Tetraphenylmethane based tectons in dendrimer synthesis: Preparation and energy transfer properties of a dendritic stilbene-anthracene dyad

Article abstract of DOI:10.1016/S0040-4039(02)00588-9

A topologically new stilbene-anthracene dendrimer has been prepared using a highly branched tristilbenyl dendron derived from tetraphenylmethane. Steady-state photophysical studies on this dendrimer showed energy transfer from the peripheral stilbene unit

Full text of DOI:10.1016/S0040-4039(02)00588-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 3517–3520
Tetraphenylmethane based tectons in dendrimer synthesis:
preparation and energy transfer properties of a dendritic
stilbene–anthracene dyad
Saumitra Sengupta* and Nilasish Pal
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
Received 30 January 2002; revised 13 March 2002; accepted 21 March 2002
Abstract—A topologically new stilbene–anthracene dendrimer has been prepared using a highly branched tristilbenyl dendron
derived from tetraphenylmethane. Steady-state photophysical studies on this dendrimer showed energy transfer from the
peripheral stilbene units to the anthracene core. © 2002 Elsevier Science Ltd. All rights reserved.
The photosynthetic unit of purple bacteria is a
supramolecular assembly composed of a central reac-
tion center surrounded by chlorophyll and carotenoid
chromophores.1 The latter act as the light harvesting
antennae which absorb incident photons and transfer
the energy to the central reaction center with high
efficiency. Dendrimers are highly branched macro-
molecules that have attracted considerable current
attention due to their potential application in a number
of functional devices.2 They usually consist of a multi-
armed core from which numerous branches radiate
outwards in a three dimensional array. The degree of
branching in a dendrimer increases with successive gen-
erations as a result of which higher generation den-
drimers are globular in shape and possess high
functional densities at their periphery. These features
bear close resemblance to the composition of a natural
photosynthetic unit and in recent years, have triggered
widespread activity in the synthesis of photoresponsive
dendrimers as artificial light harvesting modules.3 Pho-
toresponsive dendrimers are also gaining interest as the
active material for other photonic applications viz.
organic LEDs, optical power limiting devices and in
non-linear optics.4 In view of these, we have initiated a
program on the synthesis and photophysical studies of
dendritic bichromophoric (donor–acceptor) systems
and report here the synthesis and energy transfer prop-
erties of a dendritic stilbene–anthracene dyad.
Stilbene–anthracene dyads are a promising class of
energy transfer modules in which stilbene units act as
the donor and anthracene acts as the acceptor chro-
mophore. Spectral overlap between stilbene fluores-
cence and anthracene absorbance concurs with the
Fo¨rster’s type (SD1 “S1A) energy transfer process.5 A few
reports on cross-conjugated dendrimers comprised of
meta-distyrylbenzene dendrons and an anthracene core
have recently appeared in the literature.6 However,
effective energy transfer was only possible in a second
generation dendrimer having 12 stilbene units at the
periphery.6a We now describe a topologically new stil-
bene–anthracene dendrimer
5 in which a tetra-
phenylmethane based tristilbenyl dendron has been
used as the peripheral chromophore and show that, by
virtue of the highly branched nature of these dendrons
resulting in a greater number of energy collection sites,
energy transfer to the core is possible even in a first
generation dendrimer.
Our synthesis started with the cheap dyestuff, New
Fuchsin 1 which was easily converted to the (tri-
iodophenyl)methyl phenol 2 by a two-step procedure
previously described by us (Scheme 1).7 Threefold Heck
reaction of 2 with p-tert-butylstyrene under Jeffery’s
conditions (cat. Pd(OAc)2, Bu4NBr, KOAc, DMF,
100°C, 24 h)8 then produced the desired tristilbenyl
1
dendron 3 in 66% yield.9 The H NMR spectrum of 3
showed large coupling constants (Jꢀ16 Hz) for the
olefinic protons indicating a trans-geometry for the
stilbene moieties. The dendron 3 was then coupled to
the 9,10-bis-(chloromethyl)anthracene 4 core, under
phase transfer catalysis (10% aq. NaOH, cat. Bu4NBr,
PhCl, 90°C, 6 h),10 to produce the first generation
Keywords: dendrimer; stilbene; anthracene; energy transfer.
