Article pubs.acs.org/JPCC
White Light from a Single Fluorophore: A Strategy Utilizing ExcitedState Intramolecular Proton-Transfer Phenomenon and Its Verification Illia E. Serdiuk*,†,‡ †
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
‡
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S Supporting Information *
ABSTRACT: This article is devoted to the development of a strategy for the design of a single fluorophore emitting white light due to the excited-state intermolecular proton-transfer (ESIPT) phenomenon. The key parameters of this strategy include (i) selection of an effective blue emitter, (ii) its structural modification to enable ESIPT, and (iii) adjustment of ESIPT parameters to assess similar fluorescence intensities of both emission bands. The important factor which determines similar intensity of these bands was found to be energetic closeness of the species participating in ESIPT (Δ298G°ESIPT ≈ 0), which makes this phototransformation reversible. Verification of the proposed strategy was carried out by design and synthesis of a new ESIPT fluorophore, which exhibits white light under certain conditions. Spectral features of this compound were investigated in various liquid solutions and solid polymeric films by means of steady-state electronic absorption and steady-state, time-resolved, and temperature-dependent fluorescence spectroscopies. The conclusions on the tautomeric transformations in this compound and the origin of its spectral features are supported by quantum-chemical calculations on the density functional theory (DFT) and time-dependent DFT levels of theory. The results of investigations confirm the hypothesis that white light can be produced by a single fluorophore and evidence the applicability of the proposed strategy.
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INTRODUCTION Organic compounds and materials emitting white light (WL) have been receiving a great deal of attention recently as their application makes possible creation of simple and cheap white organic light-emitting diodes or devices (WOLEDs) which are in wide demand for illumination, lighting, and panel display fabrication. In the overwhelming majority of devices developed to date, WL is generated by combination of three or two individual luminescent components emitting red, green, and blue or orange and blue light,1 respectively.2,3 In contrast to such polycomponent devices, WOLEDs based on a single fluorophore represent technologically simpler, more stable, and profitable alternatives. The ability of a fluorophore to emit white light is, however, a very rare and poorly investigated phenomenon, because most of compounds exhibit fluorescence bands too narrow to cover the whole visual spectrum. Among a few strategies applied for the development of WL-emitting compounds, such as aggregation in the ground4,5 or excited6 states and combination of two separated chromophores in one molecule,2 substantial progress has been achieved recently in the approach utilizing the excited-state intramolecular protontransfer (ESIPT) phenomenon.7−13 ESIPT is a kind of keto− © 2017 American Chemical Society
enol or amino−imine tautomeric transformation of the initial electronically excited form (N*) to product (T*) occurring because of a specific redistribution of electronic density under excitation.14 Under certain conditions, fluorescence of the N* (blue component) and T* (yellow component) forms can appear simultaneously, and in some cases, the fluorescence covers the whole visible spectrum. The ESIPT thermodynamics and kinetics can be finely tuned by changing the strength of the acid−base centers (proton donor and acceptor) through modifications in the fluorophore structure or medium,15,16 which enables design of various multiband fluorophores including the WL-emitting ones. In the view of the rapid growth of the number of developed ESIPT fluorophores exhibiting white emission, rationalization of the gained knowledge in this field with the aim of further targeted design and synthesis of the desired compounds becomes topical. It seems to be especially useful if one takes into account that the progress mentioned above in creation of the WL ESIPT Received: January 5, 2017 Revised: February 7, 2017 Published: February 9, 2017 5277
DOI: 10.1021/acs.jpcc.7b00137 J. Phys. Chem. C 2017, 121, 5277−5286
Article
