Syntheses of rod-shaped fluorescent 1,3,2-benzodiazaboroles with phosphonium, and phosphane chalcogenide acceptor functions

Article abstract of DOI:10.1039/c2dt30666b

A series of 1,4-phenylenes X-C₆H₄-BDB with a 1,3,2-benzodiazaborolyl (BDB) and a phosphorus based end group [X = PPh ₂ (2), P(O)Ph₂ (3), P(S)Ph₂ (4), P(Se)Ph ₂ (5), P(AuCl)Ph₂ (6) and P(Me)Ph₂ (7)] as well as 2-(2′)thienyl-1,3,2-benzodiazaboroles with a second end group X [X = PPh₂ (8), P(S)Ph₂ (9), P(Se)Ph₂ (10) and P(Me)Ph₂ (11)] in the 5′ position were synthesised using established methodologies. Molecular structures of 2-9 and 11 were determined by X-ray diffraction. Compounds 3, 4, 6, 7, 9 and 11 show intense blue luminescence in cyclohexane, toluene, chloroform, dichloromethane and tetrahydrofuran with pronounced solvatochromism. Thereby Stokes shifts in the range of 8950-10440 cm^-1 and quantum yields up to 0.70 were observed in dichloromethane solutions. In contrast to this, for the selenides 5 and 10 quantum yields are small (<0.1). The absorption maxima (298-340 nm) are well reproduced by TD-DFT computations (B3LYB/G-311G(d,p)) and arise from strong HOMO-LUMO transitions. With the exception of 5 and 10 the HOMOs of the molecules under study are mainly located on the benzodiazaborole group. In 5 and 10 the HOMOs are on the selenium atoms. The LUMOs of all new neutral molecules are mainly represented by the phenylene or thiophene bridge. In the phosphonium cations the LUMOs have additional contributions from the phosphonium unit.

Full text of DOI:10.1039/c2dt30666b

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PAPER
Syntheses of rod-shaped fluorescent 1,3,2-benzodiazaboroles with
phosphonium, and phosphane chalcogenide acceptor functions†
Lothar Weber,*a Henry Kuhtz,a Lena Böhling,a Andreas Brockhinke,a Anna Chrostowska,b Alain Dargelos,b
Audrey Mazière,b Hans-Georg Stammlera and Beate Neumanna
Received 23rd March 2012, Accepted 27th June 2012
DOI: 10.1039/c2dt30666b
A series of 1,4-phenylenes X-C6H4-BDB with a 1,3,2-benzodiazaborolyl (BDB) and a phosphorus based
end group [X = PPh2 (2), P(O)Ph2 (3), P(S)Ph2 (4), P(Se)Ph2 (5), P(AuCl)Ph2 (6) and P(Me)Ph2 (7)] as
well as 2-(2′)thienyl-1,3,2-benzodiazaboroles with a second end group X [X = PPh2 (8), P(S)Ph2 (9),
P(Se)Ph2 (10) and P(Me)Ph2 (11)] in the 5′ position were synthesised using established methodologies.
Molecular structures of 2–9 and 11 were determined by X-ray diffraction. Compounds 3, 4, 6, 7, 9 and 11
show intense blue luminescence in cyclohexane, toluene, chloroform, dichloromethane and
tetrahydrofuran with pronounced solvatochromism. Thereby Stokes shifts in the range of
8950–10 440 cm−1 and quantum yields up to 0.70 were observed in dichloromethane solutions. In
contrast to this, for the selenides 5 and 10 quantum yields are small (<0.1). The absorption maxima
(298–340 nm) are well reproduced by TD-DFT computations (B3LYB/G-311G(d,p)) and arise from
strong HOMO–LUMO transitions. With the exception of 5 and 10 the HOMOs of the molecules under
study are mainly located on the benzodiazaborole group. In 5 and 10 the HOMOs are on the selenium
atoms. The LUMOs of all new neutral molecules are mainly represented by the phenylene or thiophene
bridge. In the phosphonium cations the LUMOs have additional contributions from the phosphonium
unit.
like these can display sizable second and third order non-linear
Introduction
optical (NLO) properties.5–10 Due to their low LUMO energies
Over the last two decades the development of fluorescent
organic compounds for potential application in photonic devices
has been the focus of significant interest. Among an infinite
number of molecular structures with the desired character, three-
coordinate organoboron compounds and polymers are of emer-
ging importance.1,2 Their linear and non-linear optical and elec-
tronic properties make them particularly attractive for use in
functional materials. Herein the three-coordinate boron centre
usually behaves as a π-acceptor due to its vacant p-orbital. This
field has been dominated by the use of dimesitylboryl (BMes2,
Mes = 2,4,6-Me3C6H2) and related moieties as the unsaturated
boron centre herein is sterically well shielded by methyl groups.
The π-acceptor character of the BMes2 unit is similar to NO2 and
CN based on UV3 and cyclovoltammetric data.4 Compounds
they are interesting as efficient electron-transporting or emitting
layers in organic light emitting diodes (OLEDs).11–13 Organo-
boron containing species are often strongly colored and/or lumi-
nescent14 which renders them useful as colorimetric or
luminescent sensors for anions such as fluoride15,16 and cyanide.17
Conjugated molecules with boryl substituents display very large
Stokes shifts and high quantum yields in solution as well as in the
solid state, which indicates the lack of close packing as a result of
the sterically congesting mesityl groups.18,19
In the past decade, the chemistry of 1,3,2-diazaboroles has
been rapidly developed.20–25 Some of these compounds show
strong luminescence as solids and in solution.26–34 For synthetic
reasons the 1,3-diethyl-1,3,2-benzodiazaborolyl group (1,3-Et2-
1,3,2-N2BC6H4) is frequently used and compounds having this
function are moderately air-stable.22,28–33 Calculations on 2-aryl-
ethynyl-1,3,2-benzodiazaboroles showed a localisation of the
HOMO on the diazaborole group, leading to the suggestion that
this group could act as a π-donor.33 This proposal was confirmed
by the easy oxidation of diazaboroles in cyclic voltammetric
experiments.24,31,34 In line with these observations it was
obvious to study syntheses and photophysical properties of
compounds containing different types of three-coordinate
boron centers which may function as a π-donor on one end or as
a π-acceptor on the other end of the molecule. Thereby it was
disclosed that the π-electron donating capacity of the 1,3-diethyl-
aFakultät für Chemie, Universität Bielefeld, Universitätsstraße 25,
33615 Bielefeld, Germany. E-mail: lothar.weber@uni-bielefeld.de;
Fax: (+49) 5211066146; Tel: (+49) 5211066169
bEquipe Chimie Physique, IPREM, UMR 5254, Université de Pau et des
Pays de l’Adour, 2 Av. Du Président Angot, BP 1155 64013 Pau Cedex,
France. E-mail: anna.chrostowska@univ-pau.fr;
Fax: (+33) 559407622; Tel: (+33) 559407580
†Electronic supplementary information (ESI) available: Tables of atomic
coordinates for [B3LYP/6-311G(d,p)] optimized geometries, values of
total energies and ionization energies (IEs) of 2–11. CCDC
873268–873276. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt30666b
10440 | Dalton Trans., 2012, 41, 10440–10452
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1,3,2-benzodiazaborolyl group towards the BMes2-unit lies
between that of methoxy- and dimethylamino-groups.35
reveal singlets in the range of δ = 28.6 to 29.3 ppm. The singlet
of the phosphorus atom in the 31P{1H}NMR-spectrum of 2 (δ =
−5.3 ppm) is significantly downfield shifted by oxidation (3: δ =
29.5 ppm), sulfide formation (4: δ = 43.4 ppm) and selenation
(5: δ = 35.4 ppm). The latter resonance is accompanied by 79Se-
satellites (1JP,Se = 728.8 Hz). Coordination of 2 to the gold
center in 6 gives rise to a singlet at 33.5 ppm, whereas phos-
phonium salt 7 displays a singlet at δ = 22.1 ppm. In the thio-
phene-substituted derivative 8, the 31P{1H}NMR-resonance was
observed at considerably higher field (δ = −20.4 ppm). In the
corresponding sulfide 9 (δ 31P = 33.2 ppm), selenide 10 (δ 31P =
To improve the different photophysical properties of the fluor-
ophor in terms of higher molar extinction coefficients, higher
fluorescence quantum yields and better photo-, thermo- and
electro-stabilities, we also looked at alternative acceptor systems.
