New 2,2-difluoro-1,3,2(2H)oxazaborines and merocyanines derived from them

Article abstract of DOI:10.1016/j.dyepig.2011.05.025

Synthetic methods have been developed to prepare oxazaborines, the azaanalogues of 2,2-difluoro-1,3,2(2H)dioxaborines, which can form merocyanine dyes. The first oxazaborine merocyanines with the isomeric position of the coordinating nitrogen atom have al

Full text of DOI:10.1016/j.dyepig.2011.05.025

Dyes and Pigments 92 (2011) 749e757
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New 2,2-difluoro-1,3,2(2H)oxazaborines and merocyanines derived from them
*
Konstantin Zyabrev , Marina Dekhtyar, Yurii Vlasenko, Alexander Chernega,
Yurii Slominskii, Aleksei Tolmachev
Institute of Organic Chemistry, National Academy of Sciences of Ukraine, 5 Murmanska Str., 02660 Kyiv, Ukraine
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthetic methods have been developed to prepare oxazaborines, the azaanalogues of 2,2-difluoro-
1,3,2(2H)dioxaborines, which can form merocyanine dyes. The first oxazaborine merocyanines with the
isomeric position of the coordinating nitrogen atom have also been obtained. Comparing the spectral
properties of donor-acceptor dioxa- and oxazaborine dyes, it is seen that substitution of the 3-O atom by
the NH group in the chelate ring has a slight effect on absorption and fluorescence band positions but
causes the intensity redistribution between the 0e0 and 0e1 vibronic absorption peaks and thus induces
a change in the absorption band shape due to the enhanced solvation of oxazaborines. Substitution of the
1-O ring atom by the NPh group leads to a bathochromic shift and a manifold increase in the fluorescence
quantum yield for the corresponding boron chelate dyes.
Received 7 April 2011
Received in revised form
24 May 2011
Accepted 28 May 2011
Available online 13 June 2011
Keywords:
Absorption
Dioxaborine
Fluorescence
Merocyanine
Oxazaborine
X-ray diffraction
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
reported evidence of the relationship between the nature of the
coordinating atom and the spectral properties of boron chelate
2,2-Difluoro-1,3,2dioxaborine polymethine dyes are of consid-
erable current interest due to their applications in NLO [1] and
OLED materials [2] as well as in two-photon absorption processes
polymethine compounds. Aiming at spectral controllability of
-conjugated chelate systems, here we address the optical effects
caused by the O-to-N substitution in the ligands of boron chelate
dyes. It should be noted that a number of visible-absorbing and
p
[3].
p-Conjugated systems incorporating boron chelates are
remarkable for their flexible molecular design and feasible
synthesis which afford a great diversity of dye types. As an example,
2,2-difluoro-1,3,2(2H)dioxaborines give rise to anionic and zwit-
terionic polymethines [1a,4,5] as well as to typical merocyanines
intensely fluorescing
recently been described [9]. They were synthesized by suitable
tailoring of the ligand with a highly developed -conjugated
system followed by the chelation of the BF2 group to give the
target coloured compounds. In the present work, we have devel-
oped a more general and convenient synthetic approach to oxa-
zaborine dyes which involves first the synthesis of the N,O-chelate
and then the extension of the polymethine chromophore like in
conventional cyanine chemistry.
p-conjugated oxazaborine systems have
p
[1b,6,7], linear [2a], and star-shaped uncharged
[2d,e,3a].
p-compounds
The spectral properties of quasi-linear dyes including those
containing BF2 complexes are dictated by the constitution of
heterocyclic end groups and the
p-linker between them. Previ-
ously we investigated the relationship between the structure of
the polymethine chain and the spectral behavior of dioxaborine
2. Results and discussion
dyes [4]. Ligand variations in
p-conjugated compounds of O,O-
chelated boron and their effects on the corresponding dye
spectra have also been thoroughly studied (see Refs. 1e7 and the
works cited therein). Further, the colour theory states that
heteroatom substitutions in dye auxochromes can cause signifi-
cant spectral effects [8]. At the same time, there has been no
As model and reference compounds, we have used the boron
complex of 2-acetyldimedone 1 and related unsymmetrical dyes 2
and 3 synthesized by us previously [10] (Fig. 1)
A rationale for this choice is that the exocyclic carbonyl group
conjugated with the chelate ring, activates the coordinating
carbonyl groups towards amines, just as a-trihaloalkyl substituents
do [11]. In addition, 2-acetyldimedone is a convenient starting
reagent to prepare isomeric N,O-coordinating ligands which, as
* Corresponding author. Tel.: þ380 44 5510682; fax: þ380 44 5732643.
E-mail address: zyabrev@ukr.net (K. Zyabrev).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.05.025

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K. Zyabrev et al. / Dyes and Pigments 92 (2011) 749e757
F
F
F
F
F
F
H
H
B
H
O
N
B
B
O
N
O
O
O
O
1) BF3*O(Et)2
2) Δ
N
O
O
1
5
2
O
O
6
F
F
S
F
F
B
H
O
O
N
S
B
N
O
O
S
(CH3CO)2O
N
6
N
O
7
3
O
Scheme 2. Synthetic route to oxazaborine dye 7.
