Broadband two-photon absorption characteristics of highly photostable fluorenyl-dicyanoethylenylated [60]fullerene dyads

Article abstract of DOI:10.3390/molecules21050647

We synthesized four C₆₀-(light-harvesting antenna) dyads C₆₀ (>CPAF-C_n) (n = 4, 9, 12, or 18) 1-C_n for the investigation of their broadband nonlinear absorption effect. Since we have previously demonstrated thei

Full text of DOI:10.3390/molecules21050647

molecules
Article
Broadband Two-Photon Absorption Characteristics of
Highly Photostable Fluorenyl-Dicyanoethylenylated
[60]Fullerene Dyads
Seaho Jeon 1, Min Wang 1, Wei Ji 2,*, Loon-Seng Tan 3, Thomas Cooper 3 and Long Y. Chiang 1,
*
1
Department of Chemistry, Institute of Nanoscience and Engineering Technology,
University of Massachusetts Lowell, Lowell, MA 01854, USA; baschem@hotmail.com (S.J.);
wangmin81@gmail.com (M.W.)
Department of Physics, National University of Singapore, Singapore 117542, Singapore
Functional Materials Division, AFRL/RXA, Air Force Research Laboratory, Wright-Patterson Air Force Base,
Dayton, OH 45433, USA; loon.tan@us.af.mil (L.-S.T.); thomas.cooper.13@us.af.mil (T.C.)
Correspondence: phyjiwei@nus.edu.sg (W.J.); Long_Chiang@uml.edu (L.Y.C); Tel.: +65-6516-4234 (W.J.);
+1-978-934-3663 (L.Y.C); Fax: +65-6777-6126 (W.J.); +1-978-934-3013 (L.Y.C)
2
3
*
Academic Editor: Derek J. McPhee
Received: 27 March 2016; Accepted: 6 May 2016; Published: 14 May 2016
Abstract: We synthesized four C60-(light-harvesting antenna) dyads C60 (>CPAF-Cn) (n = 4, 9, 12,
or 18) 1-Cn for the investigation of their broadband nonlinear absorption effect. Since we have
previously demonstrated their high function as two-photon absorption (2PA) materials at 1000 nm,
a different 2PA wavelength of 780 nm was applied in the study. The combined data taken at two
different wavelength ranges substantiated the broadband characteristics of
1-Cn. We proposed that the
observed broadband absorptions may be attributed by a partial -conjugation between the C60 > cage
π
and CPAF-Cn moieties, via endinitrile tautomeric resonance, giving a resonance state with enhanced
molecular conjugation. This transient state could increase its 2PA and excited-state absorption at
800 nm. In addition, a trend of concentration-dependent 2PA cross-section (σ2 ) and excited-state
absorption magnitude was detected showing a higher
σ value at a lower concentration that was
correlated to increasing molecular separation with less aggregation for dyads C60(>CPAF-C18) and
C60(>CPAF-C9), as better 2PA and excited-state absorbers.
Keywords: C60-(light-harvesting antenna) nanostructures; Z-scan measurements; ultrafast
two-photon absorption; nonlinear absorption
1. Introduction
Nonlinear optical materials have numerous applications, including photodynamic therapy,
nonlinear photonics, 3D optical data storage, frequency upconverted lasing, and fluorescence
imaging [
nonlinear absorbers [11
1
–
10]. These materials require large two-photon absorption (2PA) cross-section (σ2) of
13]. Carbon-based materials, e.g., fullerenes, nanocarbons (NC), and
–
carbon nanotubes (CNT), are suitable as the base substrate for fabricating potential 2PA absorbers.
Among them, unlike NC and CNT, fullerenes are highly versatile toward various chemical
functionalization reactions with good efficiency to form soluble derivatives that facilitate the material
engineering processing and coating fabrication.
We have recently reported dual NIR nonlinear optical absorption activities of branched
triad C60(>DPAF-C18)(>CPAF-C2M) and tetrad C60(>DPAF-C18)(>CPAF-C2M)2 nanostructures, each
consisting of hybrid light-harvesting antenna DPAF-Cn and CPAF-Cn moieties [14]. These two types
of chromophores were designed to provide linear optical absorption [or one-photon absorption
(1PA)] at 400 and 500 nm, respectively, which corresponds to 2PA excitation at 800 and 1000 nm,
Molecules 2016, 21, 647; doi:10.3390/molecules21050647
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Molecules 2016, 21, 647
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respectively. By increasing the number of CPAF-Cn to two in the corresponding tetrad, the sum of
extinction coefficients of overlapped DPAF–CPAF absorption bands at 400–550 nm led to a spectrum
profile with a nearly flat band over this wavelength range indicating broadband characteristics.
Ultrafast femtosecond (fs) 2PA cross-section values of 266 and 995–1100 GM (125 fs, at 5.0
in toluene) were reported for the hybrid tetrad at 780- and 980-nm irradiation, respectively [15].
The σ2 value was higher than that (30 GM in CS2, 1.0
10´2 M) of the dyad C60(>DPAF-C9) at
ˆ
10´3 M
ˆ
780-nm excitation. The enhancement was correlated to efficient intramolecular Förster resonance
energy-transfer events going from a high-energy DPAF-Cn antenna unit to low-energy CPAF-Cn
antenna units occurring in a cascade fashion at the C60 > cage surface.
Further investigation on the molecular structure of 7-(1,2-dihydro-1,2-methano [60]fullerene-61-
{1,1-dicyanoethylenyl})-9,9-dialkyl-2-diphenylaminofluorene C60 (>CPAF-Cn)
1-Cn led us to propose
that a partial -conjugation between the C60> cage and CPAF-Cn moieties, via endinitrile tautomeric
π
resonances or isomerization (Figure 1), may merge the 2PA wavelength of fullerene cage at 600–700 nm
together with that of CPAF-Cn (900–1100 nm) at the photoexcited state. This may simulate the 2PA
of DPAF-Cn at 700–850 nm without the attachment of this type of antenna on the fullerene cage of
1.
