Design and Emissive Features of Ionic White-Light Fluorophore

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C 2018, 122, 18615−18620

Design and Emissive Features of Ionic White-Light Fluorophore Illia E. Serdiuk*,†,‡,§ †

Faculty of Mathematics, Physics and Informatics, University of Gdańsk, Wita Stwosza 57, 80-308 Gdańsk, Poland Center for Supramolecular Optoelectronic Materials, Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea § Faculty of Chemistry, University of Gdańsk, Wita Stwosza 63, 80-308 Gdańsk, Poland ‡

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S Supporting Information *

ABSTRACT: First ionic white-light emitting fluorophore was designed and target-synthesized and its spectral features were investigated. The design stage included selection of 4′-methoxyflavylium as a prototype blue emitter followed by analysis of thermodynamics of the excited-state intramolecular proton transfer in various 4′-methoxyflavylium derivatives using time-dependent density functional theory calculations. On the basis of such a screening, the best candidate was selected, synthesized, and investigated in various solvents and polymeric films. Pure white light (WL) was attained both in solutions and solid films with fluorescence quantum yields near 20%. This research evidences that WL can be generated with reasonable quantum efficiency by an ionic emitter. The described compound or its derivatives may find applications in white light-emitting electrochemical cells.

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INTRODUCTION The heavy-metal-free organic molecular systems emitting white light (WL) have received much attention recently because of their promising features for applications in various organic light-emitting diodes and devices1−4 and light-emitting electrochemical cells (LECs).5−7 Classical approaches which enable WL emission generally utilize three or two luminescent components; thus, WL is attained as a mixture of red, green, and blue or orange and blue emissions, respectively. Conceptually, the approach which utilizes one organic compound with a multiband WL emission seems to be the easiest from the technological point of view and the most reliable.8 Such single-emitter devices are characterized by higher emission color stability, eliminated problem of layer separation, simpler fabrication process, and therefore reduced price as compared to the multicomponent ones. Apparently, the design of molecules which emit WL is a challenging and laborious task. In general, multiband emission of single-chromophore organic emitters can be assessed by various photochemical and/or photophysical phenomena such as excited-state intramolecular proton transfer (ESIPT),9−11 intramolecular12 and intermolecular13 charge transfer (ICT), intersystem crossing (ISC) enabling fluorescence, and roomtemperature phosphorescence14,15 or dual phosphorescence.16 For WL generation, the spectral parameters of forms or states participating in the above mentioned processes should be adjusted to cover the whole visual spectrum. Previously, author of this article suggested a strategy17 for design of WL fluorophores using the ESIPT phenomenon which consists in:

2 modification of its structure to enable ESIPT leading to yellow “tautomer” emission; 3 check of the ESIPT reversibility by means of the quantum-chemical calculations of the Gibbs free energies of the tautomeric forms participating in ESIPT. One of the most promising and unique features of organic optoelectronics is the possibility of fabrication of variously shaped devices by low-cost and low-consuming solutionprocessed methods. In combination with the single-chromophore white emitters, novel perspectives can be opened, for example, in the field of indoor lightning. From this point of view, polyaromatic emitters bearing ionic fragments offer very good solubility in common solvents, which enables their application by means of solution-processing methods. Hypothetically, the ionic structure of the emissive layer can also reduce the on-voltage of optoelectronic devices, which is a desired feature. To date, however, the reports on the charged fluorophores which undergo the above-mentioned photochemical or photophysical processes (ESIPT, ICT, ISC, and AIE) affording multiband emission are very scarce, thus possibility of WL generation by such systems is rather questionable. Few reports on ESIPT dual emission of charged fluorophores concerned anionic species of 2-(2′acetamidophenyl)benzimidazole,18 5-aminosalicylic acid,19 and 3,7-dihydroxyflavone20 in basic aqueous solutions. On the example of flavonol derivatives, it was proved that ionic groups not conjugated with the chromophore fragment can Received: June 15, 2018 Revised: July 19, 2018 Published: July 20, 2018

