Dynamic proton-induced emission switching in donor-functionalized dehydrobenzopyrid[15]annulenes

Article abstract of DOI:10.1021/jo070827c

(Chemical Equation Presented) An isomeric pair of 15-membered dehydrobenzopyridannulenes functionalized with -NBu₂ groups as π-electron donors was prepared and their steady-state spectroscopic parameters investigated. The property differences a

Full text of DOI:10.1021/jo070827c

Dynamic Proton-Induced Emission Switching in
Donor-Functionalized Dehydrobenzopyrid[15]annulenes
Eric L. Spitler, Sean P. McClintock, and Michael M. Haley*
Department of Chemistry and Materials Science Institute, UniVersity of Oregon,
Eugene, Oregon 97403-1253
haley@uoregon.edu
ReceiVed April 19, 2007
An isomeric pair of 15-membered dehydrobenzopyridannulenes functionalized with -NBu2 groups as
π-electron donors was prepared and their steady-state spectroscopic parameters investigated. The property
differences arising from placement of the pyridine nitrogen relative to the macrocycle, as well as the
differential effects of stepwise protonation of the acceptor and donor nitrogens, were examined. The
macrocycles exhibited dynamic shifting in the emission spectra, which is believed to correlate to induced
changes in the frontier molecular orbitals of the molecules.
Introduction
mophoric or fluorophoric systems possessing electronic and/or
photonic transfer pathways are recognized as ideal candidates
for components of analytical reagents and advanced optical
devices.4-8 Their propensity to act as bulk materials or organic
liquid crystal arrays also implies applications in OLED, organic
semiconductor, and photoelectric cell design.
We have been investigating the optoelectronic properties of
a special class of carbon-rich compounds, dehydrobenzoannu-
lenes (DBAs, Figure 1a).4a-c These fully conjugated macrocycles
composed of acetylene-linked arenes represent attractive targets
Highly conjugated, carbon-rich organic molecules have been
the focus of extensive interest in recent years because of their
unique optical and electronic properties.1-3 In particular, chro-
* Address correspondence to this author. Fax: 541-346-0487. Phone: 541-
346-0456.
(1) (a) de Meijere, A., Ed. Topics in Current Chemistry (Carbon Rich
Compounds I); Springer: Berlin, Germany, 1998; Vol. 196. (b) de Meijere,
A., Ed. Topics in Current Chemistry (Carbon Rich Compounds II);
Springer: Berlin, Germany, 1999; Vol. 201. (c) Diederich, F.; Stang, P. J.;
Tykwinski, R. R., Eds. Acetylene Chemistry: Chemistry, Biology, and
Material Science; Wiley-VCH: Weinheim, Germany, 2005. (d) Haley, M.
M.; Tykwinski, R. R., Eds. Carbon-Rich Compounds: From Molecules to
Materials; Wiley-VCH: Weinheim, Germany, 2006.
(2) Reviews, inter alia: (a) Organic Light Emitting DeVices: Synthesis,
Properties and Applications; Mu¨llen, K., Scherf, U., Eds.; Wiley-VCH:
Weinheim, Germany, 2006. (b) Chen, J.; Reed, M. A.; Dirk, S. M.; Price,
D. W.; Rawlett, A. M.; Tour, J. M.; Grubisha, D. S.; Bennett, D. W. In
NATO Science Series II: Mathematics, Physics, Chemistry (Molecular
Electronics: Bio-Sensors and Bio-Computers); Plenum: New York, 2003;
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A.; Barlow, S.; Marder, S. R.; Kippelen, B. J. Polym. Sci. Part B: Polym.
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1-25. (e) Schwab, P. F. H.; Levin, M. D.; Michl, J. Chem. ReV. 1999, 99,
1863-1933. (f) Electronic Materials: The Oligomer Approach; Mu¨llen, K.,
Wegner, G., Eds.; Wiley-VCH: Weinheim, Germany, 1998. (g) Nonlinear
Optics of Organic Molecules and Polymers; Nalwa, H. S., Miyata, S., Eds.;
CRC Press: Boca Raton, FL, 1997.
(3) Recent examples, inter alia: (a) Kang, H.; Evrenenko, G.; Dutta, P.;
Clays, K.; Song, K.; Marks, T. J. J. Am. Chem. Soc. 2006, 128, 6194-
6205. (b) Knox, J. E.; Halls, M. D.; Hratchian, H. P.; Schlegel, H. B. Phys.
Chem. Chem. Phys. 2006, 8, 1371-1377. (c) Shukla, V. K.; Kumar, S.;
Deva, D. Synth. Met. 2006, 156, 387-391. (d) Seminario, J. M. Nature
Mater. 2005, 4, 111-113. (e) Hughes, G.; Bryce, M. R. J. Mater. Chem.
2005, 15, 94-107. (f) Van der Auweraer, M.; De Schryver, F. C. Nature
Mater. 2004, 3, 507-508. (g) Special Issue on Organic Electronics. Chem.
Mater. 2004, 16, 4381-4846. (h) Simpson, C. D.; Wu, J.; Watson, M. D.;
Mu¨llen, K. J. Mater. Chem. 2004, 14, 494-504.
(4) (a) Marsden, J. A.; Miller, J. J.; Shirtcliff, L. D.; Haley, M. M. J.
Am. Chem. Soc. 2005, 127, 2464-2476. See also: (b) Marsden, J. A.; Haley,
M. M. Angew. Chem., Int. Ed. 2004, 43, 1694-1697. (c) Miller, J. J.;
Marsden, J. A.; Haley, M. M. Synlett 2004, 165-168. (d) Spitler, E. L.;
Shirtcliff, L. D.; Haley, M. M. J. Org. Chem. 2007, 72, 86-96. (e) Samori,
S.; Tojo, S.; Fujitsuka, M.; Spitler, E. L.; Haley, M. M.; Majima, T. J.
