The photophysics of pyridine-derivatized ortho-, meta-, and para-dibutylamino cruciforms

Article abstract of DOI:10.1002/chem.201300211

The photophysical properties of a series of para-substituted donor-acceptor cruciform fluorophores (p1-4) were investigated and compared with their meta and ortho isomers (m1-4 and o1-4). The structural variations were found to have a significant effect o

Full text of DOI:10.1002/chem.201300211

DOI: 10.1002/chem.201300211
The Photophysics of Pyridine-Derivatized ortho-, meta-, and
para-Dibutylamino Cruciforms
Florian Hinderer and Uwe H. F. Bunz*[a]
Abstract: The photophysical properties
quantum yields (Ffl), fluorescence life-
times (tfl), and response upon addition
of trifluoroacetic acid. The observed
spectral shifts in absorption and emis-
of a series of para-substituted donor–
acceptor cruciform fluorophores (p1–4)
were investigated and compared with
their meta and ortho isomers (m1–4
and o1–4). The structural variations
were found to have a significant effect
on the solvatochromism, fluorescence
sion caused by protonation of the cru-
ciforms make them promising candi-
dates as chemosensors. Additional
computational studies provided more
insight into the electronic structure of
the systems.
Keywords: cruciforms · distyrylben-
zene · fluorophores · solvatochrom-
ism · sensors
Introduction
which contains dibutylamino and pyridine groups, displays a
two-stage response in fluorescence when exposed to Lewis
acids. It is a promising candidate for the design of ratiomet-
ric fluorescence sensors. The potential of XFs as lumines-
cent chemosensors led to the synthesis of several types of
dyes with spatially separated FMOs, that is, paracyclo-
phanes,[2] distyrylbis(arylethynyl)benzenes,[3] tetrakis(aryl-
Herein, we describe the effect of positional mutation of the
dibutylamino (ortho, meta, and para) and the pyridyl (2-, 3-,
and 4-pyridyl) groups on the optical and electronic proper-
ties of a cross-shaped distyrylbenzene fluorophore.
1,4-Distyryl-2,5-bis(arylethynyl)benzenes, (cruciform, XF)
are attractive environment-responsive fluorophores.[1]
Donor–acceptor substituted XFs, p4 with fully conjugated
dibutylamino and pyridine groups, which feature free elec-
tron pairs at each terminus, display fascinating double-stage
ratiometric color changes upon exposure to metal cations as
model analytes.[1] A question, not yet answered, is the influ-
ence of the position of the pyridine nitrogen atom and the
dibutylamino group on the fluorescence properties of these
XFs. If one permutates, nine different positional isomers,
namely, o2–4, m2–4, and p2–4, will result. We prepared all
of them and investigated their fluorescence properties both
in the absence and in the presence of acid. We find surpris-
ing trends in this system. By variation of the substitution
pattern of the two-dimensional cross-conjugated scaffold,
the HOMOs and LUMOs can either be delocalized over the
entire molecule or localized to each 1D branch, thus ena-
bling spatial control of electronic properties such as internal
charge-transfer (ICT) pathways and the size of the band
gap. Such disjoint frontier molecular orbitals (FMOs) make
the interaction of cations either with the HOMO or LUMO
possible, thus resulting in characteristic changes in absorp-
tion and emission. The donor–acceptor substituted XF, p4,
A
R
tetraethynylethenes,[5]
ACHTUNGTRENNUNG
(oxazolo) cruciforms.[7,8] In the case
tetrakis-
N
of XFs, the +M and ꢀM-directing substituents were always
placed at the para position with respect to the connecting
styryl and alkynyl groups, respectively.
