Carbon Nanohoops: Excited Singlet and Triplet Behavior of Aza[8

Jun 30, 2015 - The excited state properties of two nitrogen-doped cycloparaphenylene molecules, or carbon nanohoops, have been studied using steady-st...
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Carbon Nanohoops: Excited Singlet and Triplet Behavior of Aza[8]CPP and 1,15-Diaza[8]CPP Douglas A. Hines,†,‡ Evan R. Darzi,§ Elizabeth S. Hirst,§ Ramesh Jasti,§ and Prashant V. Kamat*,†,‡ †

Department of Chemistry and Biochemistry and ‡Notre Dame Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, United States § Department of Chemistry and Biochemistry, University of Oregon, Eugene, Oregon 97403, United States S Supporting Information *

ABSTRACT: The excited state properties of two nitrogen-doped cycloparaphenylene molecules, or carbon nanohoops, have been studied using steady-state and time-resolved absorption and emission spectroscopies. Quantum yield of fluorescence (Φf = 0.11 and 0.13) and intersystem crossing (Φisc = 0.45 and 0.32) were determined for aza[8]CPP and 1,15diaza[8]CPP, respectively. We also present the proton transfer reaction between trifluoroacetic acid and the nitrogen-doped nanohoops, which resulted in significant modifications to the steady-state absorption and emission spectra as well as the triplet−triplet absorption spectra. From fluorescence quenching data we determine the equilibrium constant for the proton transfer reaction between aza[8]CPP (Keq = 1.39 × 10−3) and 1,15diaza[8]CPP (Keq = 2.79 × 10−3) confirming that 1,15-diaza[8]CPP is twice as likely to be protonated at a particular concentration of trifluoroacetic acid.

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INTRODUCTION Recent efforts in organic chemistry have been devoted to the synthesis of a new class of molecules called cycloparaphenylenes, or carbon nanohoops. The structure of these molecules consists of a series of para-substituted phenyl groups that bond in a continuous ring, forming a single subunit of an armchair carbon nanotube. While complicated to synthesize, they have attracted significant attention due to their unique structure (consisting of inward oriented pi bonds),1 their interesting optical properties,2,3 and the possibility that these molecules could be polymerized to form carbon nanotubes of well controlled length and diameter.1 Currently, synthetic chemists have demonstrated the synthesis of nanohoops that consist of 5−16 and 18 phenyl units,1,4−15 mastered techniques to dimerize/functionalize nanohoops,16−19 and successfully introduced extra aromatic rings onto the side of the nanohoop structure.15,20 Along with this, efforts have been made to characterize the cation and dication form of [8]CPP to better understand the charge transfer capabilities of these molecules.21,22 A number of experimental studies have described the transitions between the ground and excited states of these molecules.1,3,6,20,23 Several computational studies have elucidated the electronic structure, geometric conformations, and excited state characteristics of these molecules.3,6,11,24−28 Complementary to investigations of the singlet properties of these molecules, we have recently published a study that elucidates the triplet properties of these molecules.2 The literature is mostly composed of these types of studies that describe the excited state character of carbon nanohoops and © XXXX American Chemical Society

lacks significant investigations into their reactivity, as demonstrated in separate studies from Shinohara and Yamago, respectively.29,30 This motivates the current characterization of two nitrogen-doped carbon nanohoops as well as a demonstration of reactivity with trifluoroacetic acid, Scheme 1. Scheme 1. Unprotoanted Nanohoop on the Left (aza[8]CPP) Reacts with Trifluoroacetic Acid (Represented by [H+]) to Form the Protonated Nanohoop on the Right

Herein, we demonstrate the properties of two species of [8]cycloparaphenylene, which have been doped with one (aza[8]CPP) and two (1,15-diaza[8]CPP) nitrogen atoms, respectively. The most intriguing aspect of working with these nanohoops is their potential to be reactive toward many chemical species, specifically acids and bases. We characterize Received: May 7, 2015 Revised: June 30, 2015

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that is necessary to pursue the subsequent time-resolved measurements. The singlet states of aza[8]CPP and 1,15-diaza[8]CPP were first characterized by steady-state absorption and emission spectroscopy in the UV−visible region. Shown in Figure 1A are

both nanohoops, using various spectroscopic methods, in the absence and presence of trifluoroacetic acid. These results illustrate the reactivity of newly designed carbon nanohoops toward acid−base conditions.

