 7 (Chart 1). Influence of the plasmon resonance on the ICT system was investigated using 2-[4-(dimethylamino)phenyl]-7-hydroxy-4H-chromen-4-one (1), incapable of ESIPT. Investigations of compounds 2-[4-(dimethylamino)phenyl]-3-hydroxy-4H-chromen-4-one (2) and

2-[4-(dimethylamino)phenyl]-3,7-dihydroxy-4H-chromen-4-one

(3)

enabled

information

concerning proton transfer reactions in the conditions of the plasmon resonance. Due to ICT compound 2 is distinguished by considerably lower acidity of the hydroxyl group6 and ESIPT rate18 among other flavonol derivatives not containing amino substituents. Occurrence of both ICT and ESIPT determines sensibility of 2 to both polarity and H-bonding properties of medium.4 Compound


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3 contains an additional hydroxyl group at position 7, which can influence ESIPT and fluorescence properties of the compound. The investigations were conducted in PVA films, which due to its relatively high polarity and H-bonding ability enabled registration of the spectral effects in the picosecond and nanosecond time domains. Since repeatable results on plasmon resonance were obtained at relatively high concentrations (10–2 M) of compounds, their spectral features at high and low (10–4 M) concentrations were investigated and analyzed in order to evaluate the role of aggregation processes. Chart 1. Canonical Structures of the Compounds Investigated CH3 N CH3 HO

O

O 1

CH3 N CH3 R

O

O

O H

2, 3

2: R = H, 3: R = OH

Scheme 1. Structures of Tautomeric Forms (N, T), Possible Solvate Complex (Ns) of 2 and 3 in Protic Medium and Their Transformations in Ground and Excited Electronic States.


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EXPERIMENTAL AND THEORETICAL METHODS Reagents.

Silver

nitrate,

AgNO3

(99,99%);

trisodium

citrate

dihydrate,

(OH)C3H4(COONa)3·2H2O (99%) and poly (vinyl) alcohol (PVA) (Mw ~ 205000) as well as reagents for syntheses were purchased from Sigma Aldrich Company. All reagents used to prepare the plasmonic platform were purchased from commercial sources and used without further treatment. Deionized water used for all solutions was obtained from Hydrolab system installed in our laboratory. The gold thin films were purchased from Thin Metals Films Ltd. (UK). Compounds 119 and 220 were synthesized and identified as reported previously; their purity was controlled by 1H-NMR and elemental analysis and thin layer chromatography. For synthetic routines, procedures of preparation and results of analysis for 3 see Supporting Information. Preparation of PVA films and platforms. Stock solutions of 1, 2, and 3 of appropriate concentrations in methanol-dichloromethane (4:1, v:v) were added to 5% and 0.2% water solutions of PVA to obtain homogeneous solutions, which were used for preparation of films with low (10–4 M) and high (ca. 10–2 M) concentrations, respectively. Samples of 10–4 M concentrations were left in dark and a dust-free environment for a couple of days to allow water evaporation. Optical density of samples was low enough to neglect reabsorption and secondary emission.21 Thin films of ca. 10–2 M concentrations on quartz glass plates and plasmonic platforms were obtained by spin-coating method. The samples were spin coated at 300 rpm for 60 s to disperse the solution. After that, the samples were allowed to dry in a dark room and in air atmosphere for 24 hours. Silver colloidal nanoparticles were prepared in aqueous solutions using chemical reduction method according to the developed procedure presented earlier.15 In the first step, the sodium citrate was added dropwise to a stirred solution of AgNO3. During the process, all solutions were mixed vigorously and heated to 90-95oC for 15 min until change of color was evident (yellow). Then, the mixture was incubated in an ice bath for 15-20 min. Finally, the silver nanoparticles were purified by centrifugation at 3500 rpm for 8 min and the precipitate was then suspended in trisodium citrate.


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The plasmonic platforms were obtained according to the procedure presented earlier.22 First, gold mirrors were cleaned and drop coated with silver colloidal nanoparticles. Then, the mirrors were dried in air. When a liquid containing silver nanoparticles was evaporated, different structures were formed. In the final stage, compounds 1, 2 or 3 in 0.2% PVA were applied on gold mirror with Ag colloidal nanoparticles by spin-coating method as described above. Microscopic quartz glasses used were kept in a mixture of 33% H2O2 and H2SO4 in molar ratio 2:1 over 24 h and rinsed with deionized water. Apparatus.

The

absorption

spectra

were

measured

with

Shimadzu

1650

PC

spectrophotometer. Steady-state fluorescence intensity measurements of all the samples were carried out using a Carry Eclipse spectrofluorometer (Varian Inc., Australia) with the front face geometrical format described previously in detail.23 The emission was scanned from 400 nm to 650 nm following excitation at various wavelengths and using corresponding long pass filters on the emission side. Spectra of compounds on the plasmonic platforms free of background emission were obtained by a 366 nm excitation and using a 385 nm cut-off filter (Edmund Optics). All the spectra were corrected for the background (PVA, quartz glass and plasmonic platform) absorption/emission intensity and instrumental sensitivity. The fluorescence decays were measured in the front face mode upon the excitation ex = 378 nm using equipment constructed in our laboratory and described previously.24 As a source of excitation the laser head LDH-D-C-37 (average power 1.3 mW, peak power 557 mW) controlled by PDL 800-D module was used. Time-correlated single photon counting method was employed for data collection using PCI-board for TCSPC TimeHarp 200 (PicoQuant, Germany). Fluorescence was recorded by the H10721P-01 photomultiplier (Hamamatsu Photonics K.K., Japan) combined with the slit of Czerny-Turner spectrograph Shamrock 303i-B (Andor Technology, UK) and analysis was obtained with FluoFit Pro version (PicoQuant, Germany). All the experiments were carried out at the same instrumental conditions.


