Harvesting Fluorescence from Efficient Tk → Sj (j, k > 1) Reverse

Article pubs.acs.org/JPCC

Harvesting Fluorescence from Efficient Tk → Sj (j, k > 1) Reverse Intersystem Crossing for ππ* Emissive Transition-Metal Complexes Yuh-Chia Chang,†,∥ Kuo-Chun Tang,†,∥ Hsiao-An Pan,† Igor O. Koshevoy,*,‡ Antti J. Karttunen,§ and Pi-Tai Chou*,† †

Department of Chemistry, National Taiwan University, Taipei 106, Taiwan Department of Chemistry, University of Eastern Finland, Joensuu 80101, Finland § Department of Chemistry, University of Jyväskylä, PO Box 35, FI-40014 Jyväskylä, Finland ‡

S Supporting Information *

ABSTRACT: Using a bimetallic Au(I) complex bearing alkynyl-(phenylene)3-diphosphine ligand (A-3), we demonstrate that the fluorescence can be exquisitely harvested upon T1 → Tk (k > 1) excitation followed by Tk → Sj (j, k > 1) intersystem crossing (ISC) back to the S1 state. Upon S0 → S1 355 nm excitation, the S1 → T1 intersystem crossing rate has been determined to be 8.9 × 108 s−1. Subsequently, in a two-step laser pump− probe experiment, following a 355 nm laser excitation, the 532 nm T1 → Tk probing gives the prominent blue 375 nm fluorescence, and this time-dependent pump−probe signal correlates well with the lifetime of the T1 state. Careful examination reveals the efficiency of Tk → Sj (j, k > 1) reverse intersystem crossing to be 5.2%. The result is rationalized by a mechanism incorporating substantial involvement of metal-to-ligand charge transfer (MLCT) in the Tk (Sj) states, enhancing the rate of Tk → Sj ISC, which is competitive with the rate of Tk → T1 internal conversion. This mechanism is also proven to be operative in the A-3 solid film and should be universally applicable to the transition-metal complexes possessing a dominant ππ* configuration in the lowest-lying states. From an energy point of view, the UV fluorescence (375 nm) generated by green (532 nm) excitation can be recognized as a signal upconversion process.

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INTRODUCTION Emissive transition-metal complexes have been receiving considerable interest because they are pivotal for the fabrication of highly efficient organic light emitting diodes (OLEDs).1 The spin−orbit coupling enhanced by their central heavy metal atoms promotes an efficient singlet−triplet intersystem crossing (ISC), and facilitates strong luminescence in the as-fabricated OLEDs by harvesting both the singlet and triplet excitons.2 As a result, more emphasis has been put on the design of transitionmetal phosphors and characterizing their photophysical properties suited for fabricating practical OLEDs.3 Recent advances have inferred intersystem crossing involving dπ* and pure ππ* electronic configurations to the internal and external metal atom effect, respectively.4 For the former case, tuning the percentage of metal dπ* contribution, i.e., the percent of metalto-ligand charge transfer (MLCT%, or vice versa, the percent of ligand-to-metal charge transfer, LMCT%), in S1 by altering ligand HOMO π energy level (relative to the metal dπorbital) has demonstrated that the increase of MLCT% facilitates rate of ISC and the subsequent phosphorescence radiative decay rate. For many third-row transition-metal complexes having substantial MLCT contribution S1 → T1, kISC can even be ≫1012 s−1 with a T1 → S0 phosphorescence radiative lifetime as short as few microseconds.5 For the transition-metal complex where ππ* is the dominant configuration for both lower-lying © XXXX American Chemical Society

singlet and triplet states, the core transition metal simply executes the external heavy atom effect such that ISC, in theory, is relatively slow. Using a series of Os(II) complexes for which the dπ orbital is greatly stabilized by the π-accepting CO ligand, our recent advance6 has shown a slow S1(ππ*) → Tn(ππ*, n ≥ 1) ISC rate of subnanoseconds, so that both fluorescence and phosphorescence can be spectrally resolved in the steady-state manner in room-temperature solution. Moreover, the branching ratio for the fluorescence versus phosphorescence intensity could be excitation-energy-dependent, in which electronic transitions possessing substantial MLCT character result in a greater phosphorescence contribution. The underlying mechanism incorporates fast Si → Tj (i, j > 1) ISC that is capable of competing with the Si → S1 internal conversion (IC), defying the well-known Kasha’s rule7 and giving direct population at the triplet state and the resulting phosphorescence that bypasses the conventional pathway of slow S1 (ππ*) → T1(ππ*) ISC. This phosphorescence harvesting mechanism should be applicable for the transition-metal complexes possessing a dominant S1 (ππ*) configuration. Very recently, we have further generalized the mechanism via the studies of a Received: July 7, 2013 Revised: September 10, 2013

