Phenylethynyl-BODIPY oligomers: Bright dyes and fluorescent building blocks

Article abstract of DOI:10.1021/ol802446e

Boradiazaindacene dyes were converted into phenylethynyl-BODIPY oligomers via a cycle of reactions, notably including Sonogashira couplings. As expected, as the number, n, of repeating units increases, peak absorption and emission wavelengths are shifted

Full text of DOI:10.1021/ol802446e

ORGANIC
LETTERS
2009
Vol. 11, No. 1
85-88
Phenylethynyl-BODIPY Oligomers:
Bright Dyes and Fluorescent Building
Blocks
Yusuf Cakmak† and Engin U. Akkaya*,‡
Department of Chemistry, Middle East Technical UniVersity, 06531 Ankara, Turkey,
and Department of Chemistry and UNAM-Institute of Materials Science and
Nanotechnology, Bilkent UniVersity, 06800 Ankara, Turkey
eua@fen.bilkent.edu.tr
Received October 23, 2008
ABSTRACT
Boradiazaindacene dyes were converted into phenylethynyl-BODIPY oligomers via a cycle of reactions, notably including Sonogashira couplings.
As expected, as the number, n, of repeating units increases, peak absorption and emission wavelengths are shifted to the red end of the
visible spectrum, albeit with smaller increments as n increases. Decyl groups help to keep the solubility remarkably high, and in addition to
being very bright red-emitting fluorophores, their rigid rod-like structures could allow their use as functional building blocks.
As evidenced by the number of recent reviews,1 there has
been increased interest in the derivatization of BODIPY dyes2
and their applications3 in recent years. Part of this interest
is focused on finding novel strategies2a,b,e,4,5 to push the
absorption and emission wavelengths further into the red.
The parent dye is a green-emitting fluorophore, comparable
to fluorescein; however, with structural modifications, the
chromophore can be transformed into a red- to near-IR-
emitting dye.
In this report, we disclose our attempts to shift the dye’s
working range of wavelengths into red, by extending the
conjugation on the 2,6-axis through iterative Sonogashira
couplings, yielding the first examples of oligomeric BODIPY
dyes. The synthesis plan is shown in Scheme 1. From the
very start, we realized that as the BODIPY chain is extended,
solubility is likely to become an issue.
(2) Selected examples:(a) Rurack, K.; Kollmannsberger, M.; Daub, J.
Angew. Chem., Int. Ed. 2001, 40, 385–387. (b) Dost, Z.; Atilgan, S.; Akkaya,
E. U. Tetrahedron 2006, 62, 8484–8888. (c) Ulrich, G.; Goze, C.; Guardigli,
M.; Roda, A.; Ziessel, R. Angew. Chem., Int. Ed. 2005, 44, 3694–3698. (d)
Baruah, M.; Qin, W.; Vallee, R. A. L.; Beljonne, D.; Rohand, T.; Dehaen,
W.; Boens, N. Org. Lett. 2005, 7, 4377–4380. (e) Umezawa, K.; Nakamura,
Y.; Makino, H.; Citterio, D.; Suzuki, K. J. Am. Chem. Soc. 2008, 130, 1550–
1551. (f) Saki, N.; Dinc, T.; Akkaya, E. U. Tetrahedron 2006, 62, 2721–
2725. (g) Thivierge, C.; Bandichhor, R.; Burgess, K. Org. Lett. 2007, 9,
2135–2138.
† Middle East Technical University.
‡ Bilkent University.
(1) Recent reviews on BODIPY dyes:(a) Ulrich, G.; Ziessel, R.;
Harriman, A. Angew. Chem., Int. Ed. 2008, 47, 1184–1201. (b) Ziessel,
R.; Ulrich, G.; Harriman, A. New J. Chem. 2007, 31, 496–501. (c) Loudet,
A.; Burgess, K. Chem. ReV. 2007, 107, 4891–4932.
10.1021/ol802446e CCC: $40.75
Published on Web 12/03/2008
2009 American Chemical Society

