Novel green-yellow-orange-red light emitting donor-π-acceptor type dyes based on 1,3-indandione and dimedone moieties

Article abstract of DOI:10.1016/j.dyepig.2017.01.017

Ten novel luminescent dyes containing 1,3-indandione or dimedone as electron acceptors, amino derivatives (dimethylamino, diphenylamino, julolidine and dibiphenylamino) as electron donor units and different length olefinic linkers (1-hydroxyallylidene or 1-hydroxypenta-2,4-dien-1-ylidene) are reported in this study. Newly synthesized compound structures are proven with X-ray analysis, ¹H, ¹³C NMR spectroscopy and elemental analysis. The UV–Vis absorption, emission, solvatochromism, solvatofluorochromism, redox properties, as well as thermal stabilities and quantum chemical calculations of these dyes were systematically investigated to outline relation between structure and properties. These dyes exhibit moderate thermal decomposition temperatures above 200?°C, insignificant solvatochromism and positive, significant solvatofluorochromism, large Stokes shifts and green, yellow, orange and red light emission with quantum yields in range from 0.03 to 0.93 in non-polar solvents and in thin films. Quantum-chemical calculations (DFT) shows, that all dyes exhibit small HOMO/LUMO gaps from 2.77 to 3.22?eV, which is in agreement with experimental data.

Full text of DOI:10.1016/j.dyepig.2017.01.017

Accepted Manuscript
Novel green-yellow-orange-red light emitting Donor-π-Acceptor type dyes based on
1,3-indandione and dimedone moieties
Ilze Malina, Valdis Kampars, Baiba Turovska, Sergey Belyakov
PII:
S0143-7208(16)31074-9
10.1016/j.dyepig.2017.01.017
DYPI 5716
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 27 October 2016
Revised Date: 5 January 2017
Accepted Date: 9 January 2017
Please cite this article as: Malina I, Kampars V, Turovska B, Belyakov S, Novel green-yellow-orange-red
light emitting Donor-π-Acceptor type dyes based on 1,3-indandione and dimedone moieties, Dyes and
Pigments (2017), doi: 10.1016/j.dyepig.2017.01.017.
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Graphical Abstract

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Novel Green-Yellow-Orange-Red Light
Emitting Donor-π-Acceptor Type Dyes Based
on 1,3-Indandione and Dimedone moieties
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Ilze Malina , Valdis Kampars , Baiba Turovska , Sergey Belyakov
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aRiga Technical University, Institute of Applied Chemistry, Paula Valdena Str. 3, Riga, LV-1048,
Latvia
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Latvian Institute of Organic Synthesis, Aizkraukles Str. 21, Riga, LV-1006, Latvia
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Abstract
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Ten novel luminescent dyes containing 1,3-indandione or dimedone as
electron acceptors, amino derivatives (dimethylamino, diphenylamino, julolidine and
dibiphenylamino) as electron donor units and different length olefinic linkers (1-
hydroxyallylidene or 1-hydroxypenta-2,4-dien-1-ylidene) are reported in this study.
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Newly synthesized compound structures are proven with X-ray analysis, H-, C-
NMR spectroscopy and elemental analysis. The UV-Vis absorption, emission,
solvatochromism, solvatofluorochromism, redox properties, as well as thermal
stabilities and quantum chemical calculations of these dyes were systematically
investigated to outline relation between structure and properties. These dyes exhibit
moderate thermal decomposition temperatures above 200 °C, insignificant
solvatochromism and positive, significant solvatofluorochromism, large Stokes shifts
and green, yellow, orange and red light emission with quantum yields in range from
0.03-0.93 in non-polar solvents and in thin films.
Quantum-chemical calculations (DFT) shows, that all dyes exhibit small
HOMO/LUMO gaps from 2.77 to 3.22 eV, which is in agreement with experimental
data.
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corresponding author
E-mail adress: malina.ilze1@gmail.com (I.Malina)
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Keywords: D-π-A dyes; 2-cinnamoyl-1,3-indandione, 2-cinnamoyldimedone,
fluorescent dyes, amino donors, positive solvatofluorochromism.
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1. Introduction
Among many types of fluorescent organic materials, compounds with π-
conjugated system between electron donor (D) and electron acceptor (A) have been
intensively investigated and found applications in several fields, including medicine
for fluorescent markers [1], photovoltaic technologies for dye-sensitized solar cells [2]
and optoelectronic devices for example, in organic light emitting diodes (OLEDs) [3-
6], which can be utilized in full-color flat-panel displays for mobile phones and
television. The main advantage for compounds with D-π-A structure is the variety of
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donor, acceptor and π-bridge groups, which allows to adjust the fluorescent material
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desirable properties, for example emission color. The commonly investigated D-π-A
compounds consist of N,N-dialkyl, N,N-diphenyl substituted anilines [3-9], N-
substituted carbazoles [8] as D parts and dicyanomethylene pyran [3,6,10],
dicyanomethylene furan [11], pyridine [8,12] and benzothiadiazole [13] as electron A
groups. As for the π-bridge, it is possible to use olefinic linkers, such as vinyl or
butadienyl fragments [8,10] or different cyclic linkers as thiophene bridge [14].
However, there are not a sufficient data in the literature about comprehension of
organic compound structure-luminescence property relationship, which would
facilitate new suitable material derivation. Some studies deals with the comparison of
different donor [3,4,9], acceptor [14] group or π-linker length [14,15] effect on
luminescence properties, however continuous work to understand structure-property
relationships is necessary.
2-Acyl-β-diketones, for example, 2-acetyl-1,3-indandione or 2-
acetyldimedone containing three electron withdrawing carbonyl groups are excellent
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starting materials for D-π-A luminophores. These compounds easily react in aldol
condensation reactions with electrophiles, thus forming compounds with extending
conjugation system. The most interesting are the 2-acetyl-β-diketone condensation
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with aromatic aldehydes, which results in 2-cinnamoyl-β-diketone compounds, but
only a few studies deal with the investigation of their physical and chemical properties
[16-21]. Continuing our previous research work about derivatives of 2-cinnamoyl-1,3-
indandione O-methyl ethers [22], we investigate new luminophores on the basis of
two well-known cyclic β-diketones - 1,3-indandione and dimedone. These acceptor
units are linked to various amino donors – N,N-dimethyl (N(Me) ) and N,N-diphenyl
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(N(Ph) ), N,N-dibiphenyl (N(BPh) ) and julolidine through different length of olefinic
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linkers, such as 1-hydroxyallylidene or 1-hydroxypenta-2,4-dien-1-ylidene. Our goal
is to examine structure-property relationship of these luminophores to gain clearer
understanding of how π-bridge length, different donor and acceptor units influences
the thermal, electrochemical, absorption and fluorescent properties. DFT calculations
were performed to gain understanding about electronic structures of newly
synthesized compounds.
