Spectral Features and Excited-State Transformations of Hydroxy

Article pubs.acs.org/JPCC

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 ‡

S Supporting Information *

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poly(vinyl 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 the polarity and hydrogen-bonding ability of the medium, various tautomeric species localized in different domains of PVA can be distinguished in the ground and electronically excited states, which 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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of the ESIPT fluorophores can afford generation of various emission colors from blue to red and even white light.1,5 Two 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).7−9 In most 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 for relatively rare examples of applications compared to the photoinert ones.

INTRODUCTION Excited state intramolecular proton transfer (ESIPT) represents one of the fundamental photochemical reactions. ESIPT takes place in compounds which contain a proton donor in the vicinity of a 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 also 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 © 2016 American Chemical Society

Received: July 23, 2016 Revised: December 16, 2016 Published: December 19, 2016 636

DOI: 10.1021/acs.jpcc.6b07386 J. Phys. Chem. C 2017, 121, 636−648

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The Journal of Physical Chemistry C 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 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 the distance between the chromophores and the metallic structure.14 It is worth adding that the enhancement is a result of a resonance interaction of localized surface plasmons in metallic nanoparticles with propagating surface plasmon polaritons induced in the metal film through the nearfield interactions.15 In this study, we investigated for the first time the plasmon resonance effect on flavonols (3-hydroxy-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 a century.5 Proton transfer in these compounds represents an example of a keto−enol tautomerization (Scheme 1). Flavonol

Chart 1. Canonical Structures of the Compounds Investigated

mation 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 the sensitivity of 2 to both polarity and Hbonding properties of the medium.4 Compound 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.

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 the 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 the 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 a dark and dust-free environment for a couple of days to allow water evaporation. The 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 the 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 h. Silver colloidal nanoparticles were prepared in aqueous solutions using the chemical reduction method according to the developed procedure presented earlier.15 In the first step, the

derivatives containing a 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 a high sensitivity of fluorescent features on polarity. Moreover, in protic environments, intermolecular Hbonding leads to formation of species without an intramolecular hydrogen bond (Scheme 1), which can result in the appearance of an 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). The influence of the plasmon resonance on the ICT system was investigated using 2-[4-(dimethylamino)phenyl]-7hydroxy-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 infor637


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The Journal of Physical Chemistry C

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 three-dimensional array with x, y, and z axes represented by emission wavelength, impulse delay time, and photon count, respectively. Deconvolution of the thus obtained fluorescence decay surface using emission bands of fluorescent components (available from separation of the steady-state or time-resolved fluorescence spectra) afforded their individual fluorescence decays. Mutual correlation of these individual fluorescence decays together with deconvolution using the instrument response function enabled determination of lifetimes (τ) and “apparent” rate constants of transformations of the excited-state species (k′relax and k′ESIPT)

sodium citrate was added dropwise to a stirred solution of AgNO3. During the process, all solutions were mixed vigorously and heated to 90−95 °C 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. 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, compound 1, 2, or 3 in 0.2% PVA was applied on a gold mirror with Ag colloidal nanoparticles by the spincoating method as described above. The 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 a 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 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 cutoff filter (Edmund Optics). All of 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 λex = 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 the PDL 800-D module was used. The time-correlated single photon counting method was employed for data collection using PCIboard 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 of the experiments were carried out at the same instrumental conditions. 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 the Siano− Metzler function.26 The signal of the excitation pulse in the 3Dfluorescence emission−excitation spectra was eliminated numerically. In the view of polycomponent fluorescence decay of 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

τ = (k f + kd + k relax + kESIPT)−1 k′ESIPT = kESIPT·

k′relax =

(1)

k fi ·Sj k fj·Si

k fi ·Sj k relax · j k f ·Si

(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*, 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. The ratio Sj/Si was obtained by deconvolution of steady-state spectra. The ratio kif/kjf for each pair of excited forms was estimated on the basis of values of the oscillator strength ( f) of the S1 → S0 transition predicted at the TD DFT (PCM: ethanol) level of theory k f ≈ 0.661·νf 2·f

(4)

where v is the 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 was observed in the nanosecond domain, according to the relationship

φ = τ ·k f

(5)

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 the 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 the DFT/TD DFT levels of theory,28 with the B3LYP29,30 hybrid functional and the cc-pVDZ 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 638


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The Journal of Physical Chemistry C Table 1. Steady-State Spectral Parameters of Forms of 1−3 in PVA Films under Various Conditionsa SPl/S10−2M comp 1

conditions −4

1 × 10

M

1 × 10−2 M 1 × 10−2 M on platform 2

1 × 10−4 M

1 × 10−2 M

1 × 10−2 M on platform

3

1 × 10−4 M

1 × 10−2 M


form

λabs

λex

λfl

N/Nr Np N/Nr + Np D N/Nr + Np D N/Nr T Ns N/Nr T Ns N/Nr T Ns N/Nr T Ns N/Nr T Ns N/Nr T Ns

