Synthesis and spectroscopic study of highly fluorescent β-enaminone based boron complexes

Article abstract of DOI:10.1016/j.saa.2015.03.044

The newly synthesized 1, 1, 2-trimethyl-1H benzo[e]indoline based β-enaminone boron complexes exhibited the intense fluorescence (F_max = 522-547 nm) in solution as well as in solid state (F_max = 570-586 nm). These complexes exhibited large stoke shift, excellent thermal and photo stability when compared to the boron dipyrromethene (BODIPY) colorants. Optimized geometry and orbital distribution in ground states were computed by employing density functional theory (DFT). The cyclic voltammetry study revealed the better electron transport ability of these molecules than current electroluminescent materials like tris(8-hydroxyquinoli-nato)-aluminium (Alq₃) and BODIPY, which can find application in electroluminescent devices.

Full text of DOI:10.1016/j.saa.2015.03.044

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 146 (2015) 80–87
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and spectroscopic study of highly fluorescent b-enaminone
based boron complexes
⇑
Haribhau S. Kumbhar, Balu L. Gadilohar, Ganapati S. Shankarling
Dyestuff Technology Department, Institute of Chemical Technology, Matunga, Mumbai 400 019, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
Synthesis of novel boron complexes
bearing b-enaminone ligand.
Highly fluorescent in solvents as well
as in solid state with large Stokes
shift.
ꢀ
ꢀ
These boron complexes exhibits
excellent thermal and photostability.
Electrochemical study reveals better
electron transport ability compared
to Alq3.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 August 2014
Received in revised form 28 February 2015
Accepted 2 March 2015
Available online 12 March 2015
The newly synthesized 1, 1, 2-trimethyl-1H benzo[e]indoline based b-enaminone boron complexes
exhibited the intense fluorescence (Fmax = 522–547 nm) in solution as well as in solid state
(Fmax = 570–586 nm). These complexes exhibited large stoke shift, excellent thermal and photo stability
when compared to the boron dipyrromethene (BODIPY) colorants. Optimized geometry and orbital
distribution in ground states were computed by employing density functional theory (DFT). The cyclic
voltammetry study revealed the better electron transport ability of these molecules than current
electroluminescent materials like tris(8-hydroxyquinoli-nato)-aluminium (Alq3) and BODIPY, which
can find application in electroluminescent devices.
Keywords:
Organoboron complex
b-Enaminone
Solid state fluorescence
Large stoke shift
Photostability
Ó 2015 Elsevier B.V. All rights reserved.
Introduction
dye-sensitized solar cells [15,16], as donor materials for bulk
hetero-junction solar cells [17,18], and in photodynamic therapy
Organoboron complexes are one of the most important types of
fluorescent dyes. Remarkably, boron dipyrromethene (BODIPY)
dyes [1–4], have attracted significant attention due to their
excellent optical properties with high fluorescence quantum
yields, high molar extinction coefficients, sharp absorption bands,
high photo and chemical stability [5]. BODIPY dyes are widely used
in biolabeling [6,7], as a chemosensors [8–14], as sensitizers for
[19,20]. BODIPY’s are rarely used as electroluminescent material
due to their small Stokes shifts leading to self-quenching and mea-
surement error due to excitation and scattering of light [21,22].
High planarity of BODIPY dyes also leads to stacking of molecules
causing quite strong inter molecular interactions resulting in the
concentration quenching in the solid state [23–27]. Hence, they
hardly exhibit fluorescence in the solid state [28]. Poor photosta-
bility of the BODIPY is one of the major disadvantage for their prac-
tical application in electroluminescence and lasers [29–32]. Other
than BODIPY NBF2N [33,34], NBF2O [35–38], OBF2O [39,40], and
NBF2S [41] moieties have also asymmetrically chelated BF2 moiety.
⇑
Corresponding author. Tel.: +91 2233612708; fax: +91 22 33611020.
