Synthesis of C60-diphenylaminofluorene dyad with large 2PA cross-sections and efficient intramolecular two-photon energy transfer

Article abstract of DOI:10.1039/b202681c

The first, highly two-photon active C₆₀ derivative comprised of a A-sp³-D conjugate structure was synthesized showing effective two-photon absorption cross-sections (σ₂′ = 196×10^-48 cm⁴ sec^-1

Full text of DOI:10.1039/b202681c

Synthesis of C60-diphenylaminofluorene dyad with large 2PA
cross-sections and efficient intramolecular two-photon energy transfer
Long Y. Chiang,*a Prashant A. Padmawar,b Taizoon Canteenwala,b Loon-Seng Tan,*c Guang S. He,d
Ramamurthi Kannan,c Richard Vaia,c Tzu-Chau Lin,d Qingdong Zhengd and Paras N. Prasadd
a Department of Chemistry, Institute of Nanoscience and Engineering, University of Massachusetts Lowell,
Lowell, MA 01854, USA. E-mail: Long_Chiang@uml.edu
b Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
c
Polymer Branch, AFRL/MLBP, Air Force Research Laboratory, Wright-Patterson Air Force Base, Dayton,
OH 45433, USA
d Institute for Lasers, Photonics and Biophotonics, SUNY at Buffalo, Buffalo, NY 14260, USA
Received (in Cambridge, UK) 20th March 2002, Accepted 6th June 2002
First published as an Advance Article on the web 24th July 2002
The first, highly two-photon active C60 derivative comprised
of a A-sp3-D conjugate structure was synthesized showing
effective two-photon absorption cross-sections (s2A = 196
310248 cm4 sec21 molecule21) in the nanosecond regime
among the best values for diphenylaminofluorene-based
AFX chromophores.
fluorene 5 (Rf = 0.6, SiO2, hexane+toluene 3+2) as a greenish
brown solid in 52% yield (or 71% based on recovered C60). A
clear large down-field shift of the a-proton (H61) peak with
1
more than 1.2 ppm to d 5.70 was observed in the H NMR
spectrum (CDCl3, 400 MHz). It was also accompanied with
down-field shifts of all phenyl proton peaks at C5, C6, and C8 to
d 7.83 (d, J = 8.0 Hz), 8.48 (dd, J = 8.0 and 1.3 Hz), and 8.32
(d, J = 1.3 Hz), respectively. The corresponding shifts were
0.18, 0.53, and 0.39 ppm. Interestingly, the value of the d shift
is reverse proportional to the distance between the proton
affected and the fullerene cage with a larger value for H61 > H6
> H8 > H5. It can be correlated to de-shielding of these protons
by the fullerene ring currents indicating active interactions
occurring between C60 and the 2PA-active fluorene.
Single crystals of 5 were successfully obtained from CS2–
EtOH by a slow diffusion method in a H-tube. The X-ray
structural analysis of this crystal† gave verification of the
molecular structure and indicated a monoclinic unit cell with a
space group of P21/n and dimensions of a = 10.00 Å, b = 19.58
Å, c = 25.72 Å, a = 90°, b = 93.05°, and g = 90° with final
The concept of multi-dimensional conjugation and dendrimer
structure as an approach for enhancing the cross-sections of
2PA molecules was recently demonstrated.1–3 As the first step
toward new multidimensional conjugated 2PA chromophores,
we prepared, to our best knowledge, the first, highly two-photon
active C60 derivative possessing a one-dimensional A-sp3-D
conjugate system with coupling of the electron-accepting (A)
fullerene cage and the electron-donating (D) diphenylamino-
fluorene (DPAF).
