C O M M U N I C A T I O N S
which was calculated by comparing the integration of donor
emission in the absence of the core (3) and in the target dendrimer
4 at the same excitation wavelength.12 Furthermore, a 6.2-fold
increase in the core emission relative to the emission of the core
lacking the peripheral donors (2, λex ) 345 nm) is observed. This
amplification effect is calculated by obtaining the ratio of the
integrated emissions of both 2 and 4 when each is excited at 354
nm. The significant increase in core emission is due to the light-
harvesting properties of the donor coumarin 2 chromophores located
at the periphery of the dendrimer and to their ability to very
efficiently transfer their absorbed energy to the core.12,13
To the best of our knowledge, dendrimer 4 represents the first
reported organic-based nonionic material that transforms UV and
visible light directly to NIR radiation based on FRET. In addition
to possibly being applicable to optoelectronic devices, this light-
harvesting antenna14 might find use in increasing the performance
of dye-sensitized solar cells,15 a current focus of our investigations.
Figure 2. Energy level diagram illustrating the photophysical processes
of 2, 3, and 4. Excitation of 4 at the coumarin 2 λmax (λex ) 345 nm) results
in FRET to the higher excited state of the perylene core. This state is rapidly
converted to S1, from which emission is observed. This entire process is
highlighted with bold arrows. Emission of the coumarin 2 donor at 445 nm
occurs only when the perylene acceptor chromophore is not present.
Acknowledgment. Support of this research by the AFOSR
(FDF-49620-01-1-0167) is gratefully acknowledged. D.W.B. ac-
knowledges NSERC (Canada) for a postdoctoral fellowship.
Supporting Information Available: Experimental details and
chemical characterization data (PDF). This material is available free
References
(1) (a) Dittmer, J. J.; Marseglia, E. A.; Friend, R. H. AdV. Mater. 2000, 12,
1270-1274. (b) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.;
Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119-
1122. (c) Ferrere, S.; Gregg, B. J. Phys. Chem. B 2001, 105, 7602-7605.
(d) Tian, H.; Liu, P.; Zhu, W.; Gao, E.; Wu, D.; Cai, S. J. Mater. Chem.
2000, 10, 2708-2715. (e) Ferrere, S.; Gregg, B. J. Phys. Chem. B 1997,
101, 4490-4493. (f) Wang, Z.; Huang, C.; Li, F.; Weng, S.; Yang, S. J.
Photochem. Photobiol., A 2001, 140, 255-262.
(2) (a) Weil, T.; Wiesler, U. M.; Herrmann, A.; Bauer, R.; Hofkens, J.; De
Schryver, F. C.; Mu¨llen, K. J. Am. Chem. Soc. 2001, 123, 8101-8108.
(b) 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, 7668-7676. (c) Gronheid, R.; Hofkens, J.;
Ko¨hn, F.; Weil, T.; Reuther, E.; Mu¨llen, K.; De Schryver, F. C. J. Am.
Chem. Soc. 2002, 124, 2418-2419.
(3) (a) Gvishi, R.; Reisfeld, R. Chem. Phys. Lett. 1993, 213, 338-344. (b)
Sadria, M.; Hadel, L.; Sauers, R. R.; Husain, S.; Krogh-Jespersen, K.;
Westbrook, J. D.; Bird, F. R. J. Phys. Chem. 1992, 96, 7988.
(4) (a) Seybold, G.; Wagenblast, G. Dyes Pigm. 1989, 11, 303. (b) Langhals,
H. Nachr. Chem. Tech. Lab. 1980, 28, 716. (c) O’Neil, M. P.; Niemczyk,
M. P.; Svec, W. A.; Gosztola, D.; Gaines, G. L.; Wasielewski, M. R.
Science 2002, 257, 63.
Figure 3. Normalized steady-state emission of 2, 3, and 4 (λex ) 345 nm)
in CHCl3 at room temperature. Complete quenching of coumarin 2
fluorescence emission is observed when 4 is excited at the coumarin 2
(donor) λmax, resulting in a 6.2-fold increase in the core emission. The inset
shows an expansion of the region from 700 to 850 nm.
component, along with two smaller absorption bands at 414 and
435 nm, which are indicative of higher level excited states (i.e., S2
and/or S3). As with most analogous compounds, the excitation of
2 at these higher excited states results in rapid internal conversion
to the first singlet excited state (S1, Kasha’s rule).11 Therefore, no
fluorescence was observed in the visible region when either 2 or 4
is excited anywhere in the visible. The broad absorption band
centered at 685 nm is due to excitation of the core from S0 to S1.
Figure 2 shows an energy level diagram that gives a generalized
illustration of these photophysical processes.
The emission spectra of 2, 3, and 4, all excited at 345 nm, are
shown in Figure 3. A comparison of Figures 1 and 3 shows that
the emission of coumarin 2 overlaps well with the higher level
excited-state absorption of the core, thus creating a pathway for
FRET to occur from coumarin 2 to the perylene core derivative.12
Indeed, when dendrimer 4 is excited at the donor λmax (λex ) 345
nm), the donor emission is completely quenched due to energy
transfer to the core. This results in a 99% energy transfer efficiency,
(5) Geerts, Y.; Quante, H.; Platz, H.; Mahrt, R.; Hopmeier, M.; Bo¨hm, A.;
M u¨llen, K. J. Mater. Chem. 1998, 8, 2357-2369 and references therein.
(6) Quante, H.; Geerts, Y.; Mu¨llen, K. Chem. Mater. 1997, 9, 495-500 and
references therin.
(7) Quante, H.; Schlichting, P.; Ulrike, R.; Geerts, Y.; Mu¨llen, K. Macromol.
Chem. Phys. 1996, 197, 4029-4044.
(8) Zhao, Y.; Wasielewski, M. R. Tetrahedron Lett. 1999, 40, 7047-7050.
(9) Bo¨hm, A.; Arms, H.; Henning, G.; Blaschka, P. DE 19547210 (U.S. Patent
6184378), 2001.
(10) Gilat, S. L.; Adronov, A.; Frechet, J. M. J. J. Org. Chem. 1999, 64, 7474-
7484.
(11) Turro, N. J. Modern Molecular Photochemistry; University Science
Books: 1991; pp 76-148.
(12) Adronov, A.; Gilat, S. L.; Frechet, J. M. J.; Ohta, K.; Neuwahl, F. V. R.;
Fleming, G. R. J. Am. Chem. Soc. 2000, 122, 1175-1185.
(13) Gilat, S. L.; Adronov, A.; Frechet, J. M. J. Angew. Chem., Int. Ed. 1999,
38, 1422-1427.
(14) Adronov, A.; Frechet, J. M. J. Chem. Commun. 2000, 1701-1710.
(15) (a) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737-740. (b) For a review,
see: Kalyanasundaram, K.; Gra¨tzel, M. Coord. Chem. ReV. 1998, 77,
347-414.
JA027564I
9
J. AM. CHEM. SOC. VOL. 124, NO. 40, 2002 11849