White Light from a Single Fluorophore: A Strategy Utilizing Excited

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White Light from a Single Fluorophore: a Strategy Utilizing Excited State Intramolecular Proton Transfer Phenomenon and Its Verification Illia E. Serdiuk J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00137 • Publication Date (Web): 09 Feb 2017 Downloaded from http://pubs.acs.org on February 16, 2017

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The Journal of Physical Chemistry

White Light from a Single Fluorophore: a Strategy Utilizing Excited State 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

*Corresponding author Phone: +48 58 523 22 44, Phone/fax: +48 58 523 22 66, e-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT This article is devoted to the development of a strategy for 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, steadystate, time-resolved and temperature-dependent fluorescence spectroscopies. The conclusions on the tautomeric transformations in this compound and origin of its spectral features are supported by quantum-chemical calculations on DFT and TD DFT levels of theory. The results of investigations confirm the hypothesis that white light can be produced by a single fluorophore and evidence applicability of the proposed strategy.


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INTRODUCTION Organic compounds and materials emitting white light (WL) have been receiving a lot of attention recently as their application makes possible creation of simple and cheap white organic light emitting diodes or devices (WOLEDs) widely demanded for illumination, lightning and panel display fabrication. In the overwhelming majority of devices developed so far, 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, like 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 Proton Transfer (ESIPT) phenomenon.7–13 ESIPT is a kind of ketoenol or amino-imine tautomeric transformation of the initial electronically excited form (N*) to product (T*) occurring due to 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, cover the whole visible spectrum. The ESIPT thermodynamics and kinetics can be finely tuned by changing 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 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 demanded compounds becomes topical. It seems to be especially useful if one takes


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into account that the progress mentioned above in creation of the WL ESIPT fluorophores was achieved owing to laborious screening within a reasonable number of related compounds. Taking into account the responsivity functions of a human eye17 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/annulated aromatic rings. With the prospective of further application in WL-emitting 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 less as possible (the νSt value above 4000 сm–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 so far,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 mentioned above 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 electron-releasing effect (substituted amino groups10 and deprotonated hydroxyl groups20) and extended π-system12 enable close intensities of the N* and T*


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bands. As reasons favoring close intensities of the blue and yellow components seem to be poorly understood yet, what impeds target design of WL ESIPT fluorophores, this article focuses on investigations of these reasons on a particular example. To verify if the mentioned above criteria are actually applicable, they were tested on the 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) since 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 in the result of the intramolecular charge transfer (ICT), which enables tuning its fluorescence maxima via 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 On its turn, ESIPT occurring in flavones containing hydroxyl group at position 5 causes extremely low φ values of these compounds.23 Recent investigations of 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 of 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). In the result of such considerations, 2-

Chart 1. The compound designed as WL fluorophore (1) and its prototype (2) 3' 2'

[4-(dimethylamino)phenyl)-7-hydroxy-6-(3phenylpropanoyl)-4H-chromen-4-one (1, Chart

R'

7

8

O 4

O

6

R

5

4'

N(CH 3) 2

1

O

2

5' 6' 3

R = (CH2)2Ph R' = OH (1); OCH3 (2)

1) was designed and target-synthesized as a


5


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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 a substrate 3 (Scheme 1) with one equivalent of 4-(dimethylamino)benzaldehyde in the Claisen-Schmidt condensation.25 Spectral features of the 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.

EXPERIMENTAL AND THEORETICAL METHODS Reagents. All reagents, solvents of relevant grades and polymers were purchased from SigmaAldrich. Solvents for spectroscopic measurements were dried and distilled before use. Synthesis. Following the approach reported previously,25 1 was synthesized starting from 1-(5acetyl-2,4-dihydroxyphenyl)-3-phenylpropan-1-one

(3),

which

was

condensed

with

4-

(dimethylamino)benzaldehyde (Claisen-Schmidt condensation) followed by the intramolecular cyclisation 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 dimethylsulphate in the presence of K2CO3. The detailed synthetic procedures and results of analyses for 1 (Figures S1–S3) and 2 are described in Supporting Information. Scheme 1. Synthesis of 1 and 2 N(CH3) 2

