Carbon Nanohoops: Excited Singlet and Triplet Behavior of [9]- and

Article pubs.acs.org/JPCA

Carbon Nanohoops: Excited Singlet and Triplet Behavior of [9]- and [12]-Cycloparaphenylene Douglas A. Hines,‡,† Evan R. Darzi,§ Ramesh Jasti,§ and Prashant V. Kamat*,‡,† †

Notre Dame Radiation Laboratory and ‡Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States § Department of Chemistry, Boston University, Boston, Massachusettes 02215, United States ABSTRACT: Cycloparaphenylene molecules, commonly known as “carbon nanohoops”, have the potential to serve as building blocks in constructing carbon nanotube architectures. The singlet and triplet excited-state characteristics of [9]-cycloparaphenylene ([9]CPP) and [12]cycloparaphenylene ([12]CPP) have now been elucidated using time-resolved transient absorption and emission techniques. The fluorescence quantum yields (Φ) of [9]CPP and [12]CPP were determined to be 0.46 and 0.83, respectively. Rates of nonradiative recombination (knr), radiative recombination (kr), and intersystem crossing (kisc) determined in this study indicate that radiative decay dominates in these nanohoop structures. The triplet extinction coefficient was determined through energy transfer with biphenyl, and the triplet quantum yield (ΦT) was calculated to be 0.18 and 0.13 for [9]CPP and [12]CPP, respectively. The rate of triplet state quenching by oxygen was measured to be 1.7 × 103 ([9]CPP) and 1.9 × 103 s−1 ([12]CPP). The excited-state dynamics established in this study enable us to understand the behavior of a carbon nanotube-like structure on a single subunit level.

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INTRODUCTION Graphitic materials such as carbon nanotubes (CNTs),1−3 graphene,4−6 and C607−10 are of considerable interest to the fields of materials science and chemistry. Recent research in materials science has focused on the integration of graphitic materials into electrodes for photovoltaics,10−12 field-effect transistors (FETs),13−15 and batteries.16−18 Chemists have concentrated on characterizing the excited-state interactions between semiconductors and graphitic materials, focusing on the ability of these composite materials to transport and store charge.19−21 Specifically, the interest in CNTs has fueled the design of a new class of materials called cycloparaphenylenes or “carbon nanohoops”. Synthetic chemists have made advances toward synthesizing CNTs from a bottom-up approach that would allow the polymerization of a single subunit to a CNT of desired length.22−32 This class of CNT subunits is being referred to as cycloparaphenylene molecules. The emergence of these molecules has given chemists a unique opportunity to study the smallest fundamental unit of the CNT to better understand the chemistry of these “graphitic” materials. At the same time, the development of carbon nanohoops allows synthetic chemists an opportunity to rationally construct a CNT from a single subunit with a well-controlled diameter. Strides have been made to synthesize nanohoops of various diameters with control over surface functional groups as well. The synthesis of cycloparaphenylene molecules of [6] through [16] and [18] subunits have already been achieved.22−33 Other reports show that nanohoops can be dimerized,34 phenylsubstituted,35 and nitrogen-substituted.36 Despite these synthetic achievements, no major effort has been made to © 2014 American Chemical Society

characterize the excited-state behavior of these molecules. Ground-state absorption and emission spectra24,25,36,37 are understood, and some theoretical calculations have attempted to understand the transition to, and relaxation from, the singlet excited state.25,32,37−39 Presented herein are the characteristics of the singlet and triplet excited state of two CNT subunits, [9]-cycloparaphenylene ([9]CCP) and [12]-cycloparaphenylene ([12]CCP).

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EXPERIMENTAL SECTION Materials. Toluene (ACS, reagent grade) was used as received without further purification. [9]CPP and [12]CPP were synthesized by the Jasti group at Boston University.22 Quantum Yield Measurements. UV−visible absorption measurements were taken with a Varian Cary 50 Bio spectrophotometer. Emission spectra were recorded with a Jobin Yvon Fluorolog using an excitation wavelength of 350 nm with a 390 nm long-pass filter placed between the nanohoop sample and the emission monochromator. The absorption of each nanohoop solution was held to approximately 0.1 at the excitation wavelength in order to reduce self-absorption from the emission spectrum. Quinine sulfate dye (ϕ = 0.55 in 0.5 M H2SO4) was used as a reference for fluorescence quantum yield calculations. Emission Lifetime. Emission lifetime measurements were recorded with a Jobin Yvon Fluorocube. A pulsed LED source (371 nm, 200 ps fwhm, 1 MHz repetition rate) was incident on Received: December 17, 2013 Revised: February 4, 2014 Published: February 6, 2014 1595

