pubs.acs.org/NanoLett
Triplet States and Electronic Relaxation in Photoexcited Graphene Quantum Dots Mallory L. Mueller,† Xin Yan,† John A. McGuire,‡ and Liang-shi Li*,† †
Department of Chemistry, Indiana University, Bloomington, Indiana 47405 and ‡ Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824 ABSTRACT Electronic relaxation in photoexcited graphenes is central to their photoreactivity and their optoelectrical applications such as photodetectors and solar cells. Herein we report on the first ensemble studies of electronic energy relaxation pathways in colloidal graphene quantum dots with uniform size. We show that the photoexcited graphene quantum dots have a significant probability of relaxing into triplet states and emit both phosphorescence and fluorescence at room temperature, with relative intensities depending on the excitation energy. Because of the long lifetime and reactivity of triplet electronic states, our results could have significant implications for applications of graphenes. KEYWORDS Graphene, excited states, electronic relaxation, triplet states
T
compete with internal conversion (Scheme 1)14-20 due to a large singlet-triplet splitting (∼ 1 eV) and weak spin-orbit coupling.21-25 As a result, intersystem crossing occurs mostly from the lowest singlet or triplet excited states (S1 and T1 in Scheme 1, respectively), leading to a quantum yield of fluorescence or phosphorescence independent of excitation wavelength.26 However, in very large aromatic hydrocarbons such as graphenes this may not be valid, as the delocalization of electrons over a large area significantly reduces the singlet-triplet splitting and thus considerably increases the probability of intersystem crossing.15,21-25 According to perturbation theory, the amplitude of mixing between singlet and triplet state wave functions due to the spin-orbit coupling is given by15,21-25
he large optical absorptivity and widely tunable band gap of graphene make it an attractive material for optical and optoelectrical devices.1,2 This has led to emerging applications ranging from saturable absorbers for laser pulse shaping3,4 to ultrafast photoresistors5,6 and photodetectors.7,8 Recently, as a part of our efforts to create carbon-based materials for photovoltaics, we have developed a solution-chemistry approach to stable colloidal graphene quantum dots for light harvesting.9,10 The size of the quantum dots can be precisely controlled, so that they can have an absorption edge of 1.4 eV, the optimal value for single-junction solar cells. Since electronic relaxation in photoexcited graphenes is central to these applications,11 it is of critical importance to understand the processes occurring after graphenes absorb light. Herein we report on the first ensemble photophysical studies of electronic relaxation in graphene quantum dots. We show that the photoexcited graphene quantum dots have a significant probability of relaxing into triplet states and can emit phosphorescence as well as fluorescence with intensities dependent on excitation energy. Because triplet states have very long lifetimes and are often reactive, our results could have important implications in applications of graphenes and their photostability in devices. Graphene quantum dots are natural extensions of polycyclic aromatic hydrocarbons, the photophysics of which has been extensively studied.12,13 However, the large size of graphene could lead to novel photophysical phenomena because of the reduced spacing between the electronic energy levels. For example, in aromatic hydrocarbons intersystem crossing from high excited electronic states with a given spin multiplicity generally is not efficient enough to
cST ≈
(1)
where C is the spin-orbit coupling matrix element and ES and ET the energy of the singlet and triplet states, respectively. If ES and ET approach each other, e.g., due to a weaker singlet-triplet splitting, the mixing amplitude becomes significant and the probability of intersystem crossing gets enhanced, which is known as the “energy gap law” for nonradiative energy relaxation.22-24 To understand the photophysical behavior of graphenes, we have conducted studies on monodisperse graphene quantum dots in solution. The graphenes we studied contain 132 conjugated carbon atoms (1, the graphene core is marked in blue) and were synthesized through solution chemistry.10 Stepwise, controlled solution chemistry enables us to obtain identical, well-characterized precursors for the graphenes, leading to graphene quantum dots with uniform size and shape. The graphenes are enclosed in all three dimensions by three solubilizing 2′,4′,6′-trialkyl phenyl groups
* To whom correspondence should be addressed,
[email protected]. Received for review: 04/26/2010 Published on Web: 06/24/2010 © 2010 American Chemical Society
C ES - ET
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DOI: 10.1021/nl101474d | Nano Lett. 2010, 10, 2679–2682