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2. Experimental
2.1. Materials and instrument
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All chemicals used in synthesis and analysis are commercially available and
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are used without further purification unless otherwise stated. The H- and C-NMR
spectra were recorded in CDCl solutions on a Bruker Avance 300 MHz spectrometer
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at 300 MHz for H- and 75 MHz for C- nuclei. Chemical shifts were expressed in
parts per million (ppm) relative to solvent signal. The IR spectra in the range from
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4000 to 650 cm were recorded on a Perkin-Elmer Spectrum 100 FTIR spectrometer
using KBr pellets. Elemental analyses were performed on Euro Vector EA 3000
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analyzer. Thermal properties were determinated on a Perkin Elmer STA 6000
instrument. Each sample was heated from 30-550 °C with heating rate 10 °C/min in
nitrogen atmosphere. The purity of prepared organic compounds was established on
Waters 2695 HPLC using Waters 2996 Photodiode array UV-Vis detector. The
chromatographic analyses were performed using XTerra MS C18 (5µm 2.1×100
mm) column, 50% Acetonitrile/0,1% formic acid solution as mobile phase and flow
rate 0.2 ml/min. Low resolution mass spectra were acquired on a Waters EMD
1000MS mass detector (ESI+ mode, voltage 30 V). The UV-Vis absorption spectra of
all compounds in solutions and in thin films were acquired using Perkin-Elmer 35
UV/Vis spectrometer. Emission spectra in solutions and in thin films were measured
on QuantaMaster 40 steady state spectrofluorometer (Photon Technology
International, Inc.). Absolute photoluminescence quantum yields in solutions and in
thin films were determined using QuantaMaster 40 steady state spectrofluorometer
(Photon Technology International, Inc.) equipped with 6 inch integrating sphere by
LabSphere. The cyclic voltammograms were recorded using a computer
controlled electrochemical system PARSAT 2273. The measurements were carried
out using a three-electrode cell configuration. Stationary glassy carbon disk (Ø 0.5
cm) was used as a working electrode, Pt wire - as an auxiliary electrode. Potentials
were measured vs saturated calomel electrode (SCE) and recalculated vs normal
hydrogen electrode (NHE). The potential scan rate was 100 mV/sec. Electrochemical
redox reactions were studied in 0,1 M tetra butylammonium tetrafluorophosphate
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(TBAPF ) solution in acetonitrile under Ar atmosphere. Density functional theory
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(DFT) has been performed using ORCA program [23] and Gaussian 09 [24] package.
For auxiliary tasks Avogadro program [25] was used. The geometry optimization of
all compounds were obtained using non-local functional B3LYP with 6-311G** [26]
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basis set. Diffraction data was collected on a Bruker-Nonius KappaCCD
diffractometer using graphite monochromated Mo-Kα radiation (λ=0.71073 Ǻ). The
crystal structure was solved by direct methods [27] and refined by full-matrix least
squares [28] using maXus complex of programs [29]. Crystal data or compound 16:
monoclinic, a=12.6089(2), b=10.5952(2), c=25.3021(5) Å; β=104.0991(8) °;
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V=3278.33 (10) Å ; Z=8, µ=0.09 mm ; d =1.270 g/cm ; space group – P2 /a. For
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structure refinement 5716 independent reflections with I>2σ(I) were used; the final R-
factor is 0.137.
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2.2. Synthesis
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2-Acetyl-1,3-indandione (8) was synthesized as described in the literature
[30], 2-acetyldimedone (9) as in [31], 4-(bis(4-iodophenyl)amino)benzaldehyde (5)
and 4-(di([1,1'-biphenyl]-4-yl)amino)benzaldehyde (7) as in [32]. Synthesis of starting
materials 3, 7 are shown in Scheme 1 and in Scheme 2 the synthesis of D-π-A
compounds 11-20 are depicted.
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2.2.1. Synthesis of (E)-3-(4-(N,N-dimethylamino)phenyl)acrylaldehyde (3)
A mixture of 4-N,N-dimethylaminobenzaldehyde (1) (5.00 g, 34 mmol) and 25
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ml concentrated sulfuric acid was cooled to 0 °C. Then 1 ml distilled water was added
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slowly. The solution was cooled to 0 °C and acetaldehyde (2) (5.6 ml, 102 mmol) was
added dropwise. During the addition temperature of reaction mixture was not allowed
to exceed 2 °C. After that dark colored reaction mixture was stirred at 0 °C for 0.5 h
and then poured into ice water and neutralized with 10% NaOH solution until pH 7.
The obtained brown solution was filtered and crude product was washed with distilled
water, crystallized from ethanol twice to give orange solid. Yield: 56%, m.p.=134-136
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°C. FT-IR (KBr, cm ): 2921, 2801, 2738, 1662, 1599, 1527, 1456, 1373. H-NMR
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(300 MHz, CDCl , ppm): δ= 9.62 (1H, d, J=7.5 Hz), 7.48 (2H, d, J=7.5 Hz), 7.41
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(1H, d, J=13.1 Hz), 6.71 (2H, d, J=7.5 Hz), 6.57 (1H, dd, J=7.5 Hz, 13.1 Hz), 3.08
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(6H, s). MS (C H NO), m/z: 176.2 (M ).
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Scheme 1. Reagents and conditions: (a) – concentrated H SO , 0 °C, 0.5 h; 10% NaOH. (b)
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KI, KIO , (CH COO) O, 80 °C, 4 h. (c) – 2M Na CO , Pd(PPh ) , dry toluene, 80 °C, 24 h.
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2.2.2. Synthesis of derivatives of 2-cinnamoyl-1,3-indandiones (11-15)
To 2-acetyl-1,3-indandione 8 (1.88 g, 10 mmol) and corresponding derivative
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of benzaldehyde 1,3,4,7,10 (10 mmol) piperidine (5 mmol) was added. Reaction
mixture was refluxed at 110 °C for 4 hours, then it was cooled to 80 °C and 8 ml of
ethanol was added. Solution was boiled for 30 minutes, and after cooling, the formed
crystals were filtered off and washed with ethanol. Products were recrystallized from
CH Cl and EtOH mixture.
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(E)-2-(3-(4-(N,N-dimethylamino)phenyl)-1-hydroxyallylidene)-1H-indene-1,3(2H)-
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dione (11), purple crystals, Yield 67%, m.p.=210-211 °C. FT-IR, (KBr, cm ): 3019,
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2904, 1698, 1631, 1588, 1567, 1523. H-NMR (300 MHz, CDCl , ppm): δ=13.30
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(1H, s), 7.93 (1H, d, J=15.0 Hz), 7.88-7.78 (2H, m), 7.80 (1H, d, J=15.0 Hz), 7.73-
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7.67 (2H, m), 7.65 (2H, d, J=7.5 Hz), 6.81 (2H, d, J=7.5 Hz), 3.10 (6H, s). C-NMR
(75 MHz, CDCl , ppm): δ=197.31, 189.01, 174.35, 152.52, 146.30, 140.95, 138.76,
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134.31, 133.51, 131.40, 122.67, 122.07, 111.83, 106.49, 40.23. MS, m/z: 320.2 (M ),
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318.2 (M ). Anal. Calcd. (%) for C H NO : C, 75.22; H, 5.37; N, 4.39; found: C,
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75.22; H 5.33; N, 4.31.