1 selection of a highly emissive blue emitter; © 2018 American Chemical Society

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DOI: 10.1021/acs.jpcc.8b05727 J. Phys. Chem. C 2018, 122, 18615−18620

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The Journal of Physical Chemistry C

Figure 1. (A). Chemical structures of 4′-methoxyflavylium (MF), its derivatives 1−6 with ESIPT sites, and the target-designed compound MSF in ° values of 1-6 with the S1−S0 transition their cationic C forms. (B). Calculated relative S1-state energies of C* and CT* forms, Δ298GC*→CT* energies (TDDFT, B3LYP, cc-pVDZ).

fluorescence lifetime spectrometer equipped with a LDH-P-C375 laser head for picosecond pulses and analysis was obtained with FluoFit Pro version software. No change of optical properties of the investigated samples was observed during measurements. Quantum-Chemical Calculations. Unconstrained geometry optimizations of tautomeric forms of 1−6 in the ground (S0) and excited singlet (S1) electronic states were performed at density functional theory (DFT)/time-dependent DFT (TDDFT) level of theory,29 with the B3LYP30 hybrid functional with the cc-pVDZ basis set using the Gaussian 09 program package.31 For calculations of MSF, the 6-311+ +g(d,p) basis set was used. After completion of each optimization, the Hessian matrix was calculated to find out whether the obtained structures were stationary. The energies and oscillator strengths of the S1 → S0 transitions were obtained by single-point calculations using the optimized S1 state geometries. To calculate Gibbs free energy change during the ESIPT transformation in MSF, geometry optimizations of C* form with different constrained O−H distances were performed, followed by computation of vibrational frequencies.

noticeably modify spectral features and ESIPT thermodynamics because of the electrochromic effect. 21 Dual fluorescence of sodium 4-(N,N-dimethylamino)naphthalene1-sulfonate originating from the local-excited and chargetransfer states was observed in solutions with a wide range of pH.22 Weak multiband emission of protonated Schiff base derivatives containing triphenylamine and quinoline fragments was observed in solutions and was attributed to emission from ICT or twisted ICT (TICT) states.23 A rare case of dual fluorescence originating from S2 and S1 states of carbazoleimidazole ionic liquids was also reported.24 Unfortunately, most of these reports concern multiband emission with low quantum yield occurring only in liquid solutions, which casts doubt on the possibility of their use as light-emitting materials. For these reasons, the research described here was aimed to verify if generation of WL with an ionic emitter with a reasonable fluorescence quantum yield is possible in principle. This article also presents a development of the abovementioned strategy utilizing ESIPT phenomenon toward design of such fluorophores.

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EXPERIMENTAL AND THEORETICAL METHODS Reagents and Compounds. All reagents, solvents of relevant grades, and polymers were purchased from SigmaAldrich. Detailed synthetic procedures and results of analysis are presented in the Supporting Information. Spectroscopic Measurements. Absorption and fluorescence spectra were recorded on a PerkinElmer Lambda UV/vis 40 spectrophotometer and a HORIBA-Jobin Yvon FluoroMax4 spectrofluorimeter, respectively. The obtained spectra were corrected on the background absorption/emission and smoothed. Fluorescence spectra were corrected on the instrumental sensitivity. Relative values of fluorescence quantum yields (φ) were obtained using quinine bisulfate in 0.1 N H2SO4 as a reference (φ = 0.52)25 and corrected for refractive indices of solvents. Absolute φ values were measured using PTFE-coated integrating sphere following the procedure described in ref 26. Fluorescence quantum yield ratio of cationic and cation-tautomeric forms (φC*/φCT*) were determined by deconvolution of the steady-state fluorescence spectra into individual components using Siano−Metzler function27 implemented into the Spectral Data Lab software.28 Fluorescence lifetimes were measured on a FluoTime 300