Synthesis of C60-diethyl-2-diphenylaminofluorene dyad was
made using ketomethane as a linker analogous to the mono-
functionalized C60 malonate derivative forming a cyclopropane
moiety at the [6,6]-bond, as demonstrated by Bingel.4 The
resulting phenylketone structure may allow better carbonyl
interaction with p-orbitals of the C60 cage with a higher density
of p-electrons at a close vicinity than the corresponding
methano[60]fullerene malonate esters. As the phenyl ring is
being replaced by a larger conjugated chromophore, the partial
extended p-electron interaction through space with the C60 cage
may be enhanced despite a small steric gap of the sp3 carbon in
between. Accordingly, diethyl-2-diphenylaminofluorene 3 was
synthesized in a yield of 98% by the reaction of 2-bromo-
9,9-diethylfluorene 2 with diphenylamine (1.0 equiv.) under
catalytic condensation conditions in refluxing toluene in the
presence of tris(dibenzylideneacetone)dipalladium(0) catalyst
(0.25 mmol%), rac-BINAP ligand (0.75 mmol%), and sodium
tert-butoxide (1.5 equiv.).5 Compound 2 was prepared from
2-bromofluorene 1, iodoethane, and NaOH in toluene using
tetrabutylammonium bromide as a phase transfer catalyst
(Scheme 1).6 Friedel–Crafts condensation of 3 with a mixture of
bromoacetyl bromide and aluminium chloride in 1,2-dichloro-
ethane at 0–10 °C afforded 7-bromoacetyl-9,9-diethyl-2-diph-
enylaminofluorene 4 (Rf = 0.3, SiO2, hexane+toluene 3+2, m/z
510 as shown in the negative ion DCI-MS) as a yellow
crystalline solid in 66% yield. The chemical shift of the a-
residues of the refinement [I > 2s (I)] R1 = 0.052 and wR2
=
0.128. The unit cell packing structure is shown in Fig. 1. The
molecular packing of 5 is apparently dominated by the fullerene
moieties with a tightly ordered assembly in close contact,
indicating a strong hydrophobic interaction between C60 cages.
All fluorene moieties are scattered around fullerene surround-
ings showing no direct intermolecular flourene–fluorene-
overlapping. Accordingly, similar behavior of the molecular
assembly may be expected in solids or the solution in a high
1
proton (H61) was found to be d 4.49 in the H NMR spectrum
(CDCl3, 400 MHz). Assignment of the bromoacetyl group
attachment on the C7 atom of diphenylaminofluorene 4 was
made by the down-field shift of phenyl protons at C5, C6, and C8
to d 7.65 (d, J = 7.8 Hz), 7.95 (dd, J = 7.8 and 1.4 Hz), and 7.93
(d, J = 1.4 Hz), respectively. It was also confirmed by the
crystal structure of the final product 5. The reaction of C60 with
4 (1.0 equiv.) in toluene using DBU (1.0 equiv.) as a base at
ambient temperature for a period of 5 h, followed by partial
solvent removal and the subsequent work-up gave 7-(1,2-me-
thano[60]fullerene-61-carbonyl)-9,9-diethyl-2-diphenylamino-
Scheme 1 Reagents and conditions: i, Et-I, NaOH, TBA-Br, toluene, 60
3C, 8 h; ii, diphenylamine (1.0 equiv.), tris(dibenzylideneacetone)dipalla-
dium(0) (cat.), rac-BINAP, t-BuONa, toluene, 110 °C, 8 h; iii, bromoacetyl
bromide, AlCl3, CH2CH2, 0 °C to rt, 4 h; iv, C60, DBU, toluene, rt, 5 h.
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Fig. 1 Unit cell packing structure of 5 showing a well-organized assembly
of fullerene cages with scattered diethyldiphenylaminofluorene moieties.
Fig. 2 Relative 2PA-induced emission intensity of (a) AF-284 in THF (1.0
3 106 M), (b) AF-370 in THF (1.0 3 106 M), and (c) 5 in CS2 (1.0 3 104
concentration, which was used in the 2PA absorption measure-
ments. Solubility of DPAF moieties in CS2 and ODCB is high.
As the concentration of 5 in CS2 increases, we propose that
strong interaction and possible aggregation of fullerene cages
should lead to formation of nano-clusters with DPAF segments
dispersed in CS2.