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EXPERIMENTAL SECTION Materials. Trifluoroacetic acid (TFAA, Alfa Aesar, 99%), triethylamine (TEA, Aldrich, 99.5%), and toluene (ACS, reagent grade) were used as received without further purification. Carbon nanohoops (aza[8]CPP and 1,15-diaza[8]CPP) were synthesized and purified by the Jasti group at Boston University.4 Absorption and Emission Measurements. A Cary-50 Bio spectrophotometer was used to take absorption measurements. Emission measurements (350 nm excitation, 390 nm long pass filter) were recorded with a Jobin Yvon Flurolog-3 Spectrofluorometer. Each nanohoop sample was kept relatively dilute to minimize self-absorption of emitted photons. Quantum yield calculations employed the use of quinine sulfate dye (ϕ = 0.55 in 0.5 M H2SO4) as a reference.31 Emission Lifetime. Emission lifetime data was recorded on a Jobin Yvon Fluorocube using a pulsed LED excitation source (371 nm, 200 ps fwhm, and a 1 MHz repetition rate) and a 420 nm long pass filter. Each trace was recorded at the peak wavelength for that particular sample, which shifted to the red for the highest concentration of TFA. Peak wavelengths are noted in the figures containing each decay trace. Flash Photolysis. Flash photolysis experiments employed a 355 nm pump beam (fwhm = 10 ns, 10 Hz repetition rate, 2.5 mJ/pulse) originating from a Spectra Physics Nd:YAG laser (Quanta-Ray Pro230). A 1000 W xenon lamp (Hanovia) pulsed by a lamp pulser (Sorensen Power Supplies) was used as the probe source. The detection system consisted of a monochromator (Digikrom 240, CVI Laser Corporation) coupled to a photomultiplier tube with the output recorded on a 1 GHz Lecroy oscilloscope. All nanohoop solutions (∼4 mL) were purged with N2 for approximately 20 min prior to experiments. Pulse Radiolysis. We employed a model Titan Beta-8/161S Electron Linear Accelerator (LINAC) coupled with a xenon lamp to carry out pulse radiolysis measurements. High energy electron pulses (∼8 MeV, 50 ns duration) were incident on biphenyl (0.05 M) and nanohoop solutions (∼20 μM in toluene and purged with N2) contained in a 1 cm quartz cuvette. The concentration of each nanohoop solution was determined by our calculation of the singlet extinction coefficient at ∼348 nm. We dissolved 0.7 mg of each nanohoop in 50 mL of toluene and calculated the concentration using the Beer−Lambert law. The singlet extinction coefficient of aza[8]CPP (68 300 M −1 cm −1 ) and 1,15-diaza[8]CPP (50 400 M−1 cm−1) were determined from the absorption spectrum of a solution of 0.7 mg of nanohoop powder dissolved in 50 mL of toluene.

Figure 1. (A) Normalized absorption (solid lines) and emission spectra (dashed lines) for aza[8]CPP (traces a and c) and 1,15diaza[8]CPP (traces b and d). (B) Emission lifetime traces for aza[8]CPP and 1,15-diaza[8]CPP generated by TCSPC measurements at the peak wavelength for each nanohoop.

the normalized absorption and emission spectra of both nanohoops. The absorption spectra of these nanohoops are similar in shape to those previously reported in the literature with the addition of a more pronounced shoulder from ∼375− 475 nm. We see an absorption peak at 348 nm for aza[8]CPP and 350 nm for 1,15-diaza[8]CPP and note the same 2 nm shift for the emission spectra, from 543 to 545 nm, respectively. The fluorescence quantum yield (Φf) of both nanohoops was determined from eq 1 ⎛ F ⎞⎛ 1 − 10 Astd ⎞⎛ nNH 2 ⎞ ⎟ Φf = Φstd⎜ NH ⎟⎜ ⎟⎜ ⎝ Fstd ⎠⎝ 1 − 10 ANH ⎠⎝ nstd 2 ⎠