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compared to that of [12]CPP (452 nm). This large red shift for [9]CPP is attributed to [9]CPP being a smaller diameter nanohoop and having a larger ring strain than [12]CPP. Due to the more strained and less mobile configuration, excited [9]CPP has access to a larger number of vibrational relaxations when compared to excited [12]CPP.24,32,41 The fluorescence quantum yields for [9]CPP and [12]CPP were determined to be 0.46 and 0.83, respectively. Fluorescence quantum yield measurements were made with reference to quinine sulfate dye, which has a quantum yield of 0.55 in 0.05 M H2SO4.42 Quantum yields were calculated using eq 1

nanohoop solutions in toluene. Lifetime data was recorded at the maximum emission peak for [9]CPP (495 nm) and [12]CPP (451 nm). A 420 nm long-pass filter was placed between the nanohoop sample and the detector in order to prevent scattered photons originating at the excitation source from reaching the detector. Flash Photolysis. Nanohoop solutions were excited with a 355 nm pump beam (fwhm = 10 ns, 10 Hz repetition rate, 2.5 mJ/pulse) using a Nd:YAG laser from Spectra Physics (Quanta-Ray Pro230). The white light probe consisted of a 1000 W xenon lamp (Hanovia) pulsed by a lamp pulser (Sorensen Power Supplies). The probe beam was collected with a monochromator (Digikrom 240, CVI Laser Corporation) coupled to a photomultiplier tube and recorded with a 1 GHz Lecroy oscilloscope. In a typical experiment, 3 mL of a 5 μM nanohoop/toluene solution was purged with N2 for 20 min in a quartz cuvette prior to all measurements. Triplet Quenching with Oxygen. Toluene was placed in an airtight container and saturated with oxygen. On the basis of the literature value,40 the concentration of oxygen-saturated toluene was considered to be 9.88 mM; further calculations of the oxygen concentration in this article are derived from this value. Aliquots of 10−50 μL of saturated toluene were injected into the nanohoop solution, and the nanohoop kinetics were monitored for each change in O2 concentration at the maximum of the triplet absorbance spectrum. Care was taken to minimize the diffusion of O2 out of the solution and into the head space of the cuvette. Pulse Radiolysis. A model Titan Beta-8/16- 1S electron linear accelerator (LINAC) was used to carry out pulse radiolysis measurements. Electron pulses (∼8 MeV, 50 ns duration) were incident on nanohoop/biphenyl solutions in toluene (purged with N2) contained in a 1 cm quartz cuvette. A xenon lamp was used to generate a white light continuum for differential absorption measurements. Triplet extinction coefficients for [9]CPP and [12]CPP were generated by energy transfer with biphenyl (0.05 M). Note that the concentration of each nanohoop solution was calculated from the UV−vis absorption spectra and the reported values of the singlet molar extinction coefficient for [9] and [12]CPP.37

2 ⎞ ⎛ F ⎞⎛ 1 − 10 A nh ⎞⎛ nnh ⎜ 2 ⎟ Φf = Φstd⎜ nh ⎟⎜ A std ⎟ ⎝ Fstd ⎠⎝ 1 − 10 ⎠⎝ nstd ⎠

(1)

where Φf is the quantum yield of fluorescence, F is the integrated fluorescence intensity, A is the absorbance of the solution at the excitation wavelength, n is the refractive index of the solvent, and the abbreviations “nh” and “std” represent the nanohoop sample and quinine sulfate standard, respectively. The values of Φf presented in this article agree well with measurements in previous studies. The slight difference in these values arises from previous measurements being made in the crystalline form or dissolved in chloroform, whereas our measurements are in toluene.37 Emission Lifetime. Time-correlated-single photon counting (TCSPC) was used to probe the excited-state dynamics of a sample through the fluorescence of the singlet state. Using this method, we have measured the lifetime of the singlet state, τs, for [9]CPP (τs = 5.3 × 10−9 s) and [12]CPP (τs = 1.9 × 10−9 s), where τs is derived from the fit of the observed data in Figure 2 to a single-exponential function, eq 2 ⎛ −t ⎞ y = A* exp⎜ ⎟ ⎝ τs ⎠

(2)

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RESULTS AND DISCUSSION Excited Singlet State Characterization. Both [9]CPP and [12]CPP show absorption maxima at 340 nm (Figure 1). The emission maximum of [9]CPP (490 nm) is red-shifted Figure 2. Emission lifetime of (a) [9]CPP and (b) [12]CPP as recorded by TCSPC. Trace (c) represents the instrument response time (IRT).

where τs is the lifetime of the singlet state. The rate of radiative recombination for the nanohoop samples can be derived by rearranging eq 3 Φf =

kr (k r + k nr + k isc)