N(CH3) 2 HO

OH

O

O (H3 C) 2 N

O R

CH3 3

R = (CH2)2Ph

H2O/NMP, KOH, RT, 36 h (39%)

HO

OH I2

O

O R

DMSO, 90' C, 2 h (43%)

HO O

(CH3)2SO4 CH3CN,

R 4

O

O

K2CO3, RT, 10 h (87%)

1

H3CO

O

O R

O 2

N(CH3) 2

Preparation of Polymeric Films. Polymeric films with ca. 5 μM of 1 were prepared by drying


6

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homogeneous solutions of corresponding polymers in dichloromethane or tetrahydrofuran. 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 nitrogen cryostat OptistatDN2 (Oxford Instruments) equipped with a Mercury iTC controller. The obtained spectra were corrected on the background absorption/emission and smoothed. Fluorescence spectra were corrected on the instrumental sensitivity. 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, SI) of individual fluorescence bands of 1 were determined by deconvolution of the steady-state fluorescence spectra into individual components using 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-C-375 and LDH-P-C-390 laser heads for picosecond pulses. Rate constants of the radiative (kf) and non-radiative deactivation (kd) were calculated using equations:

  kf 

1 k f  kd


(1), (2),

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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 DFT / TD DFT level of theory,29 with the B3LYP30 hybrid functional with the ccpVDZ basis set using the GAUSSIAN 09 program package.31 After completion of each optimization, the Hessian matrix was calculated to find out whether the obtained structures were stationary.29 Gibbs free energy contributions at 298.14 K and standard pressure were then calculated according to statistical thermodynamic routines.32 The unspecific solvent effect was included at the level of the Polarized Continuum Model (PCM).33

RESULTS AND DISCUSSION Absorption and Fluorescence Spectroscopy of Solutions of 2. Since one of the aims of current work was to develop and verify a methodology for design of a WL ESIPT fluorophore, firstly, spectral properties of 2 were investigated in various solvents to check if the prototype fluorophore was selected properly. Maximum of the long-wavelength absorption band of 2 falls in the 355–390 nm range (Figure 1c, Table S2, SI) and shows positive solvatochromy typical for 4'dimethylaminoflavone derivatives.34 In terms of Catalán and Kamlet-Taft solvent parameters, the absorption maximum position is influenced to a large extent by solvent polarity/polarizability, whereas solvent basicity has minor or negligible effect (Table 3S, SI). 2 exhibits single-band fluorescence in all the solvents investigated (Figure 1d). Its fluorescence maximum falls in the 400–500 nm range (Table S2, SI) showing positive solvatofluorochromism (Table S3, SI). On the basis of the comparison of the slope values of solvatochromic

dependences,

sensitivity

of

the

fluorescence


maxima

to

the

solvent

8

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polarity/polarizability is about twice larger than that of the absorption maxima, whereas solvent basicity has negligible effect. At 298 K, the maximal fluorescence quantum yield (φ) value up to 80% is observed in chloroform and chlorobenzene (Table 1), in which fluorescence band is centered at 446 and 438 nm, respectively. These solvents represent media with a dielectric constant (ε) range of 4.7–5.8. In more polar (acetonitrile) and especially less polar (methylcyclohexane) solvents, the φ value decreases (Table 1).

a

b

c

d

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).


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Table 1. Fluorescence quantum yields (in %) of 1 and 2 and Gibbs free energy changes (in kJ/mol) for ESIPT (N*→T*) in 1 in various solvents at 298 K 1 Solvent

ε

φ (φN*; φT*)

Methylcyclohexane

2.0

1.1 (0; 1.1)

Benzene

2.3

13 (9; 4)

Chloroform

4.7

Chlorobenzene Dichloromethane

2 Δ298G°N*→T*

exp

a

b

theor

φN*

8.3

1.3

1.0

7.9

68

23 (15; 8)

0.1

6.7

80

5.8

20 (12; 8)

–0.1

6.6

77

9.1

3 (1; 2)

–2.4

6.9

70

Acetonitrile

White Light from a Single Fluorophore: A Strategy Utilizing Excited

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