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Data processing and analysis. Processing of the measured steady-state and time-resolved spectra, and fluorescence decay surface was performed using Spectral Data Lab software.25 Deconvolution of absorption and fluorescence emission spectra into individual bands and evaluation of their partial intensity and area was performed using Siano-Metzler function.26 Signal of the excitation pulse in the 3D-fluorescence emission-excitation spectra was eliminated numerically. In the view of polycomponent fluorescence decay of the compounds 1–3, investigations of their excited-state deactivation dynamics were conducted by means of analysis of the whole fluorescence decay surface, following procedures previously described and applied for similar systems in liquid media.19,27 Besides evaluation of lifetimes and transformation rate constants of the excited species in a polycomponent system, the applied “multidimentional fluorescence phase plane” method allows recognizing transformations which occur and which do not.27 Briefly, fluorescence decays were measured in the 400–650 nm observation range in a 10 nm step, affording a threedimensional array with x, y, and z axes represented by emission wavelength, impulse delay time, and photon count, respectively. Deconvolution of thus obtained fluorescence decay surface using emission bands of fluorescent components (available from separation of the steady-state or timeresolved fluorescence spectra) afforded their individual fluorescence decays. Mutual correlation of these individual fluorescence decays together with deconvolution using instrument response function enabled determination of lifetimes (τ) and “apparent” rate constants of transformations of the excitedstate species (k'relax and k'ESIPT) τ = (kf + kd + krelax + kESIPT)–1, k 'ESIPT  k ESIPT  k 'relax  krelax 

k if  S j k fj  Si

k if  S j k fj  Si

(1) (2)

,

(3) ,

where krelax is the rate constant of excited-state relaxation (N*→Nr* for 1, 2, and 3, Nr*→N* for 2 and 3) or excimer formation (Nr*+N→D* for 1), kESIPT is the rate constant of ESIPT (N*→T*,


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T*→N*, Nr*→T* and T*→Nr* for 2 and 3), kf is the rate constant of radiative deactivation, S is the area of relevant fluorescence bands and superscripts or subscripts i and j indicate initial (substrate) and final (product) species. Ratio Sj/Si was obtained by deconvolution of steady-state spectra. Ratio kif /kjf for each pair of excited forms was estimated based on values of oscillator strength (f) of the S1→S0 transition predicted at the TD DFT (PCM: ethanol) level of theory (4)

k f  0.661  2f  f where v is experimental wavenumber of the S1→S0 transition.

The experimental kf values of initial species were obtained using quantum yields of compounds in water/methanol (1:4, v:v) solutions, where no relaxation or ESIPT were observed in the nanosecond domain, according to relationship (5)

  kf .

The kf values of T* forms were then evaluated from the computationally predicted kN*f /kT*f ratio. The krelax values were assumed to be close to apparent ones k'relax and used uncorrected due to similar parameters of the N* and Nr* forms and unavailability of the kf values for the D* species. Values of the rate constants of other nonradiative deactivation processes (kd) were obtained as differences between τ–1 and sum of deactivation rate constants (kf, krelax and kESIPT) from eq. 1. Quantum-chemical calculations. Unconstrained geometry optimizations of tautomeric forms of the compounds 2 and 3 in the ground (S0) and excited singlet (S1) electronic states were carried out at DFT/TD DFT levels of theory,28 with the B3LYP29,30 hybrid functional and the ccpVDZ basis set, using the GAUSSIAN 09 program package.31 After completion of each optimization, the Hessian matrix was calculated and, as all vibrational frequencies were positive, it was concluded that each optimized structure was stationary and corresponded to the relevant state in true energetic minimum.28 Gibbs free energy contributions at 298.14K and standard pressure were then included following statistical thermodynamic routines.32 The solvent effect (ethanol) was


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included at the level of the Polarized Continuum Model (PCM).33,34 The GAUSSIAN 09 program package was also used for calculations of the oscillator strength of the S1→S0 transition by utilizing the optimized S1 state geometry from the TD DFT optimization.35 Dipole moments for the nonrelaxed ground state (S0non-relaxed) were obtained by single point calculations using the optimized S1 state geometry.