In a number of informative papers triarylphosphane-oxides and
-sulfides were successfully employed as acceptor units in highly
fluorescent push–pull chromophores in extended organic π-elec-
tron systems.36,37 Thus, it was obvious to also consider diaryl-
phosphanes, their chalcogenides, phosphonium salts and related
gold complexes as potential acceptors in our push–pull chromo-
phores. p-Boryl-phenylene-phosphonium salts have been tested
as fluoride and cyanide sensors before. In contrast to our
approach these systems contain π-accepting diarylboryl units
instead of donating functions.16,17
1
21.6 ppm, JP,Se = 763.0 Hz) and phosphonium salt 11 (δ 31P =
15.3 ppm), the expected deshielding of the four-coordinate phos-
phorus atoms is registered.
1H and 13C{1H}NMR-spectra of the novel compounds are
similar to those of numerous other benzodiazaboroles.
X-Ray crystallography
Results and discussion
Molecular structures were determined for the p-phenylene-based
benzodiazaboroles 2–7 (Fig. 1, Table 1) and the three thiophene-
based derivatives 8, 9, 11 (Fig. 2, Table 2). Bond lengths and
torsion angles of interest are listed in Tables 1 and 2. These
experimental data are compared with the corresponding B3LYP/
6-311(d,p) calculated values (given in italics in Tables 1 and 2).
The unit cell of compound 3 contains two independent mol-
ecules and one molecule of water, which is involved in hydrogen
bonding to the oxygen atoms of the PvO-functions (O(1)⋯O(3)
= 2.792 Å, O(2)⋯O(3) = 2.875 Å). Bonding parameters of the
independent molecules are identical within 3 esd’s.
The bond lengths within the 1,3-diethyl-1,3,2-benzodiazabor-
olyl fragments and the B–C lengths in all structures are virtually
identical as reflected in the B–N, N–C(alkyl), N–C(aryl) and B–
C-separations of ca. 1.42, 1.46, 1.40 and 1.57 Å (Tables 1 and 2).
The geometrical parameters of the p-phenylene- and the 2,5-
thienyl-units are not exceptional and in full agreement with other
benzodiazaborolyl-arenes and -thiophenes.32 The P–C(Ph) dis-
tances in 2 [1.834(1) Å(av.)] as well as those in 3 [1.801(3) Å
(av.)], 4, 5 [1.814(1) Å(av.)], 6 [1.820(4) Å(av.)] and 7 [1.798(1)
Å(av.)] are comparable with those in related PPh3
derivatives.41–45 The agreement between experimental and calcu-
lated values for 2–7 is pretty good. For the bond lengths the
difference does not exceed 0.065 Å while the biggest deviation
from experimental of 12.9° is noted for the torsion angle C(13)–
C(14)–P(1)–C(29) in 7.
It is remarkable, however, that in the thiophene based benzo-
diazaboroles 8, 9 and 11 the P–C(thiophene) links [1.820(1),
1.798(2), 1.775(1) Å)] are shorter than the remaining P–C
(phenyl) distances within 3 esd’s [1.834(1), 1.840(1), 1.813(2),
1.818(2), 1.797(1); 1.802(1) Å]. In contrast to this, all three
arylic P–C bonds in the phenylene-based benzodiazaboroles do
not differ significantly. The bond lengths PvO in 3 (1.489(2)),
PvS in 4 and 9 (1.960(4); 1.957(1)), PvSe in 5 (2.115(4)), and
P–Au in 6 (2.225(1)) are comparable to those in Ph3PvE (E =
O: 1.46(1) Å;41 S: 1.950(3) Å;42 Se: 2.106(1) Å43), Ph3PAuCl
(2.235(3) Å),44 and Ph3P–CH3+ (1.790(2) Å).45
The new borolyl compounds 2 and 8 were formed by the estab-
lished protocol of reacting 2-bromo-1,3-diethyl-1,3,2-benzodi-
azaborole 123 with suitable organolithiums which are available
by treatment of either 4-bromophenyl-diphenylphosphane38 or
2-bromo-5-diphenylphosphanyl-thiophene39 with 2 equiv. of
tert-butyllithium at low temperature (Scheme 1).
The oxidation of 2 to the corresponding triarylphosphane
oxide 3 was effected with an excess of oxone (KHSO5·KH-
SO4·K2SO4) in the presence of silica in boiling toluene (65%
yield). The corresponding sulfide 4 and selenide 5 were formed
in good yield from 2 by reaction with excess elemental sulfur in
toluene or with grey selenium in dichloromethane, respectively
(Scheme 1).
Reaction of equimolar amounts of 2 and tetrahydrothiophene
gold(I)chloride40 in dichloromethane gives rise to the formation
of gold(I) complex 6 in 53% yield. Alkylation of compound 2
with a slight excess of dimethyl sulfate in CH2Cl2 at room temp-
erature led to phosphonium salt 7, which was isolated as a col-
ourless solid in 83% yield (Scheme 1).
Similarly, diphenylphosphanyl-thiophene 8 was treated with
S8 to give sulfide 9 (34% yield), to an oxidation with grey
selenium to furnish selenide 10 (90% yield) and to alkylation
with dimethyl sulfate to afford phosphonium salt 11 in 92%
yield (Scheme 1).
