β-Iminoenamine-BF2 complexes: Aggregation-induced emission and pronounced effects of aliphatic rings on radiationless deactivation

Article abstract of DOI:10.1002/asia.201200477

The synthesis, photophysical, and electrochemical attributes of a novel class of boron difluorides containing an aromatic-fused alicyclic/hetero- alicyclic ring built on a β-iminoenamine chromophoric backbone are reported. The compounds displayed large Stokes shifts (86-121 nm), and were emissive in the solid state. The quantum yields obtained in solution at room temperature were unusually lower by an order of magnitude compared to those in the solid state. Some of the tested compounds displayed aggregation-induced emission (AIE). Single crystal XRD analyses revealed a lack of interplanar π-π interactions, which are presumed to be absent owing to non-planarity of the alicyclic component in the molecule. For most of the studied compounds, time-dependent DFT (TD-DFT) calculations invariably reveal intramolecular charge transfer (π-π*) characteristics with the frontier orbitals concentrated on the boron-nitrogen heterocycle. The participation of boron and fluorine atoms was found to be negligible. Copyright

Full text of DOI:10.1002/asia.201200477

DOI: 10.1002/asia.201200477
b-Iminoenamine-BF2 Complexes: Aggregation-Induced Emission and
Pronounced Effects of Aliphatic Rings on Radiationless Deactivation
Karthikeyan Perumal,[b, c] Jai Anand Garg,[a] Olivier Blacque,[a] Ramanathan Saiganesh,[b]
Senthamaraikannan Kabilan,[c] Kallupattu Kuppusamy Balasubramanian,*[b, d] and
Koushik Venkatesan*[a]
Abstract: The synthesis, photophysical,
and electrochemical attributes of
a novel class of boron difluorides con-
taining an aromatic-fused alicyclic/
hetero-alicyclic ring built on a b-imi-
noenamine chromophoric backbone
are reported. The compounds displayed
large Stokes shifts (86–121 nm), and
were emissive in the solid state. The
quantum yields obtained in solution at
room temperature were unusually
lower by an order of magnitude com-
pared to those in the solid state. Some
of the tested compounds displayed ag-
gregation-induced emission (AIE).
Single crystal XRD analyses revealed
a lack of interplanar p–p interactions,
which are presumed to be absent
owing to non-planarity of the alicyclic
component in the molecule. For most
of the studied compounds, time-depen-
dent DFT (TD-DFT) calculations in-
variably reveal intramolecular charge
transfer (p–p*) characteristics with the
frontier orbitals concentrated on the
boron–nitrogen heterocycle. The par-
ticipation of boron and fluorine atoms
was found to be negligible.
Keywords: aggregation-induced
emission · boron · chromophores ·
luminescence · p–p interactions
Introduction
variety of applications,[2] the solid-state luminescence prop-
erties in most of their derivatives have been found to be
poor when compared to those in solution, primarily owing
to quenching of emission due to intermolecular p–p stacking
interactions. A prominent strategy to avoid such quenching
has been to introduce a sterically imposing substituent at
the meso-carbon of the dipyrrin core.[3] Apart from the di-
pyrrin framework, several groups have focused their atten-
tion in developing newer classes of boron-based fluoro-
phores to achieve superior solid-state luminescence proper-
ties. The groups of Yamaguchi,[1e,4] Wang,[1a,b,5] Piers,[1f,6] and
others[4c–f] have developed highly luminescent fluorophores
that are mainly based on boron. In this study, we have
aimed to explore the photophysical nature of yet a new
class of flurophores, namely, the b-iminoenamine-boron di-
fluorides with a fused aliphatic ring unit. They are akin to
the popular “nacnac” ligands or the b-diketiminate systems,
which have a “formal” 6-membered ring and a chelating m2-
H proton bridged to the nitrogen atoms.[7] The initial choice
of the chelate was for the following reasons: 1) Desymmetri-
zation of the chromophoric coordination sphere about the
nitrogen atom bound to boron (unlike in the case of the di-
pyrrin core) has been recently realized to effect larger
Stokes shifts.[6] 2) Introduction of an aliphatic ring in the
chromophoric backbone can disrupt deleterious stacking in-
teractions owing to its non-planar conformation. 3) Ease of
introduction of diverse functional groups, which can eventu-