The Journal of Physical Chemistry C fluorophores was achieved by laborious screening within a reasonable number of related compounds. Taking into account the responsivity functions of the human eye,17 determining the reception of white light for humans and previous successful attempts in creation of the WL ESIPT fluorophores, the following criteria for design of effective compounds of such a kind can be drawn. (i) Fluorescence of the blue component, represented by the N* form of an ESIPT fluorophore, should be centered in the 430−460 nm range which determines size of the chromophore π-system of three or more conjugated or annulated aromatic rings. With the prospective of further application in WLemitting devices, the compound should be preferably colorless, i.e., does not absorb light in the range above 400 nm. In the view of the self-reabsorption effect which causes considerable nonradiative deactivation of the excited state of fluorophores with small Stokes shifts (νSt),18 absorption and fluorescence bands should overlap as little as possible (the νSt value above 4000 cm−1). On the basis of these parameters, a prototype molecular system, not undergoing ESIPT, can be selected among blue fluorophores with high quantum yield of fluorescence. (ii) Fluorescence of the yellow component (T* form) should be centered in the 550−580 nm range. Generally, ESIPT causes Stokes shift values of 8000−11000 cm−1; thus, ESIPT fluorophores which absorb in the 340−400 nm should meet this criterion. (iii) Taking into account driving forces of ESIPT evaluated to date,19 the proton-transfer site should be introduced into such a position so that proton donor and acceptor receive increased acidity and basicity, respectively, in the excited state. On the other hand, relative intensity of the blue and yellow components should be close to 1. The above-mentioned relative intensity seems to be the most difficult parameter to design, as it depends on various factors, namely, ESIPT thermodynamics, kinetics, and medium effects. It can be noticed, that substituents with strong electronreleasing effect (substituted amino groups10 and deprotonated hydroxyl groups20) and extended π-system12 enable close intensities of the N* and T* bands. Because reasons favoring close intensities of the blue and yellow components seem to be poorly understood, which impedes target design of WL ESIPT fluorophores, this article focuses on investigations of these reasons in a particular example. To verify if the above-mentioned criteria are actually applicable, they were tested on a particular compound, and the WL ESIPT fluorophore was designed. 4′-Dimethylaminoflavone (2-[4-(dimethylamino)phenyl]-4H-chromen-4-one, NF) was selected as a prototype (blue fluorophore) because it meets well the first criterion: qualitative fluorescence quantum yield, fluorescence maxima at 445 nm, and high νSt value (ca. 5000 cm−1).21 The lowest excited state of NF is formed as the result of the intramolecular charge transfer (ICT), which enables tuning its fluorescence maxima via a change of the medium polarity. From the point of view of possible position of the proton-transfer site, flavones have three positions at which hydroxyl groups exhibit increased acidity in the excited state: 3, 5, and 7 (Chart 1). Introduction of the hydroxyl group at position 3 of NF proved to be an effective approach for creation of ESIPT fluorophores, but it also results in bathochromic shift of the absorption and blue-component fluorescence as compared to NF.22 In turn, ESIPT occurring in flavones containing a hydroxyl group at position 5 causes
Chart 1. Compound Designed as WL Fluorophore (1) and Its Prototype (2)
extremely low φ values of these compounds.23 Recent investigations by our group showed that introduction of a hydroxyl group at position 7 of NF does not influence considerably its spectral features,24 and introduction of a carbonyl group to position 6 of 7-hydroxyflavones affords the ESIPT-induced fluorescence centered in the 550−560 nm range25 (the second criterion). Moreover, in some ESIPT systems based on 4′-dimethylaminoflavone in specified conditions, fluorescence intensities of N* and T* bands are comparable26 (the third criterion). As a result of such considerations, 2-[4-(dimethylamino)phenyl)-7-hydroxy-6-(3-phenylpropanoyl)-4H-chromen-4-one (1, Chart 1) was designed and target-synthesized as a WL ESIPT fluorophore. The 3-phenylpropanoyl fragment was utilized in the role of both proton acceptor for ESIPT and a bulky substituent to afford selective reaction of substrate 3 (Scheme 1) with one equivalent of 4-(dimethylamino)benzaldehyde in the Claisen−Schmidt condensation.25 Spectral features of compound 1 were investigated by electronic spectroscopy methods in various solvents and polymers and analyzed to gain more knowledge on the reasons affording white fluorescence. Conclusions on the origin of spectral behavior of 1 were drawn with the help of quantum calculations and comparison of spectral features with a related compound (2, Chart 1), containing the methyl-blocked hydroxyl group.