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The gate channel of the ICCD was synchronized and opened at 50 ns after the firing of the excitation pulse. Typically, the second harmonic (400 nm, fwhm 1) is populated via T1 → Tk excitation and possesses a substantial MLCT% contribution? Then the corresponding reverse Tk → Sm ISC rate may be fast enough to compete with the Tk → T1 IC in the condensed phase. Bearing this proposal in mind, we then chose the bimetallic Au(I) complex A-3 (Scheme 1) as a prototype in an aim to prove this concept. Scheme 1. Chemical Structure of A-3

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EXPERIMENTAL SECTION Material. The complex A-3 was synthesized previously.9 Tetrahydrofuran was distilled over Na-benzophenoneketyl under a nitrogen atmosphere prior to use. Other reagents and solvents were used as received. The solution 1D 1H, 31P NMR, and 1H−1H COSY spectra were recorded on a Bruker Avance 400 spectrometer. Microanalyses were carried out in the analytical laboratory of the University of Eastern Finland. Solvents (dichloromethane and cyclohexane) of spectroscopy grade used in the photophysical study were purchased from Merck Inc. and used without further purification. An A-3 solution with a concentration of ∼6 × 10−6 M was used for steady-state spectroscopy measurement, and a ∼2 × 10−5 M A3 solution was used for time-resolved transient absorption and the two-step laser-induced emission experiments. Photophysical Measurements. Steady-state absorption and emission measurements in solution were recorded with a Hitachi (U-3310) spectrophotometer and an Edinburgh (FS920) fluorometer, respectively. Both the wavelengthdependent excitation and emission response of the fluorometer have been calibrated. To determine the photoluminescence quantum yield in solution, the samples were degassed by three freeze−pump−thaw cycles. Quinine sulfate monohydrate in sulfuric acid and coumarin 480 in methanol, with quantum yields of ∼0.57 and ∼0.87, respectively, served as the standard for measuring the quantum yield. Nanosecond lifetime studies were performed with an Edinburgh FL 900 photon-counting system using a hydrogen-filled lamp as the excitation source. The emission decays were fitted by the sum of exponential functions with a temporal resolution of ∼300 ps via the deconvolution of instrument response function. The phosphorescence lifetime (>μs) was detected by an optical spectrum analyzer, consisting of an intensified charge-coupled device (ICCD) in conjunction with a spectrograph (SP2300i, Acton).

Figure 1. Microscopic TSLIE for the solid film measurement. L1: the second harmonic (355 nm, fwhm ∼20 ns) of a Nd:YAG pumped Ti:sapphire laser (LT-2211, LOTIS TII). L2: the second harmonic (532 nm, fwhm∼8 ns) of the Nd:YAG laser (Surelite I) as the probe pulse. DG: programmable delay pulse generator (SRS Model DG535). BS1: a quartz plate used as a beam splitter. BS2: 90/10 beam splitter. BPF: band-pass filter (375−500 nm). SP: imaging spectrograph and monochromator (Acton SP-2358). ICCD: time-gated ICCD camera (PI-MAX 3, Princeton Instruments).

second harmonic (355 nm, fwhm∼20 ns) of an Nd:YAG pumped Ti:sapphire laser (LT-2211, LOTIS TII, tunable from 350 to 485 nm) was used as the pump pulse, and a second harmonic (532 nm, 8 ns) of the Nd:YAG laser (Surelite I) was applied as the probe pulse. The pump and probe pulses were synchronized, and their time delay was changed by a programmable delay pulse generator (SRS Model DG-535). The emission from the sample was then collected by microscope objectives (Olympus, LCACHN20XPH). A 90/ 10 beam splitter was used to reflect 10% of the pump and probe beam, and 90% of the emission signal was transmitted. The transmitted emission was then directed into an imaging spectrograph (Acton SP-2358) then to a time-gated ICCD camera (PI-MAX 3, Princeton Instruments), for which the gate channel was synchronized with the firing time of the probe pulse. The TSLIE system for solution measurement was similar to that of the nanosecond transient absorption (vide supra) pumped at 355 nm except that the white light probe square pulse was replaced either by a second harmonic (532 nm, 8 ns) B

