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Direct Observation of a Triplet-State Absorption-Emission Conversion in a Fullerene-Functionalized Pt(II) Metallacycle Rui-Ling Zhang,†,‡ Yang Yang,† Song-Qiu Yang,† VenKata S. Pavan K. Neti,§ Hajar Sepehrpour,§ Peter J. Stang,*,§ and Ke-Li Han*,† †

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China ‡ University of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, United States S Supporting Information *

ABSTRACT: An interesting triplet excited-state absorption-emission conversion of a fullerene-functionalized Pt(II) metallacycle (C60−Pt) caused by a concentration effect was directly observed by nanosecond transient absorption (ns TA) spectroscopy. In dilute solution, the triplet excited-state absorption (TESA) band was observed at about 750 nm with a lifetime of ca. 10.7 μs. However, with increasing the concentration, the absorption band converted to a triplet excited-state emission (TESE) band with a longer lifetime of ca. 15.4 μs. Femtosecond transient absorption experiments and quantum chemistry calculations were performed to reveal the excited-state decay pathways of C60−Pt in concentrated solution. This conversion was ascribed to the formation of a triplet excimer, which forms at localized 3C*60 states. This work demonstrates that radiative excimers with longer-lived triplet excited states can exist in concentrated solution, and this finding will provide useful information for applications of fullerene complexes, especially as photosensitizers.

1. INTRODUCTION In recent years, fullerene and its derivates have attracted extensive attention on their special photophysical properties for applications in optoelectronics1−5 and biomedicine.6−11 C60 is not only an excellent electron donor in solar cells12−15 but is also excellent in spin-conversion with high intersystem crossing (ISC) efficiency approaching 1.16 These characteristics, high ISC efficiency, and long triplet-state lifetime make C60 and its derivates promising candidates as photosensitizes, in particular for generating singlet oxygen.10,17 Among the previous investigations, most attention has been focused on the behavior of monomers. However, with increasing concentration in solution, new species, excimers,18−20 will appear with changes in the excited-state decay pathways, and hence, the performance for applications may be affected. Therefore, it is important to explore the behavior of excimers of fullerene complexes. Since the first observation of excimer fluorescence from pyrene,21 several studies have been performed to understand the nature of singlet and triplet excimers by various spectroscopic techniques, such as steady-state absorption and emission spectra and time-resolved transient absorption spectra.22−25 Previous studies have shown that excimers dissipate the excitation energy and usually increase nonradiative transition.26 Recently, the systems have been reported to show new roles, such as enhancing phosphorescence quantum yield27 and facilitating singlet fission28,29 and electron transfer.30 These © 2017 American Chemical Society

results suggest that excimers can participate in complicated photophysics processes and can play an important role in tuning the photophysical properties of compounds. Pt-containing compounds easily form excimers in solution because of their square-planar geometries that can affect the intermolecular electronic interactions.31−33 Herein, we prepared a fullerene-functionalized Pt(II) metallacycle34 C60−Pt (Figure 1a) and explored its excited-state decay pathways in concentrated solution by steady-state spectra, time-resolved fluorescence spectra, and femtosecond and nanosecond transient absorption (fs and ns TA) spectra. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were also performed to understand the origin of the low-lying excited states.

2. EXPERIMENTAL AND THEORETICAL METHODS The fullerene-functionalized Pt(II) metallacycle (C60−Pt) was prepared according to the literature procedure.34 The solvents dichloromethane (DCM) and dimethyl sulfoxide (DMSO) used in the experiments were high-performance liquid chromatography grade. The concentration of solution was determined by PerkinElmer 7300 DV inductively coupled plasma-optical emission spectroscopy (ICP-OES). Received: May 24, 2017 Published: June 14, 2017 14975

DOI: 10.1021/acs.jpcc.7b05025 J. Phys. Chem. C 2017, 121, 14975−14980

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The Journal of Physical Chemistry C

