C O M M U N I C A T I O N S
causes a substantial decrease.7,16b The larger polarity of water could
provide for a similar charge transfer stabilization in the ground and
excited states. However, there is no theoretical examination of how
hydrogen bonding from the solvent influences the strength of the
donor groups at the termini of the π system and how these changes
influence δ. A more detailed structure/optical properties relationship
analysis therefore awaits theoretical examination. From a practical
perspective, the larger ηδ values of 2C and 3C, relative to that of
1C, indicate that maximizing the donor strength of the terminal
nitrogen atoms is not necessarily a useful design parameter for TPM
applications. We propose that weaker donors, such as triarylamine,
are a better choice, because the weaker ICT character of the excited
state leads to higher η. This search for an optimal balance is likely
to be applicable to other molecular systems. Despite the gap in
relating molecular structure to optical properties, 2C and 3C show
exceptionally large action cross sections in water.
Figure 1. Normalized absorption and PL spectra of 3N in toluene (a, a′)
and 3C in water (b, b′). PL spectra were collected by exciting at the λabs of
each sample. TPA spectra of 3N in toluene (c) and 3C in water (d) (GM )
10-50 cm4‚s‚photon-1).
determines the overall absorption band shape and ultimately λabs
.
A discussion of the relatively small perturbation of solvents on λabs
and λem is therefore not appropriate at this stage. In toluene, 1N,
2N, and 3N have high η values, in the range of 0.9.
Acknowledgment. The authors are grateful to Mitsubishi
Chemical Center for Advanced Materials (MC-CAM), the NIH
(GM62958-01), and the NSF (DMR-0097611) for financial support.
A comparison of the linear spectra of the C series in water with
those of the N series in toluene reveals the following trends (Table
1). The λabs values are nearly the same (1C/1N) or slightly blue-
shifted (2C/2N, 3C/3N). The emissions are red-shifted and broader,
with no vibronic structure (see Figure 1). Most importantly for the
design of efficient TPM fluorophores, the η values in water are
inversely proportional to the donor strength of the terminal groups
(1N ∼ 1C > 2N ∼ 2C > 3N ∼ 3C).13 A substantial drop in η is
observed for 1C (0.04), while the η values for 2C (0.42) and 3C
(0.52) remain high. We also note that, in a solvent of intermediate
polarity (DMSO), there are no measurable differences in the linear
spectroscopy of the N and C series (Supporting Information). The
charged groups therefore are not interacting with the chromo-
phores.14
Two-photon excitation spectra were measured using the two-
photon induced fluorescence technique4,14 using a femtosecond
pulsed laser source (see Supporting Information). The TPA
maximum (λTPA) and δ for 1N in toluene (725 nm and 1290 GM,
respectively) are similar to those previously reported for 4,7,12,-
15-tetra(4′-dihexylaminostyryl)-[2.2]paracyclophane (720 nm and
1410 GM).10 The λTPA values for 1N, 2N, and 3N follow the trend
of λabs. Compounds 2N and 3N have higher δ than 1N. Literature
precedent shows that arylamine donor groups in D-π-D structures
can provide for larger15 or similar6b cross sections relative to their
alkylamine counterparts. Higher δ values may be expected on the
basis of additional delocalization within the extended π electron
system.
Within the range of frequencies accessible with our instrumenta-
tion (620-900 nm), the TPA measurements in water show a
substantial decrease in δ, relative to those in toluene (Table 1),
with little change in the λTPA or the general band shape (Supporting
Information). Combining these results with the determination of η
obtained by linear spectroscopy methods provides for δη values
of (in GM) 294 and 359 for 2C and 3C, respectively, which are
the highest reported action cross sections for chromophores in water.
In summary, we provide a synthetic entry to water-soluble
paracyclophane-based TPA chromophores. These molecules have
allowed us to measure a substantial decrease in δ when the solvent
is changed from toluene to water. It has been predicted theoreti-
cally,16 by looking at the effect of external fields, that δ increases
to a maximum point, after which increasing the strength of the field
Supporting Information Available: Synthetic details and char-
acterization for all compounds, TPA measurements, and spectra. This
References
(1) Denk, W.; Strickler, J. H.; Webb, W. W. Science 1990, 248, 73.
(2) So, P. T. C.; Dong, C. Y.; Masters, B. R.; Berland, K. M. Annu. ReV.
Biomed. Eng. 2000, 02, 399.
(3) Zipfel, W. R.; Williams, R. M.; Webb, W. W. Nat. Biotechnol. 2003, 21,
1369.