Accordingly, we performed several femtosecond (fs) Z-scan measurements on four derivatives of -Cn
1
under the 2PA excitation at 780 nm in this study to verify the hypothesis, as a part of our effort to
construct new nonlinear absorbing materials with broadband characteristics.
Figure 1.
C60-CPAF conjugate compound
C61, leading to the formation of a fully-conjugated form of C60> acceptor (
(
a
) Endinitrile tautomeric resonances at the bridging C61Hα-C[=C(CN)2] structure of a
-C4 and ( ) fullerenyl seven-membered ring expansion involving
) and CPAF donor ( ), as
conjugation length and absorption wavelengths,
1
b
A
D
marked in purple. This resulted in an extended
marked in burgundy red in color.
A–D
2. Results and Discussion
2.1. Materials Characterization
We have demonstrated the use of dialkyldiphenylaminofluorenyl-keto-[60]fullerene
C60(>DPAF-Cn) dyads -Cn [16], branched triads C60(>DPAF-Cn)x (x = 2) [17], and the related starburst
pentads (x = 4) [ ] for the study of simultaneous 2PA phenomena under the photoexcitation of a 780-nm
2
8
laser light in the fs region. In these compounds, DPAF-Cn was used as the light-harvesting antenna
chromophore to compensate for the low optical absorption of the fullerene cage at wavelengths
beyond 350 nm. It is also functioning as an electron donor to provide one electron capable of being
transferred intramolecularly to the C60> cage moiety upon 2PA activation that increased largely
the excited state absorptions leading to 2PA cross-section enhancement. As a general strategy
to extend the active 2PA wavelength to the longer NIR region, we modified the bridging keto
group in dyads
structure of C60(>CPAF-Cn)
electronic polarization and bathochromic shift in the optical absorption from
2
-Cn by an electron-withdrawing 1,1-dicyanoethylenyl (DCE) unit, leading to the
-Cn. The functional change resulted in the increase of molecular
400 nm for DPAF to
1
λ
max
500–550 nm for CPAF. One example was given by C60(>CPAF-C2M) dyad [18].
In the case of C60(>DPAF-C9)x (x = 1, 2, and 4), their 2PA σ2 values were found to be
concentration-dependent [
] with a higher magnitude at a lower concentration than 10´3 M.
This revealed that a minimization of the molecular aggregation should be advantageous to prevent
8

Molecules 2016, 21, 647
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the loss of σ2 magnitude, especially at a high concentration for practical use. It is crucial since a
sufficiently high 2PA material concentration may be required for the fabrication of NLO devices
to bring in significant effects. Our logic approach is to modulate the compound’s solubility by
the variation of attached alkyl chain length and shape (linearly or sterically hindered branched
structures) and to control the effective average separation distance among C60> cages when applied
in highly concentrated solutions or solid films. Accordingly, we synthesized four samples, namely,
C60(>CPAF-Cn) 1-Cn (n = 4, 9, 12, and 18, Scheme 1) for the evaluation of their alkyl chain-dependent
broadband 2PA characteristics. Since the 2PA activity of C60(>CPAF-Cn) analogous moiety by the
980-nm excitation in toluene was reported recently using examples of C60(>CPAF-C9) and hybrid
C60(>DPAF-C18) (>CPAF-C2M)n (n = 1 or 2) [15], we investigated the σ2 value and the corresponding
nonlinear absorption efficiency of the compound 1-Cn under the excitation wavelength of 780 nm to
substantiate their broadband two-photon absorbing properties.
CN
NC
N
1
-C4, C60(>CPAF-C4)
O
O
i
N
N
Br
CN
NC
N
3
-C18, BrDPAF-C18
2
-C18, C60(>DPAF-C18
)
1
-C9, C60(>CPAF-C9)
ii
CN
NC
ii
O
N
N
CN
NC
CN
5
, CPAF-C12
4
, DPAF-C12
NC
N
N
1
-C12, C60(>CPAF-C12
)
1
-C18, C60(>CPAF-C18
)
Scheme 1. Synthesis of C60(>CPAF-Cn)
1-Cn (n = 4, 9, 12, or 18) dyads. Reagents and conditions: i. C60,
DBU, toluene, rt, 5.0 h; ii, malononitrile, pyridine, TiCl4, rt, 5.0 min.
Synthesis of the compound C60(>CPAF-C9)
A similar synthetic sequence was applied for the preparation of C60(>CPAF-C4)
-C12, and C60(>CPAF-C18) -C18. Formation of a fullerenyl monoadduct
detection of its infrared spectrum displaying three typical fullerenyl signals at 753, 697, and 527 cm
corresponding to absorptions of an unfunctionalized half-cage sphere of C60>. The key chemical
modification of a keto group of C60(>DPAF-C18) -C18 to a 1,1-dicyanoethylene (DCE) group of -C18
was made by using malononitrile as a reagent. Indication of the CPAF moiety attached on a C60> cage
was seen clearly by a strong IR absorption band corresponding to cyano (–C N) stretching vibrations
centered at 2222–2224 cm with complete disappearance of the carbonyl stretching vibration of -C18
at 1680 cm´1. It was also substantiated by its 13C-NMR spectrum (Figure 2) giving chemical shifts of
three types of functional carbons, – (CN)2, in the 1,1-dicyanoethylenyl moiety
=C(CN)2, –C”N, and =
of -C18 at 169.13, 113.84, and 88.22, respectively, confirming the successful conversion reaction.
In Figure 2, it also showed chemical shifts of three aminoaryl carbons in CPAF-C18 moiety at 153.91,
1
-C9 followed the procedure described previously [18].
