Impact of the KetoâEnol Tautomeric Equilibrium on the BODIPY

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Impact of the Keto-Enol Tautomeric Equilibrium on the BODIPY Chromophore Volker Leen, Marina Laine, Joseph Molisho Ngongo, Pawel Lipkowski, Bram Verbelen, Andrzej Kochel, Wim Dehaen, Mark Van der Auweraer, Victor A. Nadtochenko, and Aleksander Filarowski J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b03489 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018

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The Journal of Physical Chemistry

Impact of the Keto-Enol Tautomeric Equilibrium on the BODIPY Сhromophore Volker Leen,a Marina Laine,b Joseph Molisho Ngongo,c Paweł Lipkowski,d Bram Verbelen,a Andrzej Kochel,b Wim Dehaen,a Mark Van der Auweraer,a Viktor Nadtochenkoe and Aleksander Filarowski*b, f a

Department of Chemistry, KU Leuven, Celestijnenlaan 200f – bus 02404, 3001 Leuven,

Belgium b

Faculty of Chemistry, University of Wroclaw, F. Joliot-Curie 14, 50-383, Wrocław, Poland.

c

Faculty of Science, Université de Kinshasa, B.P. 190, Kinshasa XI, Democratic Republic of the

Congo d

Department of Physical and Quantum Chemistry, Wrocław University of Technology, Wyb.

Wyspianskiego 27, 50-370 Wrocław, Poland e

N.N. Semenov Institute of Chemical Physics, RAS, Kosigin str. 4, 119991, Moscow, Russian

Federation f

Department of Physics, Industrial University of Tyumen, 625-000, Tyumen, Russian Federation

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: An intramolecular tautomeric fluorescent BODIPY sensor has been designed and synthesized. The obtained BODIPY dye is a combination of the 4-bora-3a,4a-diaza -s-indacene core and a diketone fragment. The study of conformational equilibria in the ground and excited states has been completed for a broad range of solvent polarity by steady state and NMR methods as well as by DFT and TD-DFT calculations. The interpretation of the unique emission observed in hydrogen bond accepting solvents upon the excitation of the fluorescent dye in the S0-S2 transition has been accomplished. The Jablonski diagram has been analyzed for the observed processes in the BODIPY dye studied on the basis of DFT and TD-DFT calculations.

1. INTRODUCTION In this study we aim to report on the influence of a tautomeric sensor on the spectral characteristics of the difluoroborondipyrromethene (BODIPY) core in the ground and excited states based on the experimental results as well as quantum-mechanical calculations. The synthesis and spectroscopic characterization of fluorescent BODIPY dyes have been investigated extensively.1-3 These compounds are used for a large number of applications such as chromogenic probes and ions,4 fluorescent switchers,5 laser dyes,6 pH indicators,7 and solar cells.8 They are also employed in biochemical and biological applications.9,10 Several of these applications are based on the possibility to use fluorescence spectroscopy to measure intracellular concentrations of biologically important ions and molecules. Currently, the design, synthesis, and spectroscopic characterization of novel fluorescent probes continue to be important topics in this dynamic research area.11 In spite of extensive research on these dyes, only limited information is available on the impact of a tautomeric equilibrium and of hydrogen bonding on the charge transfer and spectral characteristics.12-15 Recently Akkaya et al.16,17 have shown the influence of protonation and


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deprotonation processes on the spectral characteristics of BODIPY dyes. Literature13,