(E)-2-(3-(4-(N,N-diphenylamino)phenyl)-1-hydroxyallylidene)-1H-indene-1,3(2H)-
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dione (12), purple crystals,Yield 60%, m.p.=220-222 °C. FT-IR, (KBr, cm ): 3049,
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3033, 1696, 1632, 1617, 1548, 1504, 1489. H-NMR (300MHz, CDCl , ppm):
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δ=13.25 (1H, s), 7.90 (1H, d, J=13.5 Hz), 7.85-7.80 (3H, m), 7.75-7.66 (2H, m), 7.55
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(2H, d, J=7.5 Hz), 7.37-7.32 (4H, m), 7.20-7.13 (6H, m), 7.04 (2H, d, J=7.5 Hz). C-
NMR (75 MHz, CDCl , ppm): δ=197.59, 188.83, 173.92, 151.01, 145.30, 145.00,
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139.10, 138.85, 133.80, 133.50, 130.30, 129.61, 126.00, 124.58, 122.50, 122.06,
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120.90, 114.73, 107.33. MS, m/z: 444.2 (M ), 442.3 (M ). Anal. Calcd. (%) for
C H NO : C, 81.25; H, 4.77; N, 3.16; found: C 81.64; H, 4.82; N, 3.54.
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(E)-2-(3-(1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinolin-9-yl)-1-hydroxyallylidene)-
1H-indene-1,3(2H)-dione (13), dark green crystals, Yield 50%, m.p.=225-226 °C. FT-
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IR, (KBr, cm ): 3014, 2939, 2843, 1693, 1627, 1567, 1543, 1513, 1463. H-NMR
(300MHz, CDCl , ppm): δ=13.25 (1H, s), 7.84 (1H, d, J=14.3 Hz), 7.82-7.76 (2H, m),
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7.69 (1H, d, J=14.3 Hz), 7.67-7.61 (2H, m), 7.20 (2H, s), 3.31 (4H, t, J=2.6 Hz), 2.78
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(4H, t, J=2.6 Hz), 1.99 (4H, quint, J=2.6 Hz). C-NMR (75 MHz, CDCl , ppm):
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δ=197.51, 189.00, 174.58, 147.30, 147.00, 140.80, 139.00, 133.93, 133.54, 129.70,
122.30, 122.20, 122.00, 121.95, 110.00, 106.30, 50.00, 27.10, 21.35. MS, m/z: 372.4
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(M ). Anal. Calcd. (%) for C H NO : C, 77.61; H, 5.70; N, 3.77; found: C, 77.62; H,
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(E)-2-(3-(4-(N,N-di([1,1'-biphenyl]-4-yl)amino)phenyl)-1-hydroxyallylidene)-1H-
indene-1,3(2H)-dione (14), dark red solid, Yield: 40%, m.p.=202-203 °C. FT-IR
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(KBr, cm ): 3063, 3027, 1700, 1622, 1584, 1558, 1505, 1483. H-NMR (300 MHz,
CDCl , ppm): δ=13.28 (1H, s), 7.96-7.83 (4H, m), 7.73-7.66 (3H, m), 7.73-7.69 (2H,
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m), 7.64-7.58 (10H, m), 7.50-7.45 (4H, m), 7.37 (2H, d, J=6.0 Hz), 7.30-7.27 (4H,
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m), 7.16 (2H, d, J=6.0 Hz). C-NMR (75 MHz, CDCl , ppm): δ=197.39, 188.73,
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173.65, 150.57, 145.69, 144.83, 141.05, 140.33, 138.81, 137.38, 134.65, 133.81,
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107.26. MS, m/z: 596.3 (M ), 594.6 (M ). Anal. Calcd. (%) for C H NO : C, 84.68;
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H, 4.91; N, 2.35; found: C, 84.23; H, 4.92; N, 2.52.
2-((2E,4E)-5-(4-(N,N-dimethylamino)phenyl)-1-hydroxypenta-2,4-dien-1-ylidene)-
1H-indene-1,3(2H)-dione (15), purple solid, Yield: 30%, m.p.=230-231 °C. FT-IR
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(KBr, cm ): 3017, 2913, 2903, 2821, 1690, 1634, 1618, 1572, 1572, 1554, 1522. H-
NMR (300MHz, CDCl , ppm): δ=13.19 (1H, s), 7.85-7.78 (2H, m), 7.73-7.66 (3H,
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m), 7.45 (2H, d, J=7.5 Hz), 7.40 (1H, d, J=13.5 Hz), 7.06-6.92 (2H, m), 6.70 (2H, d,
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J=7.5 Hz) 3.09 (6H, s). C-NMR (75 MHz, CDCl , ppm): δ=197.41, 188.82, 173.72,
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151.51, 147.30, 144.83, 140.93, 138.82, 134.43, 133.61, 129.61, 124.02, 122.98,
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122.34, 121.88, 118.01, 112.01, 106.63, 40.15. MS, m/z: 346.3 (M ), 344.3 (M ).
Anal. Calcd. (%) for C H NO : C, 76.50; H, 5.54; N, 4.06; found: C, 76.45; H, 5.54;
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N, 4.20.
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2.2.3. Synthesis of derivatives of 2-cinnamoyl-1,3-dimedones (16-20)
Compounds 16-20 were synthesized by same procedure as for compounds 11-
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15, except 2-acetyldimedone (9) was used instead of 2-acetyl-1,3-indandione (8).
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(E)-2-(3-(4-(N,N-dimethylamino)phenyl)-1-hydroxyallylidene)-5,5-
dimethylcyclohexane-1,3-dione (16), pink crystals, Yield 56%, m.p.=154-155 °C. FT-
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IR, (KBr, cm ): 3093, 2957, 2923, 2866, 2807, 1647, 1612, 1580, 1532, 1280, 1160.
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H-NMR (300 MHz, CDCl , ppm): δ=8.17 (1H, d, J=15.0 Hz), 8.00 (1H, d, J=15.0
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(2H, s), 1.11 (6H, s). C-NMR (75 MHz, CDCl , ppm): δ=201.85, 196.08, 186.76,
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7.40; N, 4.47; found: C, 72.90; H, 7.38; N, 4.41.
(E)-2-(3-(4-(N,N-diphenylamino)phenyl)-1-hydroxyallylidene)-5,5-
dimethylcyclohexane-1,3-dione (17), orange crystals, Yield 45%, m.p.=162-164 °C.