Bulky ortho substituents can distort the planarity of the
p-system resulting in a less distinctive electronic conjugation
and limited orbital overlap. Although, for meta-bridged p-
systems, relatively low electronic delocalization is expected
in the ground state, rearrangement of the p-orbitals in the
excited state can enhance the electronic coupling of the sub-
units. The photophysical and photochemical behavior of
trans-stilbenes with either donor or donor–acceptor substitu-
ents in the para position has been well described in litera-
ture, but comprehensive studies for the corresponding meta
and ortho isomers are rare.[9,10] Greater charge-transfer char-
acter, longer fluorescence lifetimes, larger fluorescence
quantum yields, and smaller isomerization quantum yields
were found for meta-substituted cis- and trans-aminostil-
benes relative to those found for nonsubstituted and para-
substituted derivatives.[11] This “meta-amino effect” origi-
ꢀ
nates from a high torsional barrier for the C C double bond
in the excited state.[12] The same behavior was also observed
for other donor-substituted trans-stilbenes.[13]
[a] Dipl.-Chem. F. Hinderer, Prof. Dr. U. H. F. Bunz
Organisch-Chemisches Institut
We explored the photophysical behavior of the cruciforms
p1–4, m1–4, and o1–4 by positional variation of the nitrogen
atoms within the aniline and pyridine subunits. We report
absorption and fluorescence spectra as well as fluorescence
quantum yields and lifetimes of the XFs, p1, m1, o1, and the
nine donor–acceptor-substituted XFs, p2–4, m2–4, and o2–4.
Ruprecht-Karls-Universitꢀt Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Tel : (+49)6221-548401
E-mail: uwe.bunz@oci.uni-heidelberg.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300211.
8490
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 8490 – 8496

FULL PAPER
Results and Discussion
Synthesis: The general synthesis route for both the donor-
and donor–acceptor-substituted cruciform, p1–4, m1–4, and
o1–4, as reported previously,[1] is outlined in Scheme 1. The
precursors for the donor substituents can be afforded in a
straightforward two-step synthesis (Scheme 2). While para-
(dibutylamino)benzaldehyde is commercially available, alky-
lation of the corresponding bromoanilines, 1 and 2, followed
by a subsequent formylation using N,N-dimethylformamide
(DMF) as the electrophile precursor provides the ortho- and
meta-isomers, 5 and 6. All alkynylated pyridines are com-
mercially available as their trimethylsilyl-substituted deriva-
tives.
Scheme 2. Synthesis of ortho- and meta-(dibutylamino)benzaldehydes 5
and 6.
A double Horner olefination of 7 with (dibutylamino)al-
dehyde 5 and 6 and the para-isomer affords 1,4-diiodo-2,5-
distyrylbenzenes 8–10, respectively. The ethynyl arms are at-
tached then through a Sonogashira–Hagihara cross-coupling
reaction, which employed the 2,3,4-ethynylpyridines and
was carried out in THF/diisopropylamine at room tempera-
ture with [PdACHTUNGTERNU(NG PPh3)2Cl2] and CuI as catalysts (see the Sup-
porting Information for details).
UV/Vis absorption and fluorescence spectra: Representative
for all XFs, the normalized UV/Vis absorption and fluores-
cence spectra of p4, m4, and o4 in n-hexane and CH2Cl2 are
shown in Figure 1. The UV/Vis absorption and fluorescence
spectra for all cruciforms are given in the Supporting Infor-
mation. Whereas the para isomer, p4, displays two broad ab-
sorption bands between 300 nm and 550 nm, the ortho and
meta isomers, o4 and m4, show a single band at approxi-
mately 325 nm with low-energy shoulders. The high intensity
low-energy band of p4 is attributed to an internal charge-
transfer (ICT) transition; the aniline ring in o4 is probably
severely twisted out of planarity, owing to steric hindrance,
thus leading to a blue-shifted absorption spectrum. Overall,
the data show that, in the ground state, everything is as ex-
Scheme 1. General synthesis of cruciforms. In the compound labels, the
italicized letter (o, m, and p) refers to the position (ortho, meta, and para,
respectively) of the dimethylamino group on the styryl axis, whereas the
number (2–4) refers to the position of the pyridine nitrogen atom with
respect to the alkyne group. DIPA=diisopropylamine.
Figure 1. Normalized UV/Vis absorption and fluorescence spectra of p4,
m4, and o4 in n-hexane (top) and CH2Cl2 (bottom).
Chem. Eur. J. 2013, 19, 8490 – 8496
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8491

U. H. F. Bunz and F. Hinderer
Table 1. UV/Vis absorption and emission maxima, absorption coefficient
(e), Stokes shift (nStokes), fluorescence quantum yield (Ffl), fluorescence
lifetime (tfl), fluorescence rate (kfl), and nonradiative deactivation (knr)
constants in CH2Cl2.
pected: meta substitution suppresses conjugation of the ani-
line to the styryl nucleus, and the ortho-substituted deriva-
tives are twisted, thus pinching off conjugation.