ally modulate the ground/excited states, which lead to intri-
guing luminescence properties. Gardinier et al. have elegant-
ly compared the electronic properties of difluoroboron com-
plexes bearing b-diketonate, b-ketoiminate, and b-diiminate
Stable tetracoordinate organoboron complexes are of cur-
rent scientific interest owing to their potential utility as
functional luminophores for an array of applications.[1] This
is further evident from the significant amount of research di-
rected towards the boron–dipyrromethene (BODIPY) class
of molecules. Incorporation of a boron atom is perceived to
stabilize the N,N-monoanionic chelate by an effective deloc-
alization of the negative charge. Although BODIPY dyes
have evolved as a privileged class of molecules for a wide
[a] J. A. Garg,+ Dr. O. Blacque, Dr. K. Venkatesan
Institute of Inorganic Chemistry
University of Zꢀrich
Winterthurerstrasse 190, CH-8057, Zꢀrich (Switzerland)
Fax : (+41)446354806
E-mail: venkatesan.koushik@aci.uzh.ch
[b] K. Perumal,+ Dr. R. Saiganesh, Prof. Dr. K. K. Balasubramanian
Shasun Research Centre, 27 Vandaloor-Kelambakkam Road
Keelakottaiyur, Chennai-600048 (India)
E-mail: kkbalu@hotmail.com
[c] K. Perumal,+ Prof. Dr. S. Kabilan
Department of Chemistry
Annamalai University
Annamalai Nagar, Chidambaram-600002 (India)
[d] Prof. Dr. K. K. Balasubramanian
Department of Chemistry
B. S. Abdur Rahman University
Seethakathi Estate, Vandalur, Chennai-600048 (India)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200477.
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FULL PAPER
ligands,[8] which bear close resemblance to the system under
investigation. Similarly, Mu et al. and other research groups
have investigated the photoluminescence (PL) properties of
anilido-imine-BF2 compounds.[9] The observed luminescence
in this case was attributed to p*–p transition of their conju-
gated chelating anilido-imine ligands. Very recently, Piers
et al. have described similar compounds that exhibit large
Stokes shifts and good photostability.[6]
Herein, we have tried to exploit the basic skeleton of the
borylated b-iminoenamine by encompassing it with either
1,2-dihydronaphthalene, 2H-chromene, or 2H-thiochromene
in its chromophoric backbone. Functional variation of the
substituents in the aromatic unit and also changes in the het-
eroatom of the fused aliphatic unit led to novel compounds
with tunable emission properties. In line with our expecta-
tions, some of the synthesized compounds exhibited bright
luminescence in the crystalline state and in the aggregated
state in solution (AIE phenomenon), but were almost non-
emissive in various solvent media. Combined photophysical,
NMR spectroscopy, and single crystal X-ray diffraction stud-
ies have suggested that the aliphatic part of the molecule is
primarily responsible for the varied emission behavior in
this class of compounds.
This route was previously established in our group and was
used for the synthesis of condensed heterocycles such as ac-
ridines/quinolines.[10]
Detailed synthetic procedures for the preparation of new
iminoenamines (2a–2n) are provided in the Supporting In-
formation. We envisaged that the iminoenamines that are
potential systems for electrocyclic ring closure should readi-
ly react with Lewis acidic BF3·Et2O to form stable boron
compounds. As expected, treatment of iminoenamines with
BF3·Et2O in the presence of excess base such as Et3N or di-
isopropylamine (DIPA; to mop up the nascent HF generat-
ed) yielded the desired products in modest to good yields of
70–95% (Scheme 2). The reaction was also found to pro-
ceed with the hydrochloride salt of the iminoenamines in
a one-pot sequence, which does not require prior neutraliza-
tion. All of the products 3a–3 f, 4a–g, and 5 were stable to
Results and Discussion
Synthesis and Characterization
Iminoenamines suitable for complexation were realized in
two steps starting from the appropriate a-tetralone/
ACHTUNGTNER(NUNG 2H)-
benzopyran-4-one/(2H)-benzthiopyran-4-one (Scheme 1).
ACHTUNGTRENNUNG
Vilsmeier–Haack formylation of the ketones furnished their
respective b-chlorovinyl aldehydes (1a–1e), which upon
treatment with 2.0 equivalents of substituted aryl amines in
the presence of p-toluenesulfonic acid (PTSA) gave the de-
sired iminoenamines (2a–2n) in the yield range of 65–92%.