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RESULTS AND DISCUSSION Singlet State Characterization. A useful way to characterize the excited state of organic molecules is through UV−visible absorption and emission spectroscopies. These techniques elucidate the behavior of the singlet-state of chemical species by describing the electronic transitions between the ground and excited state. In the present experiments, these techniques give us basic information about aza[8]CPP and 1,15-diaza[8]CPP

(1)

where F is the integrated fluorescence intensity, A is the absorbance, and n is the refractive index of the solvent used to dissolve the standard (std) and nanohoop (NH). Using eq 1 we calculate a fluorescence quantum yield of 0.11 and 0.13 for aza[8]CPP and 1,15-diaza[8]CPP, respectively. These values are summarized in Table 1. The relatively high fluorescence quantum yield of these samples motivates studies of their excited state dynamics. We B

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Table 1. Summary of Quantum Yields and Rate Constants for Radiative Recombination and Internal Conversion to the Triplet Excited State Φf aza[8]CPP 1,15-diaza[8]CPP

0.11 0.13

ΦT

kr (s−1)

τs (s) −9

1.4 × 10 1.6 × 107

8 × 10 8 × 10−9

0.45 0.32

kisc (s−1) 7

5.7 × 107 4.0 × 107

first probe the dynamics of the nanohoop excited state using time correlated single photon counting measurements (TCSPC). Shown in Figure 1B are emission lifetime traces recorded by TCSPC. We see little variation in the singlet lifetime between the two nanohoop samples and fit this data to a single exponential function y = A e(−t / τs)

(2)

where τs represents the lifetime of the excited state and A is a weighted coefficient representing the contribution of the exponent to the fit (relevant in biexponential fitting). From this fit we determine an excited state lifetime of 8 ns for both samples, confirming the observed similarity in the traces. Note that the τs parameter represents a direct measure of the lifetime of the singlet excited state. For aza[8]CPP and 1,15diaza[8]CPP we assume that deactivation from the singlet excited state is governed by the competition between three processes, each having a characteristic rate constant (k): (i) radiative recombination, kr, (ii) nonradiative recombination, knr, and (iii) internal conversion to the triplet excited state, kisc. By making this assumption, we can calculate each rate constant as long as we know the corresponding quantum yield of the process. For instance, we calculate the radiative rate of recombination in aza[8]CPP and 1,15-diaza[8]CPP using eq 3

kr =

Φf τs

Figure 2. Triplet−triplet absorption spectrum of aza[8]CPP and 1,15diaza[8]CPP generated by flash photolysis. Note that triplet absorption spectra were produced by monitoring the ΔA of each kinetic trace at ∼1 μs time delay.

generated through the absorption of an electron beam (see experimental section). This excited species participated in an energy transfer process to the excited state of nanohoop (NH). The maximum of the biphenyl T−T absorbance (ΔODBP at t = 0 μs) and the nanohoop absorbance (ΔODNH t ≈ 20 μs, Figure 3A) were used to estimate the triplet extinction coefficient of the nanohoops using eq 5

(3)

εNH = εBP

where τs and Φf are the experimental determinations of the singlet lifetime and the fluorescence quantum yield. Using eq 3, we determine kr for aza[8]CPP and 1,15-diaza[8]CPP to be 1.4 × 107 (s−1) and 1.6 × 107 (s−1), respectively. Rate constants (kr and kisc) are summarized in Table 1 with separate quantum yields for each process. We further deconvolute the excited state behavior of these molecules by examining their excited triplet properties. Triplet State Characterization. Previously, we reported the triplet characteristics of [9]CPP and [12]CPP, which have a broad triplet−triplet (T−T) absorbance extending the full width of the visible spectrum.2 Analysis of the T−T absorption spectrum of the two nitrogen doped nanohoops, Figure 2, reveals a broad T−T absorption spectrum that is similar to our previous reports. The two N-doped nanohoops have triplet spectra that are similar in shape, both having maxima at 410 and 740 nm. To better characterize the triplet state, we determine the triplet extinction coefficient using pulse radiolysis. Triplet extinction coefficients for aza[8]CPP and 1,15diaza[8]CPP were determined through energy transfer with an excited biphenyl (εBP = 27 100 M−1 cm−1, 0.05 M) triplet, Figure 3A,B, in toluene.2,32 The energy transfer reaction between these two species is illustrated in eq 4 3