The fluorescence spectra of all isomers display blue emis-
sion in nonpolar solvents (e.g. n-hexane) with a well-defined
vibronic structure for p1–4, and somewhat structured for
m1–4 and o1–4 (see the Supporting Information). All spec-
tra were independent of excitation wavelength. Whereas the
emission maxima of the meta isomers, m1–4, in nonpolar
solvents are blue shifted (435–451 nm) compared to their
para isomers, p1–4, (465–500 nm) and the ortho isomers in
polar solvents lie in the range, 461–474 nm, the emission
maxima of the meta and ortho isomers are switched. In con-
trast to the absorption maxima, the fluorescence maxima
reveal a significant positive solvatochromism on going from
the nonpolar solvent, n-hexane, to the polar solvent, DMF.
Large bathochromic shifts for p1–4 (87–109 nm) and o1–4
(93–118 nm), and m1–4 (138–152 nm) are observed, that of
the latter being especially remarkable. The data indicate
that these species undergo absorption that leads to their
conversion from a less polar ground state to a locally excited
Franck–Condon state, followed by their relaxation to an en-
ergetically low-lying polar ICT state. The small bathochro-
mic shift in absorption (40 nm) for p1–4 with increasing sol-
vent polarity is attributed to an expected polar quinoidal
resonance structure in the ground state. The broadened and
featureless fluorescence spectra for all isomers in polar sol-
vents also provides evidence for the ICT character of the ex-
cited state. The loss of vibrational structure is explained by
structural relaxation of the excited state through solute–sol-
vent interactions.
XF labs,max
lfl,max
e
nStokes Ffl tfl
[cmꢀ1 [109 s]
kfl
[108 sꢀ1
knr
[108 sꢀ1
]
[nm]
[nm]
A
]
G
]
E
G
]
U
p1 340, 440 524
p2 335, 450 561
p3 335, 445 548
p4 338, 441 583
58480
46801
29492
41904
91156
93544
45942
60233
76672
60830
60968
59756
3643 0.40 1.6
4397 0.26 3.9
4224 0.17 3.0
6322 0.06 4.4
11344 0.12 9.2
12372 0.06 8.6
12353 0.05 8.2
12165 0.02 7.6
11649 0.28 3.5
12277 0.21 5.1
12268 0.23 4.8
12778 0.18 4.0
2.50
0.67
0.57
0.14
0.13
0.07
0.06
0.03
0.80
0.41
0.48
0.45
3.75
1.90
2.77
2.14
0.96
1.09
1.16
1.29
2.06
1.55
1.60
2.05
m1 331
m2 328
m3 325
m4 327
530
552
543
543
o1 325, 388 523
o2 324, 388 538
o3 323, 390 535
o4 323, 388 550
the meta isomers, the long fluorescence lifetimes could be
also explained by a slow charge recombination in going
from an excited state with an enhanced electronic coupling
to an electronically decoupled ground state. As charge re-
combination is a radiationless process, the low fluorescence
quantum yields for m1–4 could also be explained. Another
reason for the low quantum yields of the meta isomers could
also be the existence of one or more conformers, that, with
respect to the mono exponential fluorescence decay and the
emission spectra being independent of excitation wave-
length, are nonfluorescent and only play a negligible role.
On the basis of these results, both twisted-intramolecular
charge transfer (TICT) and photoisomerization seem im-
plausible.
The fluorescence quantum yields (Ffl) and lifetimes (tfl)
were measured in CH2Cl2 and used to estimate the radiative
ꢀ1
(kfl =Ffl/tfl) and nonradiative rate constants (knr =tfl ꢀkfl)
Solvatochromism: The absorption and emission spectra of
all cruciform, p1–4, m1–4, and o1–4 were measured in ten
solvents of different polarity, polarizability, hydrogen-bond
donor (HBD) and acceptor (HBA) strength. Representa-
tively, Figure 2 shows the absorption and emission spectra of
o4 (all spectra can be found in the Supporting Information).
The solvent-induced shifts, depending on processes such as
nonspecific and specific solute–solvent interactions, were an-
alyzed by a multiple linear-regression analysis (Table 2) ac-
cording to Kamlet and Taft [Eq. (1)]:[14]
for all isomers (Table 1). Although moderate fluorescence
quantum yields were found for the para (0.06–0.40) and
ortho (0.18–0.28) isomers, p1–4 and o1–4, the values were
substantially lower for the meta isomers, m1–4 (0.02–0.12).