RESULTS AND DISCUSSION In the PVA films (concentration 1×10–4 M) all the compounds investigated absorb light in the 350–470 nm range. The long-wavelength absorption maxima of 1, 2 and 3 are centered at 396, 417 and 408 nm, respectively (Table 1, Figure 1a,b,c). Number of fluorescence bands and the maxima locations of the compounds investigated in PVA films depend on the excitation wavelength (Figure 2a,c,e and 3a,c,e). Number of the fluorescent components present in the ground state was investigated by means of 3D fluorescence emission-excitation spectroscopy. Spectral features of 1 in 0.1 mM PVA films. Maximum of the peak observed in the 3D fluorescence spectra of 1 at c=1×10–4 M (Figure 2a) shifts batochromically with the rise of excitation wavelength: emission at wavelengths lower than 450 nm is excited mainly at wavelengths lower than 400 nm, whereas emission centered at near 475 nm is excited mainly in the 420–440 nm range. On the basis of this data and fluorescence emission spectra at different excitation wavelengths (Figure 3a,b) one can conclude that 1 in PVA contains two fluorescent components. The first one is characterized by the fluorescence excitation maximum at 386 nm and emission maximum at 450 nm (Table 1). Excitation and emission maxima of the second component are more than 1200 cm–1 batochromically shifted to 410 and 473 nm, respectively. Separation of the absorption spectra using the excitation ones (Figure 1a), on the basis of the assumption that the components are characterized by similar extinction coefficients, allowed to estimate that contribution of the red-shifted component is 39 % in the ground state.


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Table 1. Steady-State Spectral Parameters of Forms of 1-3 in PVA Films at Various Conditionsa Comp Conditions

Form

λabs

λex

λfl

1×10–4 M

N/Nr

396

386

447

410

473

402

463 599 473 607

1

Np 1×10–2 M

2

N/Nr + Np D –2 1×10 M N/Nr + Np on platform D –4 1×10 M N/Nr T Ns 1×10–2 M

N/Nr T Ns

–2

1×10 M on platform

3

–4

1×10 M

1×10–2 M

394

417

408 424

414

407 425

N/Nr T Ns

408

N/Nr T Ns

410

405 434 409 435

total partial

5.3

5.3 5.5

7.2

7.5 6.5

481 567 510 509 570 507 528 566 507

N/Nr T Ns

SPl/S10-2M

3.5

468 570 501 504 575 510

–2

1×10 M on platform

514 2.7 2.8 N/Nr 584 2.6 T 507 3.7 Ns a λabs and λex – positions of maxima of the long-wavelength bands in absorption and fluorescence excitation spectra, respectively, in nm; λfl – position of the maximum of the fluorescence band, in nm; SPl/S10-2M – area under fluorescence spectra (total) or band (partial) measured on plasmonic platform divided by area of the same quantity measured in the absence of the platform.

Previous investigations of the acid-base transformations revealed that in the absence of acids or bases fluorescence of 1 in a water-methanol medium is independent of excitation, due to existence of the neutral N form only in both ground and S1 state.19 Formation of anion or cation in PVA films is even less probable due to lower concentration of proton acceptors and donors, thus one can expect


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that under such conditions the compound also exists in the N form. The observed two components of 1 in PVA may be due to different localization of the N species regarding polar and nonpolar fragments of the polymer. Since 4'-N,N-dimethylaminoflavones show positive solvatochromy, 36,37 we suggest that the blue component in absorption and fluorescence spectra originates from the N species localized in less polar regions of PVA, whereas the red-shifted bands originate from the species (Np), localized in more polar regions and surrounded by polar hydroxyl groups of PVA.

a

b

c

Figure 1. Absorption spectra and their constituent long-wavelength bands separated using fluorescence excitation spectra of 1 (a), 2 (b) and 3 (c) in PVA films, concentration 10–4 M.


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a

b

c

d

e

f

Figure 2. Projection of the 3D fluorescence excitation-emission spectra of 1 (a,b), 2 (c,d) and 3 (e,f) in PVA films at 1×10–4 M (left) and c.a. 1×10–2 M (right).


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λex = 366 nm

a

λex = 450 nm

b

λex = 366 nm

c

λex = 475 nm

d

λex = 366 nm

e

λex = 475 nm

f

Figure 3. Fluorescence emission spectra of 1 (a,b), 2 (c,d) and 3 (e,f) in PVA films with concentration 10–4 M (magenta), 10–2 M (green), and 10–2 M on plasmonic platform (blue) at various excitation wavelengths (λex).


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Fluorescence of 1 in the PVA films excited at 378 nm (mainly N species are excited) decays biexponentially. The short-living fluorescent component appears with positive amplitudes at the blue part of the emission band, and with negative ones at the red part (Table S1, Supporting Information). The long-living component appears with positive amplitudes at the whole length of the band. In the fluorescence emission spectra the batochromic shift of the initial band at 445 nm to 462 nm is observed during 10 ns (Figure 4a). Deconvolution of the whole fluorescence decay surface using the separated bands with the mentioned above maxima reveals that the 462 nm band arises from the 445 nm (Figure 5a). The rate constant of the transformation is 0.20 ns–1 (Table 2). As 4'-N,N-dimethylaminoflavones in the S1 state are sensitive to the change of medium polarity, we suppose that the observed batochromic shift of the fluorescence spectra of 1 with time can be caused by a slow solvent relaxation of PVA. The latter process most probably results in the change of unpolar environment of 1 (N* species) to more polar one (Nr* species, Scheme 2a). Taking into account that the Nr* fluorescence maximum is close to that of Np*, existing in the ground state and distinguished by means of the steady-state fluorescence spectroscopy, and that transformation of N* to Nr* is irreversible it can be concluded that in the excited state 1 is more stabilized in the polar fragments of PVA compared to the ground state. Spectral features of 1 in 10 mM PVA films. At higher concentrations (c.a. 1×10–2 M) in thin PVA films on quartz plates, the long-wavelength absorption band of 1 becomes broader (Figure S1a, Supporting Information). In the steady-state fluorescence spectra, the N*/Np* band at 468 nm also becomes broader and an additional band appears near 599 nm (Figure 2b and 3a,b). The latter band is excited at 395 nm, similarly to that of the N*/Np* one (Figure 2b). The fluorescence decays of 1 at such conditions are polyexponential at various observation wavelengths. In the time-resolved fluorescence spectra, solvent relaxation of the N* species (460 nm) to the Nr* ones (480 nm) can be observed (Figure 4b) similarly to the behavior of 1 at lower concentration discussed above. Batochromic shifts of the N* and Nr* fluorescence bands, compared to those at 1×10–4 M, indicate


