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Article

Broadband Two-Photon Absorption Characteristics of Highly Photostable Fluorenyl-Dicyanoethylenylated [60]Fullerene Dyads

1
Department of Chemistry, Institute of Nanoscience and Engineering Technology, University of Massachusetts Lowell, Lowell, MA 01854, USA
2
Department of Physics, National University of Singapore, Singapore 117542, Singapore
3
Functional Materials Division, AFRL/RXA, Air Force Research Laboratory, Wright-Patterson Air Force Base, Dayton, OH 45433, USA
*
Authors to whom correspondence should be addressed.
Molecules 2016, 21(5), 647; https://doi.org/10.3390/molecules21050647
Submission received: 27 March 2016 / Revised: 27 April 2016 / Accepted: 6 May 2016 / Published: 14 May 2016
(This article belongs to the Special Issue Fullerene and the Related Curved-pi Materials Chemistry)

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 π-conjugation between the C60 > cage 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.

1. Introduction

Nonlinear optical materials have numerous applications, including photodynamic therapy, nonlinear photonics, 3D optical data storage, frequency upconverted lasing, and fluorescence imaging [1,2,3,4,5,6,7,8,9,10]. These materials require large two-photon absorption (2PA) cross-section (σ2) of nonlinear absorbers [11,12,13]. 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, 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 × 10−3 M 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 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 π-conjugation between the C60> cage and CPAF-Cn moieties, via endinitrile tautomeric 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 1-Cn 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.

2. Results and Discussion

2.1. Materials Characterization

We have demonstrated the use of dialkyldiphenylaminofluorenyl-keto-[60]fullerene C60(>DPAF-Cn) dyads 2-Cn [16], branched triads C60(>DPAF-Cn)x (x = 2) [17], and the related starburst pentads (x = 4) [8] for the study of simultaneous 2PA phenomena under the photoexcitation of a 780-nm 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 2-Cn by an electron-withdrawing 1,1-dicyanoethylenyl (DCE) unit, leading to the structure of C60(>CPAF-Cn) 1-Cn. The functional change resulted in the increase of molecular electronic polarization and bathochromic shift in the optical absorption from λmax 400 nm for DPAF to 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 [8] 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 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.
Synthesis of the compound C60(>CPAF-C9) 1-C9 followed the procedure described previously [18]. A similar synthetic sequence was applied for the preparation of C60(>CPAF-C4) 1-C4, C60(>CPAF-C12) 1-C12, and C60(>CPAF-C18) 1-C18. Formation of a fullerenyl monoadduct 1-Cn was evident by detection of its infrared spectrum displaying three typical fullerenyl signals at 753, 697, and 527 cm−1 corresponding to absorptions of an unfunctionalized half-cage sphere of C60>. The key chemical modification of a keto group of C60(>DPAF-C18) 2-C18 to a 1,1-dicyanoethylene (DCE) group of 1-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−1 with complete disappearance of the carbonyl stretching vibration of 2-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, –C=C(CN)2, –C≡N, and =C(CN)2, in the 1,1-dicyanoethylenyl moiety of 1-C18 at δ 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 δ 153.91, 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.
Similar to that of 1-C9 [18], the keto modification of 2-C18 led to a large bathochromic shift of the long-wavelength absorption band of 1-C18 to λmax 468 (ε = 4.2 × 104 L/mol·cm, toluene, 1.0 × 10−5 M) or 503 nm (ε = 2.9 × 104 L/mol·cm, CHCl3, 2.0 × 10−5 M) in nearly 58–93 nm longer than that of C60(>DPAF-C18) (2-C18) centered at λmax 410 nm, as shown in Figure 4A-a. This band was accompanied with two other absorption bands with λmax centered at 260 (ε = 1.7 × 105) and 327 (ε = 8.2 × 104) in CHCl3 or 326 (ε = 1.5 × 105 L/mol·cm) in toluene, attributed to absorptions of C60 > cage. By comparing with the λmax 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 or 900–1100 nm for 2PA) to 1(CPAF)*-Cn is followed by formation of the singlet 1C60*(>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.  

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 the coefficient by the formula σ2 = βħω/N, where ħω is the photon energy and N is the number of the molecules.
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 × 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) ≤ 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.
It is interesting to observe a higher 2PA absorption cross-section value of 6.42 × 10−48 cm4·s· photon−1·molecule−1 (or 642 GM) for C60(>CPAF-C18) at a low concentration of 5.0 × 10−4 M than that, 3.25 × 10−48 cm4·s·photon−1·molecule−1 (or 325 GM), for C60(>CPAF-C9) at the same concentration. A lower value of 1.28 × 10−48 cm4·s·photon−1·molecule−1 (or 128 GM) for C60(>CPAF-C4) than the 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 (S1Sn) 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 × 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 higher, in general, and increased more rapidly than those taken at a higher concentration of 1.0 × 10−2 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 [8]. The intensity dependence on σ2 values may 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 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.