This observation contradicts the results of Yang and Lewis,
who found that meta-aminostilbenes had larger fluorescence
quantum yields than the corresponding para isomers.[10–13]
The fluorescence lifetimes were measured by a single-
photon counting method and were all well fitted using
single-exponential decay curves. According to the results
herein, the fluorescence lifetimes of m1–4 (7.6–9.2 ns) are
longer than those of p1–4 (1.6–4.4 ns), with those of o1–4
having an intermediate value (3.5–5.1 ns). As a consequence
of the low fluorescence quantum yields and long fluores-
cence lifetimes of m1–4 in comparison to the para and ortho
isomers, only small fluorescence rate constants and large
nonradiative rate constants were determined for m1–4
(Table 1), thus resulting in low kfl/knr ratios. The long fluo-
rescence lifetimes for the meta compounds suggest that
energy transfers from a donor to an acceptor subunit within
one molecule.
*
nmax ¼ no þ sp þ aa þ bb
ð1Þ
The p* scale reflects the ability of the solvent to stabilize
either a dipole or a charge through nonspecific dielectric in-
teractions. The a and b scales describe specific solute–sol-
vent interactions, a characterizing the hydrogen-bond donor
(HBD) strength and b the hydrogen-bond acceptor (HBA)
strength of the solvents. Because the absorption maxima for
the meta and ortho cruciforms, m1–4 and o1–4, are nearly
independent of solvent polarity, the Kamlet–Taft analysis
was only used to analyze photophysical properties of the
emissive state (emission maxima). Here, nmax is the predicted
emission maximum in solution, n0 is the calculated emission
Assuming a photoinduced electron transfer (PET) from
the donor (aniline) to the acceptor (pyridine) subunit for
8492
www.chemeurj.org
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 8490 – 8496

Pyridine-Derivatized Dibutylamino Cruciforms
FULL PAPER
chromic shift with increasing hydrogen-bond donor strength
of the solvents (Table 2). The negative values of the b coeffi-
cients indicate a higher stabilization of the excited state rela-
tive to that of the ground state in basic solvents, resulting in
a red shift of the fluorescence maxima. As a resonance
structure with a positive charge located on the amino nitro-
gen atom is expected to be predominant for the excited
state, the formation of a Lewis acid–base exciplex involving
an excited cruciform and a ground-state Lewis base is plau-
sible.[15] More insight might be obtained by kinetic complex-
ation studies in binary mixtures of hydrocarbons and sol-
vents, which can act as Lewis bases.
The large negative values of the coefficients, s, provide
evidence for a strong stabilization of the relaxed excited
state in polar solvents. The red shift in emission on going
from n-hexane to DMF correlates well with the excited-
state dipole moments of both the cruciforms and the sol-
vents as well as their polarizability. The s coefficients of o1–
4 are only slightly higher than those of p1–4, but both of
these sets of s values are considerably smaller than those of
m1–4. These results indicate larger dipole moment differen-
ces between the ground and excited states and therefore
larger CT character for the ortho and meta isomers relative
to those for the para isomers. One reason for these differen-
ces might be the planarization of the more twisted ground-
state geometries of o1–4 and m1–4 upon electronic excita-
tion. The extraordinary large difference between the ground
and excited-state dipole moments for the meta isomers
might also arise from a charge separation in the excited
state. Instead of being distributed over the whole conjugated
p-system, the positive charge would be expected to be locat-
ed on the nonconjugated amino nitrogen atom and the nega-
tive charge on the pyridine subunit.
Figure 2. Normalized UV/Vis absorption (top) and emission spectra
(bottom) of o4 in a variety of solvents with different polarity.
Table 2. Kamlet–Taft coefficients for the measured emission maxima.