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FT-IR, (KBr, cm ): 3099, 3033, 2960, 2949, 2869, 1653, 1602, 1582, 1554, 1266,
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1173. H-NMR (300 MHz, CDCl , ppm): δ=8.20 (1H, d, J=13.7 Hz), 7.95 (1H, d,
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J=13.7 Hz), 7.52 (2H, d, J=6.7 Hz), 7.35-7.30 (4H, m), 7.18-7.11 (6H, m), 7.01 (2H,
13
d, J=6.9 Hz), 2.58 (2H, s), 2.44 (2H, s), 1.12 (6H, s). C-NMR (75 MHz, CDCl3,
ppm): δ=201.83, 195.97, 187.12, 150.67, 146.64, 146.31 130.56, 129.55, 127.89,
125.70, 124.38, 121.08, 119.49, 110.67, 53.27, 49.01, 30.45, 28.27. MS, m/z: 438.4
+
-
(M ), 436.6 (M ). Anal. Calcd. (%) for C H NO : C, 79.61; H, 6.22; N, 3.20; found:
2
9
27
3
C, 79.44; H, 6.27; N, 3.29.
(E)-2-(3-(1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinolin-9-yl)-1-hydroxyallylidene)-
5,5-dimethylcyclohexane-1,3-dione (18), purple solid, Yield 52%, m.p.=186-187 °C
-1
FT-IR, (KBr, cm ): 3105, 2940, 2888, 2862, 2837, 1647, 1577, 1523, 1509, 1273,
1
1137. H-NMR (300 MHz, CDCl , ppm): δ=8.10 (1H, d, J=13.3 Hz), 7.93 (1H, d,
3
J=13.3 Hz), 7.16 (2H, s), 3.28 (4H, t, J=1.8 Hz), 2.75 (4H, t, J=3.5 Hz), 2.54 (2H, s),
1
3
2.43 (2H, s), 1.97 (4H, quint, J=1.8 Hz, 3.5 Hz), 1.10 (6H, s). C-NMR (75 MHz,
CDCl , ppm): δ=201.97, 196.15, 185.95, 148.67, 146.09, 129.24, 122.11, 121.06,
3
+
114.74, 110.09, 53.31, 50.16, 49.46, 30.49, 28.36, 27.58, 21.29. MS, m/z: 366.3 (M ),
9

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2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
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2
2
2
23
24
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26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
364.5 (M ). Anal. Calcd. (%) for C H NO : C, 75.59; H, 7.45; N, 3.83; found: C,
23 27 3
75.48; H, 7.50; N, 3.93.
(E)-2-(3-(4-(N,N-di([1,1'-biphenyl]-4-yl)amino)phenyl)-1-hydroxyallylidene)-5,5-
dimethylcyclohexane-1,3-dione (19), red solid, Yield 31%, m.p.= 221-222 °C. FT-IR,
-1
1
(KBr, cm ): 3032, 2952, 2869, 1655, 1584, 1486, 1170. H-NMR (300 MHz, CDCl ,
3
ppm): δ=8.23 (1H, d, J=13.9 Hz), 7.98 (1H, d, J=13.9 Hz), 7.64-7.57 (10H, m), 7.49-
7.44 (4H, m), 7.38 (2H, d, J=6.2 Hz), 7.28-7.26 (4H, m), 7.14 (2H, d, J=6.2 Hz), 2.58
1
3
(2H, s), 2.45 (2H, s), 1.12 (6H, s). C-NMR (75 MHz, CDCl , ppm): δ=201.91,
3
196.04, 187.13, 150.32, 146.20, 145.81, 140.36, 137.08, 128.83, 128.35, 128.18,
127.18, 126.83, 125.69, 121.73, 119.75, 110.63, 53.27, 49.01, 30.48, 28.28. MS, m/z:
+
-
590.6 (M ), 588.9 (M ). Anal. Calcd. (%) for C H NO : C, 83.50; H, 5.98; N, 2.38;
4
1
35
3
found: C, 83.15; H, 5.98; N, 2.43.
2-((2E,4E)-5-(4-(N,N-dimethylamino)phenyl)-1-hydroxypenta-2,4-dien-1-ylidene)-
5,5-dimethylcyclohexane-1,3-dione (20), purple crystals, Yield 52%, m.p.= 166-167
-1
°C. FT-IR, (KBr, cm ): 3089, 2963, 2944, 2825, 1655, 1584, 1548, 1415, 1400, 1146.
1
H-NMR (300 MHz, CDCl , ppm): δ=7.88-7.71 (2H, m), 7.42 (2H, d, J=6.5 Hz),
3
7.03-6.88 (2H, m), 6.69 (2H, d, J=6.5 Hz), 3.05 (6H, s), 2.55 (2H, s), 2.43 (2H, s),
1
3
1.10 (6H, s). C-NMR (75 MHz, CDCl , ppm): δ=202.06, 195.96, 186.35, 151.38,
3
148.74, 148.69, 144.47, 129.43, 124.10, 123.30, 122.61, 112.01, 110.37, 53.28, 49.26,
+
-
40.19, 30.42, 28.27.MS, m/z: 340.3 (M ), 338.5 (M ). Anal. Calcd. (%) for
C H NO : C, 74.31; H, 7.42; N, 4.13; found: C, 73.98; H, 7.51; N, 4.23.
21
25
3
2
2
45 3. Results and Discussion
46
3.1. Synthesis and characterization of luminophores
2
2
47
Schemes 1 and 2 show synthetic routes of luminophores 11-20. (E)-3-(4-(N,N-
48
dimethylamino)phenyl)acrylaldehyde (3) was obtained with moderate yield in a single
10

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step acid catalyzed aldol condensation between acetaldehyde (2) and 4-N,N-
dimethylaminobenzaldehyde (1). It is worth to point out, that other methods for
synthesis of cinnamaladehyde with N,N-dimethylamino donor in para position of
phenyl ring includes multiple steps (for example, Wittig reaction between
benzaldehyde derivative and Wittig reagent, which is followed by Vilsmeir reaction
between obtained alkene and Vilsmeir reagent) or expensive reagents (for example,
1,3-dioxan-2-yl-tributylphosponium bromide, which is necessary in Wittig
1
oxopropenylation reaction). H-NMR spectra of compounds 3 characterizes with large
trans constant (J=13.1 Hz) for double bond protons of cinnamoyl moiety.
2
2
58
59
Scheme 2. Reagents and conditions: (a) – piperidine, 100 °C, 4 h; ethanol, 80 °C, 0,5 h.
2
60
11

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64
65
66
67
68
69
70
71
72
73
74
75
76
77
Aldehyde 7 was obtained in two-step synthesis from 4-N,N-diphenylamino-
benzaldehyde (4). In the first stage the compound was iodinated with KIO /KI
3
mixture and the 4-(bis(4-iodophenyl)amino)benzaldehyde
(5) was obtained.
Afterwards in Pd(0) catalyzed Suzuki coupling reaction between compound 5 and
phenylboronic acid (6), aldehyde 7 with 61 % yield was synthesized.