-C4, C60(>CPAF-C12)
-Cn was evident by
1
1
1
1
´1
2
1
”
´1
2
C
C
1
δ
δ

Molecules 2016, 21, 647
4 of 13
152.18, and 149.82 along with all fullerenyl sp2 carbon peaks located within
δ
134–148, whereas two sp3
C60> carbon (CF1 and CF2) peaks were assigned at δ 73.09. Direct confirmation of the molecular mass of
1
-C18 was made by a group of sharp molecular mass ions with the maximum mass intensity centered
at m/z 1645 (M+) and 1646 (MH+) in its MALDI-TOF mass spectrum (Figure 3). This was followed by
several groups of ion peaks at m/z 1465–1550 with the group mass each separated by a –CH2– unit
(m/z 14) indicating the consecutive loss of alkyl chain carbons from the M+ peak. Full elimination of
weaker aliphatic bonds of 1-C18 led to a stable aromatic mass ion fragment at m/z 1079, matching with
+
+
the structure assigned in the Figure. Further fragmentation gave stable C60 (m/z 720) and C60H2
(m/z 722) mass ion fragments.
Figure 2. 13C-NMR spectrum of C60(>CPAF-C18) showing all sp2 ( 134–148) and two sp3 (CF1 and CF2
δ )
C60 cage carbons and 1,1-dicyanoethylenyl (DCE) carbons, as assigned.
Figure 3. Matrix-assisted laser desorption ionization mass spectrum (MALDI-MS) of C60(>CPAF-C18
)
1
-C18 using
α-cyano-4-hydroxy-cinnamic acid as the matrix material, showing the molecular ion mass
M+ (or MH+).
Similar to that of
long-wavelength absorption band of
or 503 nm ( = 2.9 cm, CHCl3, 2.0
104 L/mol
C60(>DPAF-C18) ( -C18) centered at max 410 nm, as shown in Figure 4A-a. This band was accompanied
with two other absorption bands with centered at 260 ( = 1.7 = 8.2
105) and 327 ( 104)
cm) in toluene, attributed to absorptions of C60 > cage.
1
-C9 [18], the keto modification of
-C18 to max 468 (
10´5 M) in nearly 58–93 nm longer than that of
2
-C18 led to a large bathochromic shift of the
1
λ
ε
= 4.2 cm, toluene, 1.0
ˆ
104 L/mol 10´5 M)
¨
ˆ
ε
ˆ
¨
ˆ
2
λ
λ
ε
ˆ
ε
ˆ
max
in CHCl3 or 326 (
ε
= 1.5
ˆ
105 L/mol
¨

Molecules 2016, 21, 647
5 of 13
By comparing with the
λ
max
value of CPAF-C12 5
[
19] (Figure 4A-c) antenna alone at 437 nm, a longer
absorption wavelength
λmax for all 1-C4 (Figure 4A-e), 1-C9 (Figure 4A-b), 1-C12 (Figure 4A-d), and
1
-C18 (Figure 4A-a) giving dark burgundy-red in color, clearly revealing a partial conjugation between
the CPAF moiety and a C60> cage, matching with our proposed endinitrile tautomeric resonance
isomerization described in Figure 1. In addition, pronounced solvent-dependent optical absorption
was detected that resulted in a longer wavelength in polar (CHCl3) than in non-polar (toluene) solvent.
Similar solvent polarity-dependent photophysical properties were also observed for the analogous
compound C60(>CPAF-C2M) exhibiting nanosecond transient intramolecular electron-transfer activity
from the CPAF light-harvesting antenna to the C60> acceptor moiety in polar solvents (e.g., PhCN)
upon photoexcitation while, in non-polar solvents such as toluene, intramolecular energy-transfer
activity is the major event [19]. In the latter case, initial photoactivation at either C60> (300–350 nm
for linear absorption or 600–700 nm for 2PA processes) to 1(C60>)* or CPAF-Cn (450–550 nm for 1PA
1
1
or 900–1100 nm for 2PA) to (CPAF)*-Cn is followed by formation of the singlet C60*(>CPAF-Cn)
transient state in an ultrafast rate that was capable of crossing over to the triplet 3C60*(>CPAF-Cn)
transient state, via intersystem-crossing (ISC) in a time period of roughly 1.4 nanoseconds (ns) [17].
The triplet lifetime was 34–39 microseconds (µs) [18]. Therefore, in this study, using a pulse laser light
operating at 226-fs for 2PA measurements carried out in THF or toluene, the singlet transient states
1C60*(>CPAF-Cn) (n = 4, 9, 12, or 18) should be the main targets for the consideration of excited states
and reverse saturable absorptions (RSA) [20] in correlation to the nonlinear absorption effect.
4
3
2
1
100
80
60
40
20
0
(a)
(a) C60 (>CPAF-C18
(b) C60 (>CPAF-C9)
(c) CPAF-C12
)
)
(f)
~92.4% at 780 nm
(a)
(c)
(d) C60 (>CPAF-C12
(e) C60 (>CPAF-C4)
(a) CPAF-C12
(b) C60(>CPAF-C4)
(c) C60(>CPAF-C9)
(d) C60(>CPAF-C12
(e) C60(>CPAF-C18
(f) THF
)
)
(b)
(c)
(d)
(e)
(A)
(B)
(b), (d), (e)
400
0
300
500
600
700
800
300
400
500
600
700
800
900
Wavelength (nm)
Wavelength (nm)
Figure 4. UV-vis (
) transmittance (THF, 2.0
the reference.
A
) absorption (toluene, 1.0
ˆ
10´5 M, normalized at
-C4, -C9, -C12, and 1-C18 with THF as
λ
max
486 nm) and
(
B
ˆ
10´5 M) spectra of CPAF-C12
,
1
1
1
2.2. Nonlinear Z-Scan Measurements
The open-aperture Z-scans of four C60(>CPAF-Cn) samples were carried out in THF with
femtosecond laser pulses. Z-scan measurements carried out at an ultrafast time scale of
226 femtoseconds should be able to reduce potential accumulative thermal scattering effects, normally
occurring at picosecond regions, at the wavelength of either 780 or 1000 nm. The transmittance of all
compounds studied were collected, as shown in Figure 4B, indicating a consistent level of ~92.4% at
780 nm. The data reported in Figure 5a,b were normalized to the linear transmittance for all Z-scans
by the correction of the background transmittance, T(|Z| >> Zo). The normalized transmittance ∆T(Z)
was expressed as T(Z)/T(|Z| >> Zo). Accordingly, the change in the normalized transmittance is
indicative of the nonlinear (or light-dependent) part in the compound’s absorption. Total absorption
was described by the change in the absorption coefficient ∆α
= βI, where β and I are the 2PA coefficient
and the light intensity, respectively. The absorption coefficient can be extracted from the best fitting
between the Z-scan theory [21] and the data. The 2PA cross-section value was then calculated from

Molecules 2016, 21, 647
6 of 13
the coefficient by the formula σ2
=
βh¯ ω/N, where h¯ ω is the photon energy and N is the number of
the molecules.