Org. Chem. 2007, 72, 2785-2793.
10.1021/jo070827c CCC: $37.00 © 2007 American Chemical Society
Published on Web 08/09/2007
6692
J. Org. Chem. 2007, 72, 6692-6699

Dehydrobenzopyrid[15]annulenes
FIGURE 1. (a) Conjugated pathways present in structurally related
tetrakis(phenylethynyl)benzenes and bis(dehydrobenzoannuleno)ben-
zenes and (b) donor/acceptor tetrakis(arylethynyl)benzenes with dibu-
tylaniline donors and pyridine acceptors.
of fundamental study into aromaticity and enforced planariza-
tion. We have also performed studies on donor/acceptor-
functionalized DBAs that undergo intramolecular charge transfer
upon optical excitation, as well as studies of their acyclic
tetrakis(arylethynyl)benzene analogues (TAEBs) with variation
of the acceptor group. Most recently, we reported a systematic
study of isomeric pyridine-derivatized bisdonor/bisacceptor
TAEBs (Figure 1b).4d We demonstrated that these systems
underwent two-stage fluorescence emission shifting (to varying
extents corresponding to charge-transfer pathway efficiency)
upon titration with acid or addition of assorted metal ions, and
FIGURE 2. Fifteen-membered carbon macrocycle DBAs (1), tert-
butyl functionalized DBPAs (2), and target donor-functionalized DBPAs
(3).
that these shifts correlated to the relative energies of the spatially
separate frontier molecular orbitals (FMOs). Thus, the segment-
localized FMOs in conjugated donor/acceptor fluorophores could
be manipulated independently.9
We have previously examined DBAs of the type 1, incor-
porating solublizing t-Bu groups or electron-donating NBu2
groups, as well as isomers of t-Bu-functionalized dehydroben-
zopyridannulenes (DBPAs) 2, fused at either the 2,6- or 3,5-
positions (Figure 2).10a-c Tobe et al. have also prepared
[15]DBAs in the course of examining more complex cyclophane
architectures,10d and Baxter et al. have synthesized several
related pyridannulene structures.5e We now present donor-
functionalized DBPAs of the type 3, which incorporate the
aforementioned switching behavior into 15-membered acetylenic
macrocycles. As moderate strength acceptors, pyridines can
participate in various degrees of intramolecular charge transfer,
depending on the efficiency of the conjugated pathway from
the donor to the acceptor nitrogen.4d Since both donor and
acceptor group(s) can be protonated, the ability to probe
independent manipulation of the FMOs using shifts in the
emission spectra can also be demonstrated.
(5) (a) Bunz, U. H. F.; Rubin, Y.; Tobe, Y. Chem. Soc. ReV. 1999, 107-
119. (b) Haley, M. M.; Wan, W. B. In AdVances in Strained and Interesting
Organic Molecules; Halton, B., Ed.; JAI Press: Greenwich, CT, 2000; Vol.
8, pp 1-41. (c) Watson, M. D.; Fechtenko¨tter, A.; Mu¨llen, K. Chem. ReV.
2001, 101, 1267-1300. (d) Nielsen, M. B.; Diederich, F. In Modern Arene
Chemistry; Astruc, D., Ed.; Wiley-VCH: Weinheim, Germany, 2002; pp
196-216. (e) Baxter, P. N. W.; Dali-Youcef, R. J. Org. Chem. 2005, 70,
4935-4953.
(6) Inter alia: (a) Zhao, Y.; Slepkov, A. D.; Akoto, C. O.; McDonald,
R.; Hegmann, F. A.; Tykwinski, R. R. Chem. Eur. J. 2005, 11, 321-329.
(b) Bunz, U. H. F. AdV. Polym. Sci. 2005, 177, 1-52. (c) Fasina, T. M.;
Collings, J. C.; Lydon, D. P.; Albesa-Jove, D.; Batsanov, A. S.; Howard, J.
A. K.; Nguyen, P.; Bruce, M.; Scott, A. J.; Clegg, W.; Watt, S. W.; Viney,
C.; Marder, T. B. J. Mater. Chem. 2004, 14, 2395-2404. (d) Boydston, A.
J.; Yin, Y.; Pagenkopf, B. L. J. Am. Chem. Soc. 2004, 126, 3724-3725.
(e) Gonzalo-Rodriguez, J.; Esquivias, J.; Lafuente, A.; Diaz, C. J. Org.
Chem. 2003, 68, 8120-8128. (f) Bunz, U. H. F. Chem. ReV. 2000, 100,
1605-1644. (g) Tykwinski, R. R.; Gubler, U.; Martin, R. E.; Diederich,
F.; Bosshard, C.; Gu¨nter, P. J. Phys. Chem. B 1998, 102, 4451-4465. (h)
Tykwinski, R. R.; Schreiber, M.; Carlo´n, R. P.; Diederich, F.; Gramlich,
V. HelV. Chim. Acta 1996, 79, 2249-2280. (i) Tahara, K.; Furukawa, S.;
Uji-i, H.; Uchino, T.; Ichikawa, T.; Zhang, J.; Mamdouh, W.; Sonoda, M.;
De Schryver, F. C.; De Feyter, S.; Tobe, Y. J. Am. Chem. Soc. 2006, 128,
16613-16625.