* Corresponding author. Fax: 91 033 4734266; e-mail: jusaumitra@
yahoo.co.uk
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00588-9

3518
S. Sengupta, N. Pal / Tetrahedron Letters 43 (2002) 3517–3520
Scheme 1.
The absorption and photoluminescence (PL) spectra of
the dendron 3 is shown in Fig. 1. The absorption
maximum at 318 nm is due to the stilbene units and is
red-shifted from that of trans-stilbene11 by ca. 20 nm
indicating some electronic interaction between the three
stilbene units through the central sp3-carbon. Recently,
Bazan et al. have reported similar homoconjugative
effects in some tetrastilbenylmethane derivatives.12 The
318 nm peak is also associated with a large molar
extinction coefficient (mmax 11.0×104 M−1 cm−1) which is
significantly more than three times the value of trans-
stilbene (mmax 2.7×104 M−1 cm−1),11 indicating construc-
tive excitonic coupling between the three stilbenoid
arms in 3. Such a constructive interaction is perhaps
due to the rigid tetrahedral arrangement of the chro-
mophores in 3.13 The dendron 3 produced a strong
dendrimer 5 in 45% yield after purification by prepara-
tive thin layer chromatography on silica gel.9 The cou-
pling reaction between 3 and 4 was best carried out
under the PTC conditions described above since other
methods (NaH, DMF, rt to 100°C or K2CO3, DMF, rt
to 100°C) either led to incomplete reaction or produced
1
a complex mixture of products. The H NMR spectrum
of 5 clearly showed the presence of the 9,10-bis-
(oxymethylene)anthracene core in the dendrimer struc-
ture. Thus, a four proton singlet was found at l 5.97
due to the core methylene groups together with a four
proton multiplet at l 8.38 due to the anthracene ring
a-hydrogens. The anthracene ring b-hydrogens
remained embedded along with the other aromatic pro-
tons in a broad multiplet ranging from 6.97 to 7.72
ppm.

S. Sengupta, N. Pal / Tetrahedron Letters 43 (2002) 3517–3520
3519
Figure 1. Absorption (1a) and PL spectra (1b) of 3 in
CH2Cl2.
fluorescence with a maximum at 387 nm (uexc 318 nm)
which is also red-shifted from the stilbene fluorescence
by ca. 20 nm.
The absorption spectrum of the dendrimer 5 is shown
in Fig. 2. The peaks at 234 (mmax 16.6×104 M−1 cm−1)
and 321 nm (mmax 22.8×104 M−1 cm−1) are due to the
stilbene dendrons (cf. Fig. 1). The molar absorbtivity of
the latter peak is twice that of the 318 nm peak of 3
since two dendron moieties are present in the den-
drimer. Weak absorptions were also found at 373 and
394 nm (log m 3.6) which were assigned to the anthra-
cene core [umax (nm) for the model 9,10-bis-(phen-
oxymethyl)anthracene (6):14 230 (log m 3.86), 246 (log m
3.87), 354 (log m 3.44), 374 (log m 3.62) and 394 (log m
3.62)]. The spectrum is thus a superimposition of peaks
arising out of the peripheral stilbenoid dendrons and
the anthracene core. This was as expected since the two
types of chromophores in 5 are connected by a non-
conjugated linker.
Figure 3. PL spectrum of 5 (uexc 318 nm; inset uexc 394 nm).
other hand, excitation at 318 nm, where the stilbene
moieties have strong absorption, produced peaks at 387
and 400 nm (Fig. 3). The former peak corresponds to
emission from the stilbenoid moieties (cf. Fig. 1)
whereas the latter is due to emission from the anthra-
cene unit indicating energy transfer from the surface
stilbene units to the dendrimer core. In a control exper-
iment, an equimolar mixture of 3 and the model core 6
in CH2Cl2 was irradiated at 318 nm which produced an
emission spectrum (umax 387 nm) which was fully super-
imposable on the PL spectrum of 3 showing that the
energy transfer in 5 is purely an intramolecular process.