All luminophores 11-20 were synthesized through Claisen-Schmidt
condensation reaction between 2-acetyl-1,3-indandione 8 or 2-acetyldimedone 9 and
1
13
aldehydes 1,3,4,7,10 in the presence of 5 mmol piperidine. H-, C-NMR, FT-IR
characteristics as well as X-Ray analysis data for 2-cinnamoyl-1,3-indandiones 11-13
were reported in our previous work [22].
2-Cinnamoyldimedones 16-20 showed similar characteristics to compounds
1
11-15. In H-NMR spectra large coupling constants (J=13.3-15.0 Hz) were found for
double bond protons of cinnamoyl fragment, indicating trans configuration for these
compounds. However, to get unquestionable evidence, that compounds 16-20 similar
to cinnamoyl-1,3-indandiones exist in exocyclic enol form, a single crystal of
compound 16 was obtained by slow evaporation from saturated ethanol solution for
X-ray analysis.
2
2
78
79
Table 1
Principal bond lengths in the structure 16.
Bond
Bond length, Å
1.276(7)
C1−O1
C1−C2
C2−C3
C2−C9
C3−O2
C9−O9
1.427(7)
1.478(7)
1.433(7)
1.218(6)
1.309(7)
2
2
80
81
2
2
82
83
In accordance with the X-ray diffraction data the asymmetric unit of 16
contains two independent molecules, one of them is illustrated in Fig. 1. Table 1 lists
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84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
00
01
the principal bond lengths in molecular structure 16. Due to the fact that bond lengths
in two independent molecules are identical within the errors the arithmetical mean
values are given in the Table 1. X-ray diffraction data confirms, that derivative of 2-
cinamoyldimedone 16 in solid state exist in exocyclic enol form, which is stabilized
by very strong intramolecular hydrogen bond between carbonyl group oxygen atom
(O1) from dimedone fragment and hydroxyl group (O9-H9) from allylidene fragment.
…
The hydrogen bond (O1 H9–O9) is characterized with very short bond length, which
.
..
…
is equal 2.386(7) Å (O1 H9=1.59 Å, O1 H9–O9=138°) for one of independent
.
..
…
molecules and 2.401(7) Å (O1 H9=1.48 Å, O1 H9−O9=149°) for the second. Due to
intramolecular hydrogen bond following structural parameters are presented in the
molecule − bond lengths for C3−C2 are longer than for C1−C2, meaning that bond
C3−C2 has less double bond order. Moreover, comparing carbonyl groups bond
lengths C3−O2 and C1−O1, the latter ones are significantly longer, showing more
single bond character. These data lead to conclusion, that there is extended resonance
system between atoms O1-C1-C2-C9-O9 stabilized by the intramolecular hydrogen
bond presented in the molecule, similarly that it was reported to the 2-cinnamoyl-1,3-
indandione 13 [22]. Unfortunately, a bad quality of the crystals 16 does not permit to
provide a detailed analysis of the molecular geometry.
3
3
02
03
Fig. 1. ORTEP drawing of compound 16 with the atom numbering scheme. All
3
04
nonhydrogen atoms are drawn as 50% probability ellipsoids.
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10
Interestingly, compound 16 does not show any intermolecular hydrogen bonds
between molecules, like it was observed for compound 13 [22]. Dihedral angles
between least-squares planes of dimedone and 4-N,N-dimethylaminophenyl moiety
are 7.4 and 13.2° for two independent molecules. These values indicate that molecule
16 are almost planar.
3
3
11
12
3.2. Thermal properties
3
3
3
3
3
3
3
3
3
13
14
15
16
17
18
19
20
21
In order to investigate thermal behavior of luminophores 11-20,
thermogravimetric (TG) analysis were carried out between 30 and 550 °C in nitrogen
atmosphere. Table 2 displays decomposition temperatures (T ) of investigated
5
%
compounds and TG curves of some luminophores are depicted in Fig. 2. Obtained
results from TG analysis indicate, that all 10 dyes are thermally stable up to 200 °C.
Identical order in the decrease of thermal stability depending on donor group was
found for both 2-cinnamoyl derivatives 11-14 and 16-19 and can be arranged in the
following order: N(BPh) >N(Ph) >N(Me) >Julolidyl. This sequence indicates that the
2
2
2
thermal destruction of compounds begins with the destruction of the donor part.
3
3
22
23
Table 2
Thermal and electrochemical properties of compounds 11-20.
a
Code
T5% , °C
218
E , V
E , V
ox
red
11
12
13
14
15
16
17
18
19
20
0.80
1.05
0.60
0.98
0.66
0.81
1.07
0.61
0.96
0.67
-1.17
-
1.07
1.21
1.04
1.04
-1.32
1.19
1.36
1.18
-1.15
283
-
216
290
-
-
238
233
-
271
219
-
-
288
234
a
3
24
-T5% is temperature of 5% weight loss, respectively.
14

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27
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31
32
Investigation of TG curves also confirms conclusion, that thermal degradation
pattern is influenced by amino donor group. Compounds with amino donors
containing alkyl chains, julolidine shows lower T5% values than those with phenyl or
biphenyl groups. Compounds 11,13,15 as well as 16,18,20 starts to decompose at
∼ 215-230° C and the first possible volatile fragments are the alkyl chains on the
nitrogen atom. The alkyl chain replacement with phenyl rings leads to increased T5%
(270-290 °C), similar observations was reported for tricyanovinyldihydrofuran D-π-A
dyes with different amino donors [33].
3
33
3
3
3
34
35
36
Fig. 2. TG curves of compounds 13,14,18,19 (a) and compounds 11,15,16,20 (b).
2-Cinnamoyl-1,3-indandione 14 and 2-cinnamoyl-dimedone 19 with bulky
N(BPh) substituent exhibit the highest T over 280 °C, which can be explained with
2
5%
3
3
3
37
38
39
their higher molecular mass, and additional π-π stackings. Prolonging the π-bridge by
one more double bond between donor and acceptor (11→15 and 16→20) leads to
slight increase in T , for example compound 11 T is 218 °C, but for its analogue
5
%
5%
3
3
3
40
41
42
15 decomposition temperature is increased to 238 °C. As it can be seen from Fig. 2
(a) compounds with julolidine donor (13 and 18) as well as ones with N(BPh) group
2
(14 and 19) shows two-step weight loss TG curves. On the other hand TG curves of
15

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43
44
45
46
dyes with N(Me) donor group (11,15,16,20) can be characterized with one-step
2
decomposition. The acceptor group shows no influence on the form of TG curve. The
TG analysis results show, that all compounds have good enough thermal stability for
the application in optoelectronic devices.