Results and Discussion
Synthesis. Assembly of 3a and 3b is readily accomplished
as shown in Scheme 1. Donor-functionalized alkyne segment
44a is appended to either 2,6- or 3,5-dibromopyridine 5 by using
methanolic KOH to remove the more labile trimethylsilyl (TMS)
(7) (a) Slepkov, A.; Marsden, J. A.; Miller, J. J.; Shirtcliff, L. D.; Haley,
M. M.; Kamada, K.; Tykwinski, R. R.; Hegmann, F. A. Nonlinear Optical
Transmission and Multiphoton Processes in Organics III. Proc. SPIE 2005,
5934, 29-34. (b) Zhang, X.-B.; Feng, J.-K.; Ren, A.-M.; Sun, C.-C. Opt.
Mater. 2007, 29, 955-962. (c) Slepkov, A.; Hegmann, F. A.; Tykwinski,
R. R.; Kamada, K.; Ohta, K.; Marsden, J. A.; Spitler, E. L.; Miller, J. J.;
Haley, M. M. Opt. Lett. 2006, 31, 3315-3317.
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4125. (b) Zucchero, A. J.; Wilson, J. N.; Bunz, U. H. F. J. Am. Chem. Soc.
2006, 128, 11872-11881. (c) Wilson, J. N.; Josowicz, M.; Wang, Y. Q.;
Bunz, U. H. F. Chem. Commun. 2003, 2962-2963.
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5183-5196. (b) Bazan, G. C.; Bartholomew, G. P. Synthesis 2002, 1245-
1255. (c) Nguyen, P.; Lesley, G.; Marder, T. B. Chem. Mater. 1997, 9,
406-408. (d) Meier, H.; Mu¨hling, B.; Kolshorn, H. Eur. J. Org. Chem.
2004, 1033-1042. (e) Meier, H.; Gerold, J.; Kolshorn, H.; Mu¨hling, B.
Chem. Eur. J. 2004, 10, 360-370. (f) Zimmerman, B.; Baranovic, G.;
Stefanic, Z.; Rozman, M. J. Mol. Struct. 2006, 794, 115-124. (g)
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Sugihara, Y.; Shimizu, M. J. Org. Chem. 2007, 72, 744-749.
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J. R. Organometallics 2005, 24, 1161-1172. (b) Johnson, C. A.; Baker, B.
A.; Berryman, O. B.; Zakharov, L. N.; O’Connor, M. J.; Haley, M. M. J.
Organomet. Chem. 2006, 691, 413-421. (c) Marsden, J. A.; Miller, J. J.;
Haley, M. M. Angew. Chem., Int. Ed. 2004, 43, 1694-1697. (d) Tobe, Y.;
Kishi, J.; Ohki, I.; Sonoda, M. J. Org. Chem. 2003, 68, 3330-3332.
(11) Calculated from the steady-state spectra with the techniques
described in: Drushel, H. V.; Sommers, A. L.; Cox, R. C. Anal. Chem.
1963, 35, 2166-2172.
J. Org. Chem, Vol. 72, No. 18, 2007 6693

Spitler et al.
SCHEME 1. Preparation of DBPAS 3a and 3b
bathochromic shift in the absorption spectra of 3a,b versus 6a,b,
accompanied by a significant loss of intensity of the lowest
energy band, possibly indicating weak intramolecular charge
transfer. This is in good agreement with our previously described
donor/acceptor bisannulene series, as well as recent examples
of weak donor/weak acceptor-functionalized DBAs.8f The
electron density from the donating groups results in a λmax of
the lowest energy bands in 3a and 3b that are 7 and 19 nm
greater than those for 2a and 2b, respectively. It is somewhat
surprising that the charge-transfer bands for 3a and 3b are so
similar, since 3b does not contain a direct resonance pathway
for full conjugation, implying good inductive withdrawal by
the pyridine ring as a unit.
Normalized emission spectra are shown in Figure 3b and
Table 1. There is a dramatic red shift in both systems upon
planarization. FMO plots (see next section) show significant
LUMO density localized on the diacetylene bridges, so it is
possible that cyclization has a more pronounced effect on the
excited state energies, thus affecting the emission spectra more
than the absorption. In the precursors, the more conjugated 2,6-
fused system 6a is more red-shifted, but in the annulenes the
λmax values are very similar. This may be due to the larger
predicted net dipole in 3b (8.46 D vs 4.25 D for 3a, see Table
S1 in the Supporting Information). The large increase in Stokes
shifts upon ring closure also indicates an increase in net dipole
and the efficiency of the conjugated π-system.12 Our previous
DBA studies generally show a decrease in Stokes shift upon
planarization, but in some π-electron-rich species an increase
is seen when there is significant calculated LUMO density on
the bridging pathway.4a This also seems to be the case for the
weak donor/acceptor systems here, and is likely due to the less
effective cross-conjugated pathways present in the 15-membered
macrocycles. Donor functionalization also results in maxima
significantly red-shifted relative to 2a,b,10b in agreement with
previous DBA behavior.4a
There is little difference in Φf between 3a and 3b, likely
reflecting the weak intramolecular charge-transfer transitions
dominating in both. This agrees with previous results wherein
the all-donor analogues of the systems in Figure 1 show higher
quantum yields than the donor/acceptor systems.4,13 Interestingly,
the values for 3a and 3b are comparable to those for 2a and
2b. One would expect functionaliztion with donor π-electrons
to increase Φf. The emission spectra of 2a and 2b contain
significant fine structure, however, perhaps implying more
complex vibronic interactions than are observed here. Cycles
2a and 2b also possess exceptionally long absorption cutoffs.10b
The remarkably high quantum yields of precursors 6a and 6b
help illustrate in part the effect of enforced planarization on
the conjugation, but it is likely that the increased fluorescence
is also due in part to donor electron contribution from the TIPS
groups. Examination of desilylated 6a,b, however, showed no
significant effect on either absorption or emission wavelengths
(see Figure S1 in the Supporting Information).
group in situ to afford the bisdonor precursors 6a and 6b.