The fluorescence intensity of 5 at 400 nm is nearly 500
times that of the emission peak produced by direct core
excitation (cf. inset, Fig. 3), again pointing to light
harvesting by the stilbenoid periphery and subsequent
energy transfer to the anthracene core. Further proof of
energy transfer in 5 comes from its photoluminescence
excitation (PLE) spectrum monitored at 420 nm (Fig. 4)
which showed a strong peak at 320 nm indicating
effective coupling between the stilbene units and the
anthracene core. Notably, the emission spectrum of 5
The PL spectrum of the dendrimer is shown in Fig. 3.
Excitation at 394 nm, where absorption is only due to
the anthracene core, led to a structured emission with
peaks at 402, 422 and 446 nm (see inset, Fig. 3). On the
Figure 2. Absorption spectrum of 5 in CH2Cl2.
Figure 4. PLE spectrum of 5 monitored at 420 nm.

3520
S. Sengupta, N. Pal / Tetrahedron Letters 43 (2002) 3517–3520
was devoid of much red-tailing, a phenomenon previ-
ously observed by us for a first generation m-distyryl-
benzene–anthracene dendrimer,6b suggesting that the
highly branched nature of the tetraphenylmethane
based dendrons in 5 probably discourages aggregate
formation. However, the energy transfer in 5 is less
than quantitative, perhaps due to the presence of only
six energy collection sites in the dendrimer periphery. A
higher generation dendrimer with more energy collect-
ing sites might provide improved efficiency. Moreover,
in an anthracene-cored dendrimer, the acceptor wave-
length (370–390 nm) where the energy is being trans-
ferred, has a low absorptivity which may also account
for the inefficient energy transduction in our system.
An acceptor chromophore having a strong absorbance
in the long wavelength region capped by numerous
stilbenoid energy collecting sites could provide a supe-
rior energy transfer module. We are currently investi-
gating these possibilities.
Y.-J.; Devadoss, C.; Bharathi, P.; Moore, J. S. Adv.
Mater. 1996, 8, 237; (c) Mongin, O.; Gossauer, A. Tetra-
hedron 1997, 53, 6835; (d) Halim, M.; Pillow, J. N. G.;
Samuel, I. D. W.; Burn, P. L. Adv. Mater. 1999, 11, 371;
(e) Freeman, A. W.; Fre´chet, J. M. J.; Koene, S. C.;
Thompson, M. E. Polym. Prepr. 1999, 40, 1246; (f)
Spangler, C. W.; Elandaloussi, E. H.; Reeves, B. Polym.
Prepr. 2000, 41, 789; (g) Robinson, M. R.; Wang, S.;
Bazan, G. C. Adv. Mater. 2000, 12, 1701; (h) Yokoyama,
S.; Nakahama, T.; Ohtono, A.; Mashiko, S. J. Am.
Chem. Soc. 2000, 122, 3174; (i) Maus, M.; De, R.; Lor,
M.; Weil, T.; Mitra, S.; Wiesler, U. M.; Herrmann, A.;
Hofkens, J.; Vosch, T.; Mu¨llen, K.; De Schryver, F. C. J.
Am. Chem. Soc. 2001, 123, 7668; (j) Weil, T.; Wiesler, U.
M.; Herrmann, A.; Bauer, R.; Hofkens, J.; De Schryver,
F. C.; Mu¨llen, K. J. Am. Chem. Soc. 2001, 123, 8101; (k)
Meskers, S. C. J.; Bender, M.; Hu¨bner, J.; Romanovskii,
Y. V.; Oestereich, M.; Schenning, A. P. H. J.; Meijer, E.
W.; Ba¨ssler, H. J. Phys. Chem. A 2001, 105, 10220.
5. Spieser, S. Chem. Rev. 1996, 96, 1953.
6. (a) Pillow, J. N. G.; Halim, M.; Lupton, J. M.; Burn, P.
L.; Samuel, I. D. W. Macromolecules 1999, 32, 5985; (b)
Sengupta, S.; Sadhukhan, S. K.; Singh, R. S.; Pal, N.