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that the environment of 1 in PVA at higher concentrations becomes more polar. The contribution of the 599 nm band increases with time, and 6 ns after excitation the intensity ratio of the latter and the Nr* band approaches constant value 1 : 0.75 (Figure 4b). Deconvolution of the whole fluorescence decay surface using three constituent bands mentioned above yields monoexponential decay curve for the N* emission and polyexponential decay curves for the Nr* and 599 nm bands (Figure 5b). The excited state lifetimes of the N* and Nr* species are substantially lower than those at 1×10–4 M (Table 2), which is caused by increased rate constants of nonradiative deactivation, relaxation (N*→Nr*) processes and interactions with the ground-state N species (discussed further).

a) c = 0.1 mM

b) c = 10 mM

Scheme 2. Diagrams of Transformations of Forms in the Ground and Excited States of 1


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Table 2. Kinetic Parameters of the Excited-State Deactivation of 1, 2, and 3 in PVA Filmsa Compd Conditions Form λfl τ kf kd 1

–4

1×10 M –2

1×10 M

1×10–2 M on platform 2

1×10–4 M

–2

1×10 M

1×10–2 M on platform 3

1×10–4 M

–2

1×10 M

1×10–2 M on platform

N* Nr* N* Nr* D* N* Nr* D* N* Nr* T* N* Nr* T* N* Nr* T* N* Nr* T* N* Nr* T* N* Nr* T*

445 462 460 480 599 464 504 608 467 506 571 461 510 568 473 522 569 464 489 573 478 518 573 474 516 581

1.50 ± 0.05 1.91 ± 0.02 0.30 ± 0.01 0.37 ± 0.09 0.27 ± 0.01 0.05 ± 0.01 0.60 ± 0.01 0.82 ± 0.04 0.49 ± 0.02 1.18 ± 0.08 2.53 ± 0.04 0.32 ± 0.01 0.54 ± 0.09 1.1 ± 0.2 0.09 ± 0.01 0.14 ± 0.01 0.35 ± 0.03 0.64 ± 0.01 0.59 ± 0.03 4.00 ± 0.05 0.27 ± 0.01 0.25 ± 0.04 1.2 ± 0.1 0.08 ± 0.01 0.11 ± 0.01 0.62 ± 0.03

b

0.17 [19]

b

0.14 0.10d

0.30 0.35 1.90 ~1

0.15 0.20 0.16 1.94 0.54 0.29

krelax 0.20 ± 0.01 (N*→Nr*)

1.29 ± 0.04 (N*→Nr*) 1.46 ± 0.04c (Nr*+N→D*) 2.9 ± 0.8c (D*→Nr*+N) 10.5 ± 0.04 (N*→Nr*) 0.14 ± 0.04c (Nr*+N→D*) 3.0 ± 0.4c (D*→Nr*+N) 0.09 ± 0.03 (N*→Nr*) 0.5 ± 0.1 (Nr*→N*) 0.25 ± 0.09 (N*→Nr*) 0.8 ± 0.1 (Nr*→N*) 1.3 ± 0.3 (N*→Nr*) 4.4 ± 0.3 (Nr*→N*)

0.17b 0.10d

0.38 0.19 0.10 2.78 0.29 0.29

a

kESIPT

0.62 ±0.02 (N*→Nr*) 1.34 ± 0.03 (Nr*→N*) 0.31 ± 0.06 (N*→Nr*) 1.4 ± 0.07 (Nr*→N*) 1.3 ± 0.4 (N*→Nr*) 4.0 ± 0.5 (Nr*→N*)

1.66 ± 0.08 (N*→T*) 0.13 ± 0.04 (T*→N*) 0.8 ± 0.1 (N*→T*) 0.4 ± 0.1 (Nr*→T*) 0.5 ± 0.1 (T*→Nr*) 10 ± 1 (N*→T*) 1.3 ± 0.3 (Nr*→T*) 1.9 ± 0.3 (T*→Nr*) 0.40 ± 0.02 (N*→T*) 0.04 ± 0.01 (T*→N*) 0.5 ± 0.1 (N*→T*) 2.1 ± 0.2 (Nr*→T*) 0.44 ± 0.03 (T*→Nr*) 8.3 ± 0.8 (N*→T*) 3.7 ± 0.3 (Nr*→T*) 0.69 ± 0.04 (T*→Nr*)

excitation wavelength 378 nm. λfl – fluorescence maxima, in nm; τ – lifetime of the electronically excited state calculated according to the formula τ = (kf + kd + krelax + kESIPT)–1, in ns; kf – rate constant of radiative deactivation, in ns–1; kd – rate constant of nonradiative deactivation with exception of solvent relaxation and ESIPT b processes, in ns–1; krelax and kESIPT – rate constant of relaxation, formation of excimers and ESIPT processes, respectively, in ns–1. Obtained for water:methanol d solutions. cUncorrected values. Obtained from corresponding kf values of N* species using the computationally predicted kN*f /kT*f ratio. ACS Paragon Plus Environment