The compounds 2–11 are slightly air- and moisture sensitive
due to the presence of the benzodiazaboryl group as well as the
P(III) centre in derivatives 2 and 8. All new compounds are well
soluble in common aprotic solvents such as toluene, ethers,
CH2Cl2, CHCl3, CH3CN and dimethyl sulfoxide.
Air-oxidation of 2 to form 3 was considerably accelerated
under UV irradiation as proven by 31P{1H}NMR spectroscopy.
In the phenylene-bridged derivatives, 11B{1H]-NMR-reson-
ances are observed in the narrow range of δ = 27.7 to 29.7 ppm,
whereas the corresponding signals of the thiophenes appear at
slightly higher field (δ11B = 25.4–26.6 ppm). Known com-
pounds with thienyl substituents at the boron atom of benzo-
1,3,2-diazaboroles give rise to signals at δ = 26.0–26.6 ppm in
their 11B{1H}NMR spectra. Previously prepared 1,3,2-benzodi-
azaboroles with the boron atoms substituted by benzene rings
The orientations of the (hetero)-aromatic π-systems with
respect to the benzodiazaborolyl group are important as an ideal
π-correspondence between the orbitals of boron and of the
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Scheme 1 Syntheses of 2–11.
adjacent organic scaffold is expected in the case of coplanarity
of the involved rings. The related torsion angles in 2–7 range
from 140.9° in the phosphonium ion 7 to 59.4° in the selenophos-
phoryl derivative 5. The respective torsion angles between the
two heterocycles range from −51.4° in 8 to 133.2° in the seleno-
phosphoryl derivative 10. The angles between the planes defined
by the atoms C(14), P, the chalcogen atom or the AuCl- or
methyl-substituents and the adjacent benzene plane in 3–7 range
from 147.2° in 3 to 32.6° in 7. In the thiophene derivatives the
corresponding torsion angles are −56.2° in 9 and −160.9° in 11.
The smaller angles in the phosphonium salts 7 and 11 suggest an
improved interaction between the π-orbitals on boron and on the
carbon atom attached to the boron centre.
This experimental and theoretical geometry comparison is our
starting point to the electronic structure study.
UV-visible and luminescence spectroscopy
Table 3 lists selected photophysical data for all new compounds
shown in Scheme 1 along with related D–π–A systems for com-
parison. The lowest UV-absorption maxima (in CH2Cl2) (Fig. 3)
decrease in energy on going from 2 (λmax = 298 nm) > 3
(299 nm) > 4 ≈ 5 (303 nm) > 6 (307 nm) > 8 ≈ 9 (311 nm) > 10
(318 nm) > 7 (329 nm) to 11 (340 nm).
Within the series of molecules based on the p-phenylene
linker the absorption maxima are not significantly influenced by
the change from P(O)Ph2 via the P(E)Ph2 (E = S, Se) to the
PPh2(AuCl)-substituents, whereas the introduction of the phos-
phonium unit into the ring in 7 causes a marked increase to
329 nm. Similar observations were made with the thiophene
derivatives. The replacement of the phenylene unit by the
The calculated geometrical parameters of 8, 9 and 11 fit also
very well with the experimentally determined values of bond
lengths (this difference does not exceed 0.03 Å). For the torsion
angles the deviation is more significant and probably should be
due to the difference between crystal and gas phase structure.
10442 | Dalton Trans., 2012, 41, 10440–10452
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The Stokes shifts for these compounds are large with values
from 5800–7040 cm−1 in cyclohexane. They are even larger in
the more polar solvent dichloromethane ranging from 8950 cm−1
in 5 to 10 440 cm−1 in 7.
In keeping with this, solvatochromic shifts of 2750–
3630 cm−1 are observed in the emission maxima by changing
the solvent from cyclohexane to CH2Cl2 (Fig. 4). These obser-
vations are consistent with relatively large dipole moments of
3–7, 9–11 in their singlet excited states.
The photophysical data for compounds 2 and 8 deserve a
comment. Due to the fact that the absorption and emission
spectra of 2 and 3 are nearly identical and that the quantum
yields of 2 and 8 are very small, it was suspected that these emis-
sions are caused by oxide impurities (see also ESI†).
Indeed, a CH2Cl2 solution of 8 which was irradiated for 2 h
with UV light shows improved luminescence and signals in the
31P{1H}NMR spectrum for the related phosphane oxide. This
observation also agrees with a report on quenching the emission
of a photosensitized dye through single electron transfer from tri-
valent phosphorus compounds.46 In a different paper the non-
fluorescent nature of tris(9-anthryl)phosphane is reported. It is
further mentioned that phosphanes Ph2PAr (Ar = naphthyl,
anthryl) have no fluorescence, whereas their phosphine oxides
show intense fluorescence.47
It is instructive to compare the photophysical data of 3–7 and
9–11 with those of C6H4(NEt)2-BC6H4BMes2 (12) and
C6H4(NEt)2B(C4H2S)BMes2 (13) in order to estimate the
π-acceptor capacity of phosphorus containing functions with
regard to the strong π-accepting dimesitylboryl group. The
absorption maxima in these compounds suggest that the best
P-containing acceptor in this study, namely PPh2Me, is still not
as effective as the BMes2 group.
DFTelectronic structure calculations
The TD-DFT calculated absorption maxima on compounds 2–6
and 8–10 are displayed in Table 6. The lowest energy absorption
is calculated for 2 at the λmax = 308 nm; all other absorption
maxima are red-shifted and range from 319 to 324 nm for phe-
nylene spacers containing compounds 3–5, for 6 at the λmax
=
340 nm, and from 327 to 337 nm for thiophene spacers contain-
ing analogues 8–10.
Fig. 1 Molecular structures of compounds 2–7.
The most intense absorption is for thiophene bridged com-
pounds calculated for 8 at 327 nm and corresponds to the energy
of the HOMO → LUMO transition. This band is observed in the
UV/Vis spectrum also as one of the most intense bands at λ =
311 nm. In comparison to the experiment, the calculated absorp-
tion maxima for 2–5 and 8–10 (in CH2Cl2) are red-shifted by 10
to 26 nm. The ground state dipole moments μg, molecular polar-
izabilities α and the static first-order molecular hyperpolarizabil-
ities β of compounds 2–5 and 8–10 have been calculated and are
listed in Table 8. The magnitudes of μg are all found to be rather
small, for phenylene containing series they range from 1.5 D for
the unsubstituted derivative 2, via 3.9 D for the oxygen contain-
ing 3 to 4.9 D for 5, while the highest value is obtained for
sulfur containing 4 (5.0 D). For thiophene bridged compounds,
the same tendency is noted with slightly bigger values of the
ground state dipole moments μg (8: 2.3 D; 9: 5.6 D; 10: 5.5 D).
thiophene bridge in corresponding systems, e.g. 4 vs. 9, 5 vs. 10
or 7 vs. 11, causes a red-shift of the absorption (of 11, 15 and
11 nm, respectively), which reflects an improved delocalization
of π-electrons along the rod-like scaffold of the thiophene based
benzodiazaborole derivatives. It is also obvious that within the
here designed push–pull molecules featuring a benzodiazaborole
donor the methyl-diphenylphosphonium group is the best π-elec-
tron acceptor. The negligible solvatochromism of the absorptions
agrees with a small dipole moment in the ground state.