15

investigated the influence of a tautomeric equilibrium in ortho-hydroxy aryl Schiff bases substituted at the meso- or 7- position of the BODIPY core on the position of the absorption and emission bands as well as its influence on the fluorescent quantum yield. The later report15 shows that replacement of the salicylideneaniline substituent with a naphthylideneaniline in the mesoposition of the BODIPY dye brings about a visible decrease of the fluorescent quantum yield of the BODIPY-meso-Schiff dye (in methanol from 0.38 to 0.05 and in acetonitrile from 0.48 to 0.05). The authors15 explained this phenomenon by the presence of a tautomeric equilibrium in the naphthalene derivative of the BODIPY-meso-Schiff dye, which results in a charge transfer (PET) from the Schiff fragment (in the ketoimine form) to the BODIPY core. The presence of a tautomeric equilibrium in the ground state is substantiated by changes of the absorption bands within the 300 – 450 nm region.15 It is necessary to note that the S0 – S1 band stays similar upon transfer from a non-polar solvent to a polar one (hexane to methanol). A different situation for the BODIPY-Schiff dye was observed as a result of substitution of the ortho-hydroxyaryl Schiff fragment at position 7 of the core.13 The change from a non-polar solvent to a polar ones triggers a hypsochromic shift of the main S0 – S1 absorption band. A recent paper by Lakshmi and Ravikanth18 analyzed the effect of phenylhydrazone and 2,4-dinitro-phenylhydrazone substituents on the spectral characteristics of BODIPY dyes. The paper revealed that the formation of an intramolecular hydrogen bond between the -NH-N=CHC6H5 substituent and a hydrogen bonding accepting solvent (DMSO or DMF) causes a bathochromic shift of the absorption band (50 nm) and a significant shift of the 1H NMR signal (about 3 ppm) towards low field. A stronger bathochromic shift of the absorption band (172 nm) was observed for the 2,4-dinitro-phenylhydrazone derivative of BODIPY as a result of the


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intramolecular intermolecular hydrogen bond equilibrium, but, with a much smaller 1H signal shift (∼ 1 ppm). These phenomena were explained by the authors by an increase of the electron density on the nitrogen atom and a more stable delocalized structure. To summarize, the influence of an intramolecular tautomeric equilibrium on the spectral characteristics of the BODIPY chromophore is an interesting topic for further investigations. Thus, the goal of this study is to have a deep understanding of the influence of a tautomeric equilibrium on the spectral characteristics of BODIPY dyes in the ground and excited states. The combination of the BODIPY core and a diketone substituent makes it possible to model the effect of the coupling between the quasi-aromatic O=C-C=C-O moiety and the chromophore core on the spectroscopic characteristics by changing the intramolecular hydrogen bonding (Scheme 1).

Cl

Cl

N B F

-

N

Cl

+

Cl

N

F H3C

F

N

H

CH3

F H3C

O O

+

-

B

CH3

O O

H

KETO FORM

ENOL (intra-HB) FORM Cl

Cl

+

N B F

-

N

CH3

F H3C

inter-HB FORM

O

H

...

BASE

O

Scheme 1. Chemical structure of studied BODIPY dye and scheme of the tautomeric equilibria. This investigation is expected to provide fundamental data for the effective modelling of the physicochemical properties of the chromophores. It is necessary to underline that the influence of hydrogen bonding on the fluorescence quantum yield is not unequivocal; on the one hand, hydrogen bonding diminishes the fluorescence quantum yield, but on the other hand, the hydrogen bonding influences the charge transfer between the tautomeric sensor and the


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fluorophore, which can result in an increase of the fluorescence quantum yield. The elucidation of the role of the π-electronic conjugation between the chromophore and the quasi-aromatic substituent is a significant step forward and this phenomenon is studied here by the determination of the influence of solvent polarity on the photophysical properties of the diketone BODIPY dye.

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS

2.1. Synthesis of 3-(2,4-dioxopentan-3-yl)-8-(2,6-dichlorophen-1-yl)-4,4-difluoro-4-bora3a,4a-diaza-s-indacene. All solvents used for the spectroscopic measurements were of spectroscopic grade and used as received. Chemicals were purchased from Acros Organics, Sigma Aldrich, Alfa Aesar or TCI Europe and used as received. 8-(2,6-Dichlorophenyl)BODIPY was prepared according to published literature procedures, through a water based dipyrromethane synthesis followed by oxidation and condensation (Scheme 2).19 The reaction was carried out in flame dried glassware, but no special precautions were taken for the exclusion of moisture. Solvents were not dried prior to use. 8-(2,6-Dichlorophenyl)-BODIPY (33.6 mg, 0.1 mmol) and K2CO3 (55.3 mg, 0.4 mmol, 4 equivalents) were dissolved in DMF (1 mL). To this solution acetylacetone (11 µL, 0.11 mmol, 1.1 equivalents) was added. Afterwards, the reaction mixture was stirred at room temperature under an oxygen atmosphere for 1.5 hours. Subsequently, the reaction was poured in diethyl ether, washed with aqueous HCl to an acidic pH, then washed with saturated NaCl(aq) to a neutral pH, dried over MgSO4, filtered, and evaporated to dryness. The crude product was purified by filtration over silica with diethyl ether providing a dark purple solid (43 mg, 99%). Mp. 253 °C; 1H NMR (CDCl3, 300 MHz): δ 7.89 (s,


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1H), 7.55–7.42 (m, 3H), 6.73 (d, J = 4.0 Hz, 1H), 6.70 (d, J = 4.0 Hz, 1H), 6.52 (d, J = 3.6 Hz, 1H), 6.38 (d, J = 4.0 Hz, 1H), 2.04 (s, 6H) ppm; MS (CI, m/z): 415 (M – F); HRMS (EI) m/z: calculated for C20H15BCl2F2N2O2 434.05717, found 434.05381.