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a

b

c

d

e

f

g

h

i

Figure 4.Time-resolved fluorescence spectra of 1 (a,b,c), 2 (d,e,f), and 3 (g,h,i) in PVA films at 1×10–4 M (left), 1×10–2 M (middle) and 1×10–2 M on plasmonic platform (right) normalized on the N*/Nr* emission intensity; excitation wavelength 378 nm. ACS Paragon Plus Environment

18

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a

b

c

e

f

r

d r

g

h

i

r

Figure 5. Decay curves of the separated fluorescence of forms of 1 (a,b,c), 2 (d,e,f), and 3 (g,h,i) in PVA films at 1×10–4 M (left) and 1×10–2 M (middle) and 1×10–2 M on plasmonic platform (right); excitation wavelength 378 nm ACS Paragon Plus Environment

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20 Abnormally high Stokes shift value of the fluorescence band at 599 nm (8620 cm–1 ) indicates that species responsible for this emission are formed after excitation. Taking into account that the latter band appears only at high concentrations, absence of substantial changes in the absorption spectra (Figure S1a, Supporting Information) and presence of correlation between its fluorescence decay curve with the Nr* one (Figure 5b), this emission can be attributed to the excimers (D*) formed from Nr* and N species (Scheme 2b). Based on the correlation of the fluorescence decay curves (Figure 5b), excimers coexist with the relaxed Nr* species in the same time domain (rates of the Nr* + N→D* and D*→Nr* + N transformations are 1.5 and 2.9 ns–1M–1, respectively) in which PVA relaxation (N*→Nr*) occurs (Table 2). According to earlier reports on crystal structure of 4'-N,N-dimethylaminoflavones,38 one of the important interactions among these molecules are the π-π ones. Taking into account substantial batochromic shift of the excimers fluorescence relative to the Nr* one, we assume that excimers can be formed due to interactions of π-electronic densities of monomeric species N and Nr* (Figure S2a, Supporting Information). Spectral features of 1 in 10 mM PVA films on plasmonic platform. On a plasmonic platform, consisting of silver nanoparticles on the surface of golden plate, emission intensity of the PVA film containing 1×10–2 M of 1 increases 5.3 times and the emission maxima of bands shift slightly batochromically compared to that in the absence of platform (Figure 3a, Table 1). In the time-resolved spectra, the N* emission decays fast with lifetime near 50 ps, transforming to the Nr* one with the rate constant 10.5 ns–1 (Figures 4c and 5c, Table 2). The Nr* band is centered at 504 nm, with a 1000 cm–1 batochromic shift relative to that in the absence of platform. Moreover, on the plasmonic platform, dissociation of excimer (D* → Nr* + N) proceeds more than 20 times faster than its formation (Nr* + N → D*). It can be concluded that besides the considerable fluorescence enhancement of all forms present in the excited state of 1, plasmon resonance favors relaxation and inhibits formation of excimers. The observed batochromic shifts in the steady-state spectra thus can be explained by faster and more effective relaxation processes occurring in the


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21 presence of plasmonic platform. Relaxation and formation of excimers are both intermolecular processes and they probably occur competitively. Therefore, one can suggest that the acceleration of relaxation and stabilization of the Nr* species decreases yield of D*. Spectral features of 2 and 3 in 0.1 mM PVA films. In the 3D fluorescence spectra of 2 and 3 at c=1×10–4 M (Figure 2c,e) two peaks are observed. Maximum of the first peak in the blue region shifts batochromically under increase of the excitation wavelength, similarly to that of 1, whereas maximum of the second one in the red region is independent of the excitation wavelength. On the basis of these data and emission spectra at different excitation wavelengths (Figure 3c–f), spectral properties of 2 and 3 in PVA films at 1×10–4 M are conditioned by two types of species in the ground state and three types of species in the electronically excited state. The first ones are characterized by the fluorescence excitation maxima at 408 nm (2) and 405 nm (3) (Figure 1b,c, Table 1). Excitation of these species gives dual fluorescence with maxima at 481 nm and 567 nm (2) and 468 nm and 570 nm (3). It is most probable that N species in the ground state and the N* and T* ones in the excited state coexisting due to ESIPT (Scheme 1 and 3a) are responsible for this dual fluorescence. Another type of species (Ns) is excited at 424 nm (2) and 434 nm (3) (Figure 1b,c), and fluoresce at 510 nm (2) and 501 nm (3), respectively (Figure 2c,e and 3d,f). Their single band fluorescence indicates the absence of ESIPT, which is typical for 2 in protic media of high polarity.8 Competing intermolecular H-bonding with hydroxyl groups of the polymer can disrupt intramolecular HB in 2 and 3, which prevents proton transfer (Scheme 1). Batochromic shifts of the Ns absorption and fluorescence relative to the N ones are caused by positive solvatochromic effects18 due to ICT, as was mentioned above. Therefore, similarly to 1, we conclude that different allocation of 2 and 3 molecules concerning nonpolar aprotic and polar protic fragments of PVA is the reason of fluorescence dependence on the excitation wavelength.