All the compounds 3–7 and 9–11 are luminescent; the diphe-
nyloxophosphanyl derivative 3 (Φf = 0.70), the diphenylthiophos-
phanyl derivatives 4 (Φf = 0.56) and 9 (Φf = 0.69) have
acceptable quantum yields in CH2Cl2 solution.
In contrast to this, the quantum yields of the selenium contain-
ing compounds 5 and 10 in CH2Cl2 are only 0.01, respectively.
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Table 1 Experimental and B3LYP/6-311G(d,p) selected bond lengths [Å] and torsion angles [°] for 2–7 (calculated values in italics)
2
3
4
5
6
7
B–C
B–N
B(1)–C(11)
B(1)–N(1)
B(1)–N(2)
P(1)–C(14)
P(1)–C(17)
P(1)–C(23)
1.566(2)
1.566
1.432(2)
1.440
1.433(2)
1.440
1.837(1)
1.853
1.832(1)
1.854
1.836(1)
1.854
1.574(4)
1.567
1.426(4)
1.441
1.438(4)
1.441
1.808(3)
1.833
1.801(3)
1.833
1.802(3)
1.834
1.567(2)
1.567
1.438(2)
1.441
1.432(2)
1.441
1.816(1)
1.841
1.815(1)
1.843
1.813(1)
1.843
1.570(2)
1.567
1.431(2)
1.441
1.434(2)
1.441
1.820(1)
1.842
1.814(1)
1.844
1.814(1)
1.844
1.570(5)
1.568
1.431(5)
1.440
1.421(5)
1.440
1.816(4)
1.837
1.818(4)
1.839
1.822(4)
1.839
1.566(2)
1.572
1.434(2)
1.437
1.440(2)
1.437
1.790(1)
1.805
1.798(1)
1.812
1.798(1)
1.812
P–C
PvX
P(1)–O(1)
1.489(2)
1.499
P(1)–S(1)
1.9595(4)
1.978
P(1)–Se(1)
2.1145(4)
2.138
P(1)–Au(1)
2.225(1)
2.290
P(1)–C(29)
1.793(1)
1.823
Aryl-links
C(11)–C(12)
1.405(2)
1.406
1.403(4)
1.406
1.406(2)
1.405
1.402(2)
1.407
1.397(5)
1.405
1.403(2)
1.409
C–C
C(12)–C(13)
1.389(2)
1.391
1.395(4)
1.393
1.387(2)
1.393
1.396(2)
1.390
1.395(5)
1.392
1.391(2)
1.388
C(13)–C(14)
1.400(2)
1.402
1.391(4)
1.399
1.398(2)
1.399
1.400(2)
1.399
1.397(5)
1.399
1.398(2)
1.404
C(14)–C(15)
1.402(2)
1.399
1.394(4)
1.399
1.400(2)
1.399
1.398(2)
1.398
1.403(5)
1.400
1.397(2)
1.401
C(15)–C(16)
1.388(2)
1.394
1.386(4)
1.391
1.392(2)
1.390
1.384(2)
1.393
1.394(5)
1.391
1.384(2)
1.391
C(11)–C(16)
1.406(2)
1.404
1.396(4)
1.406
1.402(2)
1.406
1.404(2)
1.405
1.399(5)
1.406
1.405(2)
1.406
Borolyl unit
N–(Et)
N(1)–C(7)
1.461(2)
1.458
1.460(2)
1.459
1.452(4)
1.459
1.455(4)
1.459
1.458(1)
1.459
1.460(1)
1.459
1.461(2)
1.459
1.463(2)
1.459
1.440(7)
1.459
1.450(4)
1.459
1.462(1)
1.460
1.464(2)
1.461
N(2)–C(9)
N–C(aryl)
N(1)–C(1)
1.398(2)
1.397
1.405(4)
1.397
1.399(1)
1.397
1.395(2)
1.397
1.398(4)
1.440
1.399(1)
1.400
N(2)–C(2)
1.401(2)
1.397
1.406(4)
1.397
1.395(1)
1.397
1.399(2)
1.397
1.394(4)
1.440
1.396(2)
1.399
C–C
C(1)–C(2)
1.408(2)
1.415
1.394(4)
1.415
1.407(2)
1.414
1.410(2)
1.414
1.408(5)
1.414
1.413(2)
1.412
Torsion angles
N(1)–B(1)–C(11)–C(12)
N(1)–B(1)–C(11)–C(16)
C(13)–C(14)–P(1)–C(17)
C(13)–C(14)–P(1)–C(23)
C(13)–C(14)–P(1)–X
−128.1
−121.4
50.7
59.1
4.6
9.5
101.1
96.4
−62.5
−57.0
−58.2
−125.0
−42.9
−52.9
−51.2
128.1
51.3
−46.4
118.4
126.2
25.0
28.8
121.6
128.4
88.8
89.9
−158.6
20.7
−34.96
−34.7
123.0
−51.4
22.0
59.4
140.9
133.9
−88.84
73.4
−128.3
−165.20
−158.6
83.70
17.7
−90.16
−91.07
152.76
164.6
−84.0
−92.9
89.8
147.2
145.4
−41.3a
34.2
32.64b
45.5
152.2
142.4
a X = Au(1). b X = C(29).
These low values are consistent with the absence of solvato-
chromism for these compounds. The situation is a little bit differ-
ent in the case of 6 and two cations 7 and 11 for which the
ground state dipole moments are bigger (6: 10.2; 7: 12.6; 11:
11.3 D). The molecular polarizabilities α of compounds 2–11
vary little in the range 5.8–6.6 × 10−23 esu.
Donor–acceptor substituted π-conjugated organic molecules
having low-lying charge transfer excited states exhibit large first-
order non-linear properties. The molecular hyperpolarizabilities
β of 2–11 were computed as analytical third derivatives of the
energy and correspond to the static hyperpolarizabilities (fre-
quency = 0). An increase in the range β = 4.6 × 10−30 esu (2) <
13.8 × 10−30 esu (8) < 16.0 × 10−30 esu (3) < 19.4 × 10−30 esu
(4) ≤ 19.6 × 10−30 esu (10) < 20.0 × 10−30 esu (9) ≤ 20.1 ×
10−30 esu (5) ≤ 25.7 × 10−30 esu (6) was calculated. For two
cations 7 and 11 very big values of static first-order molecular
hyperpolarizabilities (β) in comparison with 2–6 and 8–10 are
calculated (120.2 × 10−30 and 96.8 × 10−30 esu, respectively).