RESULTS AND DISCUSSION

Following the first item of the design strategy mentioned above, 4′-methoxyflavylium (MF, Figure 1A) was selected as an efficient blue cationic emitter. This dye and its derivatives fluoresce near 465 nm with a quantum yield above 0.75,32,33 whereas hydroxyl groups in such compounds gain increase of acidity in the S1 state34 being a driving force of ESIPT. On the next stage of design, the proton-transfer (PT) site should be introduced to afford “yellow” emission band. The PT site comprised of a hydroxyl group as proton donor and formyl group as proton acceptor was selected. Apparently, there are various possibilities of introduction of such a fragment into the MF structure. To evaluate at which position the PT site activates ESIPT, Gibbs free energies of cationic (C*) and cation-tautomeric forms (CT*) forms were calculated for the S1 state of compounds 1−6 (Figure 1A) using TDDFT method with a B3LYP functional and a cc-pVDZ basis set. Calculations on such a level of theory proved to be relatively low time-consuming and give results in good correlation with the experiment, also for organic cations.20 According to thus obtained data, the Δ298GC*→CT* ° values of compounds 1, 2, 4, 18616

DOI: 10.1021/acs.jpcc.8b05727 J. Phys. Chem. C 2018, 122, 18615−18620

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The Journal of Physical Chemistry C Scheme 1. Synthesis of MSF and Its Neutral (N) and Cationic (C) Forms

Figure 2. Absorption and fluorescence excitation spectra of MSF in a 50% sulfuric acid solution (A). Steady-state fluorescence spectra of MSF in the presence of acids in various solvents (B). Absorption spectra of PMMA films containing 0.3 mM of MSF and various amounts of 4dodecylbenzenesulfonic acid (C). Steady-state fluorescence spectra of MSF in polymeric films containing 4-dodecylbenzenesulfonic acid (D). Fragments of CIE 1931 chromaticity diagram with coordinates of MSF fluorescence in solutions (E) and polymeric films (F).

likely. One can thus conclude that compounds 1, 2, and 4−6 are not suitable for WL generation. Exceptionally, ESIPT is thermodynamically favored in 3 and its Δ298G°C*→CT* value is close to zero (−1.6 kJ/mol, Figure 1B). This indicates that in the excited state of 3, C* and CT* forms should be in equilibrium and emit light simultaneously affording dual emission with similar band intensities. The calculated S1−S0 transition energy values for C* and CT* forms are 446 and 590 nm, respectively; therefore, 3 is a promising candidate for a cationic WL fluorophore. For further investigations, namely, on the stage of preparation, formyl group in 3 was changed to the 3phenylpropanoyl one (MSF, Figure 1A) because of the similar electronic and proton-accepting effects of both these groups but higher chemical stability of the latter. The 3-phenyl-

and 5 are positive (Figure 1B), which indicates that ESIPT is thermodynamically unfavored. Nevertheless, equilibrium between C* and CT* of 1, 2, and 5 is possible with major molar contribution of C*. In such a case, the fluorescence of C* form dominates in spectra and these compounds are expected to fluoresce in the blue (2) or green (1, 4, and 5) region of spectrum. In the case of compound 5, alternative H-bond formation can occur between formyl group and hydroxyl group of the pyrylium ring (Scheme S1). Such an isomer is predicted to undergo irreversible ESIPT (Δ298G° = −33 kJ/mol) resulting in tautomeric species with fluorescence maximum at 969 nm. Compound 6 contains two active PT sites and occurrence of double ESIPT should lead to a cationic tautomer (CDT*, see Scheme S2) with low S1−S0 transition energy (942 nm). In such cases, the nonradiative deactivation is most 18617

DOI: 10.1021/acs.jpcc.8b05727 J. Phys. Chem. C 2018, 122, 18615−18620

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The Journal of Physical Chemistry C Table 1. Steady-State Spectral Features of MSF in Liquid Solutionsa C form

CT form

medium

λabs (nm)

λfl (nm)

υSt (cm−1)

λfl (nm)