E-mail addresses: gsshankarling@gmail.com, gs.shankarling@ictmumbai.edu.in
(G.S. Shankarling).
http://dx.doi.org/10.1016/j.saa.2015.03.044
1386-1425/Ó 2015 Elsevier B.V. All rights reserved.

H.S. Kumbhar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 146 (2015) 80–87
81
The BF2 chelation enhances the electron-withdrawing property as
well as the -electron delocalization. Hence, these compounds
137.30, 130.92, 129.91, 129.86, 129.46, 128.71, 128.61, 127.03,
123.59, 121.83, 111.99, 86.93, 49.70, 27.11. Mass (EI):
p
show long-wavelength absorptions and high electron affinity
[42]. Emissive solids are highly demanded for various applications,
including photoelectric conversion and OLED [43–45].
C22H18ClNO calculated 347.8 (M+), found 348.6 (M+1).
(Z)-2-(1,1-dimethyl-1H-benzo[e]indol-2(3H)-ylidene)-1-
(4-methoxyphenyl) ethanone (3c)
3c obtained from 1 and 2c (1:1 eq) using same procedure as for
3a.
In the present work, we report the synthesis and photophysical
properties of 1,1,2-trimethyl-1H-benzo[e]indole based boron
complexes bearing b-enaminone ligands, which shows the intense
fluorescence in solvents as well as in solid state with large Stokes
shift. Two methyl groups in 1,1,2-trimethyl-1H-benzo[e]indole
moiety decrease the inter molecular interaction, thereby avoiding
the re-absorption and self-quenching [46–48]. This gives rise to
more spaced packing in the solid state, resulting in solid state
luminescence with the excellent photostability.
Yield (67%). mp (156–158 °C); 1H NMR (300 MHz, CDCl3)
d(ppm): 12.02(s, 1H), 7.99–8.01(m, 3H), 7.84(d, J = 7.8 Hz, 1H),
7.75(d, J = 8.7 Hz), 7.48(t, J = 8.1, 8.4 Hz, 1H), 7.32(t, J = 8.1, 8.1 Hz,
1H), 7.25(d, J = 8.7 Hz, 1H), 6.95(d, J = 8.7 Hz, 2H), 6.16(s.1H),
3.84(s, 3H), 1.73(s, 6H). 13C NMR (CDCl3, 75 MHz) d(ppm):
188.62, 175.19, 162.24, 140.09, 132.80, 130.75, 129.93, 129.786,
129.52, 129.3, 129.08, 126.96, 123.38, 121.85, 113.66, 112.00,
86.95, 55.46, 49.51, 27.33. Mass (EI): C23H21NO2 calculated 343.4
(M+), found 344.7 (M+1).
Experimental
Materials and methods
(Z)-ethyl 4-(2-(1,1-dimethyl-1H-benzo[e]indol-2(3H)-ylidene)acetyl)
benzoate (3d)
3d obtained from 1 and 2d (1:1 eq) using same procedure as for
3a.
All reagents and other chemicals were obtained from commer-
cial suppliers and used without further purification. Nuclear
magnetic resonance spectra were recorded on Bruker 300 MHz or
Varian 300M instruments with TMS as an internal standard.
Fluorescence spectra were performed with a Varian Cary Eclipse
Yield (61%). m.p. (178–180 °C); 1H NMR (300 MHz, CDCl3)
d(ppm): 12.12(s, 1H), 8.13(d, J = 8.4 Hz, 2H), 7.99–8.04(m, 3H),
7.87(d, J = 8.1 Hz, 1H), 7.80(d, J = 8.7 Hz, 1H), 7.52(t, J = 8.4,
8.4 Hz, 1H), 7.37(t, J = 8.1, 8.1 Hz, 1H), 7.3(d, J = 8.4 Hz, 1H),
6.25(s, 1H), 4.41(q, 2H), 1.76(s, 6H), 1.42(s, 3H). 13C NMR (CDCl3,
75 MHz) d(ppm): 188.29, 176.48, 166.25, 143.76, 139.66, 132.54,
131.00, 129.92, 129.68, 129.53, 127.21, 127.08, 123.69, 121.88,
112.06, 87.52, 61.24, 49.80, 27.07, 14.39. Mass (EI): C25H23NO3 cal-
culated 385.5 (M+), found 386.7 (M+1).
fluorescence
spectrofluorometer
at
room
temperature.