Scheme 2. Chart of synthesis of studied BODIPY dye.

2.2. Measurements. Dilute solutions of the BODIPY dye in different solvents were prepared by dissolving the dry, powdered dye in an appropriate solvent so that the absorbance at the maximum of the main absorption peak was ≤ 0.1. Freshly prepared samples in 1-cm quartz cells were utilized to perform all measurements. UV–vis absorption spectra were recorded on a Perkin Elmer Lambda 40 UV–vis spectrophotometer. For the corrected steady-state excitation and emission spectra, a SPEX Fluorolog was used. Cresyl violet in methanol (Φf = 0.55) was used as fluorescence standard for the quantum yield (Φf) determinations.20 the fluorescence spectra are corrected for the wavelength dependence of the sensitivity of the detection channel. A correction for the solvent refractive index was applied. The measurements were recorded at 20°C using degassed samples. The obtained values of Φf were subjected to a relative error of 10%. The spectroscopic data are presented in Table 1.


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Table 1. Spectroscopic/photophysical data of studied compound in different solvents at 20 °C.

λabs,1 λabs,2 Solvent

λem,1

λem,2

∆ ν 1a

∆ ν 2a φf

ε (nm) (nm) (nm) (nm) (cm−1) (cm−1)

1

Toluene

2.4

524

-

545

-

735

-

0.154

2

Ethyl acetate

6.0

520

-

539

-

666

-

0.155

3

Acetic acid

6.2

519

-

538

-

689

-

0.153

4

THF

7.5

520

-

541

-

746

-

0.135

5

TFE

8.6

515

-

533

-

656

-

0.142

6

1-butanol

17.8

520

-

539

-

678

-

0.128

7

2-propanol

19.9

520

-

539

-

671

-

0.134

8

Acetone

20.7

526

415

538

-

482

-

0.023

9

Ethanol

24.5

519

-

538

-

680

-

0.126

10

Ethanol+TFAA

519

-

538

-

680

-

0.185

11

NMP

32.0

522

-

542

-

707

-

0.069

12

Methanol

33.1

518

-

537

-

683

-

0.051

13

DMF

36.7

539

414

540

483c

34

3467

0.001

14

CH3CN

37.5

520

417

537

469c

603

2659

0.027

15

DMSO

46.7

542

418

545

485c

97

3336

0.001

16

H2Ob

78.5

518

n.m.

536

n.m.

648

n.m.

n.m.

a

∆ν i = λabs,i - λem,i - Stokes shift for excitation of respectively the S0→S1 and So→S2 transition..

b

a very low solubility;

n.m. means not measurable; c

excited at maximum of λabs,2.


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2.3. Single-Crystal X-ray Diffraction. The data collection for 3-(2,4-dioxopentan-3-yl)-8-(2,6dichlorophen-1-yl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene was carried out with an Oxford diffraction Xcalibur Ruby diffractometer using MoKα radiation (λ = 0.71073 Å). The crystal structure was solved by direct methods in SHELXS21 and refined in SHELXL software.22 The images were indexed, integrated and scaled using the Oxford Diffraction data reduction package.23 Non-hydrogen atoms were refined with anisotropic thermal parameters; hydrogen atoms were included from ∆ρ maps and refined isotropically. The supplementary crystallographic data for this paper are under CCDC no. 849293 for the studied compound. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 IEZ, UK: fax: (+44) 1123-336-033. e-mail: [email protected]). The molecular structure with atom labelling (Figure S1) are Crystal data for 3-(2,4-dioxopentan-3-yl)-8-(2,6-dichlorophen-1-yl)4,4-difluoro-4-bora-3a,4a-diaza-s-indacene summarized in Table S1. 2.4. Calculations. The Gaussian suite of programs (Gaussian 09)24 was also used for the restricted density functional theory calculations at the B3LYP,25,26 M062X,27 CAM-B3LYP28 and PBE0 functionals.29-31 The use of diffuse functions was necessary to study hydrogen bonding.32 The calculations were carried out in the gas phase and the set of solvents (dichloromethane, methanol and DMSO). The interactions in the system solute-solvent was studied with Polarisable Continuum Model (PCM)33 method.

