Hydroxy-cruciforms

Article abstract of DOI:10.1039/b702883k

The synthesis of hydroxy-cruciforms 7 and 8 and their dramatically varying photophysical properties upon exposure to amines are reported. The Royal Society of Chemistry.

Full text of DOI:10.1039/b702883k

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Hydroxy-cruciforms{
Psaras L. McGrier,a Kyril M. Solntsev,a Jan Scho¨nhaber,b Scott M. Brombosz,a Laren M. Tolberta and
Uwe H. F. Bunz*a
Received (in Berkeley, CA, USA) 26th February 2007, Accepted 16th April 2007
First published as an Advance Article on the web 1st May 2007
DOI: 10.1039/b702883k
The synthesis of hydroxy-cruciforms 7 and 8 and their
dramatically varying photophysical properties upon exposure
to amines are reported.
Cruciform fluorophores (tetra-1,2,4,5-vinyl- or -ethynyl-substituted
benzenes, XFs) are p-systems with unusual frontier molecular
orbitals (FMOs). If one of the axes is donor substituted and the
other axis is acceptor substituted, species with spatially separated
FMOs can result. The HOMO is localized on the donor part of the
molecule, while the LUMO is localized on the acceptor part of the
molecule. This FMO arrangement leads to a situation in which
electronic information can be addressed spatially and for which
HOMO and LUMO are manipulated by metal cations or by
protons. We1 and others2–5 have shown that dialkylaniline- and
pyridine-containing XFs display unusually large bathochromic or
hypsochromic shifts when exposed to zinc, magnesium, calcium
Scheme 1 Synthesis of hydroxy-XFs 7 and 8.
and manganese salts or protons.1,2 The reason for the large shifts
in absorption and emission is the independently addressable
Table 1 shows the pertinent photophysical data of 7, 8 and 10.
HOMO and LUMO, enforcing large changes in the HOMO–
The XFs 7 and 8 are similar to each other, as they show blue
LUMO gap.3 Up to now, HOMO–LUMO of XF-types have been
emission with robust quantum yields. Attachment of –CF3 groups
addressed by cationic species, binding to the free electron pairs of
on the aryleneethynylene axis in 10 decreases the band gap and
pyridines and dialkylanilines. In this contribution we demonstrate
leads to significantly red-shifted absorption and emission. The
that XFs carrying strategically placed phenol functionalities show
Stokes shifts in these XFs are similar and around 3000 cm21. The
unusual photophysical effects upon deprotonation and exposure to
vibronic progression of 8 is in the expected range, while that of 7 is
amines.
smaller. The fluorescence spectrum of 10 does not show any
The synthesis of hydroxy-XFs starts with the reaction of
vibronic bands, suggesting that its excited state is structurally
phosphonate 16 with either the protected aldehyde 2 or 3 to give
different from its ground state.8
the distyrylbenzene derivatives 4 and 5 in 77% and 68% yield,
We titrated (Figs. 1, 2) 7 and 8 in a methanol–water mixture
respectively, after chromatography and crystallization. Coupling of
with aqueous base (KOH). Fig. 1 shows absorption and emission
4 or 5 with 4-tert-butylphenylacetylene in the presence of CuI–
for 8. Upon addition of hydroxide there is no significant change in
(Ph3P)2PdCl2 under standard Heck–Cassar–Sonogashira–
the absorption spectrum of 8; the emission of the phenolate of 8 is
Hagihara conditions7 in piperidine furnished the target XFs 7
largely quenched. A very weak emission band for the deprotonated
and 8, after aqueous workup, chromatography and subsequent
form of 8 is observed at 515 nm. The invariance of the absorption
deprotection with trifluoroacetic acid at 278 uC in dichloro-
spectrum is surprising and persists upon addition of a large excess
methane, as yellow or yellowish–brown solids in 41% and 40%
of hydroxide. In the case of 7, upon deprotonation (Fig. 2), the
yield respectively (Scheme 1). The relatively low coupling yield (44
absorption spectrum shows an appreciable red shift, as would be
and 48% respectively) is due to losses during the chromatography
expected for a phenolate, with a prominent absorption appearing
of the intermediate. Nevertheless, the target XFs are easily
available on a 100–200 mg scale. If 4 is coupled to 9, we obtain
Table 1 Photophysical data of 7, 8 and 10 in dichloromethane
solution
an intermediate in 53% yield, which is deprotected in 85% yield to
give 10.