υSt (cm−1)

φ (%)

φC*/φCT*

1% TFA in toluene 1% TFA in DCM 2% TFA in ACN 50% H2SO4 in H2O/MeOH

403 408 405 406

467 460 470 468

3400 2780 3420 3250

570 563 567 571

7270 6760 7060 7110

24 22 15 3.5

1.3 1.0 1.2 1.1

chromaticity 0.32, 0.30, 0.29, 0.34,

0.35 0.32 0.33 0.36

λabs, λflthe absorption and emission maxima; υStStokes shift; φtotal fluorescence quantum yield relative to quinine sulfate; φC*/φCT* fluorescence quantum yield ratio of cationic and cation-tautomeric forms.

a

Table 2. Steady-State Spectral Parameters of MSF in the Polymeric Films Doped with 4-Dodecylbenzenesulfonic Acida C form

CT form −1

polymer

wacid (%)

λabs (nm)

λfl (nm)

υSt (cm )

λfl (nm)

υSt (cm−1)

PS PS PMMA PMMA PMMA PVA PVA

0.5 5.5 3.0 9.0 13 10 15

404 414 403 405 407 410 410

462 468 453 457 462 453 453

3110 2790 2740 2810 2930 2320 2320

553 549 552 552 558 545 545

6670 5940 6700 6580 6650 6040 6040

chromaticity 0.28, 0.25, 0.29, 0.29, 0.33, 0.20, 0.20,

0.33 0.30 0.34 0.34 0.38 0.18 0.18

φ (%) 14 20 b

20 13 10 13

λabs, λflthe absorption and emission maxima; υStStokes shift; φabsolute fluorescence quantum yield. bNot determined.

a

Figure 3. (A) ESIPT diagram and frontier orbitals of C and CT forms of MSF. (B) Fluorescence decays in a PMMA film with 9% of 4dodecylbenzenesulfonic acid at various observation wavelengths. (C) Relative change of Gibbs free energy during the ESIPT transformation of C* to CT* (TDDFT/B3LYP/6-311++g(d,p)).

intensive long-wavelength band centered at 406 nm (Figure 2A). In aprotic solvents, protonation of MSF can be accomplished in the presence of 2−10% of trifluoroacetic acid (TFA). The C band maximum is negligibly affected by the change of medium: in the acidified toluene, dichloromethane (DCM), and acetonitrile (ACN), the λabs value is in the 403− 408 nm range (Table 1). In the polymeric films doped with 4dodecylbenzenesulfonic acid, C species of MSF absorbs in the similar 403−414 nm range, depending on the nature of polymer and the amount of acid (Figure 2C and Table 2). Under all the conditions investigated, MSF exhibits dual fluorescence (Figure 2B,D). In solutions, the blue band is centered in the 460−470 nm range with the Stokes shift value range of 2800−3400 cm−1, whereas the second band is centered at 560−570 nm with the Stokes shift value near 7000 cm−1 (Table 1). Fluorescence intensities of the bands are similar and the chromaticity of the emitted light is close to the pure WL (Figure 2E). Fluorescence quantum yield is near 20%

propanoyl fragment was unreactive in the reactions utilized for the preparation of MSF: Claisen−Schmidt condensation of the corresponding acetophenone derivative with 4-methoxybenzaldehyde followed by the oxidative intramolecular cyclization of thus obtained chalcone derivative (Scheme 1). The details of synthetic procedures and results of analyses can be found in the Supporting Information. Further, to check the validity of the applied design strategy, spectral features of the cationic species of MSF were investigated in various liquid and solid media. In neutral media, as most of flavones35 MSF exists in the neutral form N (Scheme 1), which absorbs light at ca. 340 nm. In water solutions, transformation of MSF to its cationic form requires the presence of large amounts of acid: for example, in water/ methanol (1/4, v/v) solutions, the N ↔ C equilibrium is completely shifted toward the cationic species when mass fraction of H2SO4 reaches 50%. Under these conditions, the absorption spectrum of C species is characterized by an 18618