Scheme 2. Synthetic route to iminoenamine-BF2 complexes.
air and moisture and were fully
characterized
by
1H NMR,
13C NMR, and HRMS experi-
ments. Some of the diagnostic
1
features in the H NMR spectra
of these boron compounds in-
clude the disappearance of the
signal intensity, owing to the
proton of the free amine and
coupling between iminomethine
3
and boron with JHꢀB around
3.7 Hz. Broad multiplets in
both 11B NMR and 19F NMR
spectra were also observed
owing to the coupling between
boron and fluorine atoms. In
certain cases, for example, the
11B NMR spectrum of 4g exhib-
ited a triplet (1JBꢀF =29.9 Hz)
Scheme 1. Synthetic route to iminoenamines.
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b-Iminoenamine-BF2 Complexes
Figure 1. X-ray structure of 4g with selective atomic numbering scheme, thermal ellipsoids are set at 50% probability (left). Crystal packing diagram of
4g (right).
and nearly the same coupling constant was observed in the
Photophysical Studies
19F NMR spectrum (1JBꢀF =30.0 Hz) with a well-resolved
quartet pattern.
All of the compounds were generally characterized by
a well-separated intense low-energy absorption band in the
range of 410–445 nm (see Table 1, Figure 2, and the Sup-
porting Information, Figure S1) with molar extinction coeffi-
cients in the order of 104 mꢀ1 cmꢀ1. In addition, less intense
X-ray Diffraction Studies
As expected, the coordinatively saturated boron atom
adopted a pseudo-tetrahedral geometry on the plane de-
fined by the imine and amido nitrogen flanked by the two
fluorine atoms, above and below the plane (Figure 1 and see
the Supporting Information, Figure S7). Selected bond dis-
tances and bond angles are provided in the Supporting In-
formation together with other relevant crystallographic data
(see the Supporting Information, Tables S1 and S2). The
N
imine-B-Namido bite angles were in the expected standard
range when compared to the previously reported[6,9] anilido-
imine complexes. Similar to boron–dipyrromethene com-
pounds,[2] the average distances of the boron atom to the ni-
trogen atom of the imine (B(1)-N(1)=1.54 ꢂ) and to the
amine nitrogen (B(1)-N(2)=1.55 ꢂ) were similar, thus sug-
gesting an effective delocalization of the positive charge of
the iminium ion. The aliphatic ring in all the three cases
adopted a puckered conformation owing to the sp3-hybrid-
ized nature of the carbon or oxygen atom, with the atoms
exhibiting torsional angles approximately in the range of
55.8–59.28. Also, the dihedral angles between the mean
planes defined by the N-bonded arene and the boron het-
erocycle were invariably greater than 358 (see the Support-
ing Information, Table S1). These facts demonstrate low ele-
ments of symmetry and planarity in these molecules. Close
examination of the crystal packing structures revealed more
interesting aspects. Although no specific pattern in the ar-
rangement or arene p–p stacking was evident (closest p–p
interplanar distance was greater than 3.8 ꢂ), significant in-
termolecular CH···F and CH···p interactions[2i,11] were ob-
served in the lattices of these compounds. Especially in com-
pound 3e, CH···F distances in the order of approximately
2.46 ꢂ were observed. In addition to conferring increased
stability, these interactions could affect photophysically rele-
vant electronic states of the molecule.
Figure 2. Electronic absorption spectra of 3a, 3d, 4c, and 4d.
bands, which indicate lower transition probability, were also
observed in the wavelength range of 249–352 nm. Steady-
state emission spectra measured in CH2Cl2 at room tempera-
ture showed very weak emission intensities with low quan-
tum yields in the range of 10ꢀ2–10ꢀ4. Compound 5 contain-
ing a sulfur atom was completely non-emissive in CH2Cl2 at
room temperature presumably owing to heavy atom photo-
induced electron transfer.[12] The other emission profiles
were Gaussian shaped, broad, and closely resembled the ab-
sorption bands (see Figure 3 and the Supporting Informa-
tion, Figure S2). In the solid state, the emission profiles
were relatively narrower and red-shifted. However, in the
cases of 3c, 4a, 4d, 4e, and 4g a distinct blue shift was ob-
served. The observed Stokes shifts were in an extraordinary
range (86–121 nm) as compared to the typical 7–15 nm shift
observed in BODIPY derivatives. Earlier observations with
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Kallupattu Kuppusamy Balasubramanian, Koushik Venkatesan et al.