CH2Cl2
7
8
10
aSchool of Chemistry and Biochemistry, Georgia Institute of
Technology, Atlanta, GA 30332, USA.
Absorption (nm)
Emission (nm)
380
432
876
3167
0.41
1.42
376
423
1219
2955
404
456
none
2822
0.57
na
Vibronic progression (cm21
)
E-mail: uwe.bunz@chemistry.gatech.edu
bInstitut fu¨r Organische Chemie, Universita¨t Heidelberg, Im
Neuenheimer Feld 270, D-69120 Heidelberg, FRG
{ Electronic supplementary information (ESI) available: Experimental and
spectral data. See DOI: 10.1039/b702883k
Stokes shift (cm21
W
t (ns)
)
0.72
2.99
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Fig. 3 Photograph of the XFs 7, 8, and 10: 1) reference; exposure to 2)
pyrrole (–), 3) quinoline (4.90), 4) pyridine (5.25), 5) imidazole (6.96;
lmax.7 = 551 nm), 6) morpholine (8.33; 555 nm), 7) piperazine (9.83;
556 nm), 8) ethylene diamine (10.7; 579 nm), 9) piperidine (10.8; 564 nm),
10) triethylamine (10.8; 555 nm), 11) diethylamine (11.0; 562 nm), 12)
diisopropylamine (11.1; 558 nm), and 13) 1,8-diazabicyclo[5.4.0]undec-7-
ene (DBU, y12; 572 nm). The numbers in parentheses are the pKa values
of the corresponding ammonium ions. The photographs are taken in
15 mL vials under a hand held black light (lmax = 366 nm).
Fig. 1 Uv–Vis (top) and emission (bottom) spectra of XF 8 in a 2 : 1 vol.
methanol–water mixture at different pH-values.
at 416 nm. At the same time, the emission changes from 476 nm
(blue) to 580 nm (yellow). Addition of an excess of KOH solution
does not change the emission wavelength further. From the
titrations, the pKa values for 7 and 8 were determined to be 9.9 and
10.0.
Changes in spectroscopic properties are not only observed upon
deprotonation of the XFs 7 and 8 in methanol–water mixtures but
also when solutions of XFs in dichloromethane are exposed to
amines. Fig. 3 shows a photograph of the XFs 7, 8 and 10 exposed
to a panel of different amines, ordered by their increasing pKa
values, while Fig. 4 (bottom) shows the corresponding emission
spectra.
Fig. 4 Absorption spectra (top) of solutions of 7 in DCM upon addition
of amine (0.1 mL). Emission spectra (bottom) of solutions of 7 in
dichloromethane (DCM, 15 mL, vial) upon addition of amine (0.1 mL).
Interestingly, the magnitude in shift and the pKa-values of the
amines do not correlate particularly well, as ethylenediamine
(pKa = 10.7), i.e. not the most basic amine, displays the largest red
shift. In the case of the exposure of 7 to quinoline or to pyridine,
fluorescence is quenched, possibly due to a back electron transfer
following proton transfer to the basic nitrogen.9 If the amine under
consideration is not very basic, as pyrrole and imidazole, either
there is no change of the emission or a mixed color (imidazole) is
observed. Similar trends are observed for XF 10, even though the
emission intensities are much lower, as is expected from the energy
gap law.10
Exposure of the XF 8 to amines in dichloromethane results in
quenching (see SI for spectra) of the emission, similar to that
observed on exposure of 8 to KOH. The above observations
demonstrate that 7 and 8 are different from hydroxystilbenes 11
and 12. The phenolate of stilbene 11 is weakly fluorescent, while
that of the meta-compound 12 is quite fluorescent.11 In 11 and 12
Fig. 2 Uv–Vis (top, 417 nm lmax deprotonated form) and emission
(bottom 474 nm, 596 nm lmax) spectra of XF 7 in a 2 : 1 vol. methanol–
water mixture at different pH-values.