DOI: 10.1021/acs.jpcc.8b05727 J. Phys. Chem. C 2018, 122, 18615−18620

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The Journal of Physical Chemistry C Δ298GC°*→ CT * = −RT ln K

in aprotic solvents with a maximal value of 24% recoded for the 1% TFA/toluene solution (Table 1). The data presented above evidence that MSF can successfully serve as a cationic WL emitter in both liquid and solid phases. Further investigations were conducted to the reveal origin of its fluorescent features. The fluorescence excitation spectra of both emission bands match each other and the absorption spectrum of C species (Figure 2A). Therefore, both excited species responsible for dual fluorescence originate from the C one in the ground state. Most likely, the first band corresponds to the excited cationic species C* and the second one corresponds to the emission of the excited cation tautomeric species CT* formed in the result of ESIPT (Figure 3A). Time-resolved measurements reveal that both C* and CT* fluorescence bands decay simultaneously and shapes of the fluorescence decay curves measured at various wavelengths are practically identical (Figures 3B, S1 and S2). In PMMA films, mean values of lifetimes obtained by single-exponential deconvolution of the fluorescence decay curves measured at various wavelengths increase with the growing mass fraction of acid content: 1.9 ± 0.1 ns (3.7%), 2.0 ± 0.1 (9.0%) and 2.76 ± 0.04 ns (13%). Such a change can be due to the changing polarity and other properties of the medium with increase of acid concentration. The fluorescence decay profile measured at 580 nm (CT* band) do not contain rise component in the subnanosecond time domain, which evidences that C* and CT* species are in a very fast excited-state equilibrium. This also explains why individual decays of C* and CT* can not be distinguished in this time domain. To estimate activation energy barrier for the ESIPT transformation, geometry optimizations of C* form with different constrained O−H distances were performed, followed by calculations of Gibbs free energies of obtained structures on the TDDFT/B3LYP/6-311++g(d,p) level of theory. The obtained results show that energy changes gradually during proton transfer (Figure 3C). Calculations thus predict no activation barrier for the ESIPT transformation of C* to CT* and support the latter assumption on its ultrafast rate and equilibrity. Because in the case of MSF, ESIPT is very fast and the equilibrium between C* and CT* species is attained prior to their radiative deactivation, thermodynamics of this transformation can be analyzed using partial fluorescence quantum yields from the steady-state measurements (values φC* and φCT*, Table 1). The chemical equilibrium constant is given by equation36 K = k fC */k fCT *(φCT */φC *)

change negligibly from −2.4 to −3.0 kJ/mol, respectively. The respective value obtained as a difference between TDDFTpredicted Gibbs free energies of CT* and C* forms is 3.5 kJ/ mol (Figure 3C), which correlates well with the experimental one.

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CONCLUSION In summary, screening of the ESIPT thermodynamics and spectral features of various derivatives of 4′-methoxyflavylium cation by means of quantum-chemical calculations enabled selection of the prototype of cationic white-light fluorophore. Designed in such a way fluorophore MSF was target− synthesized and investigated in various acidified solutions and polymeric films. In its cationic form, MSF exhibits WL because of fast and reversible ESIPT. Depending on the medium and concentration of acid, chromaticity of MSF changes from (0.25; 0.30) to (0.34; 0.36), which makes it a promising emitter for application, for example, in white LECs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b05727.

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Synthetic procedures and analyses and time-resolved emission spectra of MSF in polymeric films (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +48 58 523 22 44. Fax: +48 58 523 22 66. ORCID

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

The author declares no competing financial interest.

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ACKNOWLEDGMENTS The financial support by the Polish Ministry of Science and Higher Education within the Mobility Plus project Nr 1637/ MOB/V/2017/0 is gratefully acknowledged. Quantum chemical calculations were performed on the computers of the Wroclaw Centre for Networking and Supercomputing (WCSS), Poland.

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