analogy to the literature.6 Then, the BODIPY dye 2 was
obtained following a rather routine procedure for the
synthesis of this class of dyes. 1,4-Diethynylbenzene 3 and
the mono-TMS-protected form of it (4) are important
intermediates. The BODIPY dye 2 can be iodinated at the
electron-rich positions 2 and 6 using an I2/HIO3 iodination
procedure,7 and both monoiodinated (5) and diiodinated (6)
products can be obtained in satisfactory yields by adjusting
the dye to iodination reagent ratio (or the reaction time).
2-Iodo-BODIPY dye 5 was then subjected to Sonogashira
coupling conditions (Pd(Ph3)Cl2 and CuI catalysis) and
coupled to mono-TMS-protected 1,4-diethynylbenzene (4).
The removal of the TMS protection yielded another useful
intermediate, 8. 2,6-Diiodo derivative (6) when subjected to
coupling with this intermediate results in the dimeric
BODIPY derivative 9 where one iodo substituent is still
present. Tweaking with the reaction conditions, result in the
double coupling product which is the compound 13. The
chain was further extended by reacting the intermediate
compound 9 with mono-TMS-protected 1,4-diethynylben-
zene, followed by deprotection, to yield another intermediate
species 11. At this point, we had the parent dye 2 (n ) 0)
and the triBODIPY 13 (n ) 2) already at hand. The other
members of the oligomeric BODIPY series were obtained
as follows, all through Sonogashira couplings: 1,4- dieth-
ynylbenzene with monoiodoBODIPY 5 yielded the diBO-
DIPY 12 (n ) 1), and with the intermediate 9, the
tetraBODIPY 14 (n ) 3). Finally, the last member of the
Scheme 1
.
Synthesis of Oligomeric Phenylethynyl-BODIPY
Dyes: Dye 2 and Intermediates
Scheme 2
.
Synthesis of Oligomeric Phenylethynyl-BODIPY
Dyes 12, 13, 14, and 15
We placed two decyloxy functions on the meso-phenyl
group of the BODIPY dyes. Thus, our synthetic work starts
with the preparation of 3,5-didecyloxybenzaldehyde in
(3) (a) Atilgan, S.; Ekmekci, Z.; Dogan, A. L.; Guc, D.; Akkaya, E. U.
Chem. Commun. 2006, 4398–4400. (b) Ertan-Ela, S.; Yilmaz, M. D.; Icli,
B.; Dede, Y.; Icli, S.; Akkaya, E. U. Org. Lett. 2008, 10, 3299–3302. (c)
Coskun, A.; Akkaya, E. U. J. Am. Chem. Soc. 2005, 127, 10464–10465.
(d) Coskun, A.; Akkaya, E. U. J. Am. Chem. Soc. 2006, 128, 14474–14475.
(e) Rurack, K.; Kollmannsberger, M.; Resch-Genger, U.; Daub, J. J. Am.
Chem. Soc. 2000, 122, 968–969. (f) Zeng, L.; Miller, E. W.; Pralle, A.;
Isacoff, E. Y.; Chang, C. J. J. Am. Chem. Soc. 2006, 128, 10–11. (g)
Camerel, F.; Bonardi, L.; Ulrich, G.; Charbonniere, L.; Donnio, B.;
Bourgogne, C.; Guillon, D.; Retailleau, P.; Ziessel, R. Chem. Mater. 2006,
18, 5009–5021. (h) Coskun, A.; Yilmaz, M. D.; Akkaya, E. U. Org. Lett.
2007, 9, 607–609. (i) Park, M. S.; Swamy, K. M. K.; Lee, Y. J.; Lee, H. N.;
Jang, Y. J.; Moon, Y. H.; Yoon, J. Tetrahedron Lett. 2006, 47, 8129–8132.
86
Org. Lett., Vol. 11, No. 1, 2009