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Nd:YAG laser for A-3 in dichloromethane solution or by a second harmonic (460 nm, fwhm∼20 ns) of an Nd:YAG pumped Ti:sapphire laser for the 9,10-dibromoanthracene (DBA) in cyclohexane solution. The pump and probe pulses are directed and overlapped into a quartz cuvette (1.0 cm wide) containing the sample solution. The TSLIE is then detected by the same ICCD setup used in the above measurement for the solid film. Computational Details. The calculations were performed with the Gaussian 09 program package.10 The ground-state geometry optimization was based on density functional theory (DFT) at the B3LYP/LANL2DZ (Au) and B3LYP/6-31G* (H, C, P) levels of theory. We calculated the lowest singlet (S0 → S1) and triplet optical transitions (S0 → T1) using the timedependent density functional theory (TD-DFT) method. The solvation effect (in CH2Cl2) was based on a conductor-like polarizable continuum model (C-PCM), which is implemented in the Gaussian 09 program. In this study we also calculated the triplet-state transient absorption spectra of complex A-3. The simulation utilized the CAM-B3LYP11 functional with a LANL2DZ/6-31g(d)12 basis set. The solvent effect was also involved in this calculation using the polarizable continuum model.13 The structure of A-3 was first optimized at the lowestlying triplet state (T1), which was then followed by TD-DFT calculation to probe the triplet−triplet absorption properties.

Figure 3. Nanosecond transient absorption spectra of A-3 in aerated CH2Cl2. The spectra were obtained at different delay times between pump (355 nm) and probe (white light) pulses. Inset: time-resolved decay of the transient absorption. The theoretical calculated triplet− triplet absorption is also shown in terms of energy gap (wavelength) and transition moment (oscillator strength normalized at peak 580 nm). Note that the numbers shown in the vertical bars (gray) indicate the MLCT%.

strongly quenched with the presence of oxygen and can be fitted well by single-exponential decay kinetics with a lifetime of 680 μs and 920 ns (inset of Figure 3) in the degassed and aerated CH2Cl2, respectively. The 680 μs time constant obtained also matches the reported O2 free phosphorescence lifetime (680 μs) very well.8,9 Because the S1 → T1 intersystem crossing rate for A-3 is ≈109 s−1, the T1 state should be populated promptly within the system response time of ∼10 ns. To summarize all available data, the assignment of the transient absorption to the T1 → Tn transition is unambiguous. From a computational approach, we also fully optimized the geometry (incorporating a CH2Cl2 solvent model) of the T1 state and then performed the calculation of the T1 → Tn absorption. Various transition states calculated in terms of energy gap and transition dipole moment (oscillator strength) are marked in Figure 3 (see Table S1 in the Supporting Information). The calculated T1 → Tn absorption peak at 580 nm is consistent with that of the experimental results. In addition to the allowed ππ* transition appearing at 580 nm peak, there evidently exist several satellite transitions in the region of 450−600 nm possessing substantial MLCT% (Figure 3). We then carried out the two-step laser induced emission (TSLIE, see Experimental Section) study, in which the first pulse (355 nm, ∼20 ns) pumps A-3 to populate the T1 state, followed by the second pulse that was tuned to the T1 → Tn transient absorption region in an attempt to resolve any emission induced by the probe pulse. In this study, limited by the light source, the 532 nm laser line (2nd harmonic of Nd:YAG) was used as the probe pulse and was very likely to excite T1 → T5 (λcal, 537.76 nm; f = 0.0216; MLCT%, 4.50; see also Table S1 in the Supporting Information). As a result, Figure 4 (colored, solid lines) reveals the temporal evolution of emission spectra for A-3 acquired at different time delays between pump and probe pulses in CH2Cl2. In this study, it should be noted that the gate window of the intensified charge coupled detector (ICCD) is synchronously opened with respect to the probe pulse to eliminate any prompt fluorescence from the pump pulse. Also, any delayed fluorescence originating from the triplet−triplet annihilation has been subtracted by a pump-only control experiment. As shown in Figure 4, the resulting 375 nm emission in terms of peak wavelength and spectral profile is identical to the steady-state