The phosphorescence spectra were measured at 77 K by using liquid N2. The samples were thoroughly degassed via nitrogen bubbling for 1 h prior to obtaining phosphorescence. The timeresolved fluorescence decays were recorded using the timecorrected single photocounting (TCSPC) method. Data analysis was conducted via commercial software provided by Horiba Instruments. Nanosecond transient absorption spectra were carried out using a homemade pump−probe setup with an apparatus response time of 100 ns. The laser pulse was 355 nm with full width at half maximum of 10 ns. The laser energy used in the experiments was less than 1 mJ/pulse. All the samples were thoroughly degassed via nitrogen bubbling for 1 h. Femtosecond transient absorption measurements were carried out by using a pump−probe laser system based on a regenerative amplified Ti:sapphire laser system from Coherent (800 nm, 35 fs, and 1 kHz repetition rate). The sample was excited by a 360 nm laser pulse generated by a TOPAS optical parametric amplifier (OPA) which was pumped by the 800 nm pulse. The power of the pump beam in front of the sample was kept at 0.2 μJ/cm2. A white light continuum (WLC) generated by a 2 mm thick sapphire window from 420 to 800 nm was used as the probe beam. The delay between the pump and probe pulses was controlled by a motorized delay stage. Fs TA spectra were measured by using a quartz cuvette with a path length of 1 mm at room temperature. All quantum chemical calculations were performed using Gaussian 09 program. Ground-state geometry optimization was carried out by DFT method with the BP86 functional, and the excited states were investigated by TDDFT method with the cam-B3LYP functional.35−37 Throughout the ground-state optimization and excited-state studies, the SVP basis set was chosen for nonmetallic elements and the LANL2DZ basis set was selected for platinum atoms. To simplify calculation, the ethyl groups on phosphorus were replaced by H atoms.37,38

Figure 1. (a) Molecular structure of C60−Pt. (b) Steady-state absorption and fluorescence spectra of C60−Pt in DCM and DMSO. Inset: the absorption spectra from 400 to 500 nm.

Steady-state absorption spectra were performed on a PerkinElmer Lambda35 spectrophotometer. Steady-state fluorescence spectra and phosphorescence spectra were recorded using a Horiba JobinYvon FluoroMax-4 spectrofluorometer.

Figure 2. Nanosecond transient absorption spectra of C60−Pt in DMSO with excitation at 355 nm. (a) Top is for dilute solution (∼30 μM) and bottom is for concentrated solution (∼120 μM); (b) kinetic profiles at 735 and 750 nm are shown by blue (concentrated solution) and black (dilute solution) curves with corresponding fits (red line). 14976


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Figure 3. Femtosecond transient absorption spectra of C60−Pt in DMSO in concentrated solution (∼120 μM) upon excitation at 360 nm. (a) Contour plot of the femtosecond transient absorption spectra; (b) normalized DADS derived from global analysis; (c) femtosecond transient absorption spectra at selected decay times; (d) kinetic profiles at 750 and 510 nm with global fits.

yield. With increasing the concentration, the fluorescence intensity increases until a certain concentration and then decreases (Figure S3). No new absorption and emission bands are observed. Nanosecond transient absorption (ns TA) spectroscopy was performed to characterize features of the triplet excited states. The ns TA spectra of C60−Pt in degassed DMSO solution at different concentrations are shown in Figure 2. For the dilute solution (∼30 μM), the spectrum exhibits an excited-state absorption (ESA) band at ca. 750 nm, which is a characteristic * .44 However, with increasing the concenabsorption of 3C60 tration, the ESA band transforms to an excited-state emission (ESE) band, and a new ESA band at ca. 735 nm appears. To rule out the possibility that the emission signal of the concentrated solution at ca. 750 nm results from ground-state bleaching, the steady-state phosphorescence spectrum (Figure S4) was also measured, and it confirmed the emission behavior. All the signals, including ESA in dilute solution and ESE and ESA in concentrated solution disappeared in aerated solution, indicating that the long-lived species could be quenched by molecular oxygen and, hence, could be ascribed to the triplet excited sates. The kinetic profiles of C60−Pt in dilute and concentrated solution monitored at 750 and 735 nm are shown in Figure 2b. These decay profiles can be well fitted by a monoexponential function. The lifetime value of 750 nm in dilute solution is ca. 10.7 μs, which is much shorter than that in a concentrated solution with ca. 15.4 μs. The lifetime comparison rules out the possibility that the emission band at ca. 750 nm in concentrated solution results from a delayed fluorescence (DF). Such a DF may be due to two kinetic pathways: unimolecular tripletsinglet upconversion via reverse ISC or a bimolecular annihilation of triplet manifolds.45,46 The lifetime of the DF is expected to be equal to that of the triplet states for the former