These data were computed on the basis of the two state model,48
whereby β is proportional to the dipole moment change between
the ground and the excited state Δμ and the square of the tran-
sition moment integral for the optical transition between two
states (oscillator strength).
The hyperpolarizability is inversely proportional to the square
of the energy of the respective transition and its magnitude of β
may also be dominated by the size of the HOMO–LUMO gaps.
In keeping with this, the small β-value of 2 (4.6 × 10−30 esu)
would agree with the largest HOMO–LUMO gap (4.558 eV)
10444 | Dalton Trans., 2012, 41, 10440–10452
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Experimental
All experiments were performed under an atmosphere of dry
oxygen-free argon using Schlenk techniques. All solvents were
dried with the usual drying agents and freshly distilled prior to
use. The compounds 4-bromophenyl-diphenylphosphane,38
5-bromo-2-diphenylphosphanyl-thiophene,39 2-bromo-1,3-diethyl-
1,3,2-benzodiazaborole 123 and tetrahydrothiophene-gold(I)
chloride40 were prepared according to literature methods.
Oxone (2KHSO5·KHSO4·K2SO4) was purchased from Alfa
Aeser Johnson Matthey Comp., Karlsruhe (Germany). A 1.6 M
pentane solution of tert-butyllithium and a 1.6 M hexane solu-
tion of n-butyllithium were obtained from Aldrich.
NMR spectra were recorded in C6D6 or CDCl3 at room temp-
erature on a Bruker AM Avance DRX500 spectrometer (1H, 11B,
13C) with SiMe4 (1H, 13C) and BF3·OEt2 (11B) as external stan-
dards. Some expected 13C peaks corresponding to the boron-
bound carbon atoms were not detected above the noise levels.
The poor values of C and N in elemental analysis may be a
result of incomplete combustion due to formation of ceramic
boron carbides or nitrides. In keeping with this, varying amounts
of solid residues were found after combustion. No significant
impurities could be detected in the NMR/MS and EES-spectra,
which can be seen as a typical fingerprint of the molecules and
would show any other fluorescent species.
Fig. 2 Molecular structures of compounds 8, 9 and 11.
and the very big β-values of 7 and 11 are very nicely correlated
with the smallest HOMO–LUMO gaps (3.075 and 3.099 eV,
respectively).
Absorption spectra were measured with a UV/VIS double-
beam spectrometer (Shimadzu UV-2550). The quantum yields
were determined against POPOP (p-bis-5-phenyl-oxazolyl(2)-
benzene) (Φ = 0.93) as the standard. Single crystals were coated
with a layer of hydrocarbon oil and attached to a glass fiber.
Crystallographic data were collected with a Nonius KappaCCD
diffractometer with Mo-Kα radiation (graphite monochromator,
λ = 0.71073 Å) at 100 K for 2, 4, 5, 6, 7, 8, 9 and 11 and 200 K
for 3. Mass spectra were obtained with a VG Autospec sector
field mass spectrometer (Micromass). Crystallographic programs
used for structure solution and refinement were from
SHELX-97.54 The structures were solved by direct methods and
were refined by using full-matrix least squares on F2 of all
unique reflections and anisotropic thermal parameters for all
non-hydrogen atoms, except disordered atoms in 8 and 12. All
hydrogen atoms were refined using a riding model with U(H) =
1.5 Ueq for CH3 groups and U(H) = 1.2 Ueq for all others.
Disordered n-pentane in 6 was treated by SQUEEZE/
PLATON.55 CCDC 873268–873276 contain supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Tables 4 and 5 display [B3LYP/6-311G(d, p)] calculated MO
energies of 2–5 and 8–10, as well as HOMO–LUMO gaps and
the first four ΔSCF/TD-DFT ionization energies (IE). The
LUMOs are located at π* of phenyl in 2–5, or thiophene in 8–10
units with slight contribution of the vacant 2pz orbital of boron,
whereas the HOMOs in 2–4 and 8–9 correspond to the benzodi-
azaborole unit (π3–πNBN). In 5 and 10 the HOMOs are located
on the selenium atom. The lowest energies of the HOMOs are
noted for 2 and 5 (−5.536 and −5.553 eV, respectively) and the
highest value is obtained for 4 (−5.670 eV). The energetical pos-
ition of the HOMOs (and consequently that of IE1 which ranged
from 6.82 to 7.18 eV) is very insensitive towards structural
modification, but the nature of HOMOs is different in 5 and 10
compared to 2–4 and 8–9. For the latter ones (2–4 and 8–9), the
calculated (π3–πNBN) ionization energies are slightly influenced
by the other part of the molecule in comparison with unsubsti-
tuted 2 (IE1 = 6.83 eV) and 8 (IE1 = 6.78 eV); the oxygen with-
drawing effect is noted in the case of 3 (IE1 = 7.18 eV) and,
surprisingly, the strongest stabilizing effect of selenium in 10
(IE3 = 7.24 eV). The influence of the sulfur atom seems less sig-
nificant in this case. For sulfur and selenium containing com-
pounds (4–5, 9–10), the ionization energies of their lone pairs
correspond to the HOMO − 1 and HOMO − 2 in 4 (IE2
=
4-(Diphenylphosphanyl)-1-(1′,3′-diethyl-1′,3′,2′-
benzodiazaborol-2′yl)-benzene (2)
7.03 eV, IE3 = 7.31 eV) and 9 (IE2 = 7.05 eV, IE3 = 7.40 eV), or
to the HOMO and HOMO − 1 in 5 (IE1 = 6.82 eV, IE2 = 6.84
eV) and 10 (IE1 = 6.82 eV, IE2 = 6.86 eV), and are computed at
nearly the same value for sulfur (4, 9) or selenium (5, 10) deriva-
tives, independently of phenylene or thiophene bridging. The
HOMO–LUMO gap decreases continuously from 2 via 3, 4 to 5,
and then from 8 via 9 to 10, and mirrors the increasing red shift
in the whole series. As can be seen from Table 7, the most
important one is observed for cations 7 and 11, since it goes to
the visible region.