(a)
1.00
1.00
(b)
0.95
0.90
0.98
70 GWcm-2
150 GWcm-2
C60 (>CPAF-C4)
0.85
C60 (>CPAF-C12
)
240 GWcm-2
287 GWcm-2
421 GWcm-2
I0 = 220 GWcm-2
CPAF-C12
[c] = 2 x 10-3
M
0.80
[c] = 5 x 10-4
M
0.96
C60 (>CPAF-C9)
C60 (>CPAF-C18
λ
= 780 nm
)
λ
= 780 nm
C60(>CPAF-C18)
0.75
-6
-4
-2
0
2
4
6
-6
-4
-2
0
2
4
6
Z (mm)
Z (mm)
Figure 5. Open-aperture Z-scan curves of (a) CPAF-C12 and four C60(>CPAF-Cn) monoadducts taken
at 220 GWcm´2 in THF and (b) C60(>CPAF-C18) taken at different pulse laser intensities indicated.
Open-aperture Z-scans carried out under the irradiance of 220 GW/cm2 at 780 nm were taken
on the samples of
5
,
1
-C4,
1
-C9,
1
-C12, and
1
-C18 in THF at the concentration of 5.0
ˆ
10´4
M
with the profile plots shown in Figure 5a. These Z-scans displayed positive signs for absorptive
nonlinearities with the decrease of light-transmittance in the order of C60(>CPAF-C18) < C60(>CPAF-C9)
ď
C60(>CPAF-C12) < C60(>CPAF-C4) in solution. As a result, the 2PA cross-section values of these
compounds measured were summarized in Table 1.
Table 1. Two-photon absorption cross sections (σ2) and excited state absorption (ESA) cross-sections
(
σESA) of CPAF-C12 and C60(>CPAF-Cn) measured using laser pulses working at 780 nm with a 226-fs
duration and a repetition rate of 1.0 kHz. The light intensity I was 220 GW/cm2.
σESA/10´78 cm6¨ s2¨
Sample
[C]/M
β/cmGW´1
σ /10´48 cm4¨ s¨ photon´1¨ molecule´1
2
photon´2¨ molecule´1
5.0 ˆ 10´4
1.0 ˆ 10´3
2.0 ˆ 10´3
1.0 ˆ 10´2
0.0026
0.0048
0.0070
0.0105
2.20 (220 GM)
2.03 (203 GM)
1.48 (148 GM)
0.44 (44 GM)
CPAF-C12
6.4
1.5
5.0 ˆ 10´4
1.0 ˆ 10´3
2.0 ˆ 10´3
1.0 ˆ 10´2
0.0015
0.0027
0.0045
0.0102
1.28 (128 GM)
1.12 (112 GM)
0.95 (95 GM)
0.43 (43 GM)
C60(>CPAF-C4)
C60(>CPAF-C9)
C60(>CPAF-C12
C60(>CPAF-C18
5.9
5.0 ˆ 10´4
1.0 ˆ 10´3
2.0 ˆ 10´3
1.0 ˆ 10´2
0.0039
0.0065
0.0080
0.0110
3.25 (325 GM)
2.75 (275 GM)
1.69 (169 GM)
0.46 (46 GM)
10.2
5.4
5.0 ˆ 10´4
1.0 ˆ 10´3
2.0 ˆ 10´3
1.0 ˆ 10´2
0.0020
0.0040
0.0065
0.0124
1.65 (165 GM)
1.71 (171 GM)
1.39 (139 GM)
0.535 (54 GM)
)
)
9.1
4.4
5.0 ˆ 10´4
1.0 ˆ 10´3
2.0 ˆ 10´3
1.0 ˆ 10´2
0.0077
0.0094
0.0105
0.0139
6.42 (642 GM)
3.97 (397 GM)
2.14 (214 GM)
0.59 (59 GM)
30.1
24.7
11.3
5.1

Molecules 2016, 21, 647
7 of 13
It ´is1interesting to observe a higher 2PA absorption cross-section value of 6.42
ˆ
10´48 cm4
10´4 M than that,
¨
s
¨
photon
3.25 10
A lower value of 1.28
¨
molecule´1 (or 642 GM) for C60(>CPAF-C18) at a low concentration of 5.0
ˆ
´48
´1
´1
ˆ
cm4
¨
s¨
photon
ˆ
¨
molecule (or 325 GM), for C60(>CPAF-C9) at the same concentration.
photon´1 molecule´1 (or 128 GM) for C60(>CPAF-C4) than the
10´48 cm4
¨
s
¨
¨
un-fullerenized CPAF-C12 (220 GM) was detected, perhaps owing to its higher particle aggregation
tendency even at 10´4 M. Based on a 226-fs pulse duration is slightly longer than 130 fs required for
the intramolecular energy-transfer from the photoexcited 1(CPAF)*-Cn antenna moiety to the C60>
cage of C60(>CPAF-Cn). Completion of this energy-transfer event at the early time scale leads to the
formation of excited 1C60*(>CPAF-Cn) state. Therefore, the measured σ2 values at 226-fs should cover
partly two-photon absorptions of both CPAF-Cn and C60> moieties in the fs region and the excited
singlet state absorption (S1–Sn) of 1(C60>)* cage moiety in subsequent subpicoseconds. The initial
2PA excitation process at 780 nm represents mainly the contribution of CPAF-Cn moiety forming the
transient C60(>1CPAF*-Cn) state. The argument is valid due to the fact of low linear and nonlinear C60>
cage absorption at this wavelength as compared with the later of CPAF-Cn moiety. In addition, the
occurrence of transient conversion from 1C60*(>DPAF-C9) state to the corresponding 3C60*(>DPAF-C9)
state via inter-system crossing was reported to be effective at a much longer time scale of ~1.4 ns [17].