3. RESULTS AND DISCUSSION The absorption spectra of the studied compound were characterized and showed a narrow absorption band with a maximum between 515 nm (trifluoroethanol) and 524 nm (toluene), with


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the exception of the hydrogen accepting solvents - acetone, acetonitrile, dimethylformamide and dimethylsulfoxide (Figure 1A). The absorption band has a shoulder at around 490 nm (Figure 1A), which can be attributed to vibrational progression.3 In the same series of solvents the emission maxima consist of a relatively narrow band without a fine structure and a maximum ranging from 533 nm (trifluoroethanol) to 545 nm (toluene). For the same range of solvents, the Stokes shift amounts to 700 ± 50 cm-1 and is nearly independent from the solvent polarity. The spectral data as well as the fluorescence quantum yields (Table 1) suggest that in the solvents without hydrogen bonding character only a single absorbing and emitting species is present (prevailing the intra-HB enol tautomeric form, see discussion below, Scheme 1). However, in outspoken

intermolecular

hydrogen

bond

accepting

solvents,

acetone,

acetonitrile,

dimethylformamide and dimethylsulfoxide, the absorption spectrum consists of a broad unstructured band with maxima at respectively 539 and 542 nm. Hence, these maxima are red shifted compared to the non-hydrogen bond accepting solvents. This result suggests that hydrogen bond accepting solvents are characterized by two conformational forms of the molecule in the ground state. Excitation at the maximum of the S0-S1 band (500 – 520 nm) for all solutions results in an intense emission band within 540 – 550 nm (Figure 1). In acetone, acetonitrile, dimethylformamide and dimethylsulfoxide the features and maximum of the emission spectra still resemble those of the emitting species in non hydrogen bond accepting solvents. The comparison of the obtained results for BODIPY-diketon dye with the earlier obtained for BODIPY-meso-Schiff15 and BODIPY-7-Schiff13 dyes reveals a significant difference in the spectral behaviour. In BODIPY-meso-Schiff dye the intramolecular proton transfer (the increase in percent of the content of methanol in the solution) leads to the dramatic decrease of the


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fluorescent quantum yield, with no important shifts of the absorption and emission bands being observed. As for BODIPY-7-Schiff dye, the intramolecular proton transfer (seen in the polar solvents) causes the hypsochromic shift of the absorption band. However, for the studied BODIPY diketone solvent, the increase of the solution polarity brings about bathochromic shift of the absorption band, which draws the fundamental difference between the spectral behaviour of the studied compound and the mentioned before BODIPY-Schiff dyes.

A

1,0

1,0

0,8

0,8

0,6

0,6

F / a.u.

Normalized absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0,4

0,2

0,2 0,0

0,4

400 500 600 Wavelength / nm

0,0 500

700

600 Wavelength / nm

700

B

Figure 1. The normalized fluorescence (A) excitation spectra and (B) emission spectra of the studied BODIPY dye as function of solvent polarity: THF (black), methanol (red), CH3CN (green), toluene (blue), acetone (cyan), ethyl acetate (magenta), 2-propanol (yellow), DMSO (dark yellow), DMF (navy), 1-butanol (purple), TFE (dark cyan), acetic acid (royal). These changes in absorption and emission accurately mirror the dependences λabs/λem = f(∆f) and ∆ν = f(∆f) presented in Figure 2 (λabs/λem = f(∆ENT(30) and ∆ν = f(∆ENT(30), Figure S2, supplementary). The linear dependencies of ν abs and ν em presented in Figure 2 for the S0 → S1 transition reveal that the increase of the solvent polarity (except for the solutions with hydrogen bond accepting solvents) evokes a quite insignificant hypsochromic shift of the absorption and emission bands. The results presented above prove that the studied compound does not undergo significant structural changes in these solvents (protic in that number). The hydrogen bond


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accepting solvents are the exception due to the increase of the shift of λabs observed for these solvents.