(3)

where Φf is defined in eq 1, kr is rate of radiative recombination, knr is the rate of nonradiative recombination, and kisc is the rate of intersystem crossing. In this model, we assume that the lifetime of the singlet τs is governed by a competition of these three (kr, knr, kisc) rates and that no other deactivation pathways

Figure 1. (a) Absorption spectrum of [9]CPP, (b) absorption spectrum of [12]CPP, (c) emission spectrum of [9]CPP, and (d) emission spectrum of [12]CPP. All spectra are normalized to their respective maxima. 1596

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contribute significantly. Thus, we can substitute τs into eq 3 and rearrange to solve for kr in eq 4

kr =

Φf τs

triplet state. Triplet extinction coefficients for [9]CPP and [12]CPP were determined through energy transfer with an excited biphenyl triplet (BP, 0.05 M, ε = 27 100 M−1 cm−1 at 360 nm) in toluene. Energy transfer between these species is illustrated in eq 543

(4) 7 −1

Using eq 4, the value of kr was determined to be 8.7 × 10 s and 4.4 × 108 s−1 for [9]CPP and [12]CPP, respectively. Triplet Spectrum. To understand the excited-state dynamics of the nanohoop triplet state, we characterize the nanohoop samples with nanosecond flash photolysis. Samples were purged with nitrogen and excited with a 355 nm laser pulse. The difference absorption spectra of the nanohoop triplet state are presented in Figure 3A and show a positive

3

BP* + CPP → BP + 3CPP*

(5) 44

3

where BP* was generated in toluene by pulse radiolysis. The maximum T−T absorption of 3BP* at 360 nm (t = 0 μs) and the T−T absorption maximum of 3CPP (t ≃ 10 μs) were used to estimate the extinction coefficient using eq 645 εA = εD

(ΔODA /ΔODD) Ptr

(6)

where εA and εD are the extinction coefficients of the acceptor and donor, respectively, ΔODA and ΔODD are the respective amplitudes of the difference absorption spectra of the acceptor and the donor, and Ptr is the probability of transfer represented in eq 7. Here

Ptr =

1 ket[A] 1 (ket[A] + kD)

(7)

where ket is the bimolecular rate constant of energy transfer determined from dependence of the pseudo-first-order rate constant of 3CPP growth on the nanohoop concentration, [1A] (Figure 4A) and kD represents the excited-state decay of the donor (3BP*) in the absence of the acceptor (measured at 360 nm). The kinetic correction, Ptr, accounts for the finite probability that an energy-transfer reaction occurs. This probability is dependent on the lifetime of the excited donor and the bimolecular energy-transfer rate, determined in Figure 4B, which takes the concentration of the energy acceptor into account. It should be noted that a minimal amount of direct excitation of the nanohoop samples occurs in the pulse radiolysis experiments. This has been accounted for by subtracting the baseline ΔOD generated by direct excitation from the ΔODA value used in the calculation demonstrated in eq 6. Using this method, we have determined the triplet extinction coefficients to be 25 000 ± 4000 M−1 cm−1 for [9]CPP (λmax = 390 nm) and 31 000 ± 1300 M−1 cm−1 for [12]CPP (λmax = 680 nm). Triplet Quantum Yield. Using the experimental values for the triplet extinction coefficient, we determined the triplet quantum yield (ΦT) for [9]CPP and [12]CPP. Using C60 as an actinometry standard, we measured the quantum yield of the triplet state, calculated by eq 845

Figure 3. (A) Triplet absorption spectra of (a) [9]CPP and (b) [12]CPP. (B) Excited-state decay of the nanohoop triplet state measured at the singlet bleach minimum for (a) [9]CPP (340 nm) and (b) [12]CPP at (340 nm) and at the triplet maximum for (c) [9]CPP (390 nm) and (d) [12]CPP (680 nm).

absorption peak at 390 and 680 nm for [9]CPP and [12]CPP, respectively. Kinetic traces at the bleach minima and absorption maxima, shown in Figure 3B, reveal a monoexponential decay with lifetime τT, which is the lifetime of the triplet state. A summary of relaxation kinetics for [9]CPP and [12]CPP is shown in Table 1. The lifetimes of the nanohoop triplet states were found to be 6.7 × 10−5 and 1.1 × 10−4 s for [9]CPP and [12]CPP, respectively. Triplet Extinction Coefficient. To determine the triplet quantum yield, ΦT, of the nanohoop samples, it was first necessary to determine the extinction coefficient of the excited

⎛ ΔODnh ⎞⎛ εstd ⎞ ΦT = Φstd⎜ ⎟⎜ ⎟ ⎝ ΔODstd ⎠⎝ εnh ⎠

(8)

where ΦT is the triplet quantum yield, ΔOD is the maximum in the differential absorption spectrum taken from the kinetic traces in Figure 5, ε is the extinction coefficient and the subscripts nh and std represent the nanohoop sample and the C60 standard (ΦT = 1.0, ε = 12,000 M−1 cm−1, λmax = 750 nm), respectively.44 Shown in Figure 5 are kinetic traces of the two