An ethereal solution (30 mL) of 4-bromophenyl-diphenylphos-
phane (1.24 g, 3.63 mmol) was added to a cold solution
(−78 °C) of 7.63 mmol tert-butyllithium (4.77 mL) in a mixture
of diethyl ether (30 mL) and n-hexane (30 mL). The mixture
was stirred at −60 °C for 3 h, then re-chilled to −78 °C before a
sample of 1.01 g (3.99 mmol) of 2-bromo-1,3-diethyl-1,3,2-ben-
zodiazaborole (1) was added. The orange solution was allowed
to slowly warm up to ambient temperature, whereby a colourless
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Table 2 Experimental and B3LYP/6-311G(d,p) selected bond lengths [Å] and torsion angles [°] for 8, 9 and 11 (calculated values in italics)
8
9
11
B–C
B–N
B(1)–C(11)
B(1)–N(1)
B(1)–N(2)
P(1)–C(14)
P(1)–C(15)
P(1)–C(21)
1.557(2)
1.554
1.436(2)
1.442
1.430(2)
1.442
1.820(1)
1.834
1.840(1)
1.854
1.834(1)
1.855
1.561(3)
1.557
1.429(3)
1.440
1.435(3)
1.440
1.798(2)
1.817
1.818(2)
1.842
1.813(2)
1.842
1.5316(2)
1.563
1.433(2)
1.436
1.4314(2)
1.435
1.775(1)
1.778
1.797(1)
1.812
1.802(1)
1.811
P–C
PvX
P(1)–S(2)
1.957(1)
1.974
P(1)–C(27)
1.792(1)
1.821
Aryl-links
S–C
S(1)–C(11)
1.726(1)
1.748
1.727(2)
1.745
1.723(1)
1.742
S(1)–C(14)
1.724(1)
1.745
1.728(2)
1.740
1.723(1)
1.747
C–C
C(11)–C(12)
1.375(2)
1.379
1.373(3)
1.379
1.381(2)
1.383
C(12)–C(13)
1.423(2)
1.420
1.418(3)
1.418
1.410(2)
1.412
C(13)–C(14)
1.378(2)
1.378
1.373(3)
1.375
1.378(2)
1.381
Borolyl unit
N–(Et)
N(1)–C(7)
1.461(2)
1.459
1.455(2)
1.458
1.465(3)
1.458
1.468(3)
1.460
1.470(2)
1.461
1.459(2)
1.461
N(2)–C(9)
N–C(aryl)
N(1)–C(1)
1.393(2)
1.396
1.399(3)
1.396
1.401(2)
1.399
N(2)–C(2)
1.396(2)
1.396
1.400(3)
1.397
1.396(2)
1.398
C–C
C(1)–C(2)
1.410(2)
1.414
1.409(3)
1.414
1.408(2)
1.412
Torsion angles
N(1)–B(1)–C(11)–C(12)
N(1)–B(1)–C(11)–S(1)
S(1)–C(14)–P(1)–C(15)
S(1)–C(14)–P(1)–C(21)
S(1)–C(14)–P(1)–X
131.7
40.7
−48.9
−139.9
−141.0
33.7
41.0
−75.3
77.3
41.9
−51.4
133.2
136.0
179.4
11.6
68.1
81.2
−42.2
−18.4
92.0
−121.3
−162.4
46.3
−160.9
S(1)–C(14)–P(1)–S(2) −56.2
−44.0
S(1)–C(14)–P(1)–C(27) 164.8
−42.0
precipitate separated. It was filtered, and the filtrate was concen-
trated to a volume of ca. 2 mL. A crude product was precipitated
by the addition of n-pentane (100 mL). The slurry was stirred for
12 h at room temperature. Then the supernatant solvent was dec-
anted and the solid was crystallised from a 1 : 1 mixture of
dichloromethane and n-hexane to afford 1.14 g (72%) of colour-
less microcrystalline 2. Found C 77.52, H 6.78, N 6.18%.
C28H28BN2P (434.32) requires: C 77.43, H 6.50, N 6.45%.
with water (3 × 10 mL). The organic phase was separated and
evaporated to dryness. The crude product was purified by crys-
tallisation from a 1 : 10 mixture of CH2Cl2 and n-hexane to yield
0.25 g (65%) of 3 as a colourless solid.
Found C 73.69, H 6.34, N 6.10%. C28H28BN2OP (450.32)
requires: C 74.68, H 6.27, N 6.22%.
4-(Diphenyl-thiophosphanyl)-1-(1′,3′,-diethyl-1′,3′,2′-
benzodiazaborol-2′-yl)-benzene (4)
4-(Diphenyl-oxophosphanyl)-1-(1′,3′-diethyl-1′,3′,2′-
benzodiazaborol-2′-yl)-benzene (3)
A solution of 2 (0.40 g, 0.92 mmol) and elemental sulfur
(0.01 g, 3 mmol) in 30 mL of toluene was stirred for 1 d at room
temperature. It was filtered and the filtrate was freed from solvent
in vacuo. The crude product was washed with n-pentane (3 × 20 mL)
and then crystallised from a 1 : 1 mixture of CH2Cl2 and n-hexane.
Product 3 was obtained as colourless crystals. Yield 0.23 g (53%).
Found C 71.09, H 6.01, N 6.62%. C28H28BN2PS (466.39)
requires: C 72.11, H 6.05, N 6.01%.
Samples of solid oxone (3.14 g, 5.18 mmol) and silica (3.14 g,
51.0 mmol) were added to a solution of 2 (0.37 g, 0.85 mmol)
in toluene (30 mL). The mixture was heated for 3 d at 110 °C.
The re-cooled slurry was filtered, and solvent and volatile com-
ponents were subsequently removed in vacuo. Dichloromethane
(20 mL) was added to the residue and the mixture was washed
10446 | Dalton Trans., 2012, 41, 10440–10452
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Table 3 Selected photophysical data of compounds 2–13
Compound
Solvent
λmax, abs. [nm]
E [L mol−1 cm−1
]
λmax, em. [nm]
Stokes shift [cm−1
]
Φf
2
2
3
3
3
3
3
4
4
5
5
6
6
6
6
6
6
7
7
8
8
9
10
10
11
11
11
11
11
11
12
12
13
13
c-C6H12
CH2Cl2
c-C6H12
PhCH3
CHCl3
THF
CH2Cl2
c-C6H12
CH2Cl2
c-C6H12
CH2Cl2
c-C6H12
PhCH3
CHCl3
THF
295
298
298
292
300
299
299
300
300
303
303
308
310
308
307
307
297
315
329
310
311
311
318
318
332
337
328
340
325
328
327
329
345
350
17 981
13 802
—
—
—
—
—
—
384
399
412
416
426
370
431
369
429
385
414
430
436
442
425
469
496
—
6880
7680
8940
9080
9630
6090
9590
5700
8950
7040
7990
10 870
9500
10 220
10 600
11 000
10 440
—
0.70
0.72
0.20
0.74
0.70
0.04
0.56
0.05
0.01
—
0.68
0.09
0.33
0.54
0.21
0.03
0.57
—
9850
13 884
10 634
5664
—
11 470
12 580
11 570
12 900
11 690
5878
18 950
16 740
18 050
CH2Cl2
MeCN
THF
CH2Cl2
c-C6H12
CH2Cl2
CH2Cl2
c-C6H12
CH2Cl2
PhCH3
CHCl3
THF
CH2Cl2
MeCN
DMSO
c-C6H12
THF
—
—
—
445
389
442
476
493
488
504
517
526
408
477
439
482
9680
6110
9740
8400
9100
9130
9060
11 150
6780
6070
9440
6210
7820
0.85
0.10
0.01
0.15
0.34
0.03
0.69
0.09
9196
8490
9080
9460
8120
8390
6050
9170
29 600
31 600
19 400
20 100
0.99
0.09
0.81
0.46
c-C6H12
THF
Fig. 4 Emission spectra of 3 in different solvents (l. t. r.) (c-C6H12
toluene, THF, CHCl3, CH2Cl2).