XF
n0
[cmꢀ1
]
s
a
b
n
r
U
p1
p2
p3
p4
m1
m2
m3
m4
o1
o2
o3
o4
21716
20363
20883
21105
23307
21920
22106
21759
21682
21078
21251
21011
ꢀ2.890
ꢀ2.964
ꢀ3.127
ꢀ2.831
ꢀ5.080
ꢀ4.674
ꢀ4.762
ꢀ4.000
ꢀ3.062
ꢀ3.252
ꢀ3.288
ꢀ3.456
ꢀ1.058
ꢀ0.887
ꢀ0.647
ꢀ1.631
ꢀ1.160
ꢀ0.710
ꢀ0.328
ꢀ0.083
ꢀ0.050
ꢀ0.187
ꢀ0.135
ꢀ0.564
ꢀ1.265
ꢀ1.070
ꢀ1.006
ꢀ1.864
ꢀ2.071
ꢀ1.555
ꢀ1.733
ꢀ1.522
ꢀ1.237
ꢀ1.315
ꢀ1.182
ꢀ1.433
10
10
10
10
10
10
10
10
10
10
10
10
0.912
0.949
0.957
0.986
0.918
0.943
0.969
0.939
0.926
0.971
0.966
0.968
TFA titration: Cruciform fluorophores, sensitive to protons
and metal cations, are potentially attractive as ratiometric
chemosensors. The spectral shifts upon addition of TFA by
variation of the substituent pattern should give more insight
into the photophysical properties of these systems. Figure 3
and Table 3 show the normalized fluorescence spectra of the
trifluoroacetic acid (TFA) titrations of the cruciforms in
CH2Cl2. As expected, the positional variations of the amino
and pyridine nitrogen atoms have significant impact on the
spectral shifts in absorption and emission upon addition of
TFA. At low TFA concentrations, whereas the ICT bands
for p1–4 disappear, the absorption spectra of m1–4 and o1–4
only undergo small changes. These results seem reasonable,
as protonation of the amino groups lower their donor
strength in line with the loss of charge-transfer character.
Owing to the lower level of conjugation expected for the
ortho and meta isomers, relative to that expected for the
para isomer, the absorption and emission characteristics of
the latter should exhibit higher sensitivity to the positional
variation of the amino and pyridine nitrogen atoms. The in-
crease of TFA content at high TFA concentrations only
leads to moderate changes to the absorption spectra of p1–
4, spectra that now resemble those of m1–4 and o1–4. The
maximum in the gas phase and a, s, and b are fitted coeffi-
cients (Table 2). The negative values of the coefficients
result in a positive solvatochromism for all cruciforms with
increasing acidity, basicity, and polarity of the solvents. For
the a and b coefficients, this result is somewhat unexpected.
These systems feature four basic groups with free electron
lone pairs. However, similar to protonation of the cruci-
forms through addition of trifluoroacetic acid (TFA), hydro-
gen bonding between protic solvents and the basic nitrogen
atoms of the cruciforms should result in a net blue shift of
the emission maxima. But, interruption of that hydrogen
bonding because of the amino groups being positively charg-
ed (and thus, less basic) in the excited state, leaving only the
possibility of hydrogen-bond interactions involving the pyri-
dine groups, could be the reason for the observed batho-
Chem. Eur. J. 2013, 19, 8490 – 8496
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8493

U. H. F. Bunz and F. Hinderer
than those of the other isomers.
As Figure 4 shows, the changes
in fluorescence can be seen by
the naked eye. Remarkably, the
basicity of the second amino
group of the monoprotonated
species is obviously not much
affected by protonation of the
first one. The small difference
in the pKa values of the neutral
and monoprotonated XF spe-
cies is similar to that observed
for 4,4’-diamino-trans-stilbene,
owing to diminished conjuga-
tion of the amino groups. Con-
sidering both the absorption
and the fluorescence spectra,
also photoacidity in the excited
state is not observed. Interest-
ingly, the meta isomers exhibit
significant fluorescence turn-on
upon addition of small amounts
of TFA, a behavior that is even
more pronounced in the highly
polar
solvent,
acetonitrile
(Figure 4).
As Table 3 shows, the fluores-
cence intensity ratios, I1/I0,
upon the addition up to 100
equivalents of TFA in CH2Cl2
for m1–4, lie between 5.7 and
13.4. Additionally, the fluores-
cence lifetime, tfl, for the meta
isomers decreases markedly in
the presence of an excess of
TFA. This finding is additional
evidence for the existence of a
PET process. In conjunction
with the low fluorescence quan-
tum yields and the long fluores-
cence lifetimes of the nonproto-
nated species, these results
Figure 3. Normalized fluorescence spectra of p1–p4 (top to bottom, left column), m1–m4 (top to bottom,
middle column) and o1–o4 (top to bottom, right column) in CH2Cl2 (1.4 mm) upon addition of different num-
bers of equivalents of trifluoroacetic acid. Excitation wavelength lexc =310 nm.
position of the pyridine nitrogen atoms has little effect on
spectral shifts of the absorption spectra caused by TFA addi-
tion. Compared to the absorption spectra, the emission spec-
tra are more sensitive to TFA concentration. According to
our previous work, the donor–acceptor substituted cruci-
forms p2–4, m2–4, and o2–4 display a two-stage response in
their emission spectra depending on the TFA concentration.