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-2,2′- bis(diphenylphosphino)-1,1′-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 THF was refluxed over sodium and benzophenone overnight and distilled under reduced pressure (10−1 mmHg).

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 × 50 mL), then, the solution was dried over magnesium sulfate and concentrated in vacuo. The crude yellow oil was purified by column chromatography (SiO2, hexane‒EtOAc, 9:1) to afford 7-α-bromoacetyl-9,9-di-(n-octadecyl)- 2-diphenylaminofluorene 3-C18 (1.8 g, 78%) with its chromatographic band corresponding to Rf = 0.6 on TLC (SiO2, hexane‒EtOAc, 9:1 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 (CHCl3, 1.0 × 10−5 M) λmax (ε) 292 (1.9 × 104) and 407 (2.5 × 104 L/mol·cm); 1H-NMR (500 MHz, CDCl3, ppm) δ 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, 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) δ 190.99, 153.63, 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) νmax 3440 (m), 2920 (s), 2849 (s), 1674 (m), 1632 (m), 1593 (s), 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 × 10−5 M) λmax (ε) 260 (1.3 × 105), 325 (4.7 × 104), and 411 (3.6 × 104 L/mol·cm); 1H-NMR (500 MHz, CDCl3, ppm) δ 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, 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 solids in a yield of 50 mg (24%); MALDI-MS (TOF) calculated for 12C1261H9114N3 m/z 1647; found, m/z 655, 671, 722, 801, 867, 1079, 1290, 1465, 1479, 1647 (M+), 1648 (MH+); UV-vis (toluene, 1.0 × 10−5 M) λmax (ε) 325 (1.5 × 105), and 470 (4.3 × 104 L/mol·cm or 260 (1.7 × 105), 327 (8.2 × 104), and 503 nm (2.9 × 104 L/mol-cm) in CHCl3 (2.0 × 10−5 M); FT-IR (KBr) νmax 2960 (w), 2923 (s), 2848 (m), 2222 (m), 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) δ 8.12 (d, J = 8.12 Hz, 1H), 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) δ 169.13, 153.91, 152.18, 149.82, 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 of 7-(1,2-dihydro-1,2-methanofullerene[60]-61-{1,1-dicyanoethylene})-9,9-dibutyl-2- diphenylaminofluorene C60(>CPAF-C4), 1-C4. Similar procedures as described above for 2-C18 and 1-C18 were applied to obtain C60(>CPAF-C4) as orange-red solids in a yield of 28%; MALDI-MS (TOF) calculated for 12C981H3514N3 m/z 1254; found, m/z 1254 (M+), 1255 (MH+); UV-vis (toluene, 1.0 × 105 M) λmax (ε) 323 (1.5 × 105), and 485 (4.0 × 104 L/mol-cm); FT-IR (KBr) νmax 3027 (w), 2954 (m), 2925 (s), 2854 (m), 2224 (m), 1594 (vs), 1491 (m), 1281 (s), 1096 (s), 819 (m), 754 (s), 577 (w), 527 (m) cm−1; 1H-NMR (500 MHz, CDCl3, ppm) δ 8.15 (d, J = 8.2 Hz, 1H), 8.05 (s, 1H), 7.82 (d, J = 7.6 Hz, 1H), 7.62 (d, 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 2-C18 and 1-C18 were applied to obtain 1-C12 as orange-red solids in a yield of 24%; MALDI-MS (TOF) calculated for 12C1141H6714N3 m/z 1479; found, m/z 1479 (M+), 1480 (MH+); UV-vis (toluene, 1.0 × 105 M) λmax (ε) 326 (1.5 × 105), and 468 (4.2 × 104 L/mol·cm); FT-IR (KBr) νmax 3064 (w), 3036 (w), 2954 (m), 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) of the laser pulses was 226 ± 10 fs with the repetition rate of 1.0 kHz. In general, a sample of the 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 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 × 10−3 M and kept in 1.0-mm-thick quartz cuvette. The transmittance data were collected upon the variation of irradiance intensity from 20 to 600 GWcm−2.

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.