FULL PAPER
Table 1. Photophysical properties of compounds 3a–g, 4a–g, and 5.
core as found in the dipyrrin chelates. Solvatochromic stud-
ies performed on compounds 3d and 4e, showed only mod-
erate hypsochromic shifts in the range of approximately 3–
4 nm (see the Supporting Information, Figure S3) with an in-
crease in the solvent polarity (i.e., benzene!THF!acetoni-
trile). In a CH2Cl2 solution at room temperature, the quan-
tum yields were low (Table 1); using more viscous solvents
such as dimethyl sulfoxide (DMSO) or ethylene glycol
showed no improvement. Nevertheless, it was interesting to
see that the solid crystals of most compounds were highly
fluorescent when irradiated with a hand-held UV-lamp (ex-
citation approx. 365 nm, see the Supporting Information,
Figure S4). For compounds 3e, 4 f, and 4g, which exhibited
monoexponential decay lifetimes, the calculated[13a] radiative
(Kr) and non-radiative (Knr) rates were in the ranges of 105–
106 sꢀ1 and 108–109 sꢀ1, respectively. The magnitude of Knr
suggests a significant degree of radiationless deactivation in
solution.
RT solution (CH2Cl2)
Absorption PL
lmax[nm]
Emission at RT[d]
Eopt[f] [eV]
t
Ff[a]
Stokes lmax[nm], lmax[nm],
lemiss [ns] x 10ꢀ3 shift[b] solid
PMMA
A
/
[nm]
[nm]
(Ff
ACHTUNGTREN(NUGN Ff
PMMA)[d]
[dm3 molꢀ1
cmꢀ1])
3a 348 (7706),
429 (24996)
3b 337
541 0.3, 0.97
112
84
562
555
544
538
2.47
2.44
6.5
[e]
531
–
6.20
(14111),
447 (33670)
3c 317
G
522 0.6, 3.70
4.0
520 1.3, 2.61
9.1
101
102
116
512
514
2.58
2.59
2.48
2.45
2.65
2.53
2.51
2.62
421(22590)
ACHTUNGTRENNUNG
3d 324 (6168),
418 (21372)
3e 345 (6371),
427 (20701)
3 f 339 (1295),
445 (36301)
4a 338 (5221),
412 (15870)
4b 337 (7971),
418 (24464)
4c 335 (7849),
433 (21294)
4d 340 (7041),
410 (20904)
4e 344 (7511),
413 (22715)
543 (6.0) 522 (2.4)
552 (2.4) 547 (1.6)
540 2.0 0.57
[e]
531
515
–
–
54.01 86
4.10
–
545
510
[e]
103
121
101
112
105
110
110
–
506
The absolute quantum yield of 3e in the solid state (crys-
talline form) was found to be 2.4% when measured by an
integrating sphere apparatus. This value is significantly
higher than that observed in CH2Cl2 (Table 1). In the solid
state, one would normally expect electron-transfer quench-
ing owing to exciton–phonon coupling, which originates
from close interatomic distances, as proposed by Langhals
et al.[13b] As discussed earlier, compounds for which single-
crystal XRD data were examined, such as 3e, showed
CH···F and CH···p interactions, but were devoid of p–p in-
teractions. The presence of CH···F and CH···p interactions
and absence of p–p interactions was presumed to have some
influence on the quantum yield by altering the ground-state
properties of the luminophore. To verify this hypothesis, we
recorded the absorption spectra of 3e, 4a, 4c, 4d, and 4g in
the solid state by using a diffuse reflectance method. Indeed
bathochromic shifts of various magnitudes (5–72 nm) were
observed for these compounds (see the Supporting Informa-
tion, Figure S5). For example, the largest change in the ab-
sorption maximum (lmax) was found for 4c, which showed
a significant shift from 433 nm in CH2Cl2 to 505 nm in the
solid state. Such phenomenon has been previously attributed
to an increase in exciton cou-
539 0.9, 0.78
553 (6.4) 539 (1.7)
8.0
[e]
534
–
3.15
538
(0.003)
496
540
522 0.7, 3.20
17.0
518 0.4, 6.65
6.3
527 (6.4)
504
526
503
566
527 (10.0) 2.62
4 f 331 (8433),
413 (31059)
4g 328 (7550),
411 (26740)
523 0.6 6.91
514 (5.7)
511 (1.0)
–
2.57
2.67
2.49
521 0.1 3.60
5
352
–
–
–
(12541),
428 (31510)
[a] Quantum yield determined with fluorescein in 1N NaOH as a standard. [b] Cal-
culated from the difference of the absorption and emission maxima. [c] Quantum
yield determined in the solid state. [d] Quantum yield determined from PMMA
thin films with a 2 wt% doped concentration. [e] Multiexponential decay. [f] Optical
band-gap calculated from the absorption edge.
anilido-amine systems[1c,9] also revealed similar trends, which
could be attributed to the deviation from the C2-symmetric
pling.[14]
To ascertain if similar benefi-
cial self-organizing aggregates
can be triggered in solution, we
further carried out experiments
with 4d. For a solution of 4d in
tetrahydrofuran (THF), the PL
spectra were obtained with an
increasing concentration of de-
ionized water as shown in
Figure 4. A steady increase in
the relative emission intensity
was observed with increasing
water concentration owing to
the formation of partially solu-
ble aggregates. At 80–90% (v/
Figure 3. Emission spectra of 3d (left) and 4e (right) in CH2Cl2, thin films, and in the solid state are recorded
at room temperature.