SCHEME 1. Dot 1a
Jabłon`ski Diagram (left) and Graphene Quantum
solvent to those containing heavy atoms, such as 3-bromoiodobenzene (Figure 1b), which is known to promote nonradiative depopulation of triplet states and thus could quench phosphorescence,14,15 dramatically reduces the intensity of the 740 nm emission, suggesting its phosphorescent nature. The intensity of the 670 nm emission is also reduced, though to a much less extent. To confirm that the emission with maximum at 740 nm is phosphorescence, we studied its decay dynamics after excitation with nanosecond laser pulses. The time-dependent behavior is shown in Figure 2a, which shows a single exponential decay with a time constant of 4 µs at room temperature. The emission was monitored at 780 nm instead of 740 nm to reduce the contribution from the 670 nm emission. In contrast, the dynamics of the 670 nm emission (Figure 2b, excited with laser pulses with 50 ps width, see the Supporting Information) can be fitted with a biexponential decay, with time constants of 5.4 and 1.7 ns (weight 78% and 21%, respectively), indicating it is fluorescence. The energy spacing of ∼175 meV (∼1400 cm-1) between the fluorescence and phosphorescence peaks confirms a small singlet-triplet splitting, as expected from the size of 1. Further, it suggests significant energy overlap between the lowest singlet excited states (i.e., S1-S3) and the lowest triplet ones. Both the fluorescence and the phosphorescence show intensities dependent on excitation wavelength. In Figure 3 we show the excitation spectrum for fluorescence as the blue curve (monitored at 670 nm). The emission intensity decreases as 1 is excited to S1, S2, and S3, respectively, despite the increasing extinction coefficients of the three absorption bands, indicating nonunity efficiency of internal conversion from S2 or S3 states to S1. The emission then increases abruptly when 1 is excited to S4 or higher excited states. Similar excitation-wavelength dependence has been reported in fluorescence of benzene and other smaller aromatic compounds.27-30 The reduced emission when excited to the S2 or S3 state however has not been completely understood. Various competing pathways have been proposed, ranging from quenching by photoreaction products
a S0 represents the ground electronic state and Sn and Tn (n ) 1, 2, ...) represents singlet and triplet excited states, respectively. Key: IC, internal conversion; ISC, intersystem crossing; Fl., fluorescence; Phos., phosphorescence.
and thus are stable in various organic solvents without aggregation.9 The high degree of conjugation in 1 leads to considerable overlap among absorption bands. Figure 1a shows the absorption spectrum (black curve, left axis) of 1 dissolved in toluene at room temperature, which continuously covers most of the UV-visible range. The spectrum follows a classic pattern observed in many aromatic hydrocarbons (i.e., R, p, and β bands).12,13 By comparison with other polycyclic aromatic hydrocarbons with various symmetries, we have assigned the bands to transitions from the ground state (S0) to various excited singlet states (S1 to S4, Figure 1a).10 Vibronic broadening and the small energy spacing between the excited electronic states lead to the absorption spectrum appearing continuous. No vibronic structure could be resolved in the spectrum. Graphene 1 shows both fluorescence and phosphorescence at room temperature in toluene. Figure 1a (blue curve, right axis) shows its emission spectrum when excited at 510 nm. The solution had a concentration of 2.0 µM and was bubbled with argon to remove oxygen. Two maxima were observed at 670 and 740 nm, respectively. Changing the
FIGURE 1. Absorption and photoluminescence spectra of 1 in deoxygenated toluene (a) and 3-bromoiodobenzene (b), respectively. Absorption spectra are marked black and emission spectra blue. The quantum yield of the emission is 0.02 in panel a. The final state responsible for each of the absorption bands is marked. In panel b the intensity of the emission is dramatically reduced due to the heavy atoms in the solvent, with the peak with maximum at 740 nm reduced much more than the one at 670 nm. The fluorescence spectra in a and b are from solution with the same concentration, with intensity in scale. © 2010 American Chemical Society
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FIGURE 2. Lifetime studies of photoluminescence of 1 in toluene, monitored at 780 nm (a) and 670 nm (b), respectively. In (a) the decay is fit with a single exponential (red line), with a time constant of 3.97 µs. In (b) the decay is fit with a biexponential (red curve), with time constants of 5.4 and 1.7 ns (weight 78% and 21%, respectively). There is a third component with a much longer decay time that accounts for less than 0.5% of the signal. The lifetime measurements indicate that the emission with maxima at 740 and 670 nm are phosphorescence and fluorescence, respectively.