Therefore, the absorption contribution of 3C60*(>CPAF-C9) state can be excluded in this measurement.
These nonlinear fs absorptions may be correlated to the following nonlinear absorption measurements.
We also investigated the intensity-dependent (70–420 GWcm´2) Z-scans using the compound
1
-C18 as an example in THF at a concentration of 2.0
ˆ
10´3 M by 780-nm excitation. The resulting data
profiles were displayed in Figure 5b with the corresponding 2PA cross-sections plotted in Figure 6a
(red triangle). At this concentration, the σ2 values were h´ig2her, in general, and increased more
rapidly than those taken at a higher concentration of 1.0
ˆ
10 M (Figure 6a, blue circle) at the same
laser intensity. This trend of concentration-dependent σ2 values having a higher quantity at a lower
concentration consistent with that reported recently [ ]. The intensity dependence on σ2 values may
8
also reveal higher order absorptions, such as excited state absorption (ESA) that can be effectively
treated as three-photon absorption (3PA) for ESA, possibly taken place. In order to distinguish the
contribution of 2PA from the higher order nonlinear absorption, the ln(1–T) vs. intensity (I) relationship
was plotted, as shown in Figure 6b,c. These Z-scan curves were fitted with 2PA when the slope
is ~1.0 and fitted with ESA/3PA when the slope is ~2.0 [22]. The fitting results confirmed that, at
a low laser intensity of 74 GWcm´2 (Figure 6b), the event of 2PA process dominates, while at a
high intensity of 400 GWcm´2 (Figure 6c), the photophysical processes of ESA/3PA became the
major occurrence. Accordingly, the ESA cross-sections (σESA) of 5, 1-C4, 1-C9, 1-C12, and 1-C18 were
determined and given in Table 1. They showed the similar trend of concentration dependence in
magnitude to those of σ2 (Figure 7a) having a higher value at a lower concentration. The trend was
also coupled with their solubility where
1-C18 with two linear octadecyl chains and 1-C9 with two
branched 3,5,5-trimethylhexyl chains exhibit better solubility in solvents and a higher magnitude of σ2
.
We have examined several solvents including CS2, THF, and toluene using C60(>CPAF-C18) as the
example under 780-nm excitation and found no significant difference in the value of σ2 indicating no
solvent effect for the case.
Nonlinear absorption properties of C60(>CPAF-Cn) were investigated by irradiance-dependent
transmission measurements at the wavelength of 780 nm using the same setup as those applied in
2PA cross-section measurements conducted by fs-laser pulses. Nonlinear absorption of CPAF-C12,
C60(>CPAF-C4), C60(>CPAF-C9), C60(>CPAF-C12), and C60(>CPAF-C18) in THF measured as a function
of irradiance with 226-fs laser pulses operated at 780 nm were illustrated in Figure 7b. All the samples
showed a linear transmission (T = ~90%) with input intensity of up to 30 GW/cm2. When the incident
intensity was increased above 70 GW/cm2, the transmittance (%) began to deviate from the linear
transmission line and decrease indicating the initiation of nonlinear absorption. A systematic trend
showing higher nonlinear absorption efficiency down to 50%, 57%, 60%, and 65% for the dyads
1
-C18, 1-C9, 1-C12, and 1-C4, respectively, was observed with the increase of irradiance intensity up to

Molecules 2016, 21, 647
8 of 13
600 GW/cm2 (Figure 7b). Improvement in lowering the transmittance can be correlated to the higher
solubility of the dyads, consistent with the positive contribution of C60(>CPAF-C18) and C60(>CPAF-C9)
to a larger transient absorptions, concluded by Z-scans in Figure 5a.
10
-2
(a)
(b)
-3
-4
-5
Slope = 1.00
I0 = 74 GWcm-2
[c] = 2 x 10-3 M
λ
= 780 nm
-6
-2
3.2
3.4
3.6
3.8
4.0
4.2
4.4
1
(c)
-3
-4
-5
-6
[c][c
=
]=
2
2
.0x1
×
01-30‒3
M
M
M
C
C6060(>CPAF-C18) in THF
[c] = 1.0-2×10‒2
[c] = 2 x 10-3
M
[c]=10
M
Slope = 2.01
I0 = 400 GWcm-2
λ=780 nm
λ
= 780 nm
λ
= 780 nm
0.1
4.0
4.3
4.5
4.8
5.0
5.3
75
150
225
300 375 450
Intensity (GWcm-2)
In(I)
Figure 6.
pulse laser intensities; (
intensities Io at the focal point.
(
a
) Effective two photon absorption cross sections of C60(>CPAF-C18) plotted as a function of
b
) and ( ) show the plot of ln(1-T) vs. I for the same compound with different
c
Figure 7.
-C12, and
operated at 780 nm.
(
1
a
) Two-photon absorption cross-sections and (
b) nonlinear absorption of 5, 1-C4, 1-C9,
1
-C18 in THF (2.0
ˆ
10´3 M) measured as a function of irradiance with 226-fs laser pulses
3. Experimental Section
3.1. Materials
Reagents and solvent of n-butanol, 3,5,5-trimethylhexanol, n-dodecanol, n-octadecanol, methanesulfonyl
chloride, triethylamine, 2-bromofluorene, malononitrile, rac-2,21- bis(diphenylphosphino)-1,11-
binaphthyl (rac-BINAP), tris(dibenzylideneacetone)dipalladium(0) [Pd2(dba)3(0)], aniline, and
dichloroethane were purchased from Aldrich Chemicals (St. Louis, MO, USA) and used without
further purification. C60 (99.5%) was purchased from NeoTech Product Co. (Moscow, Russia) and
used as received. All other chemicals were purchased from Acros Ltd. (New Brunswick, NJ, USA).
The anhydrous grade solvent of TH´F1was refluxed over sodium and benzophenone overnight and
distilled under reduced pressure (10 mmHg).