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22 a) c = 0.1 mM

b) c = 10 mM

Scheme 3. Diagrams of Transformations of Forms in the Ground and Excited States of 2 and 3

Dependence of 2 fluorescence on the excitation wavelength was observed previously in poly(methyl methacrylate) (PMMA) films.39 Our data indicate that compared to PMMA, 2 in PVA represents much more intensive fluorescence from N* and Ns* relative to the T* one. The phenomenon can be explained by higher polarity and H-bonding ability of PVA than PMMA. Based on the separation of the absorption spectra using the excitation ones (Figure 1b and c) the estimated contribution of the Ns species in the ground state are 44% and 18% in the case of 2 and 3, respectively. Fluorescence decays of 2 and 3 excited at 378 nm (mainly N species are excited) are composed of three different components (Table S1, Supporting Information). The short-living components ( 470 nm, and species formed as a result of the excited-state relaxation (Nr*) from the N* ones, the main component excited at wavelengths lower than 380 nm (Figure 1b,c, Scheme 3a). We assume, that the excited-state relaxation is the reason of the mentioned above batochromic shifts of the blue bands in the timeresolved spectra (Figure 4d,g). In spite of the close positions of emission maxima, Ns* and Nr* are formed in different ways and thus their proton-transfer sites can have different environments, which can affect their features under conditions of aggregation or interactions with plasmonic platform as discussed further. Interactions of the fluorescent components were investigated by means of deconvolution of the whole fluorescence decay surface using separated emission bands of N*, Nr*, and T* (Figure 5d,g). It was found that similarly to 1, excited-state relaxation of the N* to Nr* species takes place with rate constant below 1 ns–1 (Table 2). However, the reverse transformation Nr*→N* is dominant in the case of 2 and 3. These findings indicate higher stability of N* relative to Nr* which can be due to strengthening of the intramolecular H-bonds in the S1 state, which makes formation of the intermolecular H-bonds with participation of the PVA hydroxyl groups less favorable than in the ground state. The N* and T* species co-exist via slow and reversible ESIPT,


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24 which correlates well with the observed previously reversibility of the N*↔T* transformation in various liquid solutions of 2.18 Both forward (N*→T*) and reverse (T*→N*) transformations are faster in the case of 2 (Table 2). No evidence of Nr* and T* interaction was found, which supports the mentioned above suggestion that the Nr* species, similarly to the Ns* ones, at 1×10–4 M in polar protic environment do not undergo ESIPT and most probably do not contain intramolecular HB. Spectral features of 2 and 3 in 10 mM PVA films. In thin PVA films with higher concentrations (c.a. 1×10–2 M) on quartz plates, the long-wavelength absorption bands of 2 and 3 become broader (Figure S1b,c, Supporting Information). In the case of steady-state fluorescence spectra of 2 and 3, the contribution of T* fluorescence is smaller compared to that at lower concentration (Figure 2c,d and e,f). The N/Nr* and Ns* fluorescence bands are centered near 505– 510 nm (Table 1, Figure 3 c–f). The N/Nr* bands maxima in steady-state spectra thus suffer 1150 (2) and 1500 cm–1 (3) batochromic shifts compared to those at lower concentration. Positions of the T* fluorescence maxima are practically independent of concentration for both compounds. The phenomenon is due to much lower sensitivity of T* species to changes in polarity of media, explained by their substantially lower dipole moment changes under the S1-S0 transition (1.9 D (2) and 0.9 D (3)) relative to the N* species (10.7 D (2) and 8.7 D (3)) predicted on the DFT/TD DFT level of theory (Table S2, Supporting Information). The excitation maxima of all species are close to those at lower concentrations (Figure S1b,c, Supporting Information). Separated fluorescence of forms of 2 and 3 decay faster at higher concentration (Figure 5e,h), mostly due to increased rates of nonradiative deactivation (Table 2). In the time-resolved fluorescence spectra of 2, the N* and Nr* bands appear at 461 nm and 510, respectively, similarly to those at lower concentration. In the case of 3, the N* and Nr* bands are centered at 478 nm and 518 nm, respectively, with 630 cm–1 and 1150 cm–1 batochromic shifts compared to those at lower concentration (Figure 4h). Transformation of N* to Nr* is well distinguished for both compounds