All isomers show an initial blue shift caused by protonation
of the more basic amino groups, representing a lowering of
the energy of the HOMO, followed by a subsequent red
shift upon protonation of the pyridine groups, a lowering of
the energy of the LUMO. Here, the position of the pyridine
nitrogen atom determines the magnitude of the red shift. As
expected and attributed to a smaller electronic contribution
to the LUMO, the red shifts for p3, m3, and o3 are smaller
strengthen the assumption of PET from the donor to the ac-
ceptor subunit for m1–4. But, further experimental investi-
gations will be necessary to prove this hypothesis. What
does the study of the ortho-, meta- and para-substituted
donor-acceptor cruciforms, p1–4, m1–4, and o1–4, tell us
about the effect of N-atom position on the photophysical
properties? Whereas only the absorption spectra of the
para-substituted cruciforms, p1–4, show
a low-energy
charge-transfer band, the emission spectra of all XFs reveal
internal charge-transfer character. The absorption spectra
are quite insensitive to solvent polarity, but in going from
nonpolar to polar solvents, the emission bands for all XFs
undergo a red shift, broaden, and become featureless. The
Kamlet–Taft multiple-regression analysis suggests that the
solvent-induced shifts in the emission spectra mostly corre-
8494
www.chemeurj.org
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 8490 – 8496

Pyridine-Derivatized Dibutylamino Cruciforms
FULL PAPER
Table 3. Photophysical properties of cruciforms p1–p4, m1–m4, and o1–
o4 in CH2Cl2 upon addition of trifluoroacetic acid. Fluorescence maxima
(lfl,max), fluorescence lifetime (tfl), and the ratio of the fluorescence inten-
sities (I1/I0 and I2/I0).
yields and longer fluorescence lifetimes for the meta-iso-
mers, m1–4, also suggest the operation of photoinduced
electron transfer from the amino donor to the pyridine ac-
ceptor subunit. The remarkable fluorescence turn-on and
the decreased fluorescence lifetime upon addition of tri-
fluoroacetic acid support this assumption. Whereas the fluo-
rescence intensity ratio, I1/I0, and the blue shift of the rising
band upon addition of small amounts of TFA can be fine-
tuned by the positional variation of the amino nitrogen
atom, the position of the pyridine nitrogen atom defines the
ratio, I2/I0, and the red shift upon addition of larger amounts
of TFA.
The puzzle of the red-shifted emission colors of the meta-
substituted dialkylamino-XFs, m1–4, is solved by quantum
chemical calculations on m1 (B3LYP 6-31G**, Figure 5).
The FMOs are not degenerate; HOMOꢀ1 and HOMO as
well as LUMO and LUMO+1 are energetically separated.
The meta-positioned aniline nitrogen atom contributes sig-
nificantly to the HOMO, that is, the meta-dialkylamino sub-
stituents are electronically connected to the frame of the
XF. The HOMOꢀ1 features dialkylamino substituted ben-
zenes separated from the rest of the conjugated system.
LUMO and LUMO+x (x=1, 2, and 3) display the expected
distribution, that is, the orbital coefficients are negligible at
the meta positions in the donor-substituted ring. Our crude
calculations, which show surprising delocalization of the
HOMO into the meta-positioned dialkylanilines, give only a
glimpse into the exciting properties of the excited states of
m1–m4.
50–1000 equiv TFA
excess TFA
XF
lfl,max
tfl
lfl,max
I1/I0
lfl,max
I2/I0
tfl
[nm]
[ns]
[nm]
[nm]
[ns]
p1
p2
p3
p4
m1
m2
m3
m4
o1
o2
o3
o4
524
561
548
583
530
552
543
543
523
538
535
550
1.6
3.9
3.0
4.4
9.2
8.6
8.2
7.6
3.5
5.1
4.8
4.0
466
449
447
519
439
449
432
455
455
429
443
474
0.35
0.18
0.88
0.24
5.68
5.73
12.50
13.40
2.68
2.15
1.57
0.41
443
523
466
551
444
518
463
550
457
504
454
522
0.97
0.89
0.70
0.93
3.97
1.44
1.65
8.67
2.52
0.68
0.54
1.35
3.7
3.6
2.4
2.8
3.8
6.1
2.8
3.7
3.3
4.9
3.3
4.7
Conclusion
Figure 4. The color of solutions of cruciforms in CH2Cl2 after addition of
different numbers of equivalents of TFA (horizontal line) under a black
light (l=366 nm) and normalized fluorescence spectra of m2 in acetoni-
trile (2.84 mm) upon addition of different numbers of equivalents of TFA
(right).