Fig. 1. Boron chelate 1 and related unsymmetrical dyes 2 and 3.
Compound 9 cannot form a chelate presumably due to the
decreased electrophilicity of the boron atom which, in turn, may
result from the relatively high nucleophilicity of the amino group in
ligand 8 (as compared to its isomer 5). The amino group nucleo-
philicity was sufficiently lowered on passing from compound 8 to
its phenyl-substituted derivative 10, so that the latter afforded
shown later, are further involved in the synthesis of key
oxazaborines.
To obtain oxazaborine polymethine dyes, we attempted the
classical method based on the recyclization of 4-pyrylo- to
4-pyridocyanines [12]. The corresponding reaction with amines is
quite typical for 2,2-difluoro-1,3,2(2H)dioxaborines [13]. However,
as found, merocyanines 2 and 3 are unreactive to both weakly basic
(aniline, p-anisidine) and highly basic amines (benzylamine,
chelate 11. The
enough to yield merocyanines 12 and 13 by condensations with
appropriate heterocyclic -aldehydes (Scheme 4).
a-methyl group in its oxazaborine ring proved active
u
methylamine) whereas chelate
1 recyclizes under the same
conditions to oxazaborine 4 in high yield (Scheme 1).
BF3
H
H
The attempted reaction of boron complex 4 with various elec-
trophiles did not proceed, in accordance with the literature data
[13]; an electrophilic attack on the methyl group is likely to be
hindered by the spatially close carbonyl and phenyl groups. The
steric hindrances in the oxazaborine nucleus are avoided in
complex 6 containing no phenyl substituent. As a result, it has been
successfully reacted with the benzothiazole-derived conjugated
N
O
O
H2N
O
O
BF3*O(Et)2
9
8
F
F
u
-aldehyde to provide unsymmetrical dye 7 (Scheme 2).
H
B
To obtain the isomeric oxazaborine, 2-acetyl-3-amino-
N
O
5,5-dimethylcyclohex-2-enone
8 was treated with boron tri-
9
fluoroetherate; disappointingly, the expected chelation failed to
occur, as it stopped at the stage of complex 9. Even the use of
sterically hindered strong bases (such as DBU and N,N-diisopro-
pylethylamine) as HF-abstracting agents was unsuccessful
(Scheme 3).
O
Scheme 3. Reaction of enamine 8.
F
F
F
F
B
Ph
H
Ph
B
O
O
X
N
O
N
O
RNH2
1) BF3*O(Et)2
N
2) Δ
O
O
O
10
11
2 - X = C(CH3)2
3 - X = S
F
F
X
F
F
F
F
B
O
N
X
O
B
Ph
B
N
O
N
O
O
(CH3CO)2O
11
PhNH2, C6H6
N
O
O
O
12 - X = C(CH3)2
13 - X = S
4
1
Scheme 4. Synthesis of oxazaborine 11 and related unsymmetrical dyes 12 and 13.
Scheme 1. Reactions of 1e3 with amines.

K. Zyabrev et al. / Dyes and Pigments 92 (2011) 749e757
751
F2
B1
oxazaborine ring. To compare the oxazaborine dye to its dioxa-
borine counterpart, the same structural determination was also
carried out for the crystals of dye 2. The respective ORTEP drawings
of molecular structures of 2 and 12 are shown in Figs. 2 and 3.
As found by X-ray diffraction, the 6-membered cycle C(16e21)
in molecule 2 is not planar (the maximum deviation from the least-
C1
C9
C2
F1
C10
C6
O2
C7
C8
C3
O3
C13
C17
C15
C18
ꢀ
C4
squares plane amounts to 0.314 A). The fragment C(16e18)
C5
C22
C(20e21) is almost planar (the average deviation from least-
squares plane is 0.001 A) and the dihedral angle between the
N1
C16
C12
ꢀ
C14
C(16e18)C(20e21) and C(18e20) planes is 44.16ꢀ. The dihedral
angle C(15)C(16)C(21)O(1) is 7.4ꢀ. The 6-membered cycle C(16e21)
in molecule 12 is not planar and more distorted than the C(16e21)
cycle in 2 (the maximum deviation from the least-squares ꢀplane
C21
C19
C11
O1
C23
C20
ꢀ
Fig. 2. Molecular structure of 2 (thermal ellꢀipsoids are drawn at the 50% probability
amounts to 0.362 A). The dihedral angle C(15e17)O(2) is 7.9 . The
ꢀ
level). Selected bond lengths [A] and angles [ ]: F(1)eB(1) 1.389(7), F(2)eB(1) 1.323(6),
phenyl ring C(24e29) is almost orthogonal to the plane O(1)C(15)
O(2)eB(1) 1.470(6), O(3)eB(1) 1.440(6), O(2)eC(17) 1.312(4), O(3)eC(15) 1.313(4),
C(15)eC(16) 1.444(5), C(16)eC(17) 1.368(5), O(1)eC(21) 1.219(4), C(14)eC(15) 1.383(5),
C(13)eC(14) 1.396(5), C(12)eC(13) 1.384(5), C(8)eC(12) 1.378(5); O(2)B(1)O(3)
112.2(4), C(20)C(21)C(16) 118.4(3), C(19)C(20)C(21) 114.3(3).