Removal of the triisopropylsilyl (TIPS) groups with tetrabutyl-
ammonium fluoride (TBAF) followed by oxidative intramo-
lecular homocoupling under Cu-mediated conditions furnished
macrocycles 3a and 3b in 71% and 42% overall yield,
respectively. It is notable that in both coupling steps, the more
conjugated systems a proceed in somewhat higher yields, a trend
also observed in preparation of 2a and 2b.10b
In the strong donor/strong acceptor bisannulenes,4a significant
self-associative behavior was observed in their 1H NMR spectra
due to intermolecular π-stacking and dipole attraction between
respective donor and acceptor groups. In 3a and 3b, no such
behavior was observed: NMR signals remained constant over
a several-fold concentration gradient. There were, however,
noticeable upfield shifts of the signals from protons ortho to
the monoacetylene linkages (∆δ ) 0.1-0.3 ppm) upon ring
closure of 6a,b to 3a,b, with smaller upfield shifts in other
signals (see the Experimental Section and the Supporting
Information). This effect is indicative of increased electron
delocalization along the π-conjugated pathway. Upon cyclization
to 3b, there was a dramatic (0.91 ppm) downfield shift of the
proton internal to the macrocycle, which was also observed in
2b,10b and is attributed to the anisotropy of the nearby triple
bonds.
Crystals of 3a and 3b suitable for X-ray diffraction were
obtained by evaporation from 5:1:1 and 5:3:1 solutions of
hexanes:CH2Cl2:EtOAc, respectively. Both DBPAs are planar
structures with remarkably linear diyne bridges (Csp-Csp bond
angles for 3a 177-179°and for 3b 177-178°; Figures S4a and
S5a in the Supporting Information). Most of the strain in the
molecules is contained in the monoacetylene units, which are
moderately distorted (Csp-Csp bond angles for 3a 167-174°
and for 3b 163-171°). Overall, the structures closely resemble
the purely hydrocarbon [15]DBA reported by Tobe et al.10d In
the solid state, 3b packs in slipped π-stacking arrangement,
similar to Tobe’s annulene, whereas 3a contains two sym-
metrically independent molecules arranged in zigzagging dimer-
ic pairs. The packing differences are likely due to weak
hydrogen bonding in 3b between the pyridine nitrogen atom
and an ortho pyridine proton on a neighboring molecule (N‚‚
‚H-C distance 2.62 Å, Figure S5b, Supporting Information).
Absorption and Emission Data. Electronic absorption and
emission spectra of 3a,b are shown in Figure 3a and summarized
in Table 1, along with their respective precursors 6a,b and tert-
butyl-functionalized analogues 2a,b10b for comparison. Cycliza-
tion/planarization to the macrocycle enhances π-orbital overlap,
increasing the conjugation. This results in a 17-25 nm
Molecular Orbital Plots. In our previously described ethy-
nylpyridine systems,4d the ability to protonate the donor and
acceptor segments independently resulted in a two-stage emis-
(12) (a) Anstead, G. M.; Katzenellenbogen, J. A. J. Phys. Chem. 1988,
92, 6249-6258. (b) Lackowicz, J. R. Principles of Fluorescence Spectros-
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(13) (a) Tolbert, L. M.; Nesselroth, S. M.; Netzel, T. L.; Raya, N.;
Stapleton, M. J. Phys. Chem. 1992, 96, 4492-4496. (b) Englman, R.;
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J. Phys. Chem. 1983, 87, 952-957.
6694 J. Org. Chem., Vol. 72, No. 18, 2007

Dehydrobenzopyrid[15]annulenes
FIGURE 3. (a) Absorption and (b) emission spectra of DBPAs 3a,b and precursors 6a,b in CH2Cl2 at 20-30 µM concentration. Excitation at
365 nm.
TABLE 1. Electronic Absorption and Emission Data for 2a,b,
3a,b, and 6a,b
increasing spatial separation of the orbitals. Protonation of 3a′
to 3a′+ leaves the HOMO relatively unchanged and results in
weak localization in the LUMO at the pyridine ring, but overall
has little affect. Further protonation at one of the donor sites
reveals a more separate orbital diagram in which the HOMO is
localized largely on the neutral donor while the LUMO is spread
over the protonated regions of the macrocycle, creating a more
charge transfer-like system (although the FMOs are more
separated in these cases, it is possible that both donor segments
are in fact protonated in a dynamic equilibrium that does not
lead to the orbital diagrams shown). Charge transfer is quenched
by protonation of the final donor segment, causing overlapping
molecular orbitals. Examination of 3b′ indicates that upon
protonation the system exhibits more orbital localization than
3a′, presumably due to the lack of a conjugated pathway.
Protonation of the acceptor segment localizes the HOMO onto
the two donor segments and the LUMO on the acceptor.