M) upon photoexcitation at 800 nm using a pulsed tunable dye laser.
armed oxadiazole–triarylamine system1 similar to AF-350 and
AF-380. It should be noted that the effective 2PA cross-section
value measured for 5 may involve the contribution from
excited-state absorption of the C60 cage and a reverse saturable
absorption occurring in tandem with the 2PA process similar to
the proposed sequence of intersystem crossing and excited state
absorption processes in the blend of AF-380 and C60.8
Finally, only a low 2PA-induced emission intensity was
observed in 5 (CS2, 1.0 3 1024 M, Fig. 2c) upon pulse laser
excitation at 800 nm. Whereas a relatively high contrast in
intensity of the 2PA-induced emission can be detected at lmax
480 and 484 nm for the samples of AF-284 and AF-370,
respectively, as shown in Figs. 2a and 2b, even in a low
concentration of 1.0 3 1026 M. The phenomena were
interpreted as an effective intramolecular energy transfer, after
two-photon absorption, from the diethyldiphenylaminofluorene
moiety to the fullerene cage as evidenced by the complete
quenching of fluorescence normally observed for AFX chromo-
phores. A similar quenching effect due to the energy transfer
upon linear photoexcitation of chromophore-C60 dyads was also
reported, in addition to a possible electron transfer mecha-
nism.9
Effective 2PA cross-section values of 5, AF-350, AF-284,
and AF-370 in a solution concentration of 0.02 M were obtained
by measuring the nonlinear transmissivity of a given sample as
a function of the intensity of the input laser beam. The beam was
provided by a pulsed tunable dye laser working at ~ 800 nm
wavelength, ~ 8 ns pulse duration, and 10 Hz repetition rate. An
optical path-length of 1.0 cm was applied. The technical details
of 2PA measurements in the nanosecond regime were described
elsewhere.7 Molecular two-photon absorption cross-sections
(s2 in an unit of cm4 GW21 molecule21) were obtained from the
nonlinear absorption coefficient b of the sample. Interestingly,
the effective molecular 2PA cross-section (s2A = hus2) of 5 in
CS2 under a linearly polarized 800 nm pulsed beam was found
to be 196 310248 cm4 sec21 molecule21, which is among the
highest 2PA cross-section values obtained for many chromo-
phores of the AF series to date.
The comparison of optical properties of 5 with several good
2PA chromophores at 800 nm pumping was summarized in
Table 1. For the consistency in comparison of materials optical
properties, all samples were measured under similar conditions.
Apparently, 2PA characteristics of the linear A-sp3-D conjugate
5 in the nanosecond regime are nearly double in the cross-
sections of its parent linear analogue AF-370 and in the same
range as that of the quadrupolar analogue AF-284. The value is
only less than 30% lower than s2’ (278 310248 cm4 sec21
molecule21) of the 2PA benchmark for AFX series, namely, the
octupolar analogue AF-350. Since the number density is a
significant factor in enhancing the 2PA cross-sections, the
fullerene cage should serve well as a nucleus center of the
fluorene dye in increasing the dimensionality. Furthermore, this
result also suggests that C60 with multiple AFX branches may
lead to similar cooperative interactions reported for a three-
The work was partially supported by AOARD/Airforce
Office of Scientific Research.
Notes and references
† Crystal data of 5: C91H27NO, 1150.14 (M), monoclinic, P21/n, T = 295
(2) K, a = 10.00100 (10) Å, b = 19.5790 (2) Å, c = 25.7150 (3) Å, a =
90°, b = 93.0510 (10)°, g = 90°, V = 5028.11 (9) Å3, Z = 4, Rint = 0.0396,
abs. coefficient 0.089 mm21, R1 = 0.0524 and wR2 = 0.1279 for I > 2s
(I). CCDC 183726. See http://www.rsc.org/suppdata/cc/b2/b202681c/ for
electronic files in .cif or other electronic format.
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Table 1 2PA optical properties of chromophores at 800 nm pumping
s2/310220 s2A/310248
lmax/nmb cm4 GW21 cm4sec21
Chromophore
lmax/nma emission
molecule21c molecule21c
AF-350/THF
5/CS2
AF-284/THF
AF-370/THF
400
408
403
389
485
None
480
112
79
79
278
196
196
104
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484
42
a Linear absorption of the fluorene unit. b Under an excitation wavelength of
380 nm. c At a concentration of 0.02 M under a pumping wavelength of 800
nm, uncertainty of cross-section measurement: ±15%.
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