ꢀ
C(16)C(21)N(1) (with the average deviation 0.001 A from the least-
squares plane): the dihedral angle C(29)C(24)N(1)C(21) is 84.23ꢀ.
Due to the conjugation of the N(1) electron pair with the double
bond N(1) ¼ C(21), the trigonal-planar bond configuration of N(1) is
observed, with the sum of the bond angles 359.4ꢀ. The atoms in the
polymethine chain C(15)eC(14)eC(13)eC(12)eC(8) are coplanar;
C26
C27
ꢀ
the average deviation from the least-squares plane is 0.010 A in 12
C25
ꢀ
F1
and 0.014 A in 2. The pattern and magnitude of the bond length
C28
F2
C24
alternation in this conjugated moiety appears to strongly depend
on the nature of the chelate ring. As seen from Figs. 2 and 3, the
alternation changes the sign and grows in magnitude in going from
dye 2 to 12.
B1
C10
C29
N1
C1
O1
C2
C21
C7
C9
C6
C20
C19
C15
C13
Table 1 demonstrates the spectral properties of oxazaborine
merocyanines 7, 12, and 13 as well as their dioxaborine analogues 2
and 3 (for comparison) in weakly and strongly polar aprotic
solvents (dichloromethane and acetonitrile, respectively), and in
a strongly polar protic solvent (formamide). The lmax and lfmax
denote the respective wavelengths of the absorption and fluores-
C3
C16
C17
C8
C14
C5
C22
C12
N2
C4
C18
O2
C23
C11
cence maxima, 3
the molar absorption coefficient, Dlmax and Dlfmax
the corresponding spectral shifts with reference to analogous
dioxaborine dyes, S the absorption band half-width, Dn the Stokes
Fig. 3. Molecular structure of 12 (thermal elꢀlipsoids are drawn at the 50% probability
ꢀ
level). Selected bond lengths [A] and angles [ ]: F(1)eB(1) 1.377(9), F(2)eB(1) 1.382(8),
N(1)eB(1) 1.555(8), N(1)eC(24) 1.438(7), N(1)eC(21) 1.313(9), C(16)eC(21) 1.446(9),
C(15)eC(16) 1.409(8), O(1)eC(15) 1.329(8), O(1)eB(1) 1.446(9), C(14)eC(15) 1.417(9),
C(13)eC(14) 1.358(8), C(12)eC(13) 1.406(8), C(8)eC(12) 1.388(8), N(2)eC(8) 1.374(8),
C(16)eC(17) 1.46(1), O(2)eC(17) 1.23(1); N(1)B(1)O(1) 108.9(6), C(17)C(18)C(19)
109.7(4), C(16)C(17)C(18) 117.8(6).
shift, and
F the fluorescence quantum yield.
It is challenging to elucidate the spectral effects of going from
the O,O- to N,O-chelated BF2 moiety in boron-containing hetero-
cyclic dye end groups. To this end, we invoke the concept of ideal
polymethinic and polyenic states introduced previously by Daehne
on the basis of his triad theory [14]. According to this approach, the
An alternative structure, with the BF2 moiety O,O-chelated by
two carbonyl groups, could be assumed for complex 11. To rule it
out, we performed X-ray diffraction analysis of the crystals of
related dye 12 which unequivocally confirmed the presence of the
p-properties of donor-acceptor (merocyanine) dyes are conve-
niently treated in terms of three limiting resonance structures
referring to the ground state, viz., polyenic non-charge-separated
Table 1
Spectral characteristics of merocyanines 2, 3, 7,a 12, and 13.
Dye
Solvent
lmax [nm]
10ꢁ5, Mꢁ1 cmꢁ1
)
Dlmax [nm]
S [cmꢁ1
]
lfmax [nm]
Dlfmax [nm]
Dy [cmꢁ1
]
V
(
3
2
3
CH2Cl2
CH3CN
CH2Cl2
CH3CN
HCONH2
534 (1.314)
532 (1.142)
545 (1.428)
538 (0.990)
538
505
545
538
506
e
e
e
e
e
1666
1937
992
1976
2674
556
552
559
555
556
e
e
e
e
e
741
681
460
569
602
0.03
0.01
0.04
0.01
0.02
b
b
b
b
7
CH2Cl2
0
0
856
2893
559
555
0
0
460
569
0.02
>0.01
CH3CNb
HCONH2
CH2Cl2
CH3CN
CH2Cl2
CH3CN
HCONH2
505
0
2
4
13
17
25
3182
1590
1722
1207
1264
1119
559
569
567
579
576
582
3
13
15
20
21
26
e
0.01
0.65
0.16
0.65
0.16
0.15
12
13
536 (1.276)
536 (1.251)
558 (1.516)
555 (1.525)
563
1082
1020
650
657
580
a
The analogous merocyanine with the indolenine end group could not be obtained.