BP* + (NH) → BP + (3 NH*)

(ΔOD NH /ΔODBP) Ptr

(5)

where ε represents the extinction coefficient, Ptr represents the probability of transfer, and the subscripts BP and NH represent biphenyl and nanohoop, respectively. The probability of transfer is calculated by eq 6 Ptr =

ket[NH] ket[NH] + kBP

(6)

where ket is the bimolecular rate constant of energy transfer determined from the dependence of nanohoop T−T signal growth on the nanohoop concentration, Figure 3A,B. The energy transfer rate (ket) is determined from the plot of the rate constants of acceptor growth (ka) versus concentration of acceptor, where ket is equal to the slope of the fit. The kBP term represents the rate constant of excited state decay of the 3BP* triplet in the absence of the nanohoop acceptor. This kinetic correction (Ptr) accounts for the probability that a 3BP* species will decay back to the ground state prior to the energy transfer reaction by taking the lifetime of the donor, the nanohoop concentration, and the energy transfer rate into account. Using the calculations in eqs 5 and 6 we arrive at extinction coefficients of 25 000 ± 1200 and 23 600 ± 1700 M−1 cm−1 for aza[8]CPP and 1,15-diaza[8]CPP, respectively. Singlet and triplet extinction coefficients as well as relevant values used in their calculations are summarized in Table 2. From the determination of εT we can calculate the triplet quantum yield of each nanohoop (ΦT). Similar to the

(4)

where NH and BP stand for nanohoop and biphenyl. In this set of experiments, the excited triplet state of bipehenyl, 3BP*, was C

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Figure 4. Representative kinetic traces of both nanohoops and the C60 standard to demonstrate ΦT calculations.

doped nanohoops. Substitution of ΦT for Φf returns kisc from our experimental parameters. Calculation of this parameter results in kisc values of 5.7 × 107 (s−1) and 4.0 × 107 (s−1) for aza[8]CPP and 1,15-diaza[8]CPP, respectively. Treatment with Trifluoroacetic Acid (Singlet). One of the more unique aspects of working with nitrogen-substituted derivatives of the traditional carbon nanohoops is the ability to work with them in their protonated or deprotonated states. While the ability to switch the fluorescence off using HCl has been previously demonstrated,35 the current work shows a more in depth analysis of the effect that protonation has on the excited state properties of these molecules. We characterize the proton-transfer between these molecules and trifluoroacetic acid (TFA), an acid that is readily soluble in toluene. The steady-state UV−visible absorption and emission spectra of aza[8]CPP treated with various concentrations of TFA are shown in Figure 5. We first note that both spectra demonstrate

Figure 3. (A) Growth of the 1,15-diaza[8]CPP (energy acceptor) T− T absorption signal at 450 nn. (B) Rate of T−T absorption signal growth at 450 nm for (a) aza[8]CPP and (b) 1,15-diaza[8]CPP. The slope of the straight line fit to these points is the bimolecular rate constant of energy transfer.

fluorescence quantum yield, the triplet quantum yield can be calculated from an analogous calculation in eq 733 ⎛ ΔODNH ⎞⎛ 1 − 10 A std ⎞⎛ εstd ⎞ ΦT = Φstd⎜ ⎟⎜ ⎟ ⎟⎜ ⎝ ΔODstd ⎠⎝ 1 − 10 ANH ⎠⎝ εNH ⎠