3. RESULTS AND DISCUSSION The steady-state absorption and fluorescence spectra of C60−Pt in DCM and DMSO are shown in Figure 1b. The absorption spectrum of C60 − Pt in DMSO exhibits a sharp band at 433 nm, which is characteristic of [6,6]-closed fullerene monoadducts,39,40 and a weak band at 701 nm, which is assigned to the spin-allowed zero−zero transition from the ground state to the first singlet excited state involving the fullerene moiety.41,42 Both the absorption and fluorescence spectra in DMSO have a small red shift compared to that in DCM. The fluorescence spectrum in DMSO has a peak at 713 nm and a broad shoulder at about 800 nm. The fluorescence quantum yield is very low, less than 1%. Fullerene derivatives are reported to show similar double peaks with the major peak at about 700 nm.43 Thus, the emission is assigned to the fullerene-based fluorescence. Both the fluorescence decays of C60−Pt in DMSO and DCM, which were obtained by the TCSPC method, can be fitted to biexponential functions (Figure S1). In DMSO, the longer lifetime of ca. 1.5 ns is equal to that in DCM. However, the shorter lifetime of sub-200 ps, which is limited by the resolution of the setup, is much smaller than that in DCM. The observation that the lifetime of the short-lived species will decrease with increasing solvent polarity indicates that this species possesses charge-transfer (CT) character. This inference is confirmed by DFT/TDDFT calculations (Figure S2). The calculations show that the first singlet excited state S1 has two degenerate states S1−1 and S1−2. The S1−1 state exhibits charge-transfer character (fullerene−fullerene charge transfer) and the S1−2 state shows only internal fullerene character. Hence, the short-lived species with a lifetime of sub-200 ps can be assigned to the S1−1 state and the long-lived species with a lifetime of 1.5 ns can be assigned to the S1−2 state. Both the charge-transfer character and high ISC efficiency of the fullerene moiety account for the low fluorescence quantum 14977


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Figure 4. Excited-state decay pathways of C60−Pt in concentrated solution.

less than 100 fs. The first component (τ1 = 6 ps) is ascribed to the triplet−triplet energy transfer (TTET) from the 3CT state to the 3C*60 state. Given that the lifetime of the S1−1 state is estimated to be less than 200 ps in DMSO, the second component (τ2 = 130 ps) is assigned to the decay of the S1−1 state to the ground state. The third DADS with a time constant of 1.4 ns is characterized by the strong absorption feature at 510 nm for the 1C*60 state. This time constant is not only consistent with the lifetime of 1.3 ns of 1C60 * reported in previous studies47 but also is consistent with the lifetime of the S 1−2 state which is obtained from the time-resolved fluorescence spectra. Therefore, the S1−2 state is dominated by the 1C60 * state. Because of the high ISC efficiency of C60 moiety, ISC is the main decay pathway of S1−2 state, and the relevant rate constant is estimated to be 1.4 ns. In concentrated solution, triplet excimer formation is the main decay pathway of the 3C60 * state. Therefore, the residual component which cannot be resolved within the 7.8 ns experimental window represents the rate constant of the step from the 3C*60 state to the triplet excimers. Notably, although the C60 is an excellent electron acceptor with low reorganization energy for electron transfer, the excited-state absorption bands of ions corresponding to the electron transfer from the Pt frame moiety to the C60 moiety ·+ generating C·− 60−Pt were not observed in the ns and fs TA spectra. Also, the final long-lived species forming triplet excimers was in the triplet manifolds and not in the ions. These results suggest that the triplet charge-separated state was not populated. Similar results have been reported in other fullerene-transition-metal complexes.4,51