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b-Iminoenamine-BF2 Complexes
an or chromene) in the chelating backbone. It was therefore
hypothesized that the rapid conformational changes (ring-
flipping) at ambient temperature could significantly contrib-
ute in accelerating the rates of non-radiative decay. Support-
ing this notion, low variable temperature (VT)-NMR re-
corded for 4e at 233 K revealed decoalescence of the reso-
nance signal, which corresponds to the methylene protons at
d=4.87 ppm owing to the “frozen-out” conformation (see
Figure 5a, highlighted region). Further, VT photolumines-
cence of 4b (in 2-methyltetrahydrofuran) under rigorous ex-
clusion of oxygen was also undertaken. As shown in Fig-
ure 5b, the compound exhibited maximum relative intensity
at 77 K; this decreases with an increase in temperature.
Even at 175 K, most of the emission intensity is lost, further
confirming that conformational dynamics are a key factor
Figure 4. PL spectra of 4d in THF/H2O mixtures (1.0ꢃ10ꢀ5 m, excited at
410.0 nm) with different volumes of H2O (fw).
v) ratios, the aggregates of the
compound were visible as tiny
dots when observed under the
UV lamp (see the Supporting
Information, Figure S4). A sim-
ilar phenomenon was also ob-
served in the case of 4c (see
the Supporting Information,
Figure S6). This observation is
consistent with the phenomen-
on of AIE.[15a] Aprahamian and
co-workers have recently re-
ported AIE phenomenon in
BF2-hydrazone complexes.[15b]
To gain more insight, increasing
Figure 5. a) Low VT-NMR of 4e in [D8]THF, (*) denotes solvent impurity (CH2Cl2). b) VT-photoluminescence
spectrum of 4b recorded in 2-methyltetrahydrofuran.
weight percent concentrations
of 4d were doped onto thin
films with PMMA (PMMA=
poly(methyl methacrylate)) as the host matrix. The quantum
yields obtained with 2.0, 4.0, 10.0, and 40.0 wt% loadings
were 6.4, 5.0, 4.0, and 2.0ꢃ10ꢀ3 %, respectively, thereby indi-
cating a trend opposite to aggregation induced in solution.
Based on the observations of the two above experiments,
the following tentative conclusions can be drawn: first, the
increased luminescence observed by slow incremental pre-
cipitation (AIE phenomenon) probably occurs because of
photophysically favorable preorganization due to spontane-
ous forces of self-organization; and second, under forced sit-
uations such as doping a small weight% of the dye in
PMMA, similar phenomenon need not be observed due to
the possibility of molecular motions in the matrix. The solid-
state molecular structures revealed a significant loss of pla-
narity in the molecule, and in addition, one can also expect
single bond-free rotation of the arenes about the nitrogen
atoms in solution. We reasoned that these symmetry lower-
ing factors can contribute more to the lowering of quantum
yields in solution. However, it was intriguing to note that
the anilido-difluoride system recently reported showed good
quantum yields in solution,[6] despite having similar N-
bonded arenes. A notable variation that is unique to the cur-
rent system is that it incorporates an acyclic ring (benzopyr-
responsible for quenching of fluorescence intensity in this
class of molecules. To further clarify the photoluminescence
behavior of the compounds in the crystalline state and in
the PMMA matrix, we measured the absolute quantum
yields of selected complexes 3d, 3e, and 4b both in PMMA
thin films (2 wt% doped) and in the solid state. In the
PMMA matrix, quantum yields of 2.4, 1.6, and 1.7% were
obtained for 3d, 3e, and 4b, respectively. In the case of
solids, marginally incremental values of 6.0, 2.4, and 6.4%
were observed for 3d, 3e, and 4b, respectively.