Molecules 2016, 21, 647
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3.2. Spectroscopic Measurements
1H-NMR and 13C-NMR spectra were recorded on either a Bruker Avance Spectrospin–200
or Bruker AC-300 spectrometer (Bruker, Billerica, MA, USA). UV-vis spectra were recorded on a
Hitachi U-3410 UV spectrometer (Hitachi, Chiyoda, Tokyo, Japan). Infrared spectra were recorded
as KBr pellets on a Nicolet 750 series FT-IR spectrometer (Thermo Scientific Nicolet, Waltham, MA,
USA). Mass spectroscopic measurements were performed by the use of positive ion matrix-assisted
laser desorption ionization (MALDI-TOF) technique on a micromass M@LDI-LR mass spectrometer
(Micromass, Cary, NC, USA). The matrix of 3,5-dimethoxy-4-hydroxycinnamic acid (sinapic acid)
was used.
3.3. Synthetic Procedures
Synthesis of 7-(1,2-dihydro-1,2-methanofullerene[60]-61-{1,1-dicyanoethylene})-9,9-di(3,5,5- trimethylhexyl)-
2-diphenylaminofluorene C60(>CPAF-C9), 1-C9. Similar procedures as those reported [18] were used.
Synthesis of 7-α-bromoacetyl-9,9-dioctadecanyl-2-diphenylaminofluorene BrDPAF-C18 (3-C18). To a
˝
suspension of aluminum chloride (1.30 g, 9.66 mmol) in 1,2-dichloroethane (50 mL) at 0 C was added
a solution of 9,9-dioctadecyl-2-diphenylaminofluorene [23] (2.4 g, 2.9 mmol) in 1,2-dichloroethane
(30 mL). It was then added by bromoacetyl bromide (0.56 g, 2.79 mmol) over 10 min. At the end of
addition, the mixture was warmed to ambient temperature and stirred for an additional 15.0 h.
The solution was diluted by a slow addition of water (100 mL) while maintaining the reaction
mixture temperature below 45 ˝C. The resulting organic layer was washed subsequently with dilute
hydrochloric acid (1.0 N, 50 mL) and water (2
sulfate and concentrated in vacuo. The crude yellow oil was purified by column chromatography (SiO2,
hexane-EtOAc, 9:1) to afford 7- -bromoacetyl-9,9-di-(n-octadecyl)- 2-diphenylaminofluorene -C18
(1.8 g, 78%) with its chromatographic band corresponding to Rf = 0.6 on TLC (SiO2, hexane-EtOAc, 9:1
ˆ
50 mL), then, the solution was dried over magnesium
α
3
as the eluent); FT-IR (KBr) νmax 3063 (w), 3034 (w), 2923 (s), 2852 (s), 1677 (m), 1595 (m), 1493 (m), 1466 (w),
1346 (w), 1279 (m), 1182 (w), 1027 (w), 819 (w), 753 (w), 697 (m), 620 (w), and 508 (w) cm´1; UV-vis
1
(CHCl3, 1.0
CDCl3, ppm)
1H), 7.27–7.23 (m, 4H), 7.14–7.12 (m, 5H), 7.05–7.02 (m, 3H), 4.49 (s, 2H), 1.97–1.81 (m, 4H), 1.25–1.04
(m, 66H), 0.87 (t, J = 6.78 Hz, 6H), and 0.72–0.55 (br, 4H); 13C-NMR (126 MHz, CDCl3)
190.99, 153.63,
ˆ
10´5 M)
δ
λ
(
ε
) 292 (1.9
ˆ
104) and 407 (2.5
ˆ
104 L/mol
¨
cm); H-NMR (500 MHz,
max
7.95 (d, J = 8.18 Hz, 1H), 7.93 (s, 1H), 7.64 (d, J = 7.91 Hz, 1H), 7.59 (d, J = 8.23 Hz,
δ
151.06, 148.81, 147.61, 146.89, 133.96, 131.55, 129.25, 128.80, 124.36, 123.09, 122.78, 121.61, 118.82, 118.20,
55.23, 39.96, 31.90, 31.15, 29.90, 29.67, 29.64, 29.62, 29.57, 29.55, 29.34, 29.29, 23.83, 22.67, and 14.10.
Synthesis of 7-(1,2-dihydro-1,2-methanofullerene[60]-61-carbonyl)-9,9-dioctadecanyl-2-diphenyl-
aminofluorene, C60(>DPAF-C18),
2-C18. To a mixture of C60 (0.75 g, 1.1 mmol) and 7-
α-bromoacetyl-
9,9-dioctadecanyl-2-diphenylaminofluorene
3
-C18 (0.85 g, 1.1 mmol) in dry toluene (500 mL) was
added DBU (0.18 mL, 1.2 mmol) under a nitrogen atmosphere. After stirring at room temperature
for 5.0 h, suspended solids of the reaction mixture were filtered off and the filtrate was concentrated
to a volume of 10%. Crude products were precipitated by the addition of methanol and isolated by
centrifugation (8000 rpm, 20 min). The isolated solid was a mixture of the monoadduct 2-C18 and its
bisadduct. Separation of these two products were done by column chromatography (silica gel) using a
solvent mixture of hexane-toluene (3:2) as the eluent. The first chromatographic band corresponding
to Rf = 0.7 on TLC (SiO2, hexane-toluene, 3:1) afforded C60(>DPAF-C18)
2-C18, as brown solids (1.12 g,
65% based on recovered C60); FT-IR (KBr) 3440 (m), 2920 (s), 2849 (s), 1674 (m), 1632 (m), 1593 (s),
ν
max
1491 (m), 1463 (m), 1427 (m), 1346 (w), 1331 (w), 1316 (w), 1273 (m), 1239 (w), 1200 (m), 1186 (w),
1157 (w), 1028 (w), 817 (w), 752 (m), 738 (w), 696 (m), 575 (w), 547 (w), 526 (m), and 490 (m) cm´1
UV-vis (CHCl3, 1.0 ) 260 (1.3 cm);
10´5 M) 105), 325 (4.7 104), and 411 (3.6 104 L/mol
1H-NMR (500 MHz, CDCl3, ppm)
8.43 (d, J = 6.9 Hz, 1H), 8.32 (s, 1H), 7.78 (d, J = 8.0 Hz, 1H), 7.61
(d, J = 8.0 Hz, 1H), 7.25–7.22 (m, 4H), 7.11–7.09 (m, 5H), 7.03–7.00 (m, 3H), 5.66 (s, 1H), 2.03–1.84 (m,
;
ˆ
λ
(
δ
ε
ˆ
ˆ
ˆ
¨
max

Molecules 2016, 21, 647
10 of 13
4H), 1.29–1.04 (m, 58H), 0.87 (t, J = 6.88 Hz , 6H), and 0.69 (br, 4H); 13C-NMR (126 MHz, CDCl3)
δ
188.33, 153.55, 151.20, 148.77, 147.96, 147.30, 147.20, 146.73, 145.35, 145.24, 145.06, 144.96, 144.85, 144.70,
144.52, 144.43, 144.39, 144.13, 143.74, 143.49, 143.14, 142.96, 142.91, 142.83, 142.76, 142.57, 142.32, 142.07,
142.00, 141.90, 141.06, 140.76, 139.36, 136.46, 133.57, 133.22, 129.22, 128.62, 124.40, 123.15, 122.83, 122.42,
121.71, 119.14, 117.78, 72.48, 55.09, 44.58, 40.14, 32.00, 30.16, 29.81, 29.56, 29.47, 24.11, 22.87, and 14.22.