Thermogravimetric analysis carried out on the Thermo Q-600
instrument. Electrochemical properties investigated on the CHI
instrument. ESI-MS analysis performed on Varian mass spectrome-
ter. UV–Visible spectra were performed with a Perkin-Elmer
Lamda-25 spectrophotometer at room temperature.
Synthesis of the b-enaminone ligands Compound (3a–3e) and their
boron complexes
(Z)-2-(1,1-dimethyl-1H-benzo[e]indol-2(3H)-ylidene)-1-
(4(dimethylamino) phenyl)ethanone (3e)
3e obtained from 1 and 2e (1:1 eq) using same procedure as for
3a.
1,1,2-Trimethyl-1H-benzo[e]indole (1) was synthesised by
previously reported method by using methyl isopropyl ketone
and 2-naphthylhydrazine [49].
Yield (55%). m.p. (180–182 °C); 1H NMR (300 MHz, CDCl3)
d(ppm): 12.00(s, 1H), 7.97–8.02(m,3H), 7.86(d, J = 8.4 Hz, 1H),
7.76(d, J = 8.4 Hz, 1H), 7.49(t, J = 8.1, 7.8 Hz, 1H), 7.33(t, J = 7.8,
7.8 Hz, 1H), 7.26(d, J = 8.7 Hz, 1H), 6.73(d, J = 9.3 Hz, 2H), 6.20(s,
1H), 3.04(s, 6H), 1.76(s, 6H). 13C NMR (CDCl3, 75 MHz) d(ppm):
188.73, 174.10, 152.55, 140.37, 130.56, 129.90, 129.67, 129.59,
129.19, 128.79, 127.65, 126.83, 123.12, 121.80, 111.99, 111.08,
86.90, 49.29, 40.20, 27.50. Mass (EI): C24H24N2O calculated 356.5
(M+), found 357.7 (M+1).
Synthesis of (Z)-2-(1,1-dimethyl-1H-benzo[e]indol-2(3H)-ylidene)-1-
phenylethanone (3a)
Sodium hydride (60 wt% in oil, 1.6 g, 40 mmol) was added to a
toluene (100 mL) solution of 1,1,2-trimethyl-1H-benzo[e]indole 1
(2.1 g, 10 mmol and ethyl benzoate 2a (1.5 g, 10 mmol) at room
temperature. The solution was refluxed for 1 day. After cooling to
0–5 °C, aq. NH4Cl was added to the reaction mixture and extracted
with ethyl acetate. The extract was dried over MgSO4 and concen-
trated in vacuo. Column chromatography of the residue on silica
gel gave 1 (2.6 g, 72%) as a yellow solid. mp (110–112 °C).
1H NMR (300 MHz, CDCl3) d(ppm): 12.12(s,1H), 8.03(m, 3H),
7.88(d, J = 8.1 Hz, 1H), 7.8(d, J = 8.7 Hz, 1H), 7.5(m, 4H), 7.36(t,
J = 8.1 Hz, 8.1, 1H), 7.30(d, J = 8.4 Hz, 1H), 6.22(s, 1H), 1.77(s, 6H).
13C NMR (CDCl3, 75 MHz) d(ppm): 189.51, 175.78, 140.09,
139.93, 131.18, 130.85, 129.92, 129.81, 129.47, 129.26, 128.41,
127.33, 126.96, 123.98, 121.84, 112.03, 87.33, 49.60, 27.20. Mass
(EI): C22H19NO calculated 313.4(M+), found 314.7(M+1).