Tetrahedron Lett. 2002, 43, 1117.
7. (a) Sengupta, S.; Sadhukhan, S. K. Tetrahedron Lett.
1999, 40, 9157; (b) Sengupta, S.; Sadhukhan, S. K.
Organometallics 2001, 20, 1889.
Acknowledgements
Professor Nitin Chattopadhya is thanked for access to
the
fluorescence
spectrometer
and
Pradipta
Purkayastha for some fluorescence experiments. One of
us (N.P.) thanks CSIR, New Delhi for a research
fellowship.
8. Jeffery, T. Tetrahedron 1996, 52, 10113.
1
9. 3: mp 220–224°C; H NMR (CDCl3, 300 MHz) l 1.33 (s,
27H), 2.34 (s, 9H), 4.67 (br s, 1H), 6.72 (d, 2H, J=8.5
Hz), 6.98 (d, 3H, J=16 Hz), 7.04–7.11 (m, 6H), 7.14 (d,
2H, J=8.3 Hz), 7.26 (d, 3H, J=16 Hz), 7.37 (d, 6H,
J=8.2 Hz), 7.45 (d, 6H, J=8.8 Hz), 7.48 (d, 3H, J=9.2
Hz). 5: mp>250°C; H NMR (CDCl3, 300 MHz) l 1.33
(s, 54H), 2.37 (s, 18H), 5.97 (s, 4H), 6.97–7.72 (m, 66H),
8.38 (m, 4H).
References
1. (a) McDermott, G.; Prince, S. M.; Freer, A. A.;
Hawthornthwaite-Lawless, A. M.; Papiz, M. Z.; Cogdell,
R. J.; Isaacs, N. W. Nature 1995, 374, 517; (b) Hu, X.;
Damjanovic, A.; Ritz, T.; Schulten, K. Proc. Natl. Acad.
Sci. USA 1998, 95, 5935.
2. Reviews: (a) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle,
F. Dendritic Macromolecules: Concepts, Syntheses, Per-
spectives; VCH: Weinheim, 1996; (b) Chow, H. F.;
Mong, T. K. K.; Nongrum, M. F.; Wan, C. W. Tetra-
hedron 1998, 54, 8543; (c) Smith, D. K.; Diederich, F.
Chem. Eur. J. 1998, 4, 1353; (d) Fischer, M.; Vo¨gtle, F.
Angew. Chem., Int. Ed. Engl. 1999, 38, 884; (e) Grayson,
S. M.; Fre´chet J. M. J. Chem. Rev. 2001, 101, 3819.
3. Reviews: (a) Adronov, A.; Fre´chet, J. M. J. Chem. Com-
mun. 2000, 1701; (b) Hecht, S.; Fre´chet, J. M. J. Angew.
Chem., Int. Ed. Engl. 2001, 40, 74.
1
10. Cao, D.; Meier, H. Angew. Chem., Int. Ed. Engl. 2001,
40, 186.
11. Meier, H. Angew. Chem., Int. Ed. Engl. 1992, 31, 1399.
12. (a) Oldham, W. J., Jr.; Lachicotte, R. J.; Bazan, G. C. J.
Am. Chem. Soc. 1998, 120, 2987; (b) Wang, S.; Oldham,
W. J., Jr.; Hudack, R. A., Jr.; Bazan, G. C. J. Am. Chem.
Soc. 2000, 122, 5695.
13. The molar absorbtivity of tetraphenylmethane is 2020
compared to 212 for benzene. For constructive excitonic
couplings, see: (a) Kasha, M.; Rawls, H. R.; Ashraf
El-Bayoumi, M. Pure Appl. Chem. 1965, 11, 371; (b)
Langhals, H.; Wagner, C.; Ismael, R. New. J. Chem.
2001, 25, 1047.
4. See, for example: (a) Put, E. J. H.; Clays, K.; Persoons,
A.; Biemans, H. A. M.; Luijkx, C. P. M.; Meijer, E. W.
Chem. Phys. Lett. 1996, 260, 136; (b) Wang, P.-W.; Liu,
14. Prepared by phase transfer catalyzed alkylation reaction
of 4 with excess phenol.