Synthesis of 7-(1,2-dihydro-1,2-methanofullerene[60]-61-{1,1-dicyanoethylene})-9,9-dioctadecanyl-
2-diphenylaminofluorene C60(>CPAF-C18), 1-C18. To a mixture of C60(>DPAF-C18) 2-C18 (240 mg,
0.17 mmol) and malononitrile (29 mg, 0.34 mmol) in dry chloroform (30 mL) was added pyridine
(52 mg, 0.68 mmol) with stirring under a nitrogen atmosphere. To this solution, titanium tetrachloride
(0.20 mL, excess) was added in one portion. After stirring at room temperature for 5.0 min, the reaction
mixture was quenched with water (30 mL). The resulting organic layer was washed several times with
water (100 mL each), dried over magnesium sulfate, and concentrated in vacuo to afford the crude
orange red solid product. It was purified by PTLC (SiO2, toluene-hexane, 1:1). A product fraction
collected at Rf = 0.8 (hexane-toluene, 1:1) was identified to be C60(>CPAF-C18)
1
-C18 as orange-red
1
solids in a yield of 50 mg (24%); MALDI-MS (TOF) calculated for 12C126 H9114N3 m/z 1647; found, m/z
655, 671, 722, 801, 867, 1079, 1290, 1465, 1479, 1647 (M+), 1648 (MH+); UV-vis (toluene, 1.0
ˆ
10´5 M)
104), and 503 nm
2960 (w), 2923 (s), 2848 (m), 2222 (m),
λ
(
ˆ
ε
) 325 (1.5
ˆ
105), and 470 (4.3
ˆ
1´054 L/mol
10 M); FT-IR (KBr) ν
max
¨
cm or 260 (1.7
ˆ
105), 327 (8.2
ˆ
max
(2.9
104 L/mol-cm) in CHCl3 (2.0
ˆ
1625 (s), 1596 (s), 1538 (w), 1489 (s), 1465 (w), 1345 (w), 1280 (m), 1265 (m), 1169 (m), 1089 (s), 1028 (m),
809 (w), 748 (w), 695 (w), 526 (s) cm´1; 1H-NMR (500 MHz, CDCl3, ppm)
8.01 (s, 1H), 7.78 (d, J = 7.8 Hz, 1H), 7.59 (d, J = 7.9 Hz, 1H), 7.30-7.02 (m, 12H), 5.54 (s, 1H), 2.00-1.83
(m, 4H), and 1.40–0.71 (m, 70H); 13C-NMR (200 MHz, CDCl3, ppm)
169.13, 153.91, 152.18, 149.82,
δ 8.12 (d, J = 8.12 Hz, 1H),
δ
147.92, 147.76, 146.45, 146.44, 145.74, 145.56, 145.51, 145.30, 145.18, 145.09, 144.81, 144.74, 144.22, 144.11,
143.53, 143.38, 143.32, 142.78, 142.51, 142.44, 141.78, 141.49, 137.89, 137.62, 134.00, 132.78, 129.90, 125.32,
123.82, 123.24, 123.13, 122.47, 120.02, 118.12, 113.84, 113.71, 88.22, 73.09, 50.49, 44.71, 33.92, 32.90, 31.98,
30.16, 29.82, 29.71, 29.56, 29.37, 29.31, 26.13, 25.58, 22.72, 21.53, 19.52, 17.21, and 14.07.
Synthesis
diphenylaminofluorene C60(>CPAF-C4),
-C18 were applied to obtain C60(>CPAF-C4) as orange-red solids in a yield of 28%; MALDI-MS
of
7-(1,2-dihydro-1,2-methanofullerene[60]-61-{1,1-dicyanoethylene})-9,9-dibutyl-2-
1
-C4. Similar procedures as described above for -C18 and
2
1
(TOF) calculated for 12C98 H3514N3 m/z 1254; found, m/z 1254 (M+), 1255 (MH+); UV-vis (toluene,
1.0 ) 323 (1.5 max 3027 (w), 2954 (m),
10´5 M) 105), and 485 (4.0 104 L/mol-cm); FT-IR (KBr)
2925 (s), 2854 (m), 2224 (m), 1594 (vs), 1491 (m), 1281 (s), 1096 (s), 819 (m), 754 (s), 577 (w), 527 (m) cm
1H-NMR (500 MHz, CDCl3, ppm)
8.15 (d, J = 8.2 Hz, 1H), 8.05 (s, 1H), 7.82 (d, J = 7.6 Hz, 1H), 7.62 (d,
1
ˆ
λ
max
(ε
ˆ
ˆ
ν
´1
;
δ
J = 8.2 Hz, 1H), 7.34-7.05 (m, 12H), 5.58 (s, 1H), 2.03-1.88 (m, 4H), and 1.08-0.63 (m, 14H).