396

386 410 402

447 473 463 599 473 607 481 567 510 509 570 507 528 566 507 468 570 501 504 575 510 514 584 507

394

417

408

414

424 407 425

408

405

410

434 409 435

total

partial

5.3

5.3 5.5

7.2

7.5 6.5

3.5

2.7

2.8 2.6

3.7

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 the plasmonic platform divided by the area of the same quantity measured in the absence of the platform. a

relevant state in the true energetic minimum.28 Gibbs free energy contributions at 298.14 K and standard pressure were then included following statistical thermodynamic routines.32 The solvent effect (ethanol) was 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 (S0nonrelaxed) were obtained by single point calculations using the optimized S1 state geometry.

lengths (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 the emission maximum at 450 nm (Table 1). Excitation and emission maxima of the second component are more than 1200 cm−1 bathochromically 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 it to be estimated that the contribution of the redshifted component is 39% in the ground state. 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 the ground and S1 state.19 Formation of an anion or cation in PVA films is even less probable due to the lower concentration of proton acceptors and donors; thus, one can expect 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. The 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

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RESULTS AND DISCUSSION In the PVA films (concentration 1 × 10−4 M), all of 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−c). The number of fluorescence bands and the maxima locations of the compounds investigated in PVA films depend on the excitation wavelength (Figures 2a,c,e and 3a,c,e). The number of 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. The maximum of the peak observed in the 3D fluorescence spectra of 1 at c = 1 × 10−4 M (Figure 2a) shifts bathochromically 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 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 wave639


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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.

plates, the long-wavelength absorption band of 1 becomes broader (Figure S1a, Supporting Information). In the steadystate fluorescence spectra, the N*/Np* band at 468 nm also becomes broader and an additional band appears near 599 nm (Figures 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 under 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 the lower concentration discussed above. Bathochromic shifts of the N* and Nr* fluorescence bands, compared to those at 1 × 10−4 M, indicate 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 a constant value 1:0.75 (Figure 4b). Deconvolution of the whole fluorescence decay surface using the three constituent bands mentioned above yields a 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

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 bathochromic shift of the initial band at 445 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 band (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 bathochromic 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 the unpolar environment of 1 (N* species) to a 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 (ca. 1 × 10−2 M) in thin PVA films on quartz 640


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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 ca. 1 × 10−2 M (right). 641


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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 a plasmonic platform (blue) at various excitation wavelengths (λex).

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 a Plasmonic Platform. On a plasmonic platform, consisting of silver nanoparticles on the surface of a golden plate, the 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 bathochromically compared to that in the absence of a platform (Figure 3a, Table 1). In the time-resolved spectra, the N* emission decays fast with a lifetime near 50 ps, transforming to the Nr* one with a rate constant of 10.5 ns−1 (Figures 4c and 5c, Table 2). The Nr* band is centered at 504 nm, with a 1000 cm−1 bathochromic shift relative to that in the absence of a platform. Moreover, on the plasmonic platform, dissociation of an 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

and interactions with the ground-state N species (discussed further). An 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, the absence of substantial changes in the absorption spectra (Figure S1a, Supporting Information), and the 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). On the basis of 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−1 M−1, respectively) in which PVA relaxation (N* → Nr*) occurs (Table 2). According to earlier reports on the crystal structure of 4′-N,Ndimethylaminoflavones,38 some of the most important interactions among these molecules are the π−π ones. Taking into account the substantial bathochromic shift of the excimer 642


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

(2) and 434 nm (3) (Figure 1b,c), and fluoresces at 510 nm (2) and 501 nm (3), respectively (Figures 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). Bathochromic 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 for fluorescence dependence on the excitation wavelength. 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 the higher polarity and H-bonding ability of PVA than those of PMMA. On the basis of the separation of the absorption spectra using the excitation ones (Figure 1b and c), the estimated contributions of the Ns species in the ground state are 44 and 18% in the cases 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 shortliving 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 excitedstate relaxation is the reason for the mentioned above bathochromic shifts of the blue bands in the time-resolved 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 a plasmonic platform as discussed further. 644


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The Journal of Physical Chemistry C Table 2. Kinetic Parameters of the Excited-State Deactivation of 1, 2, and 3 in PVA Filmsa compd 1

conditions 1 × 10−4 M 1 × 10−2 M


2

1 × 10−4 M

1 × 10−2 M


3

1 × 10−4 M

1 × 10−2 M


form

λfl

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

τ 1.50 1.91 0.30 0.37 0.27 0.05 0.60 0.82 0.49 1.18 2.53 0.32 0.54 1.1 0.09 0.14 0.35 0.64 0.59 4.00 0.27 0.25 1.2 0.08 0.11 0.62

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

kf 0.05 0.02 0.01 0.09 0.01 0.01 0.01 0.04 0.02 0.08 0.04 0.01 0.09 0.2 0.01 0.01 0.03 0.01 0.03 0.05 0.01 0.04 0.1 0.01 0.01 0.03