have been reported.12 Further work on this matter is
warranted for a better understanding of excitonic interactions
in this series. Normalized emission spectra (Figure 2) show
Figure 1. Normalized absorption spectra of the parent BODIPY
and the oligomeric series in dilute CHCl3 solutions (black, 2; red,
12; blue, 13; green, 14; pink, 15).
Figure 2. Normalized emission spectra of the parent BODIPY and
the oligomeric series in dilute CHCl3 solutions (black, 2; red, 12;
blue, 13; green, 14; pink, 15).
series was obtained by the reaction of 2,6-diiodoBODIPY 6
with the intermediate 11 (Scheme 2).
Absorption spectra of the dyes in chloroform solutions
were in accord with our expectations with a notable exception
of compound 13. As n was varied from 0 to 4, there was a
red shift of 120 nm. However, with each addition of the
repeating units, the additional red shifts becomes smaller: 0
to 1, +48 nm; 1 to 2, +28 nm; 2 to 3, +18 nm. The peak
position is practically unchanged for the dyes corresponding
to n ) 3 (14) and n ) 4 (15). The absorbance peak due to
compound 13 has a very peculiar shape and highly broad-
ened. The peak is clearly the sum of two distinct peaks,
which are most likely the results of Davydov (exciton)
splitting of the excited states. As judged by the magnitude
of fwhm values (Table 1), similar excitonic interactions might
an expected trend, n ) 0 emits at 520 nm, and the final
member of the series emits at 640 nm. Quantum yields and
extinction coefficients were also determined (Table 1). As a
result of the rigidity of the phenylethynyl framework in these
compounds, quantum yields remain high even at longer
wavelength emission compounds.
It is also worth mentioning that the Stokes shifts of the
new fluorophores vary in the range of 27 to 42 nm, which is
significantly larger than that of a standard BODIPY dye,
which is typically less than 10 nm.
We expect these dyes to become valuable additions to the
growing arsenal of red-emitting dyes, especially useful in
biological applications. In addition, the rigid 3D structure
of the phenylethynyl chromophores confer additional value
to these dyes as potential building blocks in the construction
of functional supramolecular assemblies. In fact, very
recently, a polymeric material incorporating BODIPY units
Table 1. Spectroscopic Properties of the Parent BODIPY and
Oligomeric Series
λmax (abs)a
λmax(em)a
(nm)
fwhma
(cm-1
)
a
dye
(nm)
εmax
Φa
2
503
551
579
597
592
95 000
69 000
104 000
118 000
251 000
512
586
621
624
620
836
1235
1107
950
0.69b
0.39c
0.45d
0.60d
0.58d
(4) (a) Wada, M.; Ito, S.; Uno, H.; Murashima, T.; Ono, N.; Urano, T.;
Urano, Y. Tetrahedron Lett. 2001, 42, 6711–6713. (b) Killoran, J.; O’Shea,
D. F. Chem. Commun. 2006, 1503–1505. (c) Zhao, W.; Carreira, E. M.
Chem. Eur. J. 2006, 12, 7254–7263.
12
13
14
15
(5) Zhang, D.; Wen, Y.; Xiao, Y.; Yu, G.; Liu, Y.; Qian, X. Chem.
Commun. 2008, 4777–4779.
1011
a In CHCl3. b Fluorescein was used as the reference dye (Φ ) 0.92 in
0.1 M NaOH(aq)).8 c Rhodamine 6G was used as the reference dye (Φ )
0.95 in EtOH).9 d Sulforhodamine 101 was used as the reference dye (Φ )
0.9 in EtOH).10
(6) Burghart, A.; Kim, H.; Welch, M. B.; Thorensen, L. H.; Reibenspies,
J.; Burgess, K. J. Org. Chem. 1999, 64, 7813–7819.
(7) Yogo, T.; Urano, Y.; Ishitsuka, Y.; Maniwa, F.; Nagano, T. J. Am.
Chem. Soc. 2005, 127, 12162–12163.
(8) Weber, G.; Teale, F. W. J. Trans. Faraday Soc. 1958, 54, 640–648.
(9) Kubin, R. F.; Fletcher, A. N. J. Lumin. 1982, 27, 455–462.
(10) Birge, R. R. Kodak Laser Dyes; Eastman Kodak Company:
Rochester, NY, 1987.
be in effect in the other members of the series (12, 14, and
15), but only in 13 do the two peaks have approximately
equal heights. This phenomenon, which is a manifestion of
dipole coupling of the chromophores, is quite common in
multichromophoric systems11 and is not limited to crystalline
or aggregate states, as many examples in dilute solutions
(11) Scholes, G. D.; Ghiggino, K. P.; Oliver, A. M.; Paddon-Row, M. N.
J. Am. Chem. Soc. 1993, 115, 4345–4349.
(12) (a) Hernando, J.; Schaaf, M. V. D.; van Dijk, E. M. H. P.; Sauer,
M.; Garcia-Parajo, M. F.; van Hulst, N. F. J. Phys. Chem. A 2003, 107,
43–52. (b) Furstenberg, A.; Vauthey, E. J. Phys. Chem. B 2007, 111, 12610–
12620. (c) Mazzoni, M.; Agati, G.; Troup, G. J.; Pratesi, R. J. Opt. A:
Pure Appl. Opt. 2003, 5, S374–S380.
Org. Lett., Vol. 11, No. 1, 2009
87

was reported13 to show interesting self-assembly properties.
All of the dyes we reported have two open positions for
further functionalization through electrophilic aromatic sub-
stitution of pyrrole moieties of the terminal BODIPYs.
Further work with these rigid oligomers is expected to yield
interesting applications in electrochromic devices and func-
tional self-assembled systems.
Acknowledgment. The authors gratefully acknowledge
support from Turkish Academy of Sciences (TUBA).
Supporting Information Available: Synthesis proce-
dures, additional spectroscopic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
(13) Nagai, A.; Miyake, J.; Kokado, K.; Nagata, Y.; Chujo, Y. J. Am.
Chem. Soc. 2008, 130, 15276–15278.
OL802446E
88
Org. Lett., Vol. 11, No. 1, 2009