Table 1. Summary of Nanohoop Relaxation Kinetics sample

kr (s−1)

[9]CPP [12]CPP

8.7 × 10 4.4 × 108 7

knr (s−1)

kisc (s−1)

τs (s)

τT (s)

6.8 × 10 2.1 × 107

4.4 × 107 6.8 × 107

5.3 × 10−9 1.9 × 10−9

6.7 × 10−5 1.1 × 10−4

7

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Table 2. Summary of Measured Extinction Coefficients and Φ Values sample [9] CPP [12] CPP

λmax (nm)

εT (M−1 cm−1)

ket (M−1 s−1)

Φf

ΦT

Φnr

390

25 000 ± 4,000

1.9 × 1010

0.46

0.18

0.36

680

31 000 ± 1,300

1.8 × 1010

0.83

0.13

0.04

3

CPP* + 3O2 → 1CPP + 1O2 *

(9)

To demonstrate the bimolecular rate of triplet-state quenching by oxygen, we monitor the lifetime of [9]CPP and [12]CPP samples in the presence of low concentrations of oxygen (see the Experimental Section for details), shown in Figure 6. The

Figure 4. (A) Kinetic traces tracking the growth of the nanohoop triplet signal at 680 nm as a result of energy transfer from biphenyl to (a) 10, (b) 13, and (c) 16 μM [12]CPP. The rate of growth of the [12]CPP triplet (ka) is extracted from the fit of the monoexponential growth of each curve. (B) The bimolecular energy-transfer rate for [9]CPP (red) and [12]CPP (blue), obtained from the slope of the linear fit to ka values plotted against various concentrations.

Figure 6. The average lifetime of the nanohoop/oxygen solution with different concentrations of dissolved oxygen. The filled dot represents the experimental data of [9]CPP, and the hollow dot represents [12]CPP. The experimental trends are fit to a straight line (red = [9]CPP; blue = [12]CPP), and the slope of the line is taken to be the bimolecular reaction rate of triplet quenching by oxygen.

plot in Figure 6 illustrates the average lifetime of each nanohoop at a specific concentration of dissolved oxygen. The slope of the linear fit of this data represents the bimolecular rate constant for the quenching reaction. From this experimental data, the rate of the quenching reaction was determined to be 1.7 × 103 s−1 M−1 for [9]CPP and 1.9 × 103 s−1 M−1 for [12]CPP. The ability of 3CPP to transfer energy to ground-state oxygen and generate singlet oxygen is a useful photophysical property of nanohoops that can be further utilized in oxidative phototransformations.

Figure 5. Kinetic traces used in the measurement of nanohoop triplet quantum yield; (a) [9]CPP at 390 nm, (b) [12]CPP at 680 nm, and (c) C60 at 750 nm.

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CONCLUSION In summary, we have characterized the singlet and triplet states of [9]- and [12]-cycloparaphenylene. Specifically, we have measured the singlet and triplet quantum yield and generated the triplet spectra via nanosecond flash photolysis. Using pulse radiolysis, we have measured the triplet extinction coefficient of the nanohoops while also demonstrating the rate of energy transfer between the samples of interest and biphenyl. The data presented in this article provide an initial characterization of the excited-state dynamics of cycloparaphenylene triplet states and is meant to lay the foundation for future work with the subject.

nanohoop samples along with the C60 standard taken at the same pump power. Absorbances at the excitation wavelength for all samples were matched (A = 0.1) so that they captured the same amount of the excitation pulse. Using the magnitude of ΔOD at short time (t ≃ 0 μs) determined by the flash photolysis experiments and the triplet extinction coefficient determined with pulse radiolysis, we calculated the triplet quantum yields for [9]CPP and [12]CPP to be 0.18 and 0.13, respectively. Quantum yield values and extinction coefficients are summarized in Table 2, including Φnr (quantum yield for nonradiative recombination, Φnr = 1 − ΦT − Φf). Quenching the Nanohoop Triplet State with Oxygen. Exposure of the nanohoop solution to ambient conditions results in a reduction of the triplet-state lifetime due to quenching of the excited triplet state by energy transfer to oxygen and the resultant formation of singlet oxygen, eq 946

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 1598

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ACKNOWLEDGMENTS The authors would like to thank Dr. Gordon Hug for useful conversations on photochemistry as well as Patrick Bennett for glass blowing work. The research described herein was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Award DE-FC02-04ER15533. This is contribution number NDRL No. 4999 from the Notre Dame Radiation Laboratory.

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