2128 | Chem. Commun., 2007, 2127–2129
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the excited state acidity of the phenolic function is significantly
enhanced, that of 12 more so than that of 11. Neither the XF 7 nor
8, on the other hand, shows dramatically enhanced photoacidity in
methanol–water mixtures with up to 50% vol. water,11 which
makes them comparable to the weak photoacid 2-naphthol with a
pKa* of 2.8.12
The absorption and emission spectra of 7 show a more complex
behavior in the presence of amines (Fig. 4). On the one hand,
except for 1,8-diaza[5.4.0]bicycloundecene (DBU, pKa y 12), the
absorption maxima show a shift of ca. 20 nm and are consistent
with a hydrogen-bonded complex.13 Upon addition of DBU a red-
shifted feature is observed, which we attribute to the fully bis-
deprotonated ground state species, as it is identical to that
observed in the KOH-promoted deprotonation of 7. On the other
hand, all of the amine complexes exhibit efficient emission from
the fully deprotonated (ion pair) state. From these observations we
conclude that in dichloromethane solutions the difference in pKa
(or DG of the proton transfer) between 7* and amines is sufficient
to produce solvent-separated ion pairs.14 In the ground state, the
observed DpKa results in the formation of hydrogen-bonded
complexes.
Fig. 5 Density of the frontier molecular orbitals (HOMO and LUMO)
of the bisphenolate anions of models of 7 and of 8 (tert-butyl groups are
omitted) as calculated by B3LYP-6-31G**//B3LYP-6-31G** using
SPARTAN.
The authors thank the National Science Foundation (grants
CHE-0548423 for UB and PM; CHE-0456892 for LMT and
KMS) for generous financial support. UB thanks Prof. Christoph
Fahrni for helpful discussions.
The observation of different, amine-dependent emission
characteristics is of potential importance since Lavigne et al.15
have shown that carboxylate-substituted polythiophenes can
discern biogenic amines when the absorption spectra of the
complexes are compared. Our approach is complementary as we
use more sensitive fluorescence spectroscopy.
Notes and references
1 J. N. Wilson and U. H. F. Bunz, J. Am. Chem. Soc., 2005, 127,
4124–4125; J. N. Wilson, M. Josowicz, Y. Q. Wang and U. H. F. Bunz,
Chem. Commun., 2003, 2962–2963; A. J. Zucchero, J. N. Wilson and
U. H. F. Bunz, J. Am. Chem. Soc., 2006, 128, 11872–11881;
W. W. Gerhardt, A. J. Zucchero, J. N. Wilson, C. R. South,
U. H. F. Bunz and M. Weck, Chem. Commun., 2006, 2141–2142.
2 E. L. Spiteler, L. D. Shirtlcliff and M. M. Haley, J. Org. Chem., 2007,
72, 86–96; J. A. Marsden, J. J. Miller, L. D. Shirtcliff and M. M. Haley,
J. Am. Chem. Soc., 2005, 127, 2464–2476.
3 A. Zen, A. Bilge, F. Galbrecht, R. Alle, K. Meerholz, J. Grenzer,
D. Neher, U. Scherf and T. Farrell, J. Am. Chem. Soc., 2006, 128,
3914–3915; A. L. Thompson, T. S. Ahn, K. R. J. Thomas,
S. Thayumanavan, T. J. Martinez and C. J. Bardeen, J. Am. Chem.
Soc., 2005, 125, 16348–16349; H. Meier, B. Mu¨hling and H. Kolshorn,
Eur. J. Org. Chem., 2004, 1033–1042.
4 H. Kang, G. Evmenenko, P. Dutta, K. Clays, K. Song and T. J. Marks,
J. Am. Chem. Soc., 2006, 128, 6194–6205; K. Hu, P. W. Zhu, Y. Yu,
A. Facchetti and T. J. Marks, J. Am. Chem. Soc., 2004, 126,
15974–15975.