Quantitative spectra of dyes could not be recorded due to their poor solubility or thermal instability in solution.
b

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K. Zyabrev et al. / Dyes and Pigments 92 (2011) 749e757
δ+
CH2Cl2
D
δ−
D
D
1.0
A
A
A
A1
A2
A3
0.8
Scheme 5. Resonance structures of donoreacceptor dyes.
HCONH2
0.6
0.4
0.2
0.0
A1, polymethinic A2, and polyenic charge-separated A3 (Scheme 5).
As predicted by intuition and supported experimentally [15],
transitions between the structures are governed by the
donoreacceptor strength of end residues and/or solvent polarity.
For weak donors and/or acceptors, the type A1 realizes which is
characterized by positive solvatochromism. By increasing the
donoreacceptor strength of end substituents to a certain value, the
ideal polymethinic structure A2 is reached which is remarkable for
practically full equalization of bond orders, just as in symmetrical
cyanine dyes; accordingly, the molecules of the type A2 are only
slightly solvatochromic, if at all (cf. to slight negative sol-
vatochromism of symmetrical cyanines). On further rise of the
donoreacceptor strength of end groups, the electronic structure of
merocyanines tends from A2 toward A3, the latter exhibiting
pronounced negative solvatochromism.
CH3CN
400
450
500
600
/nm 550
λ
Fig. 5. Normalized absorption spectra of dye 7.
For boron chelate merocyanines, the boron-containing nucleus
acts as an acceptor and the other aza heterocyclic end group as
a donor.
only due to an increase in the solvent dielectric constant but also as
a result of dye-formamide H-bonding through the unchelated
carbonyl group of merocyanine 3. One can also assume, by analogy
with previously reported studies [15a], that another effect of
H-bonding is the enhanced 0e1 vibronic absorption peak (which is
identified by its 1181 cmꢁ1 shift from the 0e0 transition band).
Replacing the NH group for the O-atom in the chelate ring, i.e.,
going from dye 3 to 7, gives rise to the second centre of H-bonding
of dye molecules by highly polar solvents (e.g., formamide) thus
leading to an inverse intensity distribution between the 0e0 and
0e1 vibronic bands (cf. to the identical absorption band shapes of 3
and 7 in weakly polar aprotic dichloromethane) Scheme 6. Like
cyanines [16], dyes 3 and 7 tending to the polyenic charge-
separated structure A3 are characterized by much weaker specific
solvation in the excited than in the ground state and, hence, by
much less pronounced solvation effects on fluorescence than on
absorption (though solvents, of course, influence the electron
density distribution in dye molecules). This is evident from the
comparison of the absorption and fluorescence band shapes for
dyes 3 and 7 (cf. Fig. 4 with Fig. 6 and Fig. 5 with Fig. 7), also taking
into account the fluorescence excitation band shapes for dye 7 (see
Fig. 8).
Comparing the spectral data for dioxaborine dye 3 and oxaza-
borine dye 7, both containing the benzothiazole nucleus as the
second end group, it is seen that substitution of the 3-O atom by the
NH group in the chelate ring has almost no effect on absorption and
fluorescence band positions as well as fluorescence intensity. At the
same time, compound 7 is more readily solvated due to the pres-
ence of the NH group, which, in turn, causes the strong intensity
redistribution between the 0e0 and 0e1 vibronic absorption
bands.
Figs. 4 and 5 demonstrate the absorption spectra of mer-
ocyanines 3 and 7 in various solvents. Negative solvatochromism of
dye 3 (see Table 1) suggests that this compound is structurally
between the types A2 and A3. Accordingly, it tends to A2 with
decreasing solvent polarity. Vice versa, switching from acetonitrile
to formamide brings the molecular structure closer to A3 and not
It is seen that the bands in the fluorescence and fluorescence
excitation spectra are narrower and more structured than in the
absorption spectra, without a strong 0e1 vibronic peak. The fluo-
rescence excitation spectra of dye 7 allow to implicitly assess the
H
H
N
H
F
F
F
F
O
B
H
B
O
N
S
O
O
S
N
N
7
3
O
O
H
N
H
N
H
H
H
H
O
O
Fig. 4. Absorption spectra of dye 3 (the star denotes qualitative measurement).
Scheme 6. Probable solvatation of dyes 3 and 7 by formamide.

K. Zyabrev et al. / Dyes and Pigments 92 (2011) 749e757
753
1.0
0.8
0.6
0.4
0.2
0.0
1,0
0,8
0,6
CH3CN
HCONH2
HCONH2
0,4
0,2
0,0
CH3CN
CH2Cl2
CH2Cl2
500
550
600
650
700
750
λ
/nm
350
400
450
500
550
600
650
Fig. 6. Fluorescence spectra of dye 3.
λ/nm
Fig. 8. Fluorescence excitation spectra of dye 7.
absorption of the unsolvated compound; as expected, they are
mirror images of the corresponding fluorescence spectra. Due to
the presence of an additional H-bonding centre, oxazaborine dye 7
is more sensitive to the action of polar solvents than its dioxaborine
analogue 3 (see above) and, accordingly, exhibits a broader fluo-
rescence band in formamide.