3
3
47
48
3.3. Electrochemical properties
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
Cyclic voltammetry was employed for all synthesized compounds to
investigate their redox behavior. The results are summarized in Table 2 and some
compounds cyclic voltammograms in acetonitrile are shown in Fig. 3. For all studied
compounds both electrode processes - cathodic reduction as well as anodic oxidation
are electrochemically irreversible in acetonitrile.
The structure of the electron acceptor moieties (1,3-indandione or dimedone)
has a negligible influence (10-20 mV) on the oxidation potentials of the dyes (11/16;
12/17; 13/18; 14/19), while prolongation of the olefinic linker facilitates anodic
oxidation for 140 mV (11/15; 16/20). Similar effects on oxidation properties caused
by elongation of planar π-bridge were reported for imidazole moiety containing dyes
[34]. The oxidation potentials of the luminophores are affected mainly by the
substituents attached directly to the nitrogen atom. Independently from acceptor
moiety (1,3-indandione or dimedone) electron-rich julolidine fragment undergoes
oxidation at 0.60 V. Furthermore, dyes 11 and 16 with two electron-donating methyl
groups at the nitrogen atom undergo oxidation 200 mV more anodically. For the
luminophores 12, 17, 14 and 19 conjugation between the nitrogen lone electron pair
and the π-electron system of the aromatic rings results in the highest oxidation
potential values.
16

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67
68
69
70
Fig. 3. Cyclic voltammograms of compounds 12,13,16 in acetonitrile solutions.
The electrochemical reduction process of compounds 11-20 are localized on
their acceptor moieties. Cathodic peak potentials of the compounds 16-20 are 110-
150 mV more negative than those of 11-15, confirming that 1,3-indandione possesses
stronger electron-acceptor strength than dimedone moiety.
3
3
3
3
71
72
73
74
3
3
75
76
3.4. Absorption properties
In order to gain information about compounds structure-property relationship,
-
5
3
3
3
3
3
3
3
3
3
3
77
78
79
80
81
82
83
84
85
86
UV-Vis absorption spectra (c∼ 1.5·10 M) in a series of solvents with different
dielectric constants and thin films were investigated. The data of absorption band
maxima (λ ) are summarized in Table 3 and some compounds absorption spectra are
abs
depicted in Fig. 4. All compounds 11-20 in chloroform show intense visible light
absorption with band maxima from 466 to 540 nm. The absorption bands are wide and
correspond to intramolecular charge transfer (ICT) transitions π(D)→π*(A) between
donor and acceptor groups in the molecule. For compounds 14 and 19 with N(BPh)2
substituent in cinnamoyl fragment second absorption band around 341 nm are
observed and can be attributed to π→π* transitions of the aminodibiphenylamino
moiety. Due to the difference in acceptor groups the ICT band occurrs in the different
17

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01
02
spectral range – from 466 to 509 nm for dimedone derivatives and from 502 to 540
nm for 1,3-indandione derivatives. Amino donor groups based on their structure and
strength also shows some effect on ICT band position. By varying the donor moiety
on 1,3-indandione and dimedone derivatives, ICT band is red shifted in the order:
N(Me) ∼ N(Ph) <N(BPh) <Julolidyl. N(Me) and N(Ph) shows similar donor strength
2
2
2
2
2
based on absorption data. However, it is known, that N(Ph) could have stronger
2
donor character than N(Me) group [9], but due to nitrogen atom lone pair conjugation
2
with π-electrons of phenyl rings, the donor strength is reduced and λabs does not show
the expected red shift. Furthermore, change of the donor unit from N(Ph) to N(BPh)
2
2
results in bathochromic shift of λabs (+10 nm) for both dimedone and 1,3-indandione
compounds. This fact could be explained with nitrogen atom lone pair weaker
conjugation with biphenyl than phenyl group, which results in enhanced donor
character for N(BPh) group. This assumption was confirmed by theoretical chemical
2
calculations (Section 3.6). Among all investigated amino groups julolidyl moiety
shows the most red shift of λ , which can be explained with its rigid character and
abs
probably, more effective p-π conjugation, which ensures stronger electron-donating
effect than other amino substituents. Furthermore, prolongation of the π-bridge
between D and A moieties (11→15 and 16→20) also shows some effect on ICT bands
4
4
03
04
4
4
4
4
4
4
05
06
07
08
09
10
position in the spectra. As expected red shift of the λabs was observed in the
absorption spectra - 1,3-indandione derivative (15) shows +17 nm shift, but dimedone
derivative (20) +20 nm bathochromic shift. It was concluded, that the strongest
impact on absorption spectra position is due to the acceptor unit and π-bridge length
between A and D moieties and amino donor group strength shows lesser effect.
18

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12
13
14
15
Fig. 4. Absorption spectra of compounds 12,14,17,19 in chloroform (c∼ 1.5·10 M) solutions
(a) and in thin films (b)
Only four luminophores (12,14,17,19) with N(Ph) and N(BPh) donor groups
2 2
4
4
4
4
4
4
4
4
4
4
4
16
17
18
19
20
21
22
23
24
25
26
formed thin films from chloroform solutions. As it can be seen from Fig. 4 (b) in solid
state compounds 12,14,17,19 absorption bands are wider and slight effect on
absorption band maxima are observed compared to data obtained from solution
samples.
The investigation of solvatochromic behavior of the newly synthesized dyes
was also conducted, λabs in different solvents are summarized in Table 3.
Unfortunately, some compounds were insoluble in polar solvents (MeCN and MeOH)
and full solvatochromic outlook was not obtained. From the data depicted in Table 3 it
was concluded, that these dyes did not show notable solvatochromism and upon
changing the solvents polarity from non-polar toluene to polar MeCN, only small λabs
shifts (1-12 nm) were observed.
4
4
27
28
3.5. Emission properties
4
4
29
30
Emission properties of synthesized compounds were examined in series of
-5
solvents solutions (c∼ 1.5·10 M) and in thin films. The data of emission band maxima
19

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4
31
32
(λ ), Stokes shifts (λsts), and absolute photoluminescence quantum yields (Φ) are
em
summarized in Table 3.