Synthesis of 8,8-difluoro-12,12-dimethyl-10-phenyl-
8,12dihydrobenzo[e][1,3,2]oxazaborinino [3,4-a]indol-7-ium-8-uide
(4a)
Compound 3a (626 mg, 2 mmol) was dissolved in dry dichloro-
methane (12 mL), triethylamine (0.5 mL, 4 mmol) and boron
trifluoride diethyl ether complex (1.41 mL, 4 mmol) was added to
the solution and stirred at room temperature for 5 h. Then water
was added to the solution. The solution was extracted with
CH2Cl2. The organic layer was washed with water and dried over
MgSO4. The crude obtained, after evaporation of the solvent, was
purified by silica gel column chromatography to afford compound
4a (580 mg, 81%) as a yellow solid.
(Z)-1-(4-chlorophenyl)-2-(1,1-dimethyl-1H-benzo[e]indol-2(3H)-
ylidene)ethanone (3b)
3b obtained from 1 and 2b (1:1 eq) using same procedure as for
3a.
Yield (84%). m.p. (140–142 °C); 1H NMR (300 MHz, CDCl3)
d(ppm): 12.08(s, 1H enolic), 7.99(d, J = 8.7 Hz, 2H), 7.93(d,
J = 8.4 Hz, 2H), 7.8(d, J = 8.4 Hz, 1H), 7.79(d, J = 8.7 Hz, 1H),
7.51(m, J = 8.4, 1.5 Hz, 2H), 7.43(d, J = 9 Hz, 2H), 7.36(m, J = 8.1,
8.1, 1.2 Hz, 1H), 7.29(d, J = 8.4, 1H), 6.14(s, 1H), 1.75(s, 6H). 13C
NMR (CDCl3, 75 MHz) d(ppm): 187, 176.17, 139.71, 138.38,
Yield (81%) m.p. (252–254 °C); 1H NMR (300 MHz, CDCl3)
d(ppm): 7.91–8.09(m, 6H), 7.47–7.62(m, 6H), 6.48(s, 1H), 1.78(s,
6H). 13C NMR (CDCl3, 75 MHz) d(ppm): 182.78, 172.77, 140.82,
134.48, 133.51, 132.68, 132.50, 130.36, 130.03, 128.86, 128.59,
127.66, 127.24, 125.27, 122.31, 115.79, 87.89, 51.67, 24.27. Mass
(EI): C22H18BF2NO calculated 361.2, found 361.4 (M+).

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10-(4-Chlorophenyl)-8,8-difluoro-12,12-dimethyl-8,12-dihydrobenzo
[e][1,3,2]oxazaborinino [3,4-a]indol-7-ium-8-uide (4b)
triethyl amine as base, to give the desired organoboron compounds
4a–4e with good yields. These compounds were column purified
and characterized by mass, 1H NMR, and 13C NMR spectra.
The compounds 3a–3e were found to be present in the b-enam-
inone form rather than b-iminoenol or b-ketoimine. This was con-
firmed by the Mass spectroscopy [ESI-MS] which showed a unique
fragmentation pattern by formation of [MꢁAr]+ ion (see Fig. S1 and
Scheme S1) rather than [ArCO]+ ion, this may be due to the
intramolecular hydrogen bonding in b-enaminone form [57].
Also, from 1H NMR, a peak at 6.22 ppm for an allylic proton and
another peak at 12.12 ppm for the N–H proton can be observed
which further confirms 100% stability in b-enaminone form
because of the intramolecular hydrogen bonding. Hence the forma-
tion of [ArCO]+ from the b-enaminone is least favoured [58,59]. The
disappearance of the signal at 12.12 ppm for N–H proton and shift-
ing of signal at 6.22–6.48 ppm in 1H NMR confirms the formation
of boron complex in compound (4c) (Fig. 1).
4b obtained from 3b by the same procedure as for 4a.