0.17b[19]

0.14b 0.10d

kd 0.30 0.35 1.90 ∼1

0.15 0.20 0.16 1.94 0.54 0.29

krelax

1.29 1.46 2.9 10.5 0.14 3.0 0.09 0.5

± ± ± ± ± ± ± ±

0.04 (N* → Nr*) 0.04c (Nr* + N → 0.8c (D* → Nr* + 0.04 (N* → Nr*) 0.04c (Nr* + N → 0.4c (D* → Nr* + 0.03 (N* → Nr*) 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

kESIPT

0.20 ± 0.01 (N* → Nr*)

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*)

D*) N) D*) N) 1.66 ± 0.08 (N* → T*) 0.13 0.8 0.4 0.5 10 1.3 1.9 0.40

± ± ± ± ± ± ± ±

0.04 (T* → N*) 0.1 (N* → T*) 0.1 (Nr* → T*) 0.1 (T* → Nr*) 1 (N* → T*) 0.3 (Nr* → T*) 0.3 (T* → Nr*) 0.02 (N* → T*)

0.04 0.5 2.1 0.44 8.3 3.7 0.69

± ± ± ± ± ± ±

0.01 (T* → N*) 0.1 (N* → T*) 0.2 (Nr* → T*) 0.03 (T* → Nr*) 0.8 (N* → T*) 0.3 (Nr* → T*) 0.04 (T* → Nr*)

a 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 processes, in ns−1; krelax and kESIPT - rate constant of relaxation, formation of excimers, and ESIPT processes, respectively, in ns−1. bObtained for water:methanol solutions. cUncorrected values. dObtained from corresponding kf values of N* species using the computationally predicted kN*f/kT*f ratio.

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

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

fluorescence bands are centered near 505−510 nm (Table 1, Figure 3c−f). The N/Nr* band maxima in steady-state spectra thus suffer 1150 (2) and 1500 cm−1 (3) bathochromic shifts compared to those at lower concentration. The positions of the T* fluorescence maxima are practically independent of concentration for both compounds. The phenomenon is due 645


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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 a 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*/N r * bands suffer a 700 and 400 cm −1 bathochromic 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 bathochromically 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 nearly 15 times in the presence of a plasmonic platform and reach 10 ns−1 (Table 2). Rates of both forward and reverse ESIPT 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 the 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 transformations. 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

to the 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 timeresolved fluorescence spectra of 2, the N* and Nr* bands appear at 461 and 510 nm, respectively, similarly to those at lower concentration. In the case of 3, the N* and Nr* bands are centered at 478 and 518 nm, respectively, with 630 and 1150 cm−1 bathochromic shifts compared to those at lower concentration (Figure 4h). Transformation of N* to Nr* is well distinguished for both compounds (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, the 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 πelectronic density of 2 with increasing concentration. Since comparison of the absorption spectra of 2 at 1 × 10−4 and 1 × 10−2 M allows one to conclude that there are 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 the hydrogen atom of the hydroxyl group of N* participates in bifurcated hydrogen bonding, similar to what was found in the 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 and 1 × 10−2 M, whereas the fluorescence maximum of the N* species shifts bathochromically 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 Hbonded 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 Hbonds 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 5 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 bathochromically at 1 × 10−2 M in the case of 3, the differences in Nr* → T* ESIPT rates can be explained by a 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

Δ298G° = −RT ln(kN*→ T */k T*→ 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 the thermodynamics of ESIPT in 2 was found to be polaritydependent,18 these findings can serve as an additional proof of 646


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The Journal of Physical Chemistry C the assumed different structure of the Nr* + N excimers in 2 and 3. Importantly, plasmon resonance does not substantially affect the 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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AUTHOR INFORMATION

Corresponding Author

*Phone: +48 58 523 51 13. Phone/fax: +48 58 523 50 12. Email: [email protected].

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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 an 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 a plasmonic platform, the excited-state relaxation processes occur faster and, in the case of compounds 1 and 2, more effectively, based on the sufficient bathochromic shifts of the relaxed species emission. Formation of excimers in the excited state of 1 is less favorable in the presence of a plasmonic platform. Plasmonic resonance causes the 15-fold increase of the 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 one to conclude that plasmonic resonance has no substantial effect on the ESIPT thermodynamics under the conditions applied. Due to these reasons, the fluorescence intensity of the ESIPT product (T*) increases symbatically in the plasmon 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.

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predicted values of N and T dipole moments and Δ298G° for the N → T transformation in the S0 and S1 states (PDF)

ORCID

Illia E. Serdiuk: 0000-0002-4563-0773 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the grants 2014/13/N/ST4/ 04105 (I.E.S.) and 2015/17/B/ST5/03143 (P.B., A.S., B.G.) 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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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07386. 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 647


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Spectral Features and Excited-State Transformations of Hydroxy

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