Now consider the spectral properties of the dyes derived from
oxazaborine 11 in which the 1-O atom of the dioxaborine ring is
substituted by the NPh group. Comparing the spectral character-
istics of dyes 2 and 3 to those of 12 and 13, respectively (see
Table 1), one can see that going from the O,O- to N,O-chelate ring is
accompanied by a bathochromic effect, slight for indolenine dyes
(2e4 nm for absorption and 13e15 nm for fluorescence) and more
pronounced for their benzothiazole analogues (13e25 and
oxazaborine dyes 12 and 13 (respective blue shifts of 0 and 3 nm on
passing from dichloromethane to acetonitrile) almost vanishes as
compared to their dioxaborine counterparts (2 and 7 nm) and even
changes its sign to positive in some media (a red shift of 5 nm for 13
on passing from dichloromethane to formamide) (Fig. 9). Moreover,
12 and 13 show a some tendency towards absorption band nar-
rowing relative to the corresponding dioxaborine dyes 2 and 3
(except dye 13 in dichloromethane), in contrast to what is observed
for dye 7 vs. 3 except their dichloromethane solutions (trace the
values S in Table 1). The trends in fluorescence spectra are quite
similar (cf. Fig. 6 with Fig. 10). All these features consistently
suggest that dyes 12 and 13, though falling in the same negatively
solvato(fluoro)chromic region A2eA3 as dyes 2, 3 and 7, are closer
to the polymethinic type A2 and can, under certain conditions,
move to the region A1eA2 accordingly changing the sign of sol-
vato(fluoro)chromism. The specificity found is likely the result from
20e26 nm).
Interestingly,
negative
solvatochromism
of
Fig. 7. Fluorescence spectra of dye 7.
Fig. 9. Absorption spectra of dye 13 (the star denotes qualitative measurement).

754
K. Zyabrev et al. / Dyes and Pigments 92 (2011) 749e757
HCONH2
CH2Cl2
1.0
0.8
0.6
0.4
0.2
0.0
CH3CN
Fig. 12. Excited singlet and triplet state energies in dyes 2 and 12 obtained at the RHF/
6-31G** level of theory, with 10 occupied and 10 virtual MOs included in the config-
uration interaction space.
substituents in heterocyclic systems led to large Stokes shifts and
high quantum yields of the resulting molecules due to their excited-
state planarization [17], here we can hardly attribute the
enhancement of fluorescence to the specific effects of the phenyl
group. First, the 84ꢀ-degree twisted phenyl ring participates very
550
600
700
65λ0 /nm
Fig. 10. Fluorescence spectra of dye 13.
slightly in the frontier p-MOs which are mainly involved in the first
electronic transition (see Figs. 3 and 11). Second, this group is
unlikely to achieve significant coplanarization with the rest of the
molecule in the relaxed S1 state, since the Stokes shifts of 12 and 13
are only a little, if at all, larger than those of 2 and 3, respectively
(see Table 1).
the lower electron-acceptor strength of N,O-chelate nucleus 11
than that of its O,O analogue 1 or N,O-chelate 6 with the isomeric
position of the N atom. This assumption is also corroborated by the
X-ray analytical data for compounds 2 and 12 presented in Figs. 2
and 3: in the solid state, a smaller bond alternation in the poly-
methine chain of 2 suggests that it is closer than 12 to the ideal
polymethinic bond-equalized type A2 (whereas the opposite is
inferred for the dissolved dyes from the magnitudes of their sol-
vatochromism). As indicated by the opposite alternation patterns of
the two compounds, they are positioned on different sides of A2:
charge-separated dye 2 in the region A2eA3 and non-charge-
separated dye 12 in A1eA2. Going from the former to the latter
region implies a decrease in the donoreacceptor strength of end
substituents (in our case, a weakening of the acceptor character of
the chelate ring). Another evidence of the decreased acceptor
strength of the NPh-substituted chelate is provided by the ab initio
calculated frontier MO energies of dyes 2 and 12: they are notice-
ably higher for the latter, as shown in Fig. 11.
We have also suggested intersystem crossing, a possible channel
of the fluorescence quenching, to be present in dye 2 and absent in
12. However, the calculated energies of the excited singlet and
triplet states do not support this assumption, since S1 depopulation
via the close-lying triplet state(s) appears even more probable in
the oxazaborine than in the dioxaborine merocyanine (see Fig. 12).
As another plausible explanation for the enhanced fluorescence
of dyes 12 and 13 compared to 2 and 3 (and also to most of the
merocyanines [18]), we can assume that the oxazaborine dyes have
a reduced probability for the S0eS1 conical intersections associated
with bond twisting. Conical intersections causing photo-
isomerizations or radiationless returns of reagents to the initial S0
state act as photochemical funnels; a driving force for their occur-
rence is, as a rule, a S1 potential surface curvature in the vicinity of
the FranckeCondon region caused by electron density localization
in certain parts of the excited molecule (this is typical of push-pull
merocyanines as well as of short polymethines) [19]. It can be that
substitution of the 1-O ring atom by the less electron-acceptor NPh
group in the chelate merocyanines concerned leads to a flattened S1
potential surface thereby preventing the interstate conical inter-
section in dyes 12 and 13.