24

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25 (Figure 4e,h), rate constants of the direct transformation of N* to Nr* are at least twice higher compared to those at lower concentration, whereas values of the same quantity for the reverse transformation Nr*→N* remain practically unchanged within experimental error (Table 2). In the case of 2, rate constant of the N*→T* ESIPT is reduced at 1×10–2 M, which is the main reason for the observed lower impact of the T* fluorescence. The mentioned above practically the same positions of maxima of the N* fluorescence bands can evidence that there is no considerable change in the influence of the molecular environment on the π-electronic density of 2 with increasing concentration. Since comparison of the absorption spectra of 2 at 1×10–4 M and 1×10–2 M allows to conclude negligible aggregation effects in the ground state (Figure S1b, Supporting Information), one of the reasons for the observed inhibition of the N*→T* ESIPT rate can be formation of excimers N*+N at high concentration in which hydrogen atom of hydroxyl group of N* participates in bifurcated hydrogen bonding, similarly as was found in crystal phase (Figure S2b, Supporting Information). In contrast, the rate constant of the N*→T* ESIPT in 3 was found to be the same at 1×10–4 M and 1×10–2 M, whereas fluorescence maximum of the N* species shifts batochromically at higher concentration. For these reasons, we assume that the N* species of 3 can participate rather in the formation of π-bonded, but not H-bonded excimers N*+N (Figure S2a, Supporting Information). On the basis of correlation of fluorescence decay curves of the forms at 1×10–2 M (Figure 5e,h), it was found that the Nr* species of 2 and 3 undergo ESIPT Nr*→T* (Scheme 3b). This finding may indicate that at high concentrations the Nr* species are less exposed to intermolecular H-bonding interactions with PVA molecules and contain intramolecular H-bond after relaxation. Compared to the discussed above N*→T* transformation, the rate constant of the Nr*→T* one is twice lower in the case of 2 and five times higher in the case of 3 (Table 2). Taking into account that fluorescence maxima of the Nr* bands are practically independent of concentration in the case of 2 and shifted batochromically at 1×10–2 M in the case of 3, the differences in Nr*→T* ESIPT


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26 rates can be explained by certain influence of the H-bonded and π-bonded excimers Nr*+N in the case of 2 and 3, respectively. Such an assumption supports the one mentioned above, made for the N* species and ESIPT in them. The assumed certain contributions of the N*+N and Nr*+N interactions to the excited-state deactivation dynamics also correlate with the above mentioned rate differences in the N*↔Nr* transformations under increase of concentration. Spectral features of 2 and 3 in 10 mM PVA films on plasmonic platform. On a plasmonic platform, intensity of the total N*/Nr* and T* fluorescence of 2 and 3 excited at 366 nm increases 7.2 and 2.7 times, respectively (Figure 3c,e, Table 1). The N*/Nr* bands suffer 700 and 400 cm–1 batochromic shift to 528 and 514 nm for 2 and 3, respectively, compared to those in the absence of the platform. Fluorescence of the Ns* species excited at 475 nm increases near 3.6 times and their maxima remain practically unchanged for both compounds compared to those in the absence of the platform (Figure 3d,f). In the time-resolved fluorescence spectra of 2 and 3, similar changes occur in the absence and presence of the plasmonic platform, but in the latter case, these changes proceed faster (Figure 4f,i). Maxima positions of the N* emission are unaffected by the plasmon resonance for both compounds. In the case of 2, the Nr* band is shifted batochromically to 522 nm on the platform, whereas no substantial shift of the similar band is observed in the case of 3. Rate constants of both N*→Nr* and Nr*→N* transformations increase more than 3 times for both compounds (Table 2). The mutual ratio of the latter rate constants remains, however, similar as in the absence of the platform within experimental error, which indicates that under conditions of plasmonic resonance the mutual energy of the N* and Nr* species is not affected but the stationary state is attained faster. The evaluated rates of N*→T* ESIPT in 2 and 3 increase near 15 times in the presence of plasmonic platform and reaches 10 ns–1 (Table 2). Rates of both forward and reverse ESIPT


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27 Nr*↔T* on the platform are enhanced near 3.5 and 1.7 times for 2 and 3, respectively, so the equilibrium between the latter species is attained faster. Plasmon resonance was found to enhance fluorescence intensity of the electronically excited molecules due to acceleration of the radiative deactivation.40 We assume that the effect is similar for the directly excited (N*) and photoinduced species (Nr* and T*). However, if the rate of only radiative deactivation increased, the yield of photoinduced species would decrease, which is not the case according to the steady-state fluorescence measurements. Our findings indicate that plasmon resonance accelerates both radiative deactivation, relaxation processes, and proton transfer transforations. The evaluated rates of direct and reverse proton transfer enabled estimation of thermodynamic parameters of transformations of this kind in the excited state. Using the relationship Δ298G° = –RTln(kN*→T*/ kT*→N*),

(6)

changes of Gibbs free energy (Δ298G°) of the N*→T* transformation were found to be similar for 2 and 3 at 1×10–4 M: –6.4 and –5.7 kJ/mol, respectively. The corresponding theoretically predicted values are –13.7 and –19.1 kJ/mol, respectively (Table S3, Supporting Information). The Δ298G° values of Nr*→T* transformation which is observed at higher concentration are, however, different for 2 and 3: +0.6 and –3.9 kJ/mol, respectively, and remain practically unchanged under conditions of plasmon resonance. Taking into account that thermodynamics of ESIPT in 2 was found to be polarity-dependent,18 these findings can serve as an additional proof of the assumed different structure of the Nr*+N excimers in 2 and 3. Importantly, plasmon resonance does not affect substantially relative energy of species participating in ESIPT under the conditions applied, thus combination of these two phenomena is promising for various applications, where enhanced fluorescence of indicators with several analytical signals can be used.