It is presumed that, although the quantum yields in-
creased roughly tenfold in moderately rigid PMMA matrix
as compared to in solution, the molecular motions are not
entirely restricted and therefore it still exhibits a lower
value as compared to that in the solid state. Such unrestrict-
ed molecular motions have been reported previously.[16] The
excited-state lifetime for many compounds could be best
fitted biexponentially containing a short subnanosecond and
a longer nanosecond term. Such biexponential decay has
been previously observed for molecules with conformational
flexibility and also for aggregation-induced luminophores
that display flipping kinetics.[9d] To gain some ideas about
the electrochemical behavior of these compounds and their
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Kallupattu Kuppusamy Balasubramanian, Koushik Venkatesan et al.
FULL PAPER
band gaps, cyclic voltammetry studies were carried out for
selected compounds 3b–3d, 3 f, 4a, 4c, 4e, and 4g. Most
compounds exhibited a prominent quasi-reversible first re-
duction wave when recorded at 258C in CH2Cl2 with 0.1m
[nBu4N]ACHTUNGTRENNUNG[PF6] as the supporting electrolyte; the anodic part
of the wave described by the anodic current (ia) was notably
suggests the involvement of the imine part of the compound
in the reduction process. From the onset of the first reduc-
tion peak,[17a] the LUMO energy values were calculated to
be in range between ꢀ2.33 eV to ꢀ2.87 eV for the measured
compounds. These values were in general agreement—both
in terms of trend and magnitude—with those obtained by
DFT calculations discussed later on.
less intense than expected. An example is shown in Figure 6
Theoretical Calculations
To better understand the luminescent properties of the syn-
thesized b-iminoenamine-BF2 complexes, DFT calculations
in the gas phase and TD-DFT calculations in CH2Cl2 (using
the CPCM polarizable conductor calculation model)[18] were
carried out for the selected molecules 3a–3c, 4c, and 4e
with the Gaussian 03 program package[19] using the hybrid
functional B3LYP[20] in conjunction with the Pople-type
basis set 6-311+GACTHNUTRGNEUNG
(2d,p) [6-311G*].[21]
According to the TDDFT calculations, the calculated UV/
Vis spectra of the studied compounds closely reproduce the
experimental results with a well-separated low-energy in-
tense absorption band appearing in the range 410–445 nm
and less intense bands in the range 249–352 nm (see the
Supporting Information, Figure S9). The calculated lmax in-
creases in the order of 393 (413 nm) for 4e >401 (421) for
3c >421 (429) for 3a >445 (433) for 4c >468 (447) nm for
3b (Table 3). The calculated values are in good agreement
with the experimental values (in brackets), the general trend
is well respected, but it is worth noting that the calculated
values are underestimated by 10–20 nm for compounds con-
taining F- or OCH3-substituted phenyl rings, while they are
overestimated by about 20 nm for the NO2-containing com-
pounds 3b and 4c.
In all cases, the S0!S1 transition (Table 3) has exclusively
a HOMO!LUMO (p/p*) character. The HOMO of the
compounds with electron-withdrawing groups (F, NO2) on
the N-coordinated phenyl rings includes large contributions
from the central b-diketiminato boron core C3N2B (61–
64%), limited participation of the phenyl rings (20–25%),
and of the fused aliphatic rings (13–19%) (Table 4 and see
the Supporting Information, Figure S10). The presence of
the electron-donating methoxy groups in 3a increases the
participation of the substituted phenyl rings in the HOMO
to 41%, thus decreasing the involvement of the rest of the
molecule to 51% for the central core and to only 8% for
the fused aliphatic cycle. The LUMO of 3a is even more lo-
calized on the central core (64%) but in a different pattern
(see the Supporting Information, Figure S11). Indeed, while
Figure 6. Cyclic voltammogram of 4a in 0.1m [nBu4N]ACTHUNTRGNENG[U PF6]; Au elec-
trode; scan rate=100 mVsꢀ1; 208C; CH2Cl2; LUMO [eV]=-(EredꢀEFc/Fc+
+4.8).[17]
and others are shown in the Supporting Information (Fig-
ure S8). Irreversible oxidation peaks were observed for 3c
and 4c within the electrochemical window of CH2Cl2; the
other less prominent oxidation waves could not be assigned
with certainty. Among the compounds with a dihydronaph-
thalene backbone (3b–3d, 3 f), those containing a p-nitro-
phenyl substituent such as 3b and 3 f, displayed reduction
peak potentials with lower negative values of ꢀ1.43 V and
ꢀ1.46 V when compared to 3c and 3d that contain a fluorine
atom as the substituent (Table 2). Among the chromene de-
rivatives (4a, 4c, 4e, and 4g), compound 4a without any
substituent on the phenyl ring showed a peak potential at
ꢀ1.95 V. In comparison, compound 4c with a p-nitrophenyl
substituent, and 4e with a m-fluorine substituent showed
relative ease of reduction with potentials at ꢀ1.45 V and
ꢀ1.80 V, respectively. Compound 4g containing a methyl
substituent shifted towards more negative reduction poten-
tials at ꢀ1.98 V as expected. The above trend tentatively
Table 2. Electrochemical data of selected complexes.