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EXPERIMENTAL AND THEORETICAL METHODS Reagents. All reagents, solvents of relevant grades, and polymers were purchased from Sigma-Aldrich. Solvents for spectroscopic measurements were dried and distilled before use. Synthesis. Following the approach reported previously,25 1 was synthesized starting from 1-(5-acetyl-2,4-dihydroxyphenyl)-3-phenylpropan-1-one (3), which was condensed with 4-(dimethylamino)benzaldehyde (Claisen−Schmidt condensation) followed by the intramolecular cyclization of the obtained chalcone derivative (4) in the presence of I2 in hot DMSO (Scheme 1). 2 was prepared from 1 in the reaction with dimethylsulfate in the presence of K2CO3. The detailed synthetic procedures and results of analyses for 1 (Figures S1−S3) and 2 are described in the Supporting Information. Preparation of Polymeric Films. Polymeric films with ca. 5 μM of 1 were prepared by drying homogeneous solutions of corresponding polymers in dichloromethane or tetrahydrofuran. The optical density of the obtained films did not exceed 0.1. Spectroscopic Measurements. Absorption and fluorescence spectra were recorded on a PerkinElmer Lambda UV/vis 40 spectrophotometer and a Horiba-Jobin Yvon FluoroMax-4 spectrofluorimeter, respectively. Fluorescence spectra at temperatures below 298 K were measured using a nitrogen cryostat OptistatDN2 (Oxford Instruments) equipped with a Mercury iTC controller. The obtained spectra were corrected by the background absorption and emission and smoothed. Fluorescence spectra were corrected by the instrumental sensitivity. 5278
DOI: 10.1021/acs.jpcc.7b00137 J. Phys. Chem. C 2017, 121, 5277−5286
Article
The Journal of Physical Chemistry C Scheme 1. Synthesis of 1 and 2
Figure 1. Normalized steady-state absorption and fluorescence spectra of 1 (a and b) and 2 (c and d) in methylcyclohexane (1, red), benzene (2, green), chloroform (3, blue), chlorobenzene (4, cyan), dichloromethane (5, magenta), and acetonitrile (6, yellow).
In all fluorimetric measurements, the optical densities of solutions were ≤0.1. For determination of fluorescence quantum yields (φ), spectra were measured in the 360−1000 nm range and the excitation signal was removed numerically. The φ values were obtained relative to quinine bisulfate in 0.1 N H2SO4 and corrected for refractive indices of solvents. Surfaces and fluorescence maxima positions (Table S1) of individual fluorescence bands of 1 were determined by deconvolution of the steady-state fluorescence spectra into individual components using the Siano−Metzler function27 implemented into the Spectral Data Lab software28 kindly provided by Prof. A. O. Doroshenko. Partial φ values of the N* and T* forms were thus available from the bands surface ratio. Fluorescence lifetimes were measured on a FluoTime 300 fluorescence lifetime spectrometer equipped with a LDH-P-C375 and LDH-P-C-390 laser heads for picosecond pulses. Rate
constants of the radiative (kf) and nonradiative deactivation (kd) were calculated using equations
φ = τ ·k f τ=
1 k f + kd
(1)
(2)
where φ and τ are fluorescence quantum yield and lifetime, respectively. The fluorescence decays of individual fluorescence bands and rates of solvate relaxation in polymeric films were obtained by deconvolution of whole fluorescence decay surface, described in detail previously.22 Quantum-Chemical Calculations. Unconstrained geometry optimizations of tautomeric and conformeric isomers of 1 and 2 in the ground (S0) and excited singlet (S1) electronic states were performed at the density functional theory/timedependent density functional theory (DFT/TD DFT) level of 5279
DOI: 10.1021/acs.jpcc.7b00137 J. Phys. Chem. C 2017, 121, 5277−5286
Article
The Journal of Physical Chemistry C
yield in this medium. The highest kf value (0.52 ns−1) is observed in benzene, and it gradually decreases with rise of the medium polarity (Table 2). Nonradiative processes prevail in media of low and high polarity. In polar dichloromethane and acetonitrile, the second decay component appears with a short lifetime value (