Subsequent protonation of a donor segment results in localiza-
tion of the HOMO around the neutral donor arene, while the
LUMO is spread out between the two protonated segments. Both
systems indicate that while the molecular orbitals are not fully
separate, stepwise protonation of the systems may allow for
manipulation of the orbitals and potentially their emission
properties. The band gap energies for 3a′ and 3b′ were also
calculated and found to be within 25 nm of the experimental
band gaps for 3a and 3b, which is closer than calculated for
previously examined DBAs (see Table S1 in the Supporting
Information).4a Thus, stepwise protonation in 3a and/or 3b
reflects an induced strong donor/acceptor species, implying
enhanced charge-transfer character more akin to that predicted
and observed in the ethynylpyridine systems, and therefore
similar emission switching behavior for 3a,b.
lowest energy abs λmax
[nm] (ꢀ [M-1 cm-1])
em λmax
[nm]
Stokes shift
[cm-1 (nm)]
c
compd
Φf
2a
2b
6a
6b
3a
3b
400 (sh) (1750)a
454b
436b
427
421
490
494
2974 (54)
2771 (47)
2222 (37)
2357 (38)
4162 (83)
∼4267 (86)
0.16
0.20
0.81
0.85
0.19
0.17
389 (3000)a
390 (49760)
383 (52910)
407 (17290)
∼408 (sh) (12410)
a Reference 10b. b Maxima corrected from ref 10b due to instrument
error. c Calculated relative to fluorescein at pH 8 (ref 11) with excitation at
lowest energy abs λmax
.
sion switching behavior, analogous to that first observed by
Bunz and co-workers in related donor/acceptor cruciforms.9a,b
Initial protonation with trifluoroacetic acid (TFA) caused a
dramatic blue shift in the spectra, followed by a more modest
red shift with subsequent protonation. Since the HOMO and
LUMO are calculated to be localized primarily on the donor
and acceptor segments, respectively, protonation of the donor
or acceptor nitrogens causes the shifts by affecting the relative
energies of the orbitals. Molecules 3a and 3b are similar
systems, but with enforced planarization and with only one
acceptor unit for two donor units.
FMO plots of 3a′ and 3b′ were calculated by using Gauss-
ian0314 (B3LYP15 level of theory, 6-31G* basis set) with
simplified structures that replaced the NBu2 donor with NMe2
(Figure 4). They predict largely overlapping HOMO and LUMO
localizations in the neutral state, but protonation of the pyridine
nitrogen and subsequent protonation of a donor nitrogen cause
(14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
Revision B.04; Gaussian, Inc.: Pittsburgh PA, 2003.
TFA Titrations. Trifluoroacetic acid (TFA) titration of 3a
in MeOH between [TFA] ) 10-5 and 10-0.3 M showed no such
switching behavior, and resulted only in qualitative quenching
of fluorescent intensity with no change in wavelength (see Figure
S2 in the Supporting Information). Repeating the titration of
3a with TFA in CH2Cl2, on the other hand, yielded far different
results (Figure 5). At [TFA] ) 10-3 M, the original charge-
transfer band at 407 nm in the absorption spectrum is greatly
enhanced, and two new bands at 494 and 619 nm appear. These
bands reach maximum intensity at [TFA] ) 10-2 M, at which
the solution attains a pale green color in ambient light. In the
emission spectra, a rapid drop-off of fluorescent intensity is
accompanied by the simultaneous appearance of a low-intensity
peak at ca. 670 nm, shifting from bright blue-green fluorescence
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J. Org. Chem, Vol. 72, No. 18, 2007 6695

Spitler et al.
FIGURE 4. Molecular orbital plots (B3LYP/6-31G*) of simplified structures 3a′ (a) and 3b′ (b) of 3a and 3b, respectively, in the neutral,
monoprotonated, diprotonated, and triprotonated states. The lower plots represent the HOMOs, and the upper plots represent the LUMOs.
to a dark, deep magenta at [TFA] ) 10-2 M. Further addition
of TFA causes a gradual loss of the new bands in the absorption,
until at [TFA] ) 10-0.3 M there are no bands past 400 nm. The
solution fades from green to very light pink and finally a pale
yellow. In the emission spectra, the red band is quenched along
with the gradual appearance of a third peak around 540 nm,
achieving a pale yellow-green fluorescence.
Titration of 3b (Figure 6) gave somewhat different results.
In the initial stage, the lowest energy band in the absorption
spectrum was suppressed and slightly red-shifted to ca. 415 nm
at 10-2 M TFA, while the fluorescence was significantly
quenched at higher than 10-4 M, with a red shift of the emission
maxima to about 560 nm. The solutions change from pale
yellow-green to darker yellow in ambient light, and under UV
illumination fade from fluorescent blue-green to a pale yellow-
orange at 10-2.7 M TFA. Additional protonation eliminates any
significant absorption past 400 nm, and at 10-2 M TFA the
emission is suddenly blue-shifted past the neutral compound to
ca. 445 nm. The solutions fade to colorless, with the generation
of a bright blue fluorescence. The behavior of both compounds
is summarized in Table 2. In contrast to our DBPAs, Baxter’s
pyridannulene systems, which lack donor π-electrons, exhibited
only red shifting and quenching with acidification.5e Titration
of acyclic precursors 6a and 6b resulted only in fluorescence
quenching with no significant change in λmax (see Figure S3 in
the Supporting Information).
We believe the behavior exhibited in 3a and 3b arises from
predominantly stepwise protonation of the molecules. Because
of the π-electron donation from the two NBu2 groups, it is
possible that the pyridine nitrogen is in general protonated first,
causing the red shifts in the absorption and emission. Ionization
of the acceptor should greatly enhance its electron withdrawal
ability, potentially forming a strong charge-transfer system with
a low optical band gap. Subsequent protonation of the donor
nitrogens then suppresses all charge transfer, leading to a system
with only π-π* transitions. The donor electrons also allow
quick but qualitative evaluation of the magnitude of the effect
with use of fluorescence spectroscopy. This easily accessible
protonated state could potentially be exploited for advanced
nonlinear optical applications. Several of our previously studied
6696 J. Org. Chem., Vol. 72, No. 18, 2007

Dehydrobenzopyrid[15]annulenes
FIGURE 5. (a) Absorption spectra of TFA titration of 3a in CH2Cl2 at ca. 60 µM concentration. (b) Emission spectra of TFA titration of 3a in
CH2Cl2. Excitation at 365-385 nm. (c) Solutions of 3a in CH2Cl2 in ambient light against a white background (top) and under illumination by
high-intensity 365 nm lamp in the dark (bottom) at indicated [TFA] (M).