Yield (55%) m.p. (242–244 °C); 1H NMR (300 MHz, CDCl3)
d(ppm): 7.94(m, 6H), 7.44–7.62(m, 4H), 6.45(s, 1H), 1.77(s, 6H).
13C NMR (CDCl3, 75 MHz) d(ppm): 182.75, 171.25, 140.68,
138.93, 134.60, 132.57, 131.97, 130.43, 130.04, 129.17, 128.90,
128.54, 127.31, 125.04, 122.31, 115.75, 87.95, 51.75, 24.16. Mass
(EI): C22H17BF2NO calculated 395.6, found 395.4 (M+).
8,8-Difluoro-10-(4-methoxyphenyl)-12,12-dimethyl-8,12-
dihydrobenzo[e][1,3,2]oxazaborinino[3,4-a]indol-7-ium-8-uide (4c)
4c obtained from 3c by the same procedure as for 4a.
Yield (41%). m.p. (246–248 °C); 1H NMR (300 MHz, CDCl3)
d(ppm): 7.89–8.09(m, 6H), 7.57(t, J = 8.1, 8.4 Hz, 1H), 7.47(t,
J = 8.1, 8.1 Hz, 1H), 6.97(d, J = 9 Hz, 2H), 6.89(s, 1H), 3.87(s, 3H),
1.76(s, 6H). 13C NMR (CDCl3, 75 MHz) d(ppm): 182.44, 172.60,
163.50, 140.98, 134.08, 132.31, 130.25, 129.99, 129.86, 128.64,
127.14, 125.83, 125.03, 122.24, 115.66, 114.24, 86.73, 55.61,
51.46, 24.48. Mass (EI): C23H20BF2NO2 calculated 391.2, found
392.7 (M+1).
Thermal stability
All organoboron compounds were subjected to the thermo-
gravimetric analysis with a ramp of 10 °C per minute from 25 to
600 °C. Compounds 4a–4e are thermally stable up to 350 °C as
shown in Fig. 2, which attributes to their good thermal stability
which is favorable for OLEDs [60,61] fabrication.
10-(4-(Ethoxycarbonyl)phenyl)-8,8-difluoro-12,12-dimethyl-8,12-
dihydrobenzo[e][1,3,2]oxazaborinino[3,4-a]indol-7-ium-8-uide (4d)
4d obtained from 3d by the same procedure as for 4a.
Yield (70%). m.p. (258–260 °C); 1H NMR (300 MHz, CDCl3)
d(ppm): 8.17(m, 8H), 7.61(t, J = 8.1, 8.4 Hz, 1H), 7.51(t, J = 7.8,
8.2 Hz, 1H), 6.54(s, 1H), 4.42(q, 2H), 1.80(s, 6H), 1.43(t, 3H). 13C
NMR (CDCl3, 75 MHz) d(ppm): 182.78, 171.10, 165.84, 140.64,
137.34, 133.77, 132.68, 130.51, 130.07, 129.94, 128.53, 127.48,
127.36, 125.52, 122.36, 115.86, 88.93, 61.54, 51.86, 24.10, 14.39.
Mass (EI): C25H22BF2NO3 calculated 433.3, found 433.5 (M+).
UV absorption and photoluminescence property
The absorption and fluorescence spectra of the compounds 4a–
4e were recorded in dichloromethane as shown in Fig. 3.
Compound 4e shows maximum absorption wavelength (kmax) at
454 nm, that shows bathochromic shift than the 4a (kmax = 406).
The fluorescence wavelength (Fmax) ranges from 522 to 547 nm
10-(4-(Dimethylamino)phenyl)-8,8-difluoro-12,12-dimethyl-8,12
dihydrobenzo[e][1,3,2]oxazaborinino[3,4-a]indol-7-ium-8-uide (4e)
4e obtained from 3e by the same procedure as for 4a.
due to
p-electron delocalization and electron withdrawing prop-
erty of BF2. Surprisingly compounds 4a–4e shows large stokes shift
of 116, 124, 109, 129 and 64 nm, respectively. This may be due to
the flexible structure of b-enaminone boron complexes in their
excited states [62].