A

B

Figure 2. A) Black circles (λabs) are in respect to the S0 → S1 transition (trend line - λabs = 14.299 ∆f + 524.3); empty squares (λabs) are in respect to the S0 → S2 transition; empty circles (λem) are in respect to the S1* → S0 transition (trend line λem = -23.258 ∆f + 546); empty squares (λem) are in respect to the Sx* → S0 transition. B) Black circles are in respect to the Stokes shift after excitation at 520 nm (trend line ∆ν = -462.01 ∆f + 806.67) and black squares are in respect to the Stokes shift after excitation at 420 nm. Therefore, the question arises: which tautomeric forms of the studied dye are observed in nonhydrogen bond accepting and an which in hydrogen bond accepting solvents in both the ground and the excited states? To answer this question 1H NMR spectra have been measured in order to further identify these species in the ground state (Figure 3). All the NMR spectra recorded in toluene, chloroform and dichloromethane show an intense singlet at 16.76 ppm (chloroform), 16.69 (dichloromethane) and 17.44 ppm (toluene). This signal is characteristic for an intramolecular hydrogen bond in the enol form of a β-diketone.34,35 This suggests that in nonhydrogen bond accepting solvents the dye is present only as the enol tautomeric form (intra-HB). Moreover, the absence of a band within the range of 4 – 5 ppm, diagnostic for the keto form of


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diketones,34,35 proves the lack of the keto form of the studied dye in non-hydrogen bond accepting solvents. However, hydrogen bond accepting solvents feature both the band within the range of 4 – 5 ppm (THF - 4.62 ppm, acetonitrile – 5.40 ppm and acetone – 5.49 ppm) and a significant decrease of the intensity of the band at 16 – 17 ppm. Notably, the 1H NMR spectra of all the solutions do not reveal an additional wide band, which could point out the presence of an intermolecular hydrogen bond between the hydroxyl group of the dye and solvent molecules. The above-mentioned data make it clear that in non-hydrogen bond solvents in the ground state only a keto-enol equilibrium occurs in the absence of a stable intermolecular hydrogen bond between BODIPY and the solvent molecules.

** Acetonitrile

*

Acetone

*

Dichloromethane

* *

Tetrahydrofuran

*

*

*

Chloroform

*

18

17

*

*

Toluene

16

15

8

7

6

5

4

3

ppm

Figure 3. NMR spectra of 3-(2,4-dioxopentan-3-yl)-8-(2,6-dichlorophen-1-yl)-4,4-difluoro-4bora-3a,4a-diaza-s-indacene

in

acetonitrile,

acetone,

dichloromethane,

tetrahydrofuran,

chloroform and toluene. The asterisks indicate the signals of the solvent and water in the solvent.


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For the verification of the observed equilibria quantum-mechanical calculations have been accomplished for the dye in the ground and excited states. The computational analysis was carried out by DFT36 and TD-DFT37 methods for the ground and excited states, respectively. To make sure that the results are highly reliable, the calculations have been performed for four different functionals namely M06-2X, PBE0, CAMB3LYP and B3LYP. All the calculations showed an analogous trend of the λabs and λem changes associated with the transition from the enol form to the keto form. The calculations provide 8 conformers for the ground and excited states (Figure 4). Besides, the calculations show that the conformer with an intramolecular hydrogen bond (conformer C1, Figure 4) is the most stable. The second most stable conformer is conformer C2 (the molecule in the keto form). The difference of energy between conformers C1 and C2 equals about 6.25 and 3.75 kcal/mol in the ground and excited states, respectively. The rest of the conformers, C3 – C8, possess very similar energy in the ground and excited states (10 – 15 kcal/mol). As to the calculated spectral characteristics, the transition from the enol form (conformer C1) to the keto form (conformer C2) brings about a bathochromic shift of the absorption band and a hypsochromic shift of the emission band (Tables S2 – S4, supplementary). The results of the calculations are in good agreement with the observed experimental data. The calculations state that the difference of energy between the conformers rapidly decreases with increasing solvent polarity (gas → dichloromethane → methanol → DMSO) especially for the exited state.