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28 CONCLUSIONS Concluding investigations of the spectral features of 1-3 in PVA films, it should be noticed that ground-state properties of the compounds are controlled by two types of forms (N and Np/Ns) situated at regions of different polarity and H-bonding ability. In the excited state, substantially increased dipole moments of the N* species induce slow solvent relaxation of PVA, which in the case of 2 and 3 occurs simultaneously with ESIPT. At low concentrations, fluorescent properties of species after PVA relaxation (Nr*) are similar to those situated in polar regions of PVA obtained by direct excitation (Ns*). At high concentrations, interactions of the excited species with the ground-state ones play important role in the excited-state deactivation dynamics. In the case of 1, excimers (D*) appear probably due to interactions of π-systems of the molecules. In the case of 2 and 3, formation of excimers affects excited-state features of molecules in nonrelaxed (N*) and relaxed (Nr*) PVA. Due to this phenomenon, ESIPT occurs in both types of species and fluorescent properties of Nr* differ from those situated in polar regions of PVA obtained by direct excitation (Ns*). In general, plasmon resonance causes fluorescence enhancement and acceleration of various processes occurring in the excited state of compounds investigated. In the presence of plasmonic platform, the excited-state relaxation processes occur faster and, in the case of compounds 1 and 2, more effectively, based on the sufficient batochromic shifts of the relaxed species emission. Formation of excimers in the excited state of 1 is less favorable in the presence of plasmonic platform. Plasmonic resonance causes the 15-fold increase of ESIPT rate in the directly excited (N*) species and the almost equal acceleration of direct and reverse proton transfer in the relaxed species (Nr*↔T*). Our experimental findings allow to conclude that plasmonic resonance has no substantial effect on the ESIPT thermodynamics under the conditions applied. Due to these reasons, fluorescence intensity of the ESIPT product (T*) increases symbatically in the plasmon-


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29 resonance enhanced fluorescence spectra. This implies that plasmon resonance can be successfully applied for enhancement of dual and multiband ESIPT fluorescence. Emission of species incapable of proton transfer (Ns*) is enhanced equally for 2 and 3.

SUPPORTING INFORMATION AVAILABLE Synthetic procedures and analyses for compound 3; absorption and fluorescence excitation spectra of 1, 2 and 3 in PVA films at various concentrations; parameters of the polyexponential fit of the fluorescence decay curves of 1, 2, and 3 in PVA at 10–4 M; fragments of 2 structure in crystal phase; absorption spectra of silver colloids in solutions and on platforms; theoretically predicted values of N and T dipole moments and Δ298G° for the N→T transformation in the S0 and S1 state. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT This research was supported by the grants 2014/13/N/ST4/04105 (I. Serdiuk) and 2015/17/B/ST5/03143 (P. Bojarski, A. Synak, B. Grobelna) financed by National Science Centre. Quantum chemical calculations were performed on the computers of the Ukrainian-American Laboratory of Computational Chemistry (UALCC, Kharkiv, Ukraine).

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31 (13) Luchowski, R.; Matveeva, E.G.; Shtoyko, T.; Sarkar, P.; Patsenker, L.D.; Klochko, O.P.; Terpetschning, E.A.; Borejdo, J.; Akopova, A.; Gryczynski, Z.; et al. Single Molecule Immunoassay on Plasmonic Platforms. Curr. Pharm. Biotechnol. 2010, 11, 96–102. (14) Cao, Y.W.; Jin, R.; Mirkin, C.A. DNA-Modified Core-Shell Ag/Au Nanoparticles. J. Am. Chem. Soc. 2001, 123, 7961–7962. (15) Luchowski, R.; Calander, N.; Shtoyko, T.; Apicella, E.; Borejdo, J.; Gryczynski, Z.; Gryczynski, I. Plasmonic Platforms of Self-Assembled Silver Nanostructures in Application to Fluorescence. J. Nanophotonics, 2010, 4, 043516. (16) Hsieh, C.-C.; Jiang C.-M.; Chou, P.-T. Recent Experimental Advances on Excited-State Intramolecular Proton Coupled Electron Transfer Reaction. Acc. Chem. Res. 2010, 43, 1364–1374. (17) Klymchenko, A.S.; Pivovarenko, V.G.; Ozturkd, T.; Demchenko, A.P. Modulation of the Solvent-Dependent Dual Emission in 3-Hydroxychromones by Substituents. New J. Chem. 2003, 27, 1336–1343. (18) 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. (19) Serdiuk, I.E.; Roshal, A.D. 7-Hydroxyflavone Revisited. 2. Substitution Effect on Spectral and Acid–Base Properties in the Ground and Excited States. J. Phys. Chem. A 2015, 119, 12672−12685. (20) Serdiuk, I.E.; Roshal, A.D.; Błażejowski, J. Quantum-Chemical Analysis of the Algar–Flynn– Oyamada Reaction Mechanism. Chem. Heterocycl. Compd. 2014, 50, 396–403. (21) Ketskemety, I.; Dombi, J.; Horvai, R.; Hevesi, J.; Kozma, L. Experimentelle Prüfung des Wawilowschen Gesetzes im Falle Fluoresziender Lösungen. Acta Physica Chem. (Szeged) 1961, 7, 17–21.


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Spectral Features and Excited-State Transformations of Hydroxy

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