4
4
4
33
34
35
Table 3
Absorption and emission characteristics of compounds 11-20 in a series in solutions and thin
films.
a
a
λ ,
λ ,
λ ,
cm
λabs
,
λem, λsts
,
nm cm
abs
em
sts
b
b
Solvent
Dye
-1
Φ
Dye
Φ
-
1
nm
499
499
503
500
nm
548
575
nm
Toluene
THF
1762 0.12
2649 <0.01
2212 0.03
499 568 2434 0.52
492 624 4300 0.49
11
13
15
17
19
12
CHCl3
MeCN
566
-
502 621 3817 0.76
c
-
-
-
-
-
-
-
-
-
-
c
MeOH
Thin film
501
-
582
-
2779 <0.01
-
-
501 643 4407 0.07
Toluene
THF
532
536
540
540
577
601
1466 0.15
2018 <0.01
1712 <0.01
506 586 2698 0.93
502 662 4815 0.10
512 663 4448 0.21
14
16
18
20
CHCl3
MeCN
595
-
-
-
-
-
-
-
-
-
494
499
-
-
-
-
-
-
MeOH
Thin film
537
-
500 667 5008 0.13
Toluene
THF
511
517
520
514
613
664
3256 0.12
4282 0.31
3775 0.25
454 528 3087 0.05
456 546 3615 0.19
467 548 3165 0.15
459 571 4273 0.02
CHCl3
MeCN
647
-
-
-
MeOH
Thin film
518
-
680
-
4599 <0.01
466 570 3915
0.01
-
-
-
-
-
-
Toluene
THF
456
452
549
624
3715
0.40
497 552 2005
0.31
6098 0.56
496 575 2770 0.32
509 577 2315 0.36
503 593 3017 <0.01
510 600 2941 <0.01
CHCl3
MeCN
466
449
451
466
612
675
643
600
5119 0.78
7457 <0.01
6621 <0.01
4793 0.17
MeOH
Thin film
-
-
-
-
Toluene
THF
465
460
476
453
572
636
4023 0.79
6016 0.16
5718 0.16
7060 <0.01
478 593 4057 0.05
476 635 5260 0.22
487 624 4508 0.15
CHCl3
MeCN
MeOH
654
666
-
475 662 5947
0.13
c
-
-
-
483 666 5689 0.05
Thin film
Stokes shifts;
Absolute photoluminescence quantum yield;
-insoluble
478
626
4946 0.13
-
-
-
-
a
4
4
4
4
36
37
38
39
b
c
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The emission color of solutions of 11-20 covers a very wide spectral range
from bright green to red light region. In non-polar toluene emission color varies from
green to red light region, but in polar solvents it shifts from yellow to red region.
Emission spectra of dyes 11-20 are characterized with wide emission bands (Fig. 5),
and emission band maxima λem is influenced by many factors, including dyes
structure. Due to the difference in acceptor groups the λem position in chloroform
solutions shifts – from 548 to 654 nm for dimedone compounds and from 566 to 663
nm for 1,3-indandione derivatives. This red shift of λem for 1,3-indandione derivatives
compared to the same analogues with dimedone cycle (11/16; 12/17; 13/18 etc) can be
explained with additional phenyl ring of 1,3-indandione moiety, which ensures more
extend π-conjugation in the molecules. Moreover, amino donor group structure and
strength show major impact on λem position in the spectra. By varying the donor
moiety λem is red shifted in the following order: N(Me) <Julolidyl< N(Ph) <N(BPh) .
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2
2
N(Me) group having the weakest donor character from all amino substituents shows
2
the λem at 566 nm (dye 11) and at 548 nm (dye 16), while dyes containing N(BPh)2
substituent shows ∼ 100 nm red shift and emits red light at 663 nm (dye 14) and 654
nm (dye 19) in chloroform solutions. All investigated compounds based on their
amino donor structures can be divided into two groups - compounds containing amino
donors with alkyl chains (Julolidyl, N(Me) ) and phenyl groups (N(BPh) , N(Ph) ).
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2
There is essential difference in the Stokes shifts for these groups – first group show
-1
smaller λsts (in the range from 1466-2779 cm for 1,3-indandione derivatives, and
-1
2005-4273 cm for dimedone derivatives) than second group. Smallest λsts was
obtained for julolidyl group containing compounds and can be explained with similar
dipole moment from ground and excited state for these dyes. Similar observations
were found for other D-π-A dyes [15]. Observed relatively large Stokes shifts (λ ) for
sts
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compounds 12,14,15 as well as for 17,19,20 indicates, that after excitation these
molecules undergo some structural reorientation, for example, excited state
intramolecular proton transfer (ESIPT), which is common for compounds with strong
intramolecular hydrogen bond in the molecules [35]. Large Stokes shifts are a huge
advantage for dyes for OLED application, because it decreases the possibility of self-
absorption, which can reduce the total emission efficiency.
The extension of the π-linker between D and A parts (11→15 and 16→20) also
has an impact on λem position in the spectra. As expected longer conjugation bridge
leads to bathochromic shift of λ , for example, λ 11→15 is red shifted for 81 nm in
em
em
chloroform solutions.
Another factor, which greatly influences the λem position in the spectra, is the
solvents polarity. Strong positive solvatofluorochromism was observed by all
investigated compounds. Upon changing the solvents polarity from non-polar
(toluene) to polar (MeOH), λem shifts in range from 24 to 126 nm. Among the
compounds investigated dyes with the diphenyl and dibiphenyl substituents exhibit
largest emission shifts due to considerably stronger donor groups in the molecules, for
example, dye 17 shows the maximum λem shift +126 nm going from toluene to
MeCN. It is common for D-π-A dyes, which exhibit ICT, to have bathochromic shift
of λem [8,9] in polar solvents and can be explained with excited state more polar
character than ground state. Therefore, λem shows larger shifts with solvents polarity
change, than λabs.
Also the absolute fluorescence quantum yields (Φ) are largely dependent on
the solvents polarity. With the increase in solvents polarity (MeOH, MeCN) the
decrease in Φ was observed and almost all dyes except 16 and 20 were non-emissive
in polar environment. In non-polar toluene dyes with N(BPh) donor group (14, 19)
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shows high Φ values (0.93 and 0.79), dyes 11-13,15,17,18 - moderate Φ ranging from
0.12 to 0.52, however remaining compounds 16,20 exhibit relatively low quantum
yields (0.05). Moreover, moderate to high quantum yields (0.10-0.78) were obtained
in THF and CHCl solvents. Obtained quantum yield data in different solvents
3
demonstrates that emission properties of investigated dyes are affected more by the
solvents polarity than nature of the donor group. Low emission intensity in polar
media can be explained with interactions between dyes and polar solvent in polar
excited state, which leads to pronounced non-radiative deactivation [9,36].
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Fig. 5. (a) - Emission spectra of compounds 11-20 in chloroform (c∼ 1.5·10 M) solutions;
(b) – solvatofluorochromism of dye 20
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Photoluminescence spectra of thin films for compounds 12,14,17 and 19 can
be characterized as wide emission bands with maximum λem in the range of 600-667
nm and with quantum yields Φ from 0.07 to 0.17. Decrease in Φ could be explained
with concentration quenching, due to intermolecular interactions in solid state. In the
future to increase Φ values in thin films, these dyes need to be doped in polymer
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matrix. However, still acquired Φ values in thin films are notable, indicating, that
compounds 12,14,17 and 19 might be the promising red light emitting optical
materials for optoelectronic devices.