,
Fig. 3 UV/Vis-spectra of 3–7 in CH2Cl2.
4-(Diphenyl-selenophosphanyl)-1-(1′,3′-diethyl-1′,3′,2′-
benzodiazaborol-2′-yl)-benzene (5)
Found C 65.34, H 5.75, N 5.34%. C28H28BN2PSe (513.28)
requires: C 65.52, H 5.50, N 4.46%.
Samples of 2 (0.075 g, 0.17 mmol) and grey selenium
(0.0136 g, 0.17 mmol) were stirred in CH2Cl2 (15 mL) for 1 d at
room temperature. It was filtered, the filtrate freed from solvent
and the colourless solid residue was washed with n-pentane (2 ×
10 mL). Product 5 was isolated as a colourless powder (0.085 g,
96.5% yield).
[κ-P{4-(diphenylphosphanyl)-1-(1′,3′-diethyl-1′,3′,2′-
benzodiazaborol-2′-yl)-benzene}-gold(I) chloride] (6)
A mixture of 2 (0.027 g, 62.4 mmol) and 0.020 g (62.4 mmol)
of tetrahydrothiophene-gold(I) chloride in 5 mL of CH2Cl2 was
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Table 4 B3LYP/6-311G(d, p) calculated MO’s (LUMO, HOMO, HOMO − 1, HOMO − 2, HOMO − 3) energies [eV] of 2, 3, 4 and 5, as well as
the HOMO–LUMO gap [eV–nm] and first four ΔSCF/TD-DFT ionization energies [eV], contour values are plotted at 0.04 (e bohr−3 1/2
)
2
3
4
5
LUMO
HOMO
HOMO − 1
HOMO − 2
HOMO − 3
Δ(HOMO–LUMO)
4.558–272.32
6.83
7.38
7.39
7.78
4.394–282.49
7.18
7.30
7.36
7.42
4.325–286.99
6.99
7.03
7.31
7.49
4.207–295.04
6.82
6.84
7.17
7.31
IE1
IE2
IE3
IE4
stirred for 20 min at room temperature. Concentrating the
mixture was followed by the addition of 10 mL of n-pentane,
whereby a colourless solid precipitated. It was filtered and the
filter-cake was washed with n-pentane (2 × 10 mL). After drying
in vacuo at 60 °C overnight, 0.022 g of product 6 (53%) was
obtained. Single crystals of 6 were grown from a 1 : 2 mixture of
CH2Cl2 and n-pentane.
Found C 63.32, H 6.48, N 4.50%. C30H35BN2O4PS (561.46)
requires: C 64.18, H 6.28, N 4.99%.
5-(Diphenylphosphanyl)-2-(1′,3′diethyl-1′,3′,2′-benzodiazaborol-
2′-yl)-thiophene (8)
A
solution of 5-(diphenylphosphanyl)-2-bromothiophene
(2.45 g, 7.17 mmol) in diethyl ether (20 mL) was added drop-
wise to a chilled (−78 °C) solution of tert-butyllithium
(15.1 mmol) in 20 mL of diethyl ether, before a sample of
Found C 50.68, H 4.26, N 4.38%. C28H28AuBClN2P (667.75)
requires: C 50.36, H 4.38, N 4.20%.
2-bromo-1,3-diethyl-1,3,2-benzodiazaborole
1
(1.99
g,
[4-(Methyl-diphenylphosphonio)-1-(1′,3′-diethyl-1′,3′,2′-
benzodiazaborol-2′-yl)-benzene]-monomethylsulfate (7)
7.89 mmol) was added. The mixture was allowed to warm to
room temperature and filtered. The filter-cake was washed with
diethyl ether (50 mL). Solvent and volatile components were
removed in vacuo, and the residue was triturated with 100 mL of
CHCl3. A colourless solid was filtered off, and the filtrate was
freed from the solvent. The solid residue was continuously
extracted with n-pentane during a period of 2 d. Product 8
(1.14 g, 35%) was isolated from the filtrate as a colourless solid.
Crystallisation of 8 was effected from a CH2Cl2–n-pentane
mixture during 2 d at −7 °C.
A sample of dimethyl sulfate (0.67 g, 5.27 mmol) was added
dropwise to a well-stirred solution of 2 (1.91 g, 4.4 mmol) in
50 mL of CH2Cl2. Stirring was continued overnight before
solvent and volatiles were removed in vacuo. The residue was
dissolved in CH2Cl2 (5 mL) and layered with n-hexane (5 mL).
Then the vessel was stored at −7 °C for 10 h. Colourless solid 7
(2.03 g, 83%) was isolated by filtration and drying in vacuo.
Single crystals of 7 were grown from a concentrated CH2Cl2-
solution which was layered with the five-fold volume of
n-pentane and stored at −7 °C.
Found C 70.51, H 6.11, N 6.29%. C26H26BN2PS (440.35)
requires: C 70.92, H 5.95, N 6.36%.
10448 | Dalton Trans., 2012, 41, 10440–10452
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Table 5 B3LYP/6-311G(d, p) calculated MO’s (LUMO, HOMO, HOMO − 1, HOMO − 2, HOMO − 3) energies [eV] of 8, 9 and 10, as well as the
HOMO–LUMO gap [eV–nm] and first four ΔSCF/TD-DFT ionization energies [eV], contour values are plotted at 0.04 (e bohr−3 1/2
)
8
9
10
LUMO
HOMO
HOMO − 1
HOMO − 2
HOMO − 3
Δ(HOMO–LUMO)
4.305–295.04
6.78
7.42
7.45
7.78
4.186–296.52
6.97
7.05
7.40
7.53
4.093–303.26
6.82
6.86
7.24
7.36
IE1
IE2
IE3
IE4
Table 6 Comparison of calculated [B3LYP/6-311G(d,p)] data for
optimized geometries of 2, 3, 4, 5, 6, 8, 9 and 10, and observed UV
absorption maxima
2 d afforded 0.66 g (34%) of colourless solid 8. The generation
of single crystals required storing a CH2Cl2–n-pentane solution
for three months at 20 °C.
Found C 66.08, H 5.58, N 5.92, S 13.84%. C26H26BN2PS2
(472.41) requires: C 66.10, H 5.55, N 5.93, S 13.57%.