5 J. K. Sorensen, M. Vestergaard, A. Kadziola, K. Kilsa and
M. B. Nielsen, Org. Lett., 2006, 8, 1173–1176.
6 J. N. Wilson, P. M. Windscheif, U. Evans, M. L. Myrick and
U. H. F. Bunz, Macromolecules, 2002, 35, 8681–8683.
7 U. H. F. Bunz, Chem. Rev., 2000, 100, 1605–1644; K. Sonogashira,
J. Organomet. Chem., 2002, 653, 46–49; E. Negishi and L. Anastasia,
Chem. Rev., 2003, 103, 1979–2017.
What is the reason for the dramatic differences in the optical
properties of 7 and 8 upon interaction with bases, i.e. proton-
dissociation induced red shift vs. quenching? A DFT calculation
(Fig. 5) of the FMO-distribution of 7 and 8 sheds light on this
issue. In the dianion of 7, HOMO and LUMO show spatial
overlap in the central ring and both absorption and emission are
Franck–Condon allowed. In the case of 8 the situation is different.
Due to the disjoint orbital structure in which the HOMO is
localized only on the two phenolate rings, while the LUMO is strictly
localized on the bisarylethynyl axis, there is a vanishingly small
spatial overlap between the two frontier molecular orbitals, resulting
in a Franck–Condon forbidden transition. Without the quantum
chemical calculations, the different spectroscopic properties of
722and 822 would be very hard to rationalize. Since the 340 nm
transition is both invariant with respect to deprotonation and
strongly allowed, we conclude that this is a HOMO–LUMO
transition in 8 but a HOMO–LUMO + n-transition in the
deprotonated form of 8, while the weaker HOMO–LUMO
transition of the dianion of 8 is hidden in the baseline.3 The
allowed transition in the dianion must have a similar gap to the
HOMO–LUMO-transition in the neutral compound, explaining
the lack of change in the absorption spectra.
8 J. S. Yang, S. Y. Chiou and K. L. Liau, J. Am. Chem. Soc., 2002, 124,
2518–2527.
9 Q. Zhou and T. M. Swager, J. Am. Chem. Soc., 1995, 117, 12593–12602;
J. G. Mu¨ller, E. Atas, C. Tan, K. S. Schanze and V. D. Kleiman, J. Am.
Chem. Soc., 2006, 128, 4007–4016.
In conclusion, we have prepared the two XFs 7 and 8 and
investigated their photophysical properties upon deprotonation or
exposure to amines. We see dramatic differences between the para
(7) and the meta (8) XF as a consequence of the different FMO
distribution. The dianion 8 suffers from a Franck–Condon
disallowed HOMO–LUMO-transition, which is responsible for
the observed fluorescence quenching. Potential applications of
such materials with separated FMOs include exciton collection and
splitting in photovoltaic devices and fluorescence sensors in cases
where the XFs are equipped with additional binding elements.
10 J. V. Caspar and T. J. Meyer, J. Phys. Chem., 1983, 87, 952–957;
L. M. Tolbert, S. M. Nesselroth, T. L. Netzel, N. Raya and
M. Stapleton, J. Phys. Chem., 1992, 96, 4492–4496.
11 F. D. Lewis and E. M. Crompton, J. Am. Chem. Soc., 2003, 125,
4044–4045; F. D. Lewis, L. E. Sinks, W. Weigel, M. C. Sajimon and
E. M. Crompton, J. Phys. Chem. A, 2005, 109, 2443–2451.
12 L. M. Tolbert and K. M. Solntsev, Acc. Chem. Res., 2002, 35, 19–27.
13 H. Baba and S. J. Suzuki, Chem. Phys., 1961, 35, 1118–1127.
14 N. K. Zaitsev, A. B. Demyashkevich and M. G. Kuzmin, High Energy
Chem., 1980, 14, 116–120.
15 T. L. Nelson, C. O’Sullivan, N. T. Greene, M. S. Maynor and
J. J. Lavigne, J. Am. Chem. Soc., 2006, 128, 5640–5641.
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