Synthesis of 7-(1,2-dihydro-1,2-methanofullerene[60]-61-{1,1-dicyanoethylene})-9,9-didodecanyl-
2-diphenylaminofluorene C60(>CPAF-C12),
1
-C12. Similar procedures as described above for
-C12 as orange-red solids in a yield of 24%; MALDI-MS (TOF) calculated
10´5 M)
3064 (w), 3036 (w), 2954 (m),
2-C18 and
1
for
-C18 were applied to obtain
1
12
C
1H6714N3 m/z 1479; found, m/z 1479 (M+), 1480 (MH+); UV-vis (toluene, 1.0
ˆ
114
λ
(
ε
) 326 (1.5
ˆ
105), and 468 (4.2
ˆ
104 L/mol
¨
cm); FT-IR (KBr) ν
max
max
2923 (s), 2851 (w), 2224 (m), 1594 (vs), 1538 (w), 1492 (s), 1279 (s), 1186 (m), 1105 (vs), 820 (w), 753 (m),
697 (m), 527 (s) cm´1; 1H-NMR (200 MHz, CDCl3, ppm)
δ
8.17 (d, J = 8.0 Hz, 1H), 8.06 (s, Hz, 1H),
7.83 (d, J = 8.0 Hz, 1H), 7.64 (d, J = 8.0 Hz, 1H), 7.40–7.06 (m, 12H), 5.59 (s, 1H), 2.4–1.7 (m, 4H), and
1.50–0.71 (m, 46H).
3.4. Z-scan and Light-Intensity Transmittance Measurements
Z-scan measurements. Open aperture Z-scan and the nonlinear transmittance experiments were
carried out with femtosecond laser pulses at 780 nm. The full width at half maximum (FWHM)

Molecules 2016, 21, 647
11 of 13
of the laser pulses was 226
compound was dissolved in various solvents (THF, toluene, or CS2) with four concentrations from
10´4 to 10´2 M studied and kept in 1.0-mm-thick quartz cuvette. The beam waist at the focal point
˘
10 fs with the repetition rate of 1.0 kHz. In general, a sample of the
was 18
˘
2 µm which corresponded to 1.0–1.6 mm diffraction length. Laser pulses were generated by a
mode-locked Ti:Sapphire laser (Quantronix, IMRA America, Inc., Detroit, MI, USA), which was seeded
by a Ti:Sapphire regenerative amplifier (Quantronix-Titan, Marlborough, MA, USA), and was focused
onto a 1.0-mm-thick quartz cuvette containing a solution of C60(>CPAF-Cn). Incident and transmitted
laser intensities were monitored as the cuvette being moved (or Z-scanned) along the propagation
direction of laser pulses.
Light-intensity transmittance measurements. Similar experimental set-up and conditions as those
of open aperture Z-scan measurements were applied for the nonlinear transmittance experiments
at 780 nm. All compounds were dissolved in THF in the concentration of 2.0
ˆ
10´3 M and kept in
1.0-mm-thick quartz cuvette. Th´e2 transmittance data were collected upon the variation of irradiance
intensity from 20 to 600 GWcm
.
4. Conclusions
Nonlinearity and the light-intensity transmittance reduction effect of four C60(>CPAF-Cn) (n = 4,
9, 12, or 18) monoadducts was substantiated by the 226-fs irradiance-dependent measurements
above the incident irradiance of 70 GW/cm2 at 780 nm. A systematic trend showing higher
efficiency in nonlinear absorption by the dyad C60(>CPAF-C18) than that of the other dyads was
correlated to its higher multiphoton absorption (MPA), including 2PA and 3PA/ESA, cross-sections.
We suggested the attachment of linear long-alkyl or branched alkyl chains being beneficial to the
enhancement of 2PA and excited-state absorption owing to the minimization of molecular aggregation,
including dimerization-induced self-quenching effect. In our analyses, excited-state absorption
(ESA) is effectively treated as three-photon absorption (3PA). A clear concentration-dependent MPA
cross-sections (σ2 and σESA) magnitude was detected showing a higher value at a lower concentration
that was correlated to an increasing molecular separation with less aggregation in solution.
By taking the σ2 values of C60(>DPAF-C9) obtained at 780 nm photoexcitation in a 160-fs time
scale previously [17] for comparison, we were able to explain a smaller fs σ2 2PA cross-sections
of C60(>CPAF-Cn) using the same excitation wavelength being due to its lower linear absorption
coefficient at 400 nm than that of C60(>DPAF-C9). Since the design of compounds 1-Cn was aimed to
extend the 2PA wavelength up to 1100 nm from that of C60(>DPAF-Cn) analogous at 800 nm, their
capability to function as 2PA and ESA/3PA absorbers even at 780 nm made them effective broadband
NLO materials. This led to the corresponding light-intensity transmittance reduction efficiency.
We suggest that the observed broadband absorptions may be attributed to a partial π-conjugation
between the C60> cage and CPAF-Cn moieties, via endinitrile tautomeric resonances, involving a fully
conjugated transient resonance state, as depicted in Figure 1. This conjugation form may enhance 2PA
absorptions of 1-Cn at 780 nm.
Acknowledgments: The authors at UML thank the financial support of Air Force Office of Scientific Research
(AFOSR) under the grant number FA9550-09-1-0183 and FA9550-14-1-0153. The authors also acknowledge that
the Z-scan measurements were conducted by Yingli Qu at National University of Singapore.
Author Contributions: All authors contribute a significant effort on this work. S.J. and M.W. carried out the
main synthetic works, spectroscopic characterization, and data analysis; W.J. performed the analyses of Z-scan
measurements; W.J., L.-S.T., T.C., and L.Y.C. all participated in the discussion of experimental studies and
contributed to a part of manuscript writing; all authors read and approved the final manuscript.
Conflicts of Interest: The authors declare no conflicts of interest.

Molecules 2016, 21, 647
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Sample Availability: Sample Availability: Samples of the compound 1-Cn are available from the authors
for collaboration.
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