As found, substitution of the 1-O ring atom by the NPh group
leads to a drastic 10e20-fold increase in the fluorescence efficiency
of the corresponding merocyanines irrespective of solvent polarity
(cf. respective
F values for dyes 2, 3 and 12,13 in Table 1). Unlike the
previously reported cases when going from alkyl to (het)aryl
3. Conclusion
We have first synthesized the 3-aza and 1-aza analogues of
2,2-difluoro-1,3,2(2H)dioxaborines as well as two series of the
corresponding oxazaborine merocyanines and studied the spec-
troscopic effects of the O-to-N substitution in both dye types. As
found, 3-NH oxazaborine dyes have much the same absorption and
fluorescence band positions as the corresponding dioxaborine
compounds but exhibit different absorption band shapes (as
a result of the enhanced solvation and hence the intensity redis-
tribution between the 0e0 and 0e1 vibronic absorption peaks).
Contrastingly, 1-NPh oxazaborine merocyanines are characterized
by a red shift in absorption and fluorescence, and also by more than
one order increase in the fluorescence quantum yield.
Fig. 11. HOMO and LUMO energies and shapes for 2 and 12 determined at the UHF/6-
31G** level of theory. (The molecular geometry is based on X-ray analytical data).

K. Zyabrev et al. / Dyes and Pigments 92 (2011) 749e757
755
4. Experiment
obtained mixture was heated at reflux for 4 h and allowed to cool to
room temperature. The resulting solid was suspended in diethyl
ether (20 ml). The precipitate was filtered off and washed with
diethyl ether (3 ꢂ 15 ml). The crude product was dissolved in MeOH
(5 ml) and the solid was filtered off. Diethyl ether (30 ml) was
added to the filtrate, followed by filtering off the precipitate and
Frontier molecular orbital energies as well as energies of the
excited singlet and triplet states for dye 2 and 12 were determined
by standard ab initio calculations with the aid of the HyperChem.
Package. 1H NMR spectra were obtained with Varian VXR 300 and
Varian Gemini 2000 instruments at 300 and 400 MHz, respectively,
using TMS as internal reference. Electronic absorption spectra were
recorded on a Shimadzu UV-3100 spectrophotometer. Fluorescence
and fluorescence excitation spectra were taken on a Cary Eclipse
instrument and were fully corrected. The fluorescence quantum
washing it with diethyl ether (2 ꢂ 10 ml). Yield 0.6 g (43%); m.p.
1
194 ꢀC; H NMR (300 MHz, [D6]DMSO, 25 ꢀC):
d
¼ 0.95 (s, 6H,
2CH3), 2.17 (s, 2H, CH2), 2.35 (s, 3H, CH3), 2.47 (s, 2H, CH2), 8.72 (br.
s.1H, NH),10.82 ppm (br. s.1H, NH); elemental analysis calcd (%) for
C10H15BF3NO2: C 48.23, H 6.07, N 5.62; found: C 48.77, H 6.00, N
yields (
F
) of dyes were determined by a known method [20] rela-
5.43.
tive to Rhodamine 6 G (
F
¼ 0.95 in EtOH).
4.5. Oxazaborine 11
4.1. Oxazaborine 4
A mixture of enamine 10 [23] (1.24 g, 5 mmol) and boron tri-
fluoride etherate (0.76 ml, 6 mmol) was heated at 80 ꢀC for 1 h and
then allowed to cool to room temperature. The resulting solid was
suspended in diethyl ether (30 ml) and then the ether layer was
removed. The crude product was suspended in i-PrOH (20 ml),
followed by filtering off the obtained precipitate, washing it twice
with i-PrOH (10 ml), and recrystallization from i-PrOH. Yield 0.64 g
Aniline (0.91 ml, 10 mmol) was added to a suspension of diox-
aborine 1 (2.3 g, 10 mmol) and benzene (15 ml). The mixture was
heated with a Dean-Stark trap at reflux for 5 h, allowed to cool to
room temperature, and left for 12 h. The solid was filtered off,
washed with benzene (2 ꢂ10 ml), and recrystallized from i-PrOH.
1
Yield 2.32 g (76%); m.p. 131 ꢀC; H NMR (300 MHz, CDCl3, 25 ꢀC):
d
¼ 1.14 (s, 6H, 2CH3), 2.44 (s, 5H, CH3 þ CH2), 2.71 (s, 2H, CH2), 7.18
(42%); m.p. 176 ꢀC; 1H NMR (300 MHz, CDCl3, 25 ꢀC):
d
¼ 0.98 (s, 6H,
(d, 3J(H,H) ¼ 8.0 Hz, 2H, ArH); 7.40e7.57 ppm (m, 3H, ArH);
elemental analysis calcd (%) for C16H18BF2NO2: C 62.98, H 5.95, N
4.59; found: C 63.12, H 6.06, N 4.69.