Meta-positioned dialkylamino groups and nitrogen atoms of
pyridine units in cruciforms do not lead to CT bands in the
absorption spectra. In the excited state, the conjugation bar-
rier that forbids meta-positioned substituents to exert their
influence, surprisingly, disappears; thus, o2–4, m2–4, and p2–
4 differ, in some respect, in their emission spectra (the meta
compounds generally having lower quantum yields and
longer lifetimes, but only slightly less red-shifted emission
features), but qualitatively they are similar. The puzzle can
be solved, at least partially, if one examines the FMOs of
m1 as a representative example. Simple analysis predicts
spond with the ability of the solvent to stabilize a dipole
through nonspecific solute–solvent interaction.
The bathochromic shifts for the meta-isomers, m1–4, were
found to be larger than those for p1–4 and o1–4, thus indi-
cating that m1–4 have higher charge-transfer character. An
explanation for this difference could be charge separation in
the excited state, with the positive charge located on the
amino nitrogen atom and the
negative charge on the pyridine
subunit. Unexpectedly, the
effect of the hydrogen-bond
donor strength on the batho-
chromic shifts is smaller than
that of hydrogen-bond accepACTHNUTRGNEUGtN or
strength. This result might be
explained by a Lewis acid–base
exciplex involving the cruciform
in the excited state and the
ground-state Lewis base. The
Figure 5. Representation of the HOMOꢀ1 (left), HOMO (middle), and LUMO (right) of m1 (B3LYP 6-
lower fluorescence quantum 31G**, Spartan).
Chem. Eur. J. 2013, 19, 8490 – 8496
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8495

U. H. F. Bunz and F. Hinderer
G. Evmenenko, P. Dutta, K. Clays, K. Song, T. J. Marks, J. Am.
Chem. Soc. 2006, 128, 6194–6205; c) M. Rumi, S. J. K. Pond, T.
Meyer-FriedACTHGUNTRNErNUG ichsen, Q. Zhang, M. Bishop, Y. Zhang, S. Barlow,
that the energetic positions of HOMO and HOMOꢀ1 re-
versed, relative to those of the ortho and para-substituted
isomers, as observed in the nonfluorescent bis(3-hydroxy-
S. R. Marder, J. W. Perry, J. Phys. Chem. C 2008, 112, 8061–8071.
[7] J. E. Klare, G. S. Tulevski, K. Sugo, A. de Picciotto, C. Nuckolls, J.
Am. Chem. Soc. 2003, 125, 6030–6031.
ACHTUNGTRENNUNG
styryl)benzenes.[16] The HOMO encompassing the meta-di-
ACHTUNGTRENNUNGalkACHTUNGTRENNUNGylamino groups conjugates into the XF frame, a situation
[8] a) J. Lim, O. S. Miljanic, Chem. Commun. 2012, 48, 10301–10303;
b) J. Lim, D. Nam, O. S. Miljanic, Chem. Sci. 2012, 3, 559–563; c) J.
Lim, T. A. Albright, B. R. Martin, O. S. Miljanic, J. Org. Chem.
2011, 76, 10207–10219; d) K. Osowska, O. S. Miljanic, Chem.
Commun. 2010, 46, 4276–4278.
that is disturbingly different from the example of the meta-
distyrylbenzenes investigated by Baumgarten, Mꢂllen et al.
in their classic paper.[17]
[9] a) K. M. Gaab, A. L. Thompson, J. Xu, T. J. Martinez, C. J. Bardeen,
J. Am. Chem. Soc. 2003, 125, 9288–9289; b) H. E. Zimmerman, J.
Am. Chem. Soc. 1995, 117, 8988–8991.
[10] J. S. Yang, Y. D. Lin, Y. H. Chang, S. S. Wang, J. Org. Chem. 2005,
70, 6066–6073.