FIGURE 6. (a) Absorption spectra of TFA titration of 3b in CH2Cl2 at ca. 30 µM concentration. (b) Emission spectra of TFA titration of 3b in
CH2Cl2. Excitation at 365-400 nm. Interference peaks from excitation wavelength removed for clarity. (c) Solutions of 3b in CH2Cl2 in ambient
light against a white background (top) and under illumination by high-intensity 365 nm lamp in the dark (bottom) at indicated [TFA] (M).
TABLE 2. Summary of TFA-Induced Emission Shifting in 3a and
3b in CH2Cl2
not observed to any significant extent in 3b, likely due to the
lack of fully conjugated pathways between the donors and the
acceptor. Only a moderate red shift, followed by a dramatic
blue shift in the emission is observed. These dynamic emission
shifts thus can also serve as a qualitative probe of conjugation
efficiency. Although it is not certain whether the red-shifted
emission peaks correspond to mono- or diprotonated species
(or an equilibrium mixture of both), in both sets of emission
spectra the most acidic solutions’ emission bands display a weak
shoulder somewhat blue-shifted from the maxima. The two
emissions may arise from states with either one or both donor
nitrogens protonated, but the lack of clearly deconvoluted bands
with distinct maxima implies that both are protonated nearly
independently or in equilibrium (rather than discretely as in the
above FMO calculations). This parallels both our previous work
initial em red-shifted em
∆λ1
(nm)
blue-shifted em
∆λ2
(nm)
compd
λ
max (nm)
λ
max (nm)
λ
max (nm)
3a
3b
490
494
670a
560b
180
66
541c
445c
129
115
a [TFA] ) 10-2 M. b [TFA] ) 10-2.7 M. c [TFA] ) 10-0.3 M.
systems have exhibited significant nonlinear optical susceptibil-
ity, with two-photon absorption cross sections that rival literature
standards.7 The greatly narrowed band gap in 3a at 10-2
M
TFA may display similar or even better results, particularly since
the donor/strong acceptor DBAs4a do not display band gaps as
small, and acidification of solutions of them causes only blue
shifts in both absorption and emission spectra. This effect is
J. Org. Chem, Vol. 72, No. 18, 2007 6697

Spitler et al.
placement in the pyridine acceptor on the degree of conjugation.
The unusually large increases in Stokes shifts upon cyclization
are believed to result from the combination of donor π-electrons,
LUMO localization on the diacetylene bridge formed in the last
synthetic step, and the cross-conjugated “kink” in the 15-
membered macocycles. It has been demonstrated that fluores-
cence spectroscopy can be used to probe the behavior of a donor/
acceptor fluorophore that can be protonated at both donor and
acceptor sites. In particular, the absorption and emission spectra
for 3a at moderate (10-2 M) acid concentration suggest a
monoprotonated state with a particularly narrow optical band
gap, prompting the question of its candidacy as a component
in nonlinear optical devices. The dynamic emission switching
bears striking resemblance to previous studies by us and others
involving independent manipulation of frontier molecular orbit-
als, leading to potential applications as fluorescent ion
sensors.9a,b,16 To our knowledge, this is the first reported instance
of such behavior in conjugated acetylenic macrocycles. The
calculated overlapping FMOs in neutral 3a and 3b further imply
that independent manipulation of HOMOs and LUMOs may
not require as dramatic of a spatial separation as previously
assumed.
FIGURE 7. 1H NMR spectra in CD2Cl2 expanded in the 6-9 ppm
region of (a) 3a, (b) 3a + 10-2 M TFA, and (c) 3a + 10-0.3 M TFA
(exact Hb/Hc shifts indeterminate due to overlap).
on the acyclic ethynylpyridines as well as Bunz’ systems, where
only one band for each protonation state was observed.9a,b In
those cases, the donors are believed to be protonated first,
followed by the acceptors. Here, the cumulative donation by
two NBu2 groups to only one pyridine likely accounts for the
opposite behavior. The interesting lack of clearly discernible
isosbestic points may imply dynamic equilibria during titration
rather than distinct species, but the emission wavelengths clearly
manifest switching behavior that takes place in predominantly
two stages within similar [TFA] ranges for both 3a and 3b.
Only in the emission spectra of 3a is there seen a very weak
iso-emissive region centered about 630 nm.
Experimental Section
General Methods. These have been described previously.4a
Precursor 6a. A mixture of 2,6-dibromopyridine (5a, 52 mg,
0.22 mmol), donor diyne 44a (317 mg, 0.66 mmol), KOH (123 mg,
2.20 mmol), THF (20 mL), i-Pr2NH (20 mL), and MeOH (1 mL)
was bubbled with Ar with stirring for 30 min. Pd(PPh3)4 (15 mg,
0.013 mmol) and CuI (5 mg, 0.026 mmol) were quickly added and
the mixture was bubbled for an additional 20 min. The temperature
was raised to 55 °C and the reaction stirred until complete by TLC
(ca. 18 h). The mixture was evaporated and the residue chromato-
graphed on silica gel (2:1 hexanes:CH2Cl2) to yield 6a (164 mg,
1
84%) as a yellow oil. H NMR (300 MHz, CDCl3) δ 7.53 (t, J )
To support the hypothesis that the pyridine nitrogen is
predominantly protonated first, we prepared samples of 3a in
CD2Cl2 and observed the TFA-induced change in proton shifts
of the 1H NMR spectra. Figure 7 shows the 6-9 ppm region of
solutions of 3a alone and at 10-2 and 10-0.3 M TFA. Assignment
of the protons is based on coupling constants and previous data
for 4,4a and is verified by correlation spectroscopy (see the
Supporting Information). Comparing the first two spectra clearly
reveals significant downfield shifts of the pyridyl protons Ha
(∆δ ) 0.49) and Hc (∆δ ) 0.18) and only slight shifts of the
donor segment protons (∆δ ) 0.01-0.07), indicative of
protonation of the pyridine nitrogen. At 10-0.3 M TFA, all
protons are shifted far downfield. The greater degree of shifting
in the donor segment protons (∆δ ) 1.00 for both Hd and He)
in this case indicates protonation at one or both of the
dibutylamino nitrogens and generation of an electron-deficient
conjugated π-system. A similar but less dramatic stepwise
shifting is observed for 3b (see the Supporting Information).