(7)

where ΔOD is the optical density at 0 μs of the kinetic trace, A is the steady state absorption at the excitation wavelength (355 nm), ε is the triplet extinction coefficient of the sample, and the subscripts “NH” and “std” correspond to nanohoop and standard, respectively. In the present experiments, we compare both nanohoop samples to C60 (employed as a triplet standard), which has a reported ΦT of unity, Figure 4.34 Using eq 7 and the results of the pulse radiolysis study we calculate a triplet quantum yield of 0.45 and 0.32 for aza[8]CPP and 1,15-diaza[8]CPP, respectively. Similar to the calculation of kr, we can use eq 3 to determine the rate constant of intersystem crossing, kisc, in the nitrogen

Figure 5. Steady-state absorption and emission spectra of aza[8]CPP treated with (a,f) 0 mM, (b,f) 0.5 mM, (c,h) 1 mM, (d,i) 2 mM, and (e,j) 9 mM TFA in toluene. Data for 1,15-diaza[8]CPP can be found in Figure S1.

an incremental decrease in peak intensity for both absorption and emission maxima, while the shape of the absorption

Table 2. Summary of Extinction Coefficients, Corresponding Peak Wavelengths, and ket Values

aza[8]CPP 1,15-diaza[8]CPP

λs (nm)

λT (nm)

εs (M−1 cm−1)

εT (M−1 cm−1)

ket (s−1)

348 350

740 740

68 300 ± 6600 50 400 ± 5800

25 000 ± 1200 23 600 ± 1700

9.7 × 103 7.7 × 103

D

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concentration of 1.4 μM. Since the concentration of TFA is ∼1.0 mM at the midway point of the titration plot (which we know from experimental parameters), we employ these four values to calculate Keq and obtain values of 1.39 × 10−3 for aza[8]CPP and 2.79 × 10−3 for 1,15-diaza[8]CPP. We highlight that Keq for 1,15-diaza[8]CPP is twice what is calculated for aza[8]CPP, which we predict to be due to the presence of an additional N in the nanohoop structure. The 2N doped ring requires a lower concentration of TFA to become at least singly protonated because the presence of two N atoms in the ring doubles the probability of the protonation reaction. Emission Lifetime of TFA Treated Nanohoops. The emission lifetime of these solutions also changes significantly as we protonate the nanohoops, demonstrated in Figure 7.

spectrum also changes as the peak intensity decreases. An isosbestic point arises at 365 nm for aza[8]CPP, suggesting that there are no intermediates or degradation-related products formed in the treatment of nanohoops with TFA. The only reaction that takes place is elucidated in eq 8 (NH) + TFA → (NH)−H+ + TFA−

(8)

which represents the proton-transfer from TFA to the carbon nanohoop (NH), forming the protonated nanhoop (NH)−H+. We also note that the emission peak red-shifts significantly (∼50 nm) with decreased intensity, suggesting that the NH−H product is slightly emissive when compared to NH. Note that similar results were found with both aza[8]CPP and 1,15diaza[8]CPP, so we have chosen to represent both molecules with the aza[8]CPP data and to include data for 1,15diaza[8]CPP (absorption and emission spectra) in the Supporting Information, Figure S1. The protonation reaction, eq 8, was observed to be reversible, which is demonstrated by the deprotonation reaction of the protonated nanohoop with triethylamine, Figure S2. The recovery of the fluorescence intensity confirms the deprotonation of the nanohoops upon addition of triethylamine. Determination of Keq for Nanohoop Protonation by TFA. To better characterize the protonation of the N-doped carbon nanohoops, eq 8, we titrate aza[8]CPP with TFA and continuously monitor the fluorescence at the peak wavelength until treatment with additional TFA results in no significant change in the emission intensity of the solution. We use the titration experiment, Figure 6, to calculate the equilibrium

Figure 7. Emission lifetime of aza[8]CPP treated with (a) 0 mM, (b) 0.5 mM, (c) 1 mM, (d) 2 mM, and (e) 9 mM TFA. Note that the decay data for the 9 mM TFA sample was measured at the peak wavelength, which had shifted to 595 nm. Emission lifetime data for 1,15-diaza[8]CPP is available in Figure S3.