[11] a) J. S. Yang, K. L. Liau, C. Y. Li, M. Y. Chen, J. Am. Chem. Soc.
2007, 129, 13183–13192.
[12] a) J. S. Yang, S. Y. Chiou, K. L. Liau, J. Am. Chem. Soc. 2002, 124,
2518–2527; b) J. S. Yang, K. L. Liau, C. M. Wang, C. Y. Hwang, J.
Am. Chem. Soc. 2004, 126, 12325–12335.
[13] a) F. D. Lewis, J. S. Yang, J. Am. Chem. Soc. 1997, 119, 3834–3835;
b) F. D. Lewis, W. Weigel, J. Phys. Chem. A 2000, 104, 8146–8153.
[14] a) M. J. Kamlet, J.-L. M. Abboud, M. H. Abraham, R. W. Taft, J.
Org. Chem. 1983, 48, 2877–2887; b) R. W. Taft, W. J. Shuely, R. M.
Doherty, M. J. Kamlet, J. Org. Chem. 1988, 53, 1737–1741.
[15] F. D. Lewis, L.-S. Li, T. L. Kurth, R. S. Kalgutkar, J. Am. Chem. Soc.
2000, 122, 8573–8574.
[1] a) A. J. Zucchero, P. L. McGrier, U. H. F. Bunz, Acc. Chem. Res.
2010, 43, 397–408; b) J. N. Wilson, U. H. F. Bunz, J. Am. Chem. Soc.
2005, 127, 4124–4125; c) M. Hauck, J. Schçnhaber, A. J. Zucchero,
K. I. Hardcastle, T. J. J. Mꢂller, U. H. F. Bunz, J. Org. Chem. 2007,
72, 6714–6725; d) P. L. McGrier, K. M. Solntsev, S. Miao, L. M. Tol-
bert, O. R. Miranda, V. M. Rotello, U. H. F. Bunz, Chem. Eur. J.
2008, 14, 4503–4510; e) C. Patze, K. Broedner, F. Rominger, O.
Trapp, U. H. F. Bunz, Chem. Eur. J. 2011, 17, 13720–13725; f) J.
Kumpf, U. H. F. Bunz, Chem. Eur. J. 2012, 18, 8921–8924; g) J.
Tolosa, K. M. Solntsev, L. M. Tolbert, U. H. F. Bunz, J. Org. Chem.
2010, 75, 523–534; h) J. Tolosa, A. J. Zucchero, U. H. F. Bunz, J.
Am. Chem. Soc. 2008, 130, 6498–6506; i) P. L. McGrier, K. M. Solnt-
ACHTUNGTRENNUNGsev, J. Schçnhaber, S. M. Brombosz, L. M. Tolbert, U. H. F. Bunz,
Chem. Commun. 2007, 2127–2129; j) J. N. Wilson, M. D. Smith, V.
Enkelmann, U. H. F. Bunz, Chem. Commun. 2004, 1700–1701.
[2] G. P. Bartholomew, G. C. Bazan, Acc. Chem. Res. 2001, 34, 30–39.
[3] J. N. Wilson, M. Josowicz, Y. Wang, U. H. F. Bunz, Chem. Commun.
2003, 2962–2963.
[16] K. M. Solntsev, P. L. McGrier, C. J. Fahrni, L. M. Tolbert, U. H. F.
Bunz, Org. Lett. 2008, 10, 2429–2432.
[4] a) E. L. Spitler, L. D. Shirtcliff, M. M. Haley, J. Org. Chem. 2007, 72,
86–96; b) J. A. Marsden, J. J. Miller, L. D. Shirtcliff, M. M. Haley, J.
Am. Chem. Soc. 2005, 127, 2464–2476.
[17] H. Gregorius, M. Baumgarten, R. Reuter, N. Tyutyulkov, K. Mꢂllen,
Angew. Chem. 1992, 104, 1621–1623; Angew. Chem. Int. Ed. Engl.
1992, 31, 1653–1655.
[5] M. Kivala, F. Diederich, Acc. Chem. Res. 2009, 42, 235–248.
[6] a) Z. I. Niazimbetova, H. Y. Christian, Y. J. Bhandari, F. L. Beyer,
M. E. Galvin, J. Phys. Chem. B 2004, 108, 8673–8681; b) H. Kang,
Received: January 20, 2013
Published online: May 13, 2013
8496
www.chemeurj.org
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 8490 – 8496