2CH3), 2.24 (s, 2H, CH2), 2.39 (s, 2H, CH2), 2.76 (s, 3H, CH3), 7.17
(d, 3J(H,H) ¼ 6.6 Hz, 2H, ArH), 7.41e7.52 ppm (n, 3H, ArH).
elemental analysis calcd (%) for C16H18BF2NO2: C 62.98, H 5.95, N
4.59; found: C 62.90, H 5.93, N 4.63.
4.2. Oxazaborine 6
4.6. Merocyanines 12, 13. General procedure
A mixture of enamine 5 [21] (1.0 g, 5.5 mmol) and boron tri-
fluoride etherate (1.4 ml, 11 mmol) was heated at 100 ꢀC for 0.5 h
and an additional portion of boron trifluoride etherate (0.5 ml,
3.9 mmol) was added. On elevating the temperature to 150 ꢀC, the
mixture was heated for 1 h and then allowed to cool to room
temperature. After the system had been evaporated to remove
volatile reagents and products, the residue was extracted by boiling
diethyl ether (5 ꢂ10 ml). The extract was evaporated and the
precipitate was recrystallized from diethyl ether. Yield 0.8 g (63%);
Acetic anhydride (0.5 ml) was added to
a mixture of
compound 11 (0.25 g, 0.8 mmol) and Fisher’s aldehyde (0.16 g,
0.8 mmol) or 3-methyl-3H-benzothiazole-2-ylidene)-acetalde-
hyde (0.15 g, 0.8 mmol). The obtained mixture was heated at
100 ꢀC for 0.5 h and then allowed to cool to room temperature.
After addition of diethyl ether (20 ml), the mixture was left for
1 h. The precipitate was filtered off, washed twice with ether
(30 ml), and recrystallized from CH3CN.
m.p. 142 ꢀC; 1H NMR (300 MHz, CDCl3, 25 ꢀC):
d
¼ 1.10 (s, 6H, 2CH3),
2.39 (s, 2H, CH2), 2.67 (s, 2H, CH2), 2.70 (s, 3H, CH3), 7.79 ppm (br. s.
1H, NH); elemental analysis calcd (%) for C10H14BF2NO2: C 52.44, H
6.16, N 6.12; found: C 52.56, H 6.21, N 6.05.
4.6.1. Merocyanine 12
Yield 0.11 g (25%); m.p. 294 ꢀC; 1H NMR (300 MHz, CDCl3,
25 ꢀC):
d
¼ 0.98 (s, 6H, 2CH3), 1.66 (s, 6H, 2CH3), 2.28 (s, 2H, CH2),
2.36 (s, 2H, CH2), 3.34 (s, 3H, NCH3), 5.85 (d, 3J(H,H) ¼ 13.5 Hz, 1H,
Ha), 6.86 (d, 3J(H,H) ¼ 7.5 Hz, 1H, ArH), 7.06 (t, 3J(H,H) ¼ 6.9 Hz, 1H,
ArH), 7.17e7.51 (m, 7H, ArH), 7.64 (d, 3J(H,H) ¼ 13.5 Hz, 1H, Hg),
8.59 ppm (t, 3J(H,H) ¼ 13.5 Hz, 1H, Hb); elemental analysis calcd (%)
for C29H31BF2N2O2: C 71.31, H 6.40, N 5.74; found: C 71.65, H 6.59, N
5.82.
4.3. Merocyanine 7
A mixture of oxazaborine 6 (0.1 g, 0.44 mmol), 3-methyl-3H-
benzothiazole-2-ylidene)acetaldehyde (0.083 g, 0.44 mmol), and
acetic anhydride (1 ml) was heated at 75 ꢀC for 15 min. After
cooling to room temperature, diethyl ether (20 ml) was added and
the resulting mixture was allowed to stand for 30 min. The
precipitate was filtered off, washed with diethyl ether (2 ꢂ 10 ml),
and recrystallized from CH3CN. Yield 0.058 g (32%); m.p. 211 ꢀC; 1H
4.6.2. Merocyanine 13
Yield 0.10 g (29%); m.p. 258 ꢀC; 1H NMR (300 MHz, CDCl3,
25 ꢀC):
d
¼ 0.97 (s, 6H, 2CH3), 2.27 (s, 2H, CH2), 2.36 (s, 2H, CH2),
NMR (400 MHz, CDCl3, 25 ꢀC):
d
¼ 1.09 (s, 6H, 2CH3), 2.38 (s, 2H,
3.56 (s, 3H, NCH3), 5.99 (d, 3J(H,H) ¼ 13.6 Hz, 1H, Ha), 7.10e7.25 (m,
3H, ArH), 7.34e7.56 (m, 6H, ArH), 7.65 (d, 3J(H,H) ¼ 13.6 Hz, 1H, Hg),
8.31 ppm (t, 3J(H,H) ¼ 13.6 Hz, 1H, Hb); elemental analysis calcd (%)
for C26H25BF2N2O2S: C 65.28, H 5.27, N 5.86; found: C 65.19, H 5.37,
N 5.94.