The synthesized compounds showed solid state fluorescence
ranging from 570 to 586 nm as shown in Fig. 3c. The presence of
methyl groups increases the steric hindrance, prevents the mole-
cules from packing compactly, avoiding the spectral broadening.
Weak molecular interactions between molecules and their large
Stokes shifts provide favorable factors that eliminate self-quench-
ing and enhance their solid state fluorescence [63].
Yield (41%). m.p. (288–290 °C); 1H NMR (300 MHz, CDCl3)
d(ppm): 8.02–7.83(m, 6H), 7.55(t, J = 7.5, 9 Hz, 1H), 7.44(t, J = 6.9,
8.1 Hz, 1H), 6.71(d, J = 8.7 Hz, 2H), 6.31(s, 1H), 3.09(s, 6H), 1.76(s,
6H). 13C NMR (CDCl3, 75 MHz) d(ppm): 181.56, 173.27, 153.38,
141.39, 132.01, 130.06, 129.89, 128.70, 126.92, 124.57, 122.14,
115.57, 111.35, 85.49, 51.07, 40.19, 24.91. Mass (EI):
C24H23BF2N2O calculated 404.3, found 405.6 (M+1).
Computational methodology
Relative fluorescence quantum yields of compounds 4a–4e
were determined by the comparative method using known using
fluorescein as a standard [64,65]. The quantum efficiency of com-
pounds 4e and 4c was found to be 0.46 and 0.08, respectively.
For compounds 4a, 4b and 4d the quantum efficiency was found
to be 0.027, 0.025 and 0.025, respectively. From the above results
it is clear that the quantum efficiency for the compounds with
the electron donating substituents on phenyl ring was good.
Relative fluorescence quantum yields for compounds 4a–4e are
summarized in Table 1.
Theoretical calculations were perform using the Turbomole-
V6.5 program [50–52] package at B3LYP (Becke, 3-parameter,
Lee–Yang–Parr) [53,54]. This is most commonly used functional
for reasonably accurate and precise results [55], in combination
with def2-SV(P) basis set [56]. Time dependant density functional
(TD-DFT) calculations were also performed with B3LYP functional
and the def2-SV(P) basis set in the gas-phase.
Results and discussion
Synthesis and characterization
Electrochemical properties
The b-enaminone and their boron complexes were synthesized
by 1,1,2-trimethyl-1H-benzo[e]indole with the ethyl benzoate
derivatives 2a–2e in presence of sodium hydride refluxing to give
b-enaminone compound 3a–3e by displacing ethoxy (–OCH2CH3)
group of benzoate ester with 1,1,2-trimethyl-1H-benzo[e]indole
active methylene (Scheme 1). Compounds 3a–3e were further
treated with BF3-etherate in dichloromethane in presence of
Cyclic voltammetry (CV) was performed in acetonitrile solution
of 0.5 mM compounds 4a–4e along with 0.1 M tetrabutyl ammo-
nium hexaflourophosphate as a supporting electrolyte. Platinum
disk, platinum wire and Ag/Ag+ were used as working electrode,
counter electrode and reference electrode, respectively. All the
measurements were recorded with 100 mV/s scan rate. The cyclic
voltammograms of the compound 4a–4e shown in Fig. 4 are

H.S. Kumbhar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 146 (2015) 80–87
83
O
H
N
Acetic acid
NaH
NH2
+
+
O
Toluene/Reflux
O
N
R
2a-e
1
2a:R= H,
2b:R= Cl,
2c:R= OCH3,
2d:R= COOEt,
2e:R= NMe2
t O/TEA
BF .E
RT
N
R
N
R
B
F
O
H
F
O
4a-e
3a-e
Scheme 1. Synthesis of organoboron complexes 4a–4e.