Interestingly, we see a significant decrease in the emission lifetime when treating the pure nanohoop with 9 mM TFA. Kinetic analysis of the emission decay of the pure nanohoop returns a single exponential fit, due to the presence of only one emissive species in solution. We then treat four separate nanohoop samples with various concentrations of TFA, resulting in samples that contain some concentration of both the unprotonated nanohoop, NH, and protonated nanohoop, (NH)−H+. Due to the presence of two emissive species, these four decay traces were fit to a biexponential equation similar to eq 2, yet containing two exponential components. The kinetic parameters from biexponential fitting of the traces in Figure 6 are summarized in Table 3. Note that the concentrations used in the emission lifetime traces (Figure 6) and absorption/emission spectra (Figure 5) were chosen from

Figure 6. Titration of (a) aza[8]CPP and (b) 1,15-diaza[8]CPP to form the protonated versions of both nanohoops.

constant, Keq, for the protonation reaction. The equilibrium constant for this reaction is defined in eq 9 Keq =

[TFA−][(NH)−H+] [TFA][NH]

Table 3. Single and Biexponential Fitting Parameters for Emission Lifetime Traces of Aza[8]CPP Treated with Various Concentrations of TFAa

(9)

as the ratio of the concentrations of the products and reactants. To calculate Keq, we first assume that the quenching of the peak emission is indicative of nearly complete (∼99%) protonation of the NH species. We know that the concentrations of [NH] and [(NH)−H+] are expected to be equal at the point corresponding to one-half of the total change in emission. Also we expect that [TFA−] is equal to [(NH)−H +] because these species have a 1:1 stoichiometry, resulting in all three species (NH, (NH)−H+, and TFA−) having an equal

A1 0 mM TFA 0.5 mM TFA 1 mM TFA 2 mM TFA 9 mM TFA a

E

0.00 0.25 0.54 0.70 0.99

A2 1.00 0.75 0.46 0.30 0.01

τ1 (s) 2.4 2.4 2.4 2.4 2.4

× × × × ×

τ2 (s) −9

10 10−9 10−9 10−9 10−9

8.0 8.0 8.0 8.0 8.0

× × × × ×

χ2 −9

10 10−9 10−9 10−9 10−9

1.01 1.11 1.32 1.15 1.04

Fitting parameters for 1,15-diaza[8]CPP can be found in Table S1. DOI: 10.1021/acs.jpca.5b04404 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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sensitivity of both aza[8]CPP and 1,15-diaza[8]CPP to treatment with acid and the subsequent protonation of the nitrogen atom. The immediate response of these molecules to protonation by acidic species illustrates their utility in sensing applications and provides a demonstration of nanohoop reactivity toward an acidic species. Demonstrating reactivity in these typically unreactive molecules is an important first step in developing mechanisms toward their polymerization into well-controlled carbon nanotubes and other graphitic structures.

the titration experiments to represent solutions that contain 0%, 25%, 50%, 75%, and 99% of the (NH)−H+ species. We first fit the trace of unprotonated aza[8]CPP to obtain a τs of 8 ns, as described previously. Next, we fit the emission lifetime trace of aza[8]CPP that is fully (∼99%) protonated to the biexponential equation and hold the long lifetime (τ2) to 8 ns. This returns a value of 2.4 ns for the short lifetime (τ1), which is then used to fit the remaining three traces (75%, 50%, and 25% NH−H) to a biexponential trace in which we allow only the weights of each lifetime to fluctuate. Interestingly, the weight coefficients (A1 and A2) of each lifetime give a good approximation of the amount of each species originally assumed to be in solution. This result indicates that our assumption that the photoluminescence quenching corresponds to the reaction described in eq 7 is appropriate and reaffirms our findings from the Keq determination. The results of the biexponential fitting, including χ2, which represents the quality of the fit, are summarized in Table 3. Triplet Characterization with TFA. The protonation of the nitrogen-doped carbon nanohoops also results in significant changes in the triplet−triplet absorption spectrum, Figure 8.

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ASSOCIATED CONTENT

S Supporting Information *

Steady-state absorption, emission spectra, emission lifetimes, and T−T absorption spectra of 1,15-diaza[8]CPP at different TFA concentrations. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b04404.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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. 5971 from the Notre Dame Radiation Laboratory.