The order of protonation is supported by computations, which
indicate that the pyridine-protonated species are 19-31 kcal
mol-1 lower in energy than the donor-protonated species
(Supporting Information, Figures S6 and S7).
7.2 Hz, 1H), 7.42 (d, J ) 9.0 Hz, 2H), 7.34 (d, J ) 7.8 Hz, 2H),
6.72 (d, J ) 2.4 Hz, 2H), 6.54 (dd, J ) 7.8, 2.4 Hz, 2H), 3.27 (t,
J ) 7.5 Hz, 8H), 1.56 (quin, J ) 7.2 Hz, 8H), 1.35 (sext, J ) 7.2
Hz, 8H), 1.15 (s, 42H), 0.96 (t, J ) 7.2 Hz, 12H). 13C NMR (75
MHz, CDCl3) δ 148.1, 144.7, 135.7, 134.2, 127.4, 125.5, 115.4,
111.9, 110.8, 106.7, 93.5, 90.2, 90.2, 50.8, 29.5, 20.5, 19.0, 14.2,
11.7. IR (NaCl) ν 2955, 2937, 2858, 2202, 2147, 1655, 1639, 1594,
1572, 1552, 1534, 1509, 1465, 1438, 1397, 1367 cm-1. MS (APCI)
m/z ([isotope], %) 894.6 ([M+], 100), 895.6 ([M+13C], 78), 896.6
([M+213C], 35). UV (CH2Cl2) λmax (log ꢀ) 346 (4.55), 390 (4.70).
Em λmax 427.
Precursor 6b. A mixture of 3,5-dibromopyridine (5b, 47 mg,
0.20 mmol), donor diyne 44a (240 mg, 0.50 mmol), KOH (94 mg,
1.67 mmol), THF (20 mL), i-Pr2NH (20 mL), and MeOH (1 mL)
was bubbled with Ar with stirring for 30 min. Pd(PPh3)4 (14 mg,
0.012 mmol) and CuI (4.6 mg, 0.024 mmol) were quickly added
and the mixture was bubbled for an additional 20 min. The
temperature was raised to 55 °C and the reaction stirred until
complete by TLC (ca. 18 h). The mixture was evaporated and the
residue chromatographed on silica gel (2:1 hexanes:CH2Cl2) to yield
1
6b (97 mg, 54%) as a yellow oil. H NMR (300 MHz, CDCl3) δ
8.55 (d, J ) 2.1 Hz, 2H), 7.84 (t, J ) 2.1 Hz, 1H), 7.32 (d, J ) 9.0
Hz, 2H), 6.73 (d, J ) 2.8 Hz, 2H), 6.55 (dd, J ) 9.0, 2.8 Hz, 2H),
3.28 (t, J ) 7.8 Hz, 8H), 1.57 (quin, J ) 7.5 Hz, 8H), 1.38 (sext,
J ) 7.8 Hz, 8H), 1.17 (s, 42H), 0.96 (t, J ) 7.2 Hz, 12H). 13C
NMR (75 MHz, CDCl3) δ 150.0, 148.0, 140.5, 136.6, 127.1, 121.1,
115.5, 111.9, 110.9, 106.5, 93.8, 93.5, 86.7, 50.8, 29.5, 20.5, 19.0,
14.2, 11.6. IR (NaCl) ν 2952, 2938, 2859, 2208, 2151, 1718, 1593,
Conclusions
We have prepared an isomeric pair of donor-functionalized
DBPAs that visibly fluoresce and display weak intramolecular
charge transfer behavior. The spectroscopic profiles of the
readily obtained molecules illustrate the effect of nitrogen
(16) Huang, J.-H.; Wen, W.-H.; Sun, Y.-Y.; Chou, P.-T.; Fang, J.-M. J.
Org. Chem. 2005, 70, 5827-5832.
6698 J. Org. Chem., Vol. 72, No. 18, 2007

Dehydrobenzopyrid[15]annulenes
1568, 1536, 1503, 1464, 1432, 1396, 1371 cm-1. MS (APCI) m/z
([isotope], %) 894.6 ([M+], 100), 895.6 ([M+13C], 70), 896.6
([M+213C], 33). UV (CH2Cl2) λmax (log ꢀ) 348 (4.65), 383 (4.72).
Em λmax 421.
solution was stirred for an additional 24 h, then evaporated to a
blue-green oil. The residue was redissolved in Et2O and washed in
H2O (3×). The organic phase was dried (MgSO4), concentrated in
vacuo, and filtered through a pad of silica (1:1 hexanes:CH2Cl2) to
1
Annulene 3a. Precursor 6a (120 mg, 0.13 mmol) was dissolved
in THF (20 mL), Et2O (20 mL), and MeOH (10 mL). TBAF (5.4
mL, 5.4 mmol, 1 M in THF) was added and the mixture was stirred
for ca. 18 h. The mixture was concentrated in vacuo, rediluted in
Et2O, and washed with H2O (2×). The organic phase was dried
(MgSO4) and concentrated in vacuo to give an amber oil. The
deprotected material was dissolved in pyridine (50 mL) and injected
into a mixture of Cu(OAc)2 (669 mg, 3.4 mmol) and CuCl (265
mg, 2.7 mmol) in pyridine (340 mL) and MeOH (50 mL) over 40
h with stirring at 65 °C. After injection was complete, the solution
was stirred for an additional 24 h, then evaporated to a blue-green
oil. The residue was redissolved in Et2O and washed with H2O
(3×). The organic phase was dried (MgSO4), concentrated in vacuo,
and filtered through a pad of silica (1:1 hexanes:CH2Cl2)to yield
yield 3b (25 mg, 77%) as a yellow solid. Mp 181-183 °C. H
NMR (CDCl3) δ 8.75 (t, J ) 1.5 Hz, 1H), 8.39 (d, J ) 1.5 Hz,
2H), 7.24 (d, J ) 9.0 Hz, 2H), 6.77 (d, J ) 2.4 Hz, 2H), 6.58 (dd,
J ) 9.0, 2.4 Hz, 2H), 3.27 (t, J ) 8.1 Hz, 8H), 1.56 (quin, J ) 8.1
Hz, 8H), 1.32 (sext, J ) 7.5 Hz, 8H), 0.97 (t, J ) 7.5 Hz, 12H).
13C NMR (CDCl3) δ 150.5, 148.1, 145.4, 131.2, 127.7, 122.0, 115.3,
112.5, 112.2, 100.2, 98.1, 90.6, 83.2, 51.0, 29.5, 20.5, 14.3. IR
(NaCl) ν 2951, 2925, 2866, 2218, 2191, 1771, 1739, 1696, 1684,
1650, 1594, 1567, 1557, 1538, 1504, 1455,1399, 1365 cm-1. MS
(APCI) m/z ([isotope], %) 580.4 ([M+], 100), 581.4 ([M+13C], 37),
582.3 ([M+213C], 11). UV (CH2Cl2) λmax (log ꢀ) 352 (4.70), 408
(4.09). Em λmax 494.
TFA Titration of 3a and 3b. Solutions of 3a and 3b were
prepared in spectrophotometric grade CH2Cl2 (ca. 1 mg in 360 µL,
UV cutoff 204 nm) and portioned equally into nine 1-dram glass
screw cap vials. Trifluoroacetic acid was diluted to the indicated
concentrations ranging from 10-5 to 10-0.3 M in 10 mL portions
of CH2Cl2. Aliquots (2 mL) of the TFA solutions were added to
each solution of analyte. The solutions were capped, shaken, and
pipetted into a four-sided quartz spectrophotometry cuvette, and
the UV-vis and fluorescence spectra were then taken immediately.
Photographs were taken within 1 h of preparation of each sample.
1
3a (66 mg, 85%) as a bright yellow solid. Mp 131-133 °C. H
NMR (CDCl3) δ 7.49 (t, J ) 7.8 Hz, 1H), 7.23 (d, J ) 8.7 Hz,
2H), 7.07 (d, J ) 7.8 Hz, 2H), 6.72 (d, J ) 2.7 Hz, 2H), 6.51 (dd,
J ) 8.7, 2.7 Hz, 2H), 3.25 (t, J ) 7.8 Hz, 8H), 1.54 (quin, J ) 7.8
Hz, 8H), 1.34 (sext, J ) 7.8 Hz, 8H), 0.96 (t, J ) 7.2 Hz, 12H).
13C NMR (CDCl3) δ 148.2, 145.4, 136.9, 131.7, 129.0, 121.2, 115.0,
112.3, 112.1, 94.1, 92.2, 82.3, 78.3, 51.0, 29.5, 20.5, 14.2. IR (NaCl)
ν 2951, 2924, 2866, 2193, 1682, 1652, 1591, 1570, 1533, 1502,
1451, 1366 cm-1. MS (APCI) m/z ([isotope], %) 580.4 ([M+], 100),
581.4 ([M+13C], 44), 582.4 ([M+213C], 8). UV (CH2Cl2) λmax
(log ꢀ) 347 (4.90), 407 (4.24). Em λmax 490.
Acknowledgment. We thank the National Science Founda-
tion (CHE-0414175 and -0718242) for financial support and
Dr. Lev N. Zakharov for crystallographic data. E.L.S. and S.P.M.
acknowledge the NSF for IGERT fellowships (DGE-0549503).
Annulene 3b. Precursor 6b (90 mg, 0.10 mmol) was dissolved
in THF (20 mL), Et2O (20 mL), and MeOH (10 mL). TBAF (6.0
mL, 6.0 mmol, 1 M in THF) was added and the mixture was stirred
for ca. 18 h. The mixture was concentrated in vacuo, rediluted in
Et2O, and washed with H2O (2×). The organic phase was dried
(MgSO4) and concentrated in vacuo to give an amber oil. The
deprotected material was dissolved in pyridine (37 mL) and injected
into a mixture of Cu(OAc)2 (499 mg, 2.5 mmol) and CuCl
(198 mg, 2.0 mmol) in pyridine (254 mL) and MeOH (37 mL)
over 40 h with stirring at 65 °C. After injection was complete, the
Supporting Information Available: Copies of 1H NMR spectra
for 3a,b and 6a,b; crystallographic data for 3a,b; computational
details; emission spectra of 3a titrated with TFA in MeOH; and
emission spectra of 6a and 6b titrated with TFA in CH2Cl2. This
material is available free of charge via the Internet at http://pubs.acs.
org.
JO070827C
J. Org. Chem, Vol. 72, No. 18, 2007 6699