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From all synthesized compounds, dye 17 with dimedone cycle as acceptor,
N(Ph) as donor and 1-hydroxyallylidene as π-bridge shows the most promising
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optical properties for application in OLED. Intense visible light absorption at ∼ 455
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nm, large Stokes shifts (3715-6098 cm ) and bright yellow or orange-light emission
in non-polar solvents as well as in thin film with moderate to high quantum yields
(0.17-0.78) is characteristic for these dye. However, due to the demand for intense
red-light emitting organic materials dye 14 with 1,3-indandione as acceptor unit,
N(BPh) as donor and 1-hydroxyallylidene as π-bridge show some potential
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application, due to its emission profile (λ ∼ 663 nm with Φ=0.13-0.21) in THF,
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CHCl and in thin film.
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3.6. Quantum-Chemical calculation of Electronic structures
To better understand the effect of the electronic structure on different
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photophysical properties of synthesized dyes, the density functional theory (DFT) was
performed using ORCA program and Gaussian 09 ab initio quantum chemical
software package. The calculated HOMO and LUMO energy levels of all dyes are
depicted in Table 4. The obtained optimized geometries reveal that compounds with
alkyl amino substituents (11,13,15 and 16,18,20) have nearly planar structure.
However, dyes with phenyl amino groups (N(Ph) , N(BPh) ) shows out of plane
2
2
twisting. In compounds 12, 14 as well as in 17, 19 phenyl and biphenyl groups are
twisted out of the plane of the cinnamoyl fragment with dihedral angles of -30.3, -
31.2, -30.1 and -32.1 °, respectively. Comparison of these dihedral angles of
luminophores with N(Ph) and ones with N(BPh) donor group (12/14, 17/19), leads
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to the fact, that latter ones have less planar donor moiety, and nitrogen atom lone pair
conjugation with biphenyl group is weaker, which results in enhanced donor character
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for N(BPh) . This conclusion is in agreement with absorption data of these
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luminophores. Moreover, such twisting out of the plane for these four compounds
could lead to lowered absorption maxima, which was observed in CHCl solutions
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(see paragraph 3.4.) and disarranged the order of donor strength based on absorption
maxima. Similar observations were reported for different amino donor containing 4-
pyridylbutadienes [8].
Fig. 6 shows the HOMO and LUMO orbitals of optimized ground state
structures. HOMO orbitals of compounds 11-20 are mainly localized on the donor
unit and cinnamoyl part of the molecule and the delocalization is not influenced by
acceptor moiety. On contrary LUMO orbitals are localized on the acceptor and
cinnamoyl moiety. Such a charge distribution leads to conclusion, that there is
noticeable intramolecular charge transfer presented in HOMO→LUMO transition.
Similar localization of molecular orbitals and charge distribution was reported for
derivatives of cinnamoyl pyrones [17].
Opt
DFT
Optical (E ) and electrochemical (Eg) bandgap, as well as bandgap (
E
g
)
g
calculated from theoretically obtained HUMO/LUMO levels are depicted in Table 4.
DFT
From the data of electronic levels, it can be seen that, band gap (
E
g
) calculated with
DFT method are in the range from 2.77 to 3.22 eV and reduces with the increase of
strength of the donor groups, for example, compound 11 has the largest difference
between HOMO-LUMO levels and weakest donor group character. Moreover, there is
relationship between conjugation length of D-π-A system and band gap – molecules
with larger HOMO-LUMO band gap have shorter conjugation length, for example,
luminophores 15 and 20 with longer π-spacer between A and D units presents the
DFT
smallest
Eg
values.
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Table 4
Calculated HOMO/LUMO energy levels and band gaps of luminophores 11-20.
DFT
a
Opt
b
c
Code
HOMO, eV
LUMO, eV
E
g
, eV
E
g
, eV
E , V
g
1
1
1
1
1
1
1
1
1
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1
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9
0
-5.37
-5.34
-5.11
-5.27
-5.22
-5.38
-5.33
-5.16
-5.26
-5.21
-2.28
-2.47
-2.19
-2.50
-2.43
-2.16
-2.37
-2.11
-2.40
-2.34
3.09
2.87
2.92
2.77
2.79
3.22
2.96
3.05
2.86
2.87
2.25
2.22
2.11
2.16
2.04
2.37
2.30
2.21
2.26
2.15
1.97
2.12
1.81
2.02
1.70
2.13
2.26
1.97
2.14
1.82
a
DFT
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-
-
Calculated from DFT calculations: E
Optical band gap Eg calculated from UV-Vis spectra in CHCl solution: Eg =1240/λonset
g
=LUMO-HOMO;
b
Opt
Opt
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(nm);
c
-Electrochemical band gap E calculated from cyclic voltammograms: E =E -E .
g
g
ox red
Optical and electrochemical band gaps are in the range 2.04-2.37 eV and 1.70-
DFT
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2.26 eV, respectively. The difference between calculated
E
g
and experimentally
Opt
obtained Eg and E band gap values can be explained with use of different methods
g
and conditions for experiments. All three method results shows, that compounds with
prolonged π-bridge and N(Me) donor (15, 20) has the smallest band gap value and,
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from the other hand compounds (11, 16) with same donor unit and shorter
conjugation length exhibit the largest band gap values.
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Fig. 6. HOMO and LUMO orbitals of optimized ground state structures of compounds 11-20,
obtained using B3LYP 6-311G** set.
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79 4. Conclusions
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In summary, the synthesis of ten D-π-A dyes containing different amino
substituents as donors (D), 1,3-indandione or dimedone as acceptors (A) and different
1-hydroxyallylidene or 1-hydroxypenta-2,4-dien-1-ylidene as π-bridge are reported.
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Donor, acceptor and π-bridge influence on absorption, emission, electrochemical and
thermal properties are investigated. Investigated dyes exhibit high thermal stability
above 200 °C, negligible solvatochromic behavior and noticeable
solvatofluorochromism changing solvents polarity as well as intense green, yellow,
orange and red light luminescence with quantum yields from 0.03 to 0.93 in non-polar
solvents. Prolongation of π-bridge by one double bond in 2-cinnamoyl-1,3-indandione
and dimedone derivatives leads to red shift of absorption and emission, enhanced
photoluminescence quantum yield and decomposition temperature, as well as reduced
oxidation potential. Substitution of a strong, bulky amino donor groups
(diphenylamino and dibiphenylamino) in dyes structures leads to higher quantum
yields, red-shifted emission, and large Stokes shifts.
For practical application in OLED dye 17 with dimedone cycle as acceptor,
N(Ph) as donor and 1-hydroxyallylidene as π-bridge, and dye 14 with 1,3-indandione
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as acceptor unit, N(BPh) as donor and 1-hydroxyallylidene as π-bridge shows the
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most promising properties among all investigated compounds. These two dyes
showed high thermal stability with T >270 °C, film forming properties and excellent
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optical properties. Compound 17 is characterized with intense visible light absorption
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at ∼ 455 nm, large Stokes shifts (3715-6098 cm ) and bright yellow (∼ 549 nm) or
orange-light (∼ 620 nm) emission in non-polar solvents as well as in thin film with
moderate to high quantum yields (0.17-0.78). However, due to the demand for intense
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