Calc. λmax Oscillator
Exp. λmax
(abs, nm)
Δλmax
(calc − exp)
Compound [nm]
strength (f)
2
3
4
5
6
8
9
10
308
319
325
324.5
340
327
337
336
0.32
0.24
0.28
0.22
0.24
0.45
0.33
0.22
298
299
303
303
308
311
311
318
10
20
22
21.5
32
16
26
18
5-(Diphenyl-selenophosphanyl)-2-(1′,3′-diethyl-1′,3′,2′-
benzodiazaborol-2′-yl)-thiophene (10)
A mixture of 8 (0.57 g, 1.29 mmol) and 0.15 g (1.94 mmol) of
grey selenium in 20 mL CH2Cl2 was stirred for 48 h at 20 °C.
Product 10 (0.60 g, 90%) was isolated from the filtrate by eva-
porating the solvent and washing the colourless solid residue
with n-pentane.
Found C 59.87, H 5.33, N 5.09%. C26H26BN2PSSe (519.31)
requires: C 60.13, H 5.05, N 5.39%.
5-(Diphenyl-thiophosphanyl)-2-(1′,3′,diethyl-1′,3′,2′-
benzodiazaborol-2′-yl)-thiophene 9
A slurry of 8 (1.81 g, 4.1 mmol) and 1.32 g (41.1 mmol) of
sulfur in CH2Cl2 (30 mL) was stirred for 24 h at room tempera-
ture. After filtration the filtrate was evaporated to dryness. The
solid residue was continuously extracted with n-pentane during
2 d, then dissolved in CH2Cl2 (10 mL) and the solution was
covered with a layer of n-pentane (20 mL). Storing at −30 °C for
[5-(Methyl-diphenylphosphonio)-2-(1′,3′-diethyl-1′,3′,2′-
benzodiazaborol-2′-yl)-thiophene]-monomethylsulfate (11)
Similar to the preparation of 7 a combination of 8 (0.05 g,
0.135 mmol) and 0.036 g (0.3 mmol) of dimethyl sulfate in
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Table 7 B3LYP/6-311G(d,p) calculated MO’s (LUMO, HOMO,
HOMO − 1, HOMO − 2, HOMO − 3) energies [eV], Δ(HOMO–
LUMO) [eV–nm] and first HOMO → LUMO UV transition for
Table 8 B3LYP/6-311G+(d,p) calculated ground state (μg) dipole
moments, molecular polarizabilities (α), static first-order molecular
hyperpolarizabilities (β) and the change of dipole moments (Δμ) of
compounds 2–11
compounds 7 and 11, contour values are plotted at 0.04 (e bohr−3 1/2
)
7
11
Compound
μg [D]
α [10−23 esu]
β [10−30 esu]
Δμ [D]
LUMO
2
3
4
5
6
7
8
9
1.5
3.9
5.0
5.9
5.8
6.2
6.3
6.6
6.1
5.9
6.1
6.1
5.8
4.6
16.0
19.4
20.1
25.7
120.2
13.8
20.0
19.6
96.8
14.2
18.6
16.9
12.4
20.0
20.7
12.6
13.2
10.9
15.5
4.9
10.2
12.6
2.3
5.6
5.5
HOMO
10
11
11.3
HOMO − 1
HOMO − 2
HOMO − 3
Conclusions
We have presented the synthesis of new molecules containing
1,3,2-benzodiazaborole moieties connected to diphenyl-phos-
phino groups via p-phenylene- or 2,5-thiophene-diyl linkers
using known methodologies.
Modification of the phosphorus atom to Ph2P(X) (X = O, S,
AuCl) functionalities leads to highly luminescent materials with
quantum yields in the range of Φf = 0.70 to Φf = 0.56, while for
X = Se only weak luminescence was detected (Φf = 0.01). This
is presumably due to dissimilarities in the frontier orbital situ-
ation of the molecules.
With the exception of the selenium derivatives 5 and 10 the
HOMOs of the new compounds are located on the benzodiaza-
borole part. In 5 and 10 the HOMOs are represented by the lone
pair on selenium. The LUMOs are of π*character and are located
at the phenylene-/thiophene-bridge with slight contributions of
the vacant 2pz orbital on boron.
Δ(HOMO–LUMO)
First HOMO →
LUMO UV transition
3.075–403.66
2.6802–462.59
3.099–400.53
2.6760–463.31
CH2Cl2 (10 mL) afforded 0.07 g (92%) of colourless crystalline
11 from CH2Cl2–n-hexane (1 : 2) at −30 °C.
Found C 58.68, H 5.62, N 4.84%. C28H33BN2O4PS2 (567.49)
requires: C 59.26, H 5.86, N 4.94%.
For all molecules under study, the most intense transition
(between 200 and 400 nm) corresponds to a charge transfer from
the benzodiazaborole group to the phenylene or thiophene
bridge, respectively. Additionally, unlike the absorption maxima,
emission maxima show large solvatochromic shifts in solvents
of increasing polarity. This is in agreement with essentially non-
polar ground states for the compounds 2–5, 8–10 with calculated
dipolar moments μg of 1.5–5.6 D, much more pronounced
polarity is conferred to compound 6 (10.2 D) and two cations 7
(12.6 D) and 11 (11.3 D). Changes of the dipole moments
between the ground state and excited state Δμ are in the range of
10.9 D in 10 to 20.7 D in 7. The strong change of the dipole
moment between the ground state and excited state is character-
istic of a charge transfer. The formal insertion of a thiophene in
place of a phenylene unit as a linker does not provide any signifi-
cant changes of the corresponding IEs of 2–6, 8–10 which
means that the electronic structure seems practically unperturbed
by the substitution pattern of these compounds.
Computational methods
All calculations were performed by using the Gaussian 0949
program package with the 6-311G(d,p) basis set. DFT has been
shown to predict various molecular properties successfully.50 All
geometry optimizations were carried out with the B3LYP51 func-
tional and were followed by frequency calculations to verify that
the stationary points obtained are true energy minima. Ionization
energies (IE) were calculated by using the B3LYP functional
with ΔSCF/TD-DFT, which means that separate SCF calculations
were performed to optimize the orbitals of the ground state and
the appropriate ionic state (IE = Ecation − Eneutral). The advan-
tages of the most frequently employed ΔSCF/TD-DFT method
of calculations of the first ionization energies have been demon-
strated previously.52 The TD-DFT53 approach provides a first
principle method for the calculation of excitation energies within
a density functional context taking into account the low-lying
ion calculated by the ΔSCF method.
None of these modifications showed a notable influence on
the absorption and emission bands, whereas the spectra of phos-
+
phonium salts (X = CH3 ) 7 and 11 showed significant red
shifts. In contrast to the other materials the LUMOs of these
compounds are lower in energy and exhibit considerable contri-
butions of the phosphonium unit. According to increased dipolar
10450 | Dalton Trans., 2012, 41, 10440–10452
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the series of compounds presented here.
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