CH2), 2.61 (s, 2H, CH2), 3.74 (s, 3H, NCH3), 6.22 (d, 3J(H,H) ¼ 13.4 Hz,
1H, Ha), 7.32e7.43 (n, 3H, 2ArH þ Hg), 7.51 (t, 3J(H,H) ¼ 7.8 Hz, 1H,
ArH), 7.69 (d, 3J(H,H) ¼ 7.8 Hz, 1H, ArH), 8.53 ppm (t, 3J(H,H) ¼
13.4 Hz, 1H, Hb); elemental analysis calcd (%) for C20H21BF2N2O2S: C
59.72, H 5.26, N 6.96; found: C 60.02, H 5.19, N 6.92.
4.7. X-ray structure determination for 2 and 12
4.4. Complex 9
Crystals of 2 and 12 suitable for X-ray diffraction analysis were
grown slowly from a saturated dye solution in i-PrOH and aceto-
nitrile, respectively.
A mixture of enamine 8 [22] (1.0 g, 5.5 mmol) and boron tri-
fluoride etherate (2.1 ml, 16.5 mmol) was heated at 100 ꢀC for 0.5 h.
After cooling to room temperature, boron trifluoride etherate
(0.5 ml, 3.9 mmol) in dichlorobenzene (10 ml) was added. The
Crystals of 2: C23H26B1F2N1O3; M ¼ 413.27; system: monoclinic,
space group: P21/c (N 14); unit-cell dimensions: a ¼ 9.636(1),

756
K. Zyabrev et al. / Dyes and Pigments 92 (2011) 749e757
3
ꢀ
ꢀ
ꢀ
[4] Zyabrev K, Doroshenko A, Mikitenko E, Slominskii Y, Tolmachev A. Design,
synthesis, and spectral luminescent properties of a novel polycarbocyanine
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type. Chem Eur J 1999;5:2511e7.
[6] Gerasov AO, Shandura MP, Kovtun YP. Series of polymethine dyes derived
from 2,2-difluoro-1,3,2-(2H)-dioxaborine of 3-acetyl-7-diethylamino-4-
hydroxycoumarin. Dyes Pigm 2008;77:598e607.
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complexes of acetylhydroxycoumarins. Dyes Pigm 2003;58:41e6.
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Structure and properties. Sofia: St. Kliment Ohridski Univ Press; 1991;
(b) Griffiths J. Colour and constitution of organic molecules. Academic Press;
1976;
b ¼ 16.562(2), c ¼ 14.445(1) A,
b
¼ 107.679(4) , V ¼ 2196.4(4) A ,
Z ¼ 4, calculated density: 1.250 g cmꢁ3
,
m
(MoK
a
) ¼ 0.092 mmꢁ1
,
F(000) ¼ 872.
Crystals of 12: C29H31B1F2N2O2; M ¼ 488.38; system: mono-
clinic, space group Cc (N9); unit-cell dimensions: a ¼ 16.034(4),
3
ꢀ
ꢀ
b ¼ 15.491(5), c ¼ 10.250(3) A,
b
¼ 94.43(1)ꢀ, V ¼ 2538.1(1)A ,
Z ¼
4
4, calculated density: 1.278 g cmꢁ3
,
m
(MoK
a
) ¼ 0.089 mmꢁ1
,
F(000) ¼ 1032.
The intensities of 24,817 (2) and 5836 (12) reflections were
measured at room temperature on a Bruker Smart Apex II
diffractometer operating in the
u and scans mode. 4500 (2) and
3336 (12) unique reflections [Rint ¼ 0.066 (2), 0.073 (12)] were
used in further refinement. Data were corrected for Lorentz and
polarization effects. The structure was solved by direct methods
and refined by the full-matrix least-squares technique in the
anisotropic approximation for non-hydrogen atoms using the
SHELXS97 and SHELXL97 programs [24], and CRYSTALS program
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Fourier maps and refined with fixed positional and thermal
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refinement converged to Rw ¼ 0.035, R1 ¼0.038, GOF ¼ 1.135 for
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observed 1276 reflections with I > 3
Rw ¼ 0.061, R1 ¼0.060, GOF ¼ 0.687 for observed 1781 reflections
with I > 2.5
s
(I) (obsd./var. ¼ 4.7); for 12, to
s
(I) (obsd./var. ¼ 5.44). CCDC reference number is
777793 for 2 and 777794 for 12.
Atomic coordinates, bond lengths, bond angles, and thermal
parameters have been deposited at the Cambridge Crystallographic
Data Centre (CCDC). These data can be obtained free of charge via
www.ccdc.cam.uk/conts/retrieving.html (or from the CCDC, 12
Union Road, Cambridge CB2 1EZ, UK; fax: þ44 1223 336 033; or
deposit@ccdc.cam.ac.uk). Any request to the CCDC for data should
quote the full literature citation and CCDC reference numbers.
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Acknowledgements
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The authors are grateful to Prof. A.A. Ishchenko and Prof. A.D.
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