and half of the triplet lifetime for the latter.46,47 Thus, the emission band at ca. 750 nm cannot result from delayed fluorescence and possibly results from a new species, triplet excimers.18,20,22 The absorption species monitored at 735 nm decays with a time constant of 7.5 μs, which is approximately half of that at 750 nm. When a trace of oxygen was added to the concentrated solution by adding a bit of aerated DMSO into the solution to perturb the kinetic time constants of the triplet states,48 the lifetimes of 735 and 750 nm were shortened to 5.5 and 12.0 μs. The former is still approximately half of the latter. This result demonstrates that the TESA band at ∼735 nm and the TESE band at ∼750 nm stem from identical species. Analogously to the delayed fluorescence due to the triplet− triplet (T-T) annihilation, we assign the TESA band at 735 nm to a delayed excimer absorption (DEA) because of the bimolecular annihilation of the triplet excimers. To provide a clear picture of the excited-state dynamics connecting the initial photoexcited state with the triplet excimers, which are probed by ns TA spectroscopy, fs TA spectroscopy was carried out on a solution of C60−Pt in DMSO. Both fs TA spectra in dilute (Figure S6) and concentrated solution (Figure 3) at short delay times (i.e., before 160 ps) are dominated by two excited-state absorption bands with peaks at 510 nm and ca. 750 nm and by a broad, unstructured absorption band from 550 to 675 nm with chargetransfer character. In the concentrated solution, at long delay times (i.e., 1.14 ns), the band at 510 nm decays to a flat, featureless absorption band but the band at 750 nm remains unchanged. The former band is assigned to the absorption of localized 1C*60 state with characteristic absorption at 510 nm,49 and the latter band is assigned to the absorption of the 3C*60 state, which is consistent with the triplet-state absorption band at ca. 750 nm observed in the ns TA spectra in dilute solution. These spectral features suggest that the triplet excimers in concentrated solution form at 3C*60 states. Global analysis was performed to estimate the rate constants of the primary excited-state decay pathways of C60−Pt. Singular value decomposition (SVD) was used for the global analysis to yield decay associated difference spectra (DADS).50 The normalized DADS of four resolved components are shown in Figure 3b. When the complex C60−Pt is irradiated with a laser pulse of 360 nm, both 1C60 excited states and 1CT (such as 1 metal-to-ligand charge transfer of the Pt frame) states will be populated. On the one hand, the photoexcited states undergo fast internal conversion (IC) to populate the low-lying excited states, S1−1 and S1−2. On the other hand, ISC from 1CT to 3CT occurs within the instrument response time with a rate constant

4. CONCLUSION In the present work, the excited-state decay pathways of a fullerene−Pt complex in concentrated solution were investigated and are shown in Figure 4. The photoexcited state eventually decays to a triplet charge transfer (3CT) state and two singlet degenerate states (S1−1 and S1−2). On the one hand, the 3CT undergoes triplet−triplet energy transfer to 3C60 * states at τ1 = 6 ps. On the other hand, ISC of the 1C60 * state occurs at τ3 = 1.4 ns to generate the 3C*60 state. Triplet excimers with a lifetime of ca. 15.4 μs form at a localized 3C*60 state and decay to the ground state by radiative transition, which accounts for the triplet-state absorption-emission conversion observed in the ns TA spectra. We show that a radiative excimer with longerlifetime triplet state can exist in a concentrated solution of fullerene−Pt complexes. Fullerene complexes can be used as photosensitizers, in particular for generating singlet oxygen for 14978


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The Journal of Physical Chemistry C photodynamic therapy.6−11 The efficiency of generating singlet oxygen is related to the lifetime of triplet states. Hence, this finding not only shows a new role of excimers but also provides useful information for fullerene complexes’ application as photosensitizers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b05025. Steady-state absorption and emission spectra with different concentrations, time-resolved fluorescence spectra, calculated frontier molecular orbitals for the low-lying excited states, phosphorescence spectrum for condensed solution, ns TA spectra with different concentrations, and fs TA spectra in low concentration (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Peter J. Stang: 0000-0002-2307-0576 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (NSFC; Grant 21403226 and 21503226) and National Key Basic Research Science Foundation of China (NKBRSF; Grant 2013CB834604). P. J. S. thanks the National Science Foundation (NSF; Grant 1212799) for financial support.

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