CsH bonds.31 The large number of atoms in graphenes would significantly increase the number of such vibrational modes. In addition, recently graphene has been shown to crumplespontaneouslyduetoathermodynamicinstability.32-34 These deviations from planarity can further increase the spin-orbit coupling and thus promote intersystem crossing. We anticipate the increased access to triplet states observed in graphene quantum dot 1 could be quite general in other graphene nanostructures and may therefore have important implications in their applications. For example, triplet states are often highly reactive,15 which could significantly affect the photostability of graphenes. We have observed photoinduced degradation of 1 in deoxygenated dichloromethane (DCM) solution. In DCM only fluorescence of 1 was observed (see the Supporting Information), which diminished within minutes under ambient light. The nature of the photoreaction is currently under investigation. Furthermore, triplet excited states have significantly longer lifetimes than singlet ones, and triplet excitons tend to have longer diffusion lengths.15 Both these features are beneficial for application of graphenes in photovoltaic devices where slower carrier recombination9 and long-distance charge transport are highly desirable. The triplet-state formation could also enable optical spin injection in the graphene quantum dots for spintronics, in which long spin decoherence time is advantageous for spin manipulation.35,36
FIGURE 3. Photoluminescence excitation spectra of 1 in toluene. The blue curve is for fluorescence, monitored at 670 nm, and the red curve for phosphorescence monitored at 780 nm. The absorption spectrum (black dotted curve) is also shown for comparison. The broad band in the red curve marked by the arrow may be the S0 f T1 transition, which is difficult to observe in the absorption spectrum due to the small extinction coefficient but is enhanced in the phosphorescence excitation spectrum.12
to formation of excimers of higher energy.27 The abrupt increase in fluorescence when states higher than S3 are excited has been attributed to autoionization of the chromophores in solution,27 the applicability of which to graphene 1 is yet to be determined experimentally. In contrast, with increasing excitation energy, the intensity of phosphorescence (red curve in Figure 3, monitored at 780 nm) roughly follows the absorption spectrum (black dotted curve in Figure 3). Its dramatic difference from the fluorescence excitation spectrum, especially the increased phosphorescence when 1 is excited to higher excited states, indicates that intersystem crossing occurs efficiently not only from the lowest excited state S1 but also from higher excited states. This is consistent with what is expected from the small energy spacing between the singlet and triplet excited states in such a large conjugated system. The exact relaxation pathways, however, can be complicated because of the number of the electronic and vibrational states involved. Besides reducing the singlet-triplet splitting, the large size of the graphenes could affect the energy relaxation through the spin-orbit coupling matrix element (C in eq 1) as well. In unsubstituted planar aromatic hydrocarbons, singlet-triplet coupling is achieved through vibronic coupling involving out-of-plane bending modes of CdC and © 2010 American Chemical Society
Acknowledgment. We thank Professors Charles Parmenter and Bogdan Dragnea for helpful discussions. We also thank Junyong Jo and Professor Dongwhan Lee for help with photoluminescence quantum yield measurements. This work is supported by Petroleum Research Fund (47677- G10) and National Science Foundation (No. 0747751). Supporting Information Available. Detailed experimental procedures, UV-vis absorption and emission of 1 in dichloromethane, and photoluminescence quantum yield determination of 1. This material is available free of charge via the Internet at http://pubs.acs.org. 2681
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