Reduction
Onset reduction
Oxidation
LUMO[a] [eV]
ꢀ
Ep,c [V]
Ered,onset [V]
Ep,a [V]
the HOMO shows antibonding p* interactions between C3
ꢀ
ꢀ
C4 C5 and N2 N6, the LUMO exhibits antibonding p* in-
3b
3c
3d
3 f
4a
4c
4e
4g
ꢀ1.43
ꢀ1.93
ꢀ1.98
ꢀ1.46
ꢀ1.95
ꢀ1.45
ꢀ1.80
ꢀ1.98
ꢀ1.26
ꢀ1.74
ꢀ1.67
ꢀ1.29
ꢀ1.72
ꢀ1.23
ꢀ1.51
ꢀ1.76
–
ꢀ2.83
ꢀ2.35
ꢀ2.43
ꢀ3.81
ꢀ2.38
ꢀ2.87
ꢀ2.61
ꢀ2.33
ꢀ
ꢀ
ꢀ
ꢀ
+1.02
–
–
–
+1.37
–
–
teractions between N2 C3, C5 N6, C3 C5, and N2 N6.
These HOMO and LUMO shapes are also observed for 3c
and 4e with contributions of the central ring of 62–64%.
The fluorescent properties are thus mainly due to an intra-
molecular charge transfer located in the central heterocycle
of these three species. It is important to note that the boron
[a] LUMO [eV]=ꢀ(EredꢀEFc/Fc++4.8).
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b-Iminoenamine-BF2 Complexes
Table 3. Selected singlet–singlet excited states with the corresponding TD-DFT verti-
cal excitation energies (nm), transition coefficients, orbitals involved in the transitions,
oscillator strengths (f), and molecular orbital energies (eV) for compounds 3a, 3b, 3c,
4c, and 4e.
served as LUMO+1 and LUMO+2 for 3a, 3c,
and 4e, become LUMO and LUMO+1 for com-
pounds 3b and 4c. The participation of the central
six-membered ring into the LUMO of 3b and 4c
drops to 16–17% and to as low as 2–4% into
LUMO+1, thereby reducing significantly the spa-
tial antibonding interactions over the molecule. On
the contrary, the electron density is delocalized at
78–79% on the NO2-substituted phenyl rings, thus
showing the influence of such an electron-withdraw-
ing group and conjugated group on the fluorescent
properties of this class of compounds. The calculat-
ed LUMO energies (see Table 3 and Table 2) were
in line with the corresponding values obtained from
the CV measurements, with the relative decrease in
energy following the order of 3c>4e>3b>4c.
Complex 3a
3b
3c
4c
4e
S0!S1
S0!S2
S0!Sn
H!L
(100%)
H!L
H!L
H!L
H!L
(100%)
(100%)
(100%)
(100%)
421.4 (0.550) 473.4 (0.606) 400.7
(0.584)
449.4 (0.533) 391.6
(0.505)
H-1!L
H!L+1
H-1!L
H!L+1
H-1!L
(100%)
(100%)
(100%)
(100%)
(100%)
363.4 (0.186) 433.5 (0.176) 328.7
(0.175)
n=10
410.6 (0.126) 345.8
(0.129)
n=3
n=8
n=3
n=3
H!L+3
H!L+2
H-2!L
H-1!L
H-2!L
(61%)
(100%)
(57%)
(65%)
(100%)
274.4 (0.115) 365.1 (0.145) 264.1
387.0 (0.128) 328.6
(0.093)
n=9
(0.146)
S0!Sn
n=10
n=4
n=4
H-6!L
H-1!L
H!L+2
H-6!L
(51%)
(82%)
(70%)
(62%)
267.9 (0.129) 351.9 (0.197)
362.3 (0.136) 263.6
(0.159)
Conclusions
Exp./
429, 348
421, 363
ꢀ2.20
447, 337
468, 355
ꢀ3.17
421, 317
401, 329
ꢀ2.33
433, 335
445, 357
ꢀ3.19
413, 344
393, 343
ꢀ2.49
Calcd[a]
LUMO
(eV)
The synthesis, photophysical, and electrochemical
aspects of a novel class of b-diketiminato-BF2 mole-
cules with an aromatic-fused alicyclic/hetero-alicy-
clic ring unit have been studied. They were found
to display large Stokes shifts as compared to the
popular boron–dipyrromethene complexes. The
solid-state emission quantum yields were found to
be superior compared to those in solvents at room
temperature. From various experiments it is under-
HOMO
(eV)
ꢀ5.61
ꢀ6.23
ꢀ5.87
ꢀ6.42
ꢀ6.17
[a] The calculated values were obtained from the TD-DFT/CPCM UV/Vis spectra
drawn by GaussView.