(4) (a) Xu, C.; Williams, R. M.; Zipfel, W.; Webb, W. W. Bioimaging 1996,
4, 198. (b) Xu, C.; Webb, W. W. J. Opt. Soc. Am. B 1996, 13, 481. (c)
Margineanu, A.; Hofkens, J.; Cotlet, M.; Habuchi, S.; Stefan, A.; Qu, J.;
Kohl, C.; Mu¨llen, K.; Vercammen, J.; Engelborghs, Y.; Gensch, T.;
Schryver, F. C. D. J. Phys. Chem. B 2004, 108, 12242.
(5) Xu, C.; Zipfel, W.; Shear, J. B.; Williams, R. M.; Webb, W. W. Proc.
Natl. Acad. Sci. USA 1996, 93, 10763.
(6) (a) Albota, M.; Beljonne, D.; Bre´das, J.-L.; Ehrlich, J. E.; Fu, J.-Y.; Heikal,
A. A.; Hess, S. E.; Kogej, T.; Levin, M. D.; Marder, S. R.; McCord-
Maughon, D.; Perry, J. W.; Ro¨ckel, H.; Rumi, M.; Subramaniam, G.;
Webb, W. W.; Wu, X.-L.; Xu, C. Science 1998, 281, 1653. (b) Rumi,
M.; Ehrlich, J. E.; Heikal, A. A.; Perry, J. W.; Barlow, S.; Hu, Z.; McCord-
Maughon, D.; Parker, T. C.; Ro¨ckel, H.; Thayumanavan, S.; Marder, S.
R.; Beljonne, D.; Bre´das, J. -L. J. Am. Chem. Soc. 2000, 122, 9500.
(7) Zojer, E.; Beljonne, D.; Kogej, T.; Vogel, H.; Marder, S. R.; Perry, J.
W.; Bre´das, J.-L. J. Chem. Phys. 2002, 116, 3646.
(8) (a) Strehmel, B.; Sarker, A. M.; Malpert, J. H.; Strehmel, V.; Seifert, H.;
Neckers, D. C. J. Am. Chem. Soc. 1999, 121, 1226. (b) Jager, W. F.;
Volkers, A. A.; Neckers, D. C. Macromolecules 1995, 28, 8153.
(9) Schuddeboom, W.; Jonker, S. A.; Warman, J. M.; Leinhos, U.; Ku¨hnle,
W.; Zachariasse, K. A. J. Phys. Chem. 1992, 96, 10809.
(10) Bartholomew, G. P.; Rumi, M.; Pond, S. J. K.; Perry, J. W.; Tretiak, S.;
Bazan, G. C. J. Am. Chem. Soc. 2004, 126, 11529.
(11) Bartholomew, G. P.; Bazan, G. C. J. Am. Chem. Soc. 2002, 124, 5183.
(12) (a) Wang, S.; Bazan, G. C.; Tretiak, S.; Mukamel, S. J. Am. Chem. Soc.
2000, 122, 1289. (b) Bazan, G. C.; Oldham, W. J.; Lachicotte, R. J.;
Tretiak, S.; Chernyak, V.; Mukamel, S. J. Am. Chem. Soc. 1998, 120,
9188.
(13) The donor strength order is consistent with the oxidation potential measured
by cyclic voltammetry (CV): the measured electrochemical E(M+/M) is
-50 mV for 1N, 160 mV for 2N, and 210 mV for 3N relative to ferrocene.
CV was carried out in 0.1 M n-Bu4NPF6 in THF with a scan rate of 100
mV/s.
(14) Pond, S. J. K.; Tsutsumi, O.; Rumi, M.; Kwon, O.; Zojer, E.; Bre´das,
J.-L.; Marder, S. R.; Perry, J. W. J. Am. Chem. Soc. 2004, 126, 9291.
(15) Cho, B. R.; Son, K. H.; Lee, S. H.; Song, Y.-S.; Lee, Y.-K.; Jeon, S.-J.;
Choi, J. H.; Lee, H.; Cho, M. J. Am. Chem. Soc. 2001, 123, 10039.
(16) (a) Luo, Y.; Norman, P.; Macak, P.; A° gren, H. J. Phys. Chem. A 2000,
104, 4718. (b) Kogej, T.; Beljonne, D.; Meyers, F.; Perry, J. W.; Marder,
S. R.; Bre´das, J. L. Chem. Phys. Lett. 1998, 298, 1. (c) Zalesny, R.;
Bartkowiak, W.; Styrcz, S.; Leszczynski, J. J. Phys. Chem. A 2002, 106,
4032.
JA0440811
9
J. AM. CHEM. SOC. VOL. 127, NO. 3, 2005 821