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RESULTS AND DISCUSSION Figure 2 shows the UV−vis absorption and emission spectra of A-3 in CH2Cl2 as well as the emission of A-3 in solid film. The

Figure 2. Absorption and photoluminescence spectra of A-3 in degassed CH2Cl2 (solid lines). Dashed line denotes photoluminescence spectrum of A-3 in solid film, prepared by casting the A-3 (in CH2Cl2) on a quartz substrate. Photoluminescence spectra of A-3 both in degassed CH2Cl2 and in solid film were obtained with a 355 nm excitation wavelength.

dominant ππ* character in both singlet and triplet manifolds leads to a slow rate of S1 → T1 intersystem crossing (8.9 × 108 s−1), the result of which is manifested by the appearance of dual emission consisting of fluorescence and phosphorescence maximized in CH2Cl2 (solid film) at 375 nm (420 nm) and 545 nm (550 nm), respectively.8 Compared to that in the solution, the red shift of the fluorescence of the solid film may imply an increase of planarity on the alkyne-(phenyl)3-alkyne moiety in A-3. In an attempt to resolve the absorption spectroscopy for the triplet manifold, we first performed nano−microsecond transient absorption measurements. After the sample was exposed to a 355 nm pump pulse (8 ns) followed by the probe of a square white light pulse (∼3 ms duration), a transient absorption spectrum maximized at 575 nm was resolved (Figure 3). The dynamics of the transient absorbance is C

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excited triplet state, and Φf the yield of the fluorescence. Because the spectral ranges for triplet transient absorptions of A-3 and DBA do not overlap well, we chose the 460 and 532 nm of the triplet−triplet transient absorption to be the probe wavelength for DBA and A-3, respectively. We thus performed the experiment under such conditions that the triplet−triplet absorbance at the probe wavelength for A-3 (532 nm) and DBA (460 nm) are kept identical, and the number of photons of the probe pulses remains unchanged for each measurement (∼1.6 × 1015 photons/pulse). At a delay time of 200 ns, within which the number of triplet-state molecules that relax to the ground state is negligible, FA‑3/FDBA can be simplified to Figure 4. TSLIE spectra of A-3 in aerated CH2Cl2 (colored, solid lines). The spectra were obtained at various delay times (as depicted) between pump (355 nm) and probe (532 nm) pulses. Inset: the integrated TSLIE signal (in logarithmic scale) versus delay time and the fitting of single exponential decay. Note that the slight deviation from the linearity is attributed to the very minor T−T annihilation and is due to the relatively higher excitation intensity (355 nm). Also shown is the scaled steady-state (SS) fluorescence spectrum (black, solid line) acquired by the same system. Note that the loss of the vibronic progression of fluorescence (see Figure 2) is due to the relatively low resolution (large slit width) used for TSLIE experiment. The TSLIE of DBA at the delay time of 200 ns in degassed cyclohexane is also shown (dotted line) for the determination of reverse ISC yield, ΦA‑3 TS , of A-3 (see text).

A‐3 A‐3 DBA DBA FA‐3/FDBA = (ΦTS Φf )/(ΦTS Φf )

When the intensity of the probe pulse at 460 nm (or 532 nm) was tuned as low as possible to avoid saturating the transient species, the ratio FA‑3/FDBA was determined to be 0.77. Thus, by knowing the values ΦA‑3 = 0.48and ΦDBA = 0.1,15 the yield of Tk f f A‑3 → S1 population for A-3, ΦTS is deduced to be 5.2% upon 532 nm T1 → Tk excitation. This value is considered to be a lower limit because the back singlet → triplet ISC is expected to take place to certain extent amid the Sm → S1 relaxation process. For practical application, we further pursued the reverse TSLIE property of the titled complex in solid film. The acquisition of TSLIE for the semitransparent thin solid film (∼200 nm prepared by spin coating in CHCl3) requires a different pump−probe setup. In this study, the TSLIE measurement in a solid utilizes an inverted microscope described in the Experimental Section (see Figure 1 for the setup). We took advantage of the spatial resolution offered by the microscope so that the overlap between pump and probe pulses could be optimized via the image taken by the microscope. Both pump (355 nm) and probe (532 nm) lasers colinearly (∼0°) excite the sample, and TSLIE was acