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Journal of Fluorescence
is based on a Knoevenagel type condensation between
selected aldehyde and a BODIPY comprising two methyl
groups in the 3 and 5 positions. However, these condensa-
tions usually proceed in low yields [15]. In view of this,
active methyl group is achieved in excellent yields [9].
There are ample reports on 8- and 3- or 3,5- substituted
BODIPYs but not much work has been done on 2-substited
BODIPYs, especially with electron acceptor moieties.
The borondifluoride complexes of acetyl acetone(acac),
benzoyl acetone(bzac) and their respective curcuminoids
congeners are the strong electron donor–acceptor (D-A)
system [16]. In the present study, we report the synthesis
dyes with conjugated electron acceptor moieties linked to
BODIPY dyes at beta position via condensation reaction
between formyl BODIPY and boron complexes of acac
and bzac. The influence of the electron accepting charac-
ter of the boron complexes of acetyl acetone and benzoyl
acetone with their respective curcuminoids congeners are
studied using UV–visible, steady state and time-resolved
fluorescence spectroscopy techniques, and supported by
quantum chemical calculations. Moreover, the formation
of intramolecular charge transfer (ICT) states is supported
by experimental results that show a sharp decrease in flu-
orescence quantum yields and lifetimes with increasing
solvent polarity.
Results and Discussion
Synthesis
The synthesis route proceeds with the known BODIPY
dye 1 [20], which was converted to dye 2 via selective
β-formylation using Vilsmeier-Haack reaction by the
synthetic procedure reported in ref [21]. The β-formyl
BODIPY 2 on further condensation with acetylacetone
difluoroboron complex 3a and benzoylacetone difluoro-
complex 3b furnished dyes 4a and 4b, respectively [22]. It
is worth noting that the acac-BF2 as well as bzac -BF2 unit
in 4a and 4b were regio-selectively removed by refluxing
in MeOH/DMSO to obtain dye 5a [23] and 5b (Scheme 1)
[22]. The BF2 unit of BODIPY moiety remained intact
under the reaction conditions, as revealed from its NMR
spectra and HRMS data. Further the dyes 4a and 4b were
synthesized in very good yield reversibly from dyes 5a
and 5b upon reaction with BF3.OEt2 in dry DCM. The
second methyl group of acetyl acetone unit of the dye 4a
was further condensed with dye 2 to get BODIPY dimer
6. Unfortunately, decomplexation of the BODIPY dimer
6 offer dye with least solubility in almost all solvents, this
limits the characterization of the dye, hence not discussed
in the present study.
Photophysical Properties
The photophysical properties such as ground state absorp-
tion, steady state as well as time resolved fluorescence
measurements of the five dyes 4a, 4b, 5a, 5b and 6 were
evaluated at 25 °C using solvents of different polarities.
The measured photophysical parameters (longest wave-
length absorption maximum (λabs), emission maximum
(λem), quantum yield of fluorescence (ΦF), and the calcu-
lated Stokes shift (ν)) of the dyes are presented in Table 1.
The normalized absorption and emission spectra of the
dyes 4a, 4b 5a, 5b and 6 in dichloromethane are shown
in Fig. 1. The acac BF2 BODIPY dye 4a shows absorp-
tion maxima at 551 nm and emission maxima at 572 nm
where as bzac BF2 BODIPY 4b shows absorption maxima
at 567 nm and emission maxima at 595 nm. The extended
conjugation of phenyl ring in dye 4b cause a bathochro-
mic shift of 16 nm in absorption and 23 nm in emission
peaks in comparison with dye 4a. Further on removal of
BF2 from OBO complex, the BODIPY dyes 5a and 5b
shows a blue shift in absorption maxima compared with
their parent dyes 4a and 4b. This concludes the function-
alization of these groups also affects to the rigidity of
the chromophore and hence, the structural changes also
have major influence on the photophysical properties. The
Materials and Methods
The chemicals and spectroscopic grade solvents were pro-
cured and used as such without further purifications unless
otherwise mentioned. All the common chemicals were of
analytical grade. The reaction progress were monitored by
TLC (thin-layer chromatography). The steady state spectro-
scopic measurements were carried out using UV–vis spec-
trophotometer and fluorimeter HORIBA fluormax Time
resolved studies were performed using HORIBA TCSPC
instrument. HRMS analysis was obtained using a 6550 iFun-
nel QTOF LC/MS, 1290 infinity Binary pump. 1H NMR, 13
C
NMR, spectra were recorded on a 500 MHz Agilent instru-
ment using TMS as an internal standard. All the quantum
chemical computations were performed using the Gaussian
09 program package [17]. The ground state (S0) geometries
of the dyes were optimized using the density functional the-
ory (DFT) method. The first excited (S1) states were opti-
mized using time dependent density functional theory (TD-
DFT) [18]. The functional used was B3LYP (the B3LYP
combines Becke’s three parameter exchange functional (B3)
with the nonlocal correlation functional by Lee, Yang and
Parr (LYP)) [19].
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More, Ankush B.