Fig. 1. 1H NMR (in CDCl3) chemical shifts comparison for the b-enaminone 3c and organoboron 4c.
obeying the quasi reversible nature. The onset of oxidation poten-
tials are 1.11, 1.02, 1.02, 1.07 and 0.5 vs. Fc/Fc+ for 4a to 4e,
respectively.
The HOMO energies for all the compounds were estimated by
using onset oxidation potential and Eq. (1) [66,67]. LUMO was esti-
mated by taking the difference between band gap energy and the
energy of HOMO level as shown in Eq. (3) and the obtained values
are given in Table 2.
EHOMO ¼ ꢁðEonset vs Fcþ=Fc þ 5:1ÞeV
ð1Þ
ð2Þ
ð3Þ
Eg opt ¼ 1240=kedge
ELUMO ¼ EHOMO þ Eg
Where E1/2 for the Fc/Fc+ 0.051 V and Optical band gap was
calculated using Eq. (2) and the absorption band edge (kedge) of
the compounds 4a–4e was summarized in Table 2.
Fig. 2. Thermo-gravimetric analysis (TGA) of compounds 4a–4e.
The HOMO and LUMO energy levels of compounds 4a–4e were
ꢁ6.21, ꢁ6.12, ꢁ6.12, ꢁ6.17, ꢁ5.6 and ꢁ3.72, ꢁ3.62, ꢁ3.64, ꢁ3.69,
ꢁ3.20, respectively (Table 2). The LUMO energies of the com-
pounds 4a–4e are comparable to the existing electroluminescent
material e.g. Alq3 (ꢁ3.0 eV) [68] and 1,3,5,7-tetramethyl-8-
phenyl-BODIPY (ꢁ3.05 eV) [22]. This proves that, the compounds
4a–4e have good electron accepting characteristic, with
enhanced charge transporting properties which can be utilized as
photoelectric functional material.
a

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H.S. Kumbhar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 146 (2015) 80–87
Fig. 3. (a) Absorption spectra in dichloromethane 1 ꢂ 10ꢁ5 M concentration. (b) Normalized fluorescence spectra of compounds 4a–4e. (c) Solid state fluorescence spectra of
compounds 4a–4e. (d) Photographs of the solid state fluorescence in UV and visible organoboron complexes 4a–4e.
Table 1
because of the electron-donating effect of methoxy and N,N-
dimethyl group, respectively. The variations in HOMO–LUMO
energies and band gap are attributed due to the modification of
substituents on basic skeleton.
Photophysical properties of compounds 4a–4e.
Compound
In dichloromethane
Solid State
kmax
Fmax
Stoke shift
(nm)
Ff
kmax
Fmax
(nm) (e)
(nm)
(nm)
(nm)
TDDFT calculations were also performed with B3LYP functional
and the def2-SV(P) basis set in gas phase. The calculated kmax
values are in well accordance with the experimental values. The
calculated kmax, the main orbital transition, and oscillator strength
(f) are shown in Table S1. Percentage deviation between experi-
mental absorption and vertical excitation computed by DFT [69]
is found to be 0–14.5%. The compound 4e shows maximum devia-
tion which is 14.5%. In all the cases, absorptions are mostly due to
the first transition from HOMO to LUMO and second transition
from HOMOꢁ1 to LUMO, respectively. HOMO orbitals delocalized
over the naphthyl part and LUMO orbitals are delocalized over
the phenacyl part of molecules (Fig. 7), which leads to effective
charge transfer which may account for large stoke shift.