Table 4. HOMO and LUMO compositions (%) for compounds 3a, 3b,
stood that, the inclusion of a non-planar aliphatic core
indeed improves the solid-state emission by preventing p–p
stacking, however, on the other hand, in solvents it simulta-
neously expedites non-radiative pathways owing to the rapid
conformational motions. The significant CH···F CH···p inter-
actions observed in some of the solid-state molecular struc-
tures, apart from conferring structural rigidity to the com-
pound, are also believed to alter the ground state of the
molecule to a certain extent.
3c, 4c, and 4e.
Atom
B1 N2 C3 C4 C5 N6 total C3BN2 phenyls
LUMO 3a
1
0
1
1
1
1
1
1
1
1
11
2
11
2
10
9
10
11
8
21
6
21
6
21
3
4
4
4
3
2
1
2
1
20
6
21
6
19
3
5
4
4
4
8
1
8
1
64
16
64
17
62
51
64
62
61
62
9
3b
3c
4c
4e
79
10
78
10
41
20
25
20
22
3
8
HOMO 3a
19
26
25
28
28
14
18
17
16
17
3b
3c
4c
4e
9
Experimental Section
Synthetic and experimental procedures are described, crystallographic
details and additional spectroscopy data of the molecules are provided in
the Supporting Information. CCDC 880782 (3d), CCDC 880783 (3e),
CCDC 880784 (4c), CCDC 880785 (4e), CCDC 880786 (4g) contain the
supplementary crystallographic data (excluding structure factors) 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.
and fluorine atoms do not participate more than 1% to
these frontier orbitals.
The lowest-energy absorption bands of 3b and 4c, experi-
mentally observed at longer wavelengths, arise from the
lowest excited states S1 and S2 that correspond to the
HOMO!LUMO
(100%)
and
HOMO!LUMO+
1 (100%) transitions, respectively. The red-shift effect of the
UV/Vis absorption is observed at the molecular orbital level
by the narrow HOMO–LUMO gaps of 3.06 and 3.23 eV for
3b and 4c, in comparison with 3.41, 3.54, and 3.68 eV for
3a, 3c, and 4e, respectively. These small energy differences
are mainly due to the low-energy LUMO and LUMO+1,
which are delocalized over the substituted phenyl rings, and
stabilized by the extended conjugation of the phenyl rings
over the NO2 groups. These two unoccupied orbitals, ob-
Acknowledgements
The authors wish to thank Dr. Thomas Fox for VT-NMR studies and PD
Dr. Laurent Bigler for HRMS measurements. Financial support from
University of Zꢀrich is gratefully acknowledged. The authors also wish to
express thanks to the management of Shasun Research Centre and the
analytical department for their support and encouragement during the
entire course of this work.
Chem. Asian J. 2012, 00, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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Kallupattu Kuppusamy Balasubramanian, Koushik Venkatesan et al.
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Received: May 29, 2012
Published online: && &&, 0000
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 0000, 00, 0 – 0
ÝÝ These are not the final page numbers!

FULL PAPER
Got it in one: A new class of b-imi-
noenamine-BF2 complexes with a p–p
stacking disruptive alicyclic core
exhibit solid-state fluorescence, large
Stokes shifts, tunable emission, and
aggregation-induced emission (AIE).
Aggregation-Induced Emission
Karthikeyan Perumal, Jai Anand Garg,
Olivier Blacque,
Ramanathan Saiganesh,
Senthamaraikannan Kabilan, Kallu-
pattu Kuppusamy Balasubramanian,*
Koushik Venkatesan*
&&&& — &&&&
b-Iminoenamine-BF2 Complexes:
Aggregation-Induced Emission and
Pronounced Effects of Aliphatic Rings
on Radiationless Deactivation
Chem. Asian J. 2012, 00, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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