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REFERENCES

(1) Jasti, R.; Bhattacharjee, J.; Neaton, J. B.; Bertozzi, C. R. Synthesis, Characterization, and Theory of 9 -, 12 -, and 18 Cycloparaphenylene: Carbon Nanohoop Structures. J. Am. Chem. Soc. 2008, 130, 17646− 17647. (2) Hines, D. A.; Darzi, E. R.; Jasti, R.; Kamat, P. V. Carbon Nanohoops: Excited Singlet and Triplet Behavior of [9]- and [12]Cycloparaphenylene. J. Phys. Chem. A 2014, 118, 1595−1600. (3) Segawa, Y.; Fukazawa, A.; Matsuura, S.; Omachi, H.; Yamaguchi, S.; Irle, S.; Itami, K. Combined Experimental and Theoretical Studies on the Photophysical Properties of Cycloparaphenylenes. Org. Biomol. Chem. 2012, 10, 5979−5984. (4) Darzi, E. R.; Sisto, T. J.; Jasti, R. Selective Syntheses of [7]−[12] Cycloparaphenylenes Using Orthogonal Suzuki−Miyaura CrossCoupling Reactions. J. Org. Chem. 2012, 77, 6624−6628. (5) Xia, J. L.; Jasti, R. Synthesis, Characterization, and Crystal Structure of 6 Cycloparaphenylene. Angew. Chem., Int. Ed. 2012, 51, 2474−2476. (6) Nishihara, T.; Segawa, Y.; Itami, K.; Kanemitsu, Y. Excited States in Cycloparaphenylenes: Dependence of Optical Properties on Ring Length. J. Phys. Chem. Lett. 2012, 3, 3125−3128. (7) Segawa, Y.; Senel, P.; Matsuura, S.; Omachi, H.; Itami, K. [9] Cycloparaphenylene: Nickel-mediated Synthesis and Crystal Structure. Chem. Lett. 2011, 40, 423−425. (8) Segawa, Y.; Miyamoto, S.; Omachi, H.; Matsuura, S.; Senel, P.; Sasamori, T.; Tokitoh, N.; Itami, K. Concise Synthesis and Crystal Structure of [12] Cycloparaphenylene. Angew. Chem., Int. Ed. 2011, 50, 3244−3248. (9) Takaba, H.; Omachi, H.; Yamamoto, Y.; Bouffard, J.; Itami, K. Selective Synthesis of [12] Cycloparaphenylene. Angew. Chem., Int. Ed. 2009, 48, 6112−6116.

Figure 8. Triplet−triplet absorption spectra for aza[8]CPP treated with (a) 0 mM, (b) 0.5 mM, (c) 1 mM, (d) 2 mM, and (e) 9 mM TFA. We consider spectra (a) to be entirely unprotonated and spectra (e) to be entirely protonated, which is confirmed by both the titration and emission lifetime experiments. Triplet−triplet absorption spectra for 1,15-diaza[8]CPP can be found in Figure S4. Note that triplet absorption spectra were produced by monitoring the ΔA of each kinetic trace at ∼1 μs time delay.

We demonstrate these changes in the spectrum using the same concentrations of TFA used in the TCSPC experiments. The most significant change in the spectrum is a decrease in the triplet−triplet absorption features between 575 and 775 nm. This indicates that the protonation of the nanohoop has a significant effect on the electronic structure or, more likely, the quantum yield of intersystem crossing for these molecules. Note that spectra (b), (c), and (d) all represent a solution that is a mixture of protonated and unprotonated nanohoop. Only spectra (a) and (e) represent triplet−triplet spectra for carbon nanohoops that we can consider to be entirely unprotonated and protonated, respectively.

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CONCLUSION We have characterized the singlet and triplet excited state of two nitrogen-doped carbon nanohoops, aza[8]CPP and 1,15diaza[8]CPP. A significant finding of the present study is the F

DOI: 10.1021/acs.jpca.5b04404 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpca.5b04404 J. Phys. Chem. A XXXX, XXX, XXX−XXX