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Figure 4. The energy and structure of conformers C1 – C8 obtained with (A) B3LYP/631+G(d,p) and (B) TD-B3LYP/6-31+G(d,p) for full optimisation parameters of molecule at ground and excited states, respectively. Gas (blue), dichloromethane (red), methanol (yellow) and DMSO (green). The most interesting spectroscopic phenomenon observed for the studied compound seems to be the appearance of an absorption band within 400 – 450 nm in the hydrogen bond accepting solvents According to literature data,3 this band should be assigned to a S0-S2 transition. It is noteworthy that the emission from S2* state has been observed earlier for BODIPY dyes.38-40 It is a surprising fact that the excitation at the λmax of the S0-S2 band of the studied dye causes a significant emission within the range of 469 – 486 nm with a large Stokes shift (2500 – 3500 cm-1, Table 1). The intensity of this emission band even exceeds the intensity of the band of S1*-S0 transition (∼540 nm, Figure 5). Hence, the results obtained here for the S2-


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S0 transition differ clearly from those observed earlier for BODIPYs38-40 due to a significant intensity of the emission and a large (in terms of BODIPY dyes) Stokes shift. The explanation of this phenomenon is presented in Scheme 3 and analysed below.

Figure 5. The fluorescence emission spectra; λext. = 420 nm (full line), λext. = 480 nm (dashed line). The inset is present the emission spectra with normalized scale.

This significant Stokes shift suggests important structural changes which are caused by the change of tautomeric form41,42 probably as a result of the relaxation from an excited S2*(keto form) state to an “unknown” S1*(X?) one. The “unknown” state S1*(X?) can be neither the S1*(keto) state nor the S1*(enol) state, otherwise one would observe the emission at λem = 540 nm, like in non-hydrogen bond accepting solvents. Therefore, the “unknown” S1*(X?) state appears to be the excited state of the dye in the tautomeric “open enol” form, S1*(“open”). It is important to mention that this tautomeric state of the dye is stabilized by the intermolecular hydrogen bonding between the hydroxyl group of the BODIPY dye and molecules of the hydrogen bond accepting solvent. Furthermore, the molecule undergoes a transformation from


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the S1*(open) state to the ground state of the dye molecule with the open form (S0(open)) followed by a tautomerization (non-radiative) transition to the S0(keto) state.

Scheme 3. Jablonski diagram of studied BODIPY dye. The energy conformers C1, C2 and C8 obtained with M06-2X/6-31+G(d,p) and TD-M06-2X/6-31+G(d,p) at ground and excited states, respectively. This mechanism also explains the high intensity of this short wavelength emission observed in hydrogen bond forming solvents. While it is highly unlikely that in the S2(keto) form fluorescence to S0 can compete with internal conversion to S1, ESIPT occurring on a timescale of a few picoseconds43 can compete efficiently with the internal conversion. This ESIPT is followed by S2→S1 interconversion in the open enol form and S1→S0 fluorescence from the relaxed open enol form. On the other hand when the S1 state of the keto form is excited the ESIPT yielding the S1 state of the open enol form is endothermic and cannot compete with the other decay processes (fluorescence, internal conversion to S0 and intersystem crossing to the T1 triplet) of the relaxed S1 state of the keto form.


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4. CONCLUSION

This paper deals with the results of the synthesis of BODIPY-diketone (3-(2,4-dioxopentan-3yl)-8-(2,6-dichlorophen-1-yl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) and the studies of its spectral properties in the ground and excited states. The analysis of the influence of the solvent polarity (2.4 < ε < 78.5) on the position of the absorption and emission bands as well as on the fluorescent quantum yield was completed. It was shown that the ground state features only ketoenol equilibrium, meanwhile in the excited state one can additionally observe the equilibrium between the intramolecular hydrogen bond and the intermolecular hydrogen bond in protonacceptor solvents. The conformational analysis for the ground and excited states of the studied dye was conducted by DFT and TD-DFT calculations. The performed quantum-mechanical calculations support the conclusions drawn on the ground of the experimental results. Jablonski diagram was completed on the basis of the computational data.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. The research data supporting this publication can be accessed at http://dx.doi.orgXXX. The following files are available free of charge. brief description (file type, i.e., docx) brief description (file type, i.e., docx)


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AUTHOR INFORMATION Corresponding author * (A.F.) E-email: [email protected] Present address: Faculty of Chemistry, University of Wroclaw, F. Joliot-Curie 14, 50-383, Wrocław, Poland. Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was supported by RFBR (no: 16-03-00405) and KNOW-12 grants as well as a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry of Wrocław University of Technology (0401/0189/17). The authors gratefully acknowledge the Wroclaw Centre for Networking and Supercomputing (WCSS) for computational facilities and Belspo for support through IAP VII/05.


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TOC GRAPHICS


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Impact of the KetoâEnol Tautomeric Equilibrium on the BODIPY

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