4a
4b
4c
4d
4e
406 (7500)
409 (9200)
412 (14,300)
418 (3000)
454 (21,200)
525
533
518
543
523
116
124
109
129
64
0.027
0.025
0.08
0.025
0.46
453
458
458
469
479
570
573
570
587
586
Photochemical stability
In order to determine the photostability of the organoboron
compounds 4a–4e, solutions in acetonitrile were subjected to the
UV-irradiation (254 nm, 0.15 mW/cm2) for 500 min. It was
observed that the optical density at excitation maxima, do not
significantly change for all the compounds (4a–4e) Fig. 5. This indi-
cates that these compounds have very good photostability than the
BODIPY dyes, may due to the highly electron rich benzo-indoline
moiety. However, the excellent photostabilities of organoboron
compounds (4a–4e) are of great importance for practical applica-
tions in photo-electronics devices.
Conclusions
The b-enaminone compounds (3a–3e) synthesized were found
to have a stable 100% enaminone form rather than ketoamine form
which was confirmed by ESI-MS and 1H, 13C NMR. These b-enam-
inone ligands formed boron complexes to give compounds
(4a–4e) respectively. The synthesized boron complexes exhibit
Computational study
fluorescence in dichloromethane (Fmax) ranging from 522 to
The minimum energy optimized geometry at ground state for
the compound 4a is shown in Fig. 6. The general trend of calculated
band gap, HOMO and LUMO energies are in well agreement with
the experimental observed values summarized in Table 2.
Compounds 4c and 4e show rise in HOMO and LUMO energy levels,
547 nm. The b-enaminone boron complex shows large stokes shifts
than that of the BODIPY (64–129 nm). Furthermore, these
complexes show solid state fluorescence (570–586 nm) and have
excellent thermal and photostability which is beneficial for
applications in OLED and fluorescent probe.

H.S. Kumbhar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 146 (2015) 80–87
85
Fig. 4. Cyclic voltammograms of 4a–4e acetonitrile solution containing 0.1 mol/L Bu4NPF6 at a scan rate of 100 mV/s.
Table 2
Electrochemical properties of the compounds 4a–4e.
E°onset vs Fc/Fc+ (V)
HOMOb
LUMOc
HOMOd
LUMOd
Eg (eV)
a
d
Compound
kedgea(nm)
Eg (eV)
4a
4b
4c
4d
4e
498
496
499
501
517
2.49
2.5
2.48
2.48
2.4
1.11
1.02
1.02
1.07
0.5
ꢁ6.21
ꢁ6.12
ꢁ6.12
ꢁ6.17
ꢁ5.6
ꢁ3.72
ꢁ3.62
ꢁ3.62
ꢁ3.69
ꢁ3.2
ꢁ5.74
ꢁ5.81
ꢁ5.58
ꢁ5.83
ꢁ5.30
ꢁ2.37
ꢁ2.50
ꢁ2.19
ꢁ2.50
ꢁ1.99
3.37
3.31
3.39
3.34
3.31
a
b
c
UV–Vis spectroscopy measurements absorption spectra kedge (nm), Optical band gap calculated from the absorption band edge of the compound, Eg opt = 1240/kedge
HOMO energy was calculated from CV using Eq. (1).
LUMO was estimated from the onset potential of cyclic voltammetry and band gap obtained by UV–Vis as shown in Eq. (2).
Theoretically calculated using DFT.
.
d
Fig. 5. Change of optical density of 4a–4e at the absorption maximum wavelength with the irradiation time. Solutions of compounds 4a–4e in Acetonitrile were irradiated
under (0.15 mW/cm2) UV light (emitting at 254 nm).
Fig. 6. (a) Front view. (b) Top view. Optimized geometry of 4a.

86
H.S. Kumbhar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 146 (2015) 80–87
Fig. 7. Molecular orbital energy diagram and isodensity surface plots of the HOMOꢁ1, HOMO, LUMO and LUMO+1 of 4a–4e, calculated at the B3LYP/def2-SV(P) level of
theory.
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Acknowledgment
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Author HSK is thankful to UGC-CAS for providing fellowship
and SAIF IIT-Bombay, Mumbai for recording Mass spectra, 1H
NMR, 13C NMR spectra.
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