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  • Sample Availability: Samples of the compound 1-Cn are available from the authors for collaboration.
Figure 1. (a) Endinitrile tautomeric resonances at the bridging C61Hα‒C[=C(CN)2] structure of a C60-CPAF conjugate compound 1-C4 and (b) fullerenyl seven-membered ring expansion involving C61, leading to the formation of a fully-conjugated form of C60> acceptor (A) and CPAF donor (D), as marked in purple. This resulted in an extended AD conjugation length and absorption wavelengths, marked in burgundy red in color.
Figure 1. (a) Endinitrile tautomeric resonances at the bridging C61Hα‒C[=C(CN)2] structure of a C60-CPAF conjugate compound 1-C4 and (b) fullerenyl seven-membered ring expansion involving C61, leading to the formation of a fully-conjugated form of C60> acceptor (A) and CPAF donor (D), as marked in purple. This resulted in an extended AD conjugation length and absorption wavelengths, marked in burgundy red in color.
Molecules 21 00647 g001
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.
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.
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Figure 2. 13C-NMR spectrum of C60(>CPAF-C18) showing all sp2 (δ 134–148) and two sp3 (CF1 and CF2) C60 cage carbons and 1,1-dicyanoethylenyl (DCE) carbons, as assigned.
Figure 2. 13C-NMR spectrum of C60(>CPAF-C18) showing all sp2 (δ 134–148) and two sp3 (CF1 and CF2) C60 cage carbons and 1,1-dicyanoethylenyl (DCE) carbons, as assigned.
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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+).
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+).
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Figure 4. UV-vis (A) absorption (toluene, 1.0 × 10−5 M, normalized at λmax 486 nm) and (B) transmittance (THF, 2.0 × 10−5 M) spectra of CPAF-C12, 1-C4, 1-C9, 1-C12, and 1-C18 with THF as the reference.
Figure 4. UV-vis (A) absorption (toluene, 1.0 × 10−5 M, normalized at λmax 486 nm) and (B) transmittance (THF, 2.0 × 10−5 M) spectra of CPAF-C12, 1-C4, 1-C9, 1-C12, and 1-C18 with THF as the reference.
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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.
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.
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Figure 6. (a) Effective two photon absorption cross sections of C60(>CPAF-C18) plotted as a function of pulse laser intensities; (b) and (c) show the plot of ln(1‒T) vs. I for the same compound with different intensities Io at the focal point.
Figure 6. (a) Effective two photon absorption cross sections of C60(>CPAF-C18) plotted as a function of pulse laser intensities; (b) and (c) show the plot of ln(1‒T) vs. I for the same compound with different intensities Io at the focal point.
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Figure 7. (a) Two-photon absorption cross-sections and (b) nonlinear absorption of 5, 1-C4, 1-C9, 1-C12, and 1-C18 in THF (2.0 × 10−3 M) measured as a function of irradiance with 226-fs laser pulses operated at 780 nm.
Figure 7. (a) Two-photon absorption cross-sections and (b) nonlinear absorption of 5, 1-C4, 1-C9, 1-C12, and 1-C18 in THF (2.0 × 10−3 M) measured as a function of irradiance with 226-fs laser pulses operated at 780 nm.
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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.
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.
Sample[C]/Mβ/cmGW−1σ2/10−48 cm4·s·photon−1·molecule−1σESA/10−78 cm6·s2· photon−2·molecule−1
CPAF-C125.0 × 10−40.00262.20 (220 GM)
1.0 × 10−30.00482.03 (203 GM)
2.0 × 10−30.00701.48 (148 GM)6.4
1.0 × 10−20.01050.44 (44 GM)1.5
C60(>CPAF-C4)5.0 × 10−40.00151.28 (128 GM)
1.0 × 10−30.00271.12 (112 GM)
2.0 × 10−30.00450.95 (95 GM)5.9
1.0 × 10−20.01020.43 (43 GM)
C60(>CPAF-C9)5.0 × 10−40.00393.25 (325 GM)
1.0 × 10−30.00652.75 (275 GM)
2.0 × 10−30.00801.69 (169 GM)10.2
1.0 × 10−20.01100.46 (46 GM)5.4
C60(>CPAF-C12)5.0 × 10−40.00201.65 (165 GM)
1.0 × 10−30.00401.71 (171 GM)
2.0 × 10−30.00651.39 (139 GM)9.1
1.0 × 10−20.01240.535 (54 GM)4.4
C60(>CPAF-C18)5.0 × 10−40.00776.42 (642 GM)30.1
1.0 × 10−30.00943.97 (397 GM)24.7
2.0 × 10−30.01052.14 (214 GM)11.3
1.0 × 10−20.01390.59 (59 GM)5.1

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MDPI and ACS Style

Jeon, S.; Wang, M.; Ji, W.; Tan, L.-S.; Cooper, T.; Chiang, L.Y. Broadband Two-Photon Absorption Characteristics of Highly Photostable Fluorenyl-Dicyanoethylenylated [60]Fullerene Dyads. Molecules 2016, 21, 647. https://doi.org/10.3390/molecules21050647

AMA Style

Jeon S, Wang M, Ji W, Tan L-S, Cooper T, Chiang LY. Broadband Two-Photon Absorption Characteristics of Highly Photostable Fluorenyl-Dicyanoethylenylated [60]Fullerene Dyads. Molecules. 2016; 21(5):647. https://doi.org/10.3390/molecules21050647

Chicago/Turabian Style

Jeon, Seaho, Min Wang, Wei Ji, Loon-Seng Tan, Thomas Cooper, and Long Y. Chiang. 2016. "Broadband Two-Photon Absorption Characteristics of Highly Photostable Fluorenyl-Dicyanoethylenylated [60]Fullerene Dyads" Molecules 21, no. 5: 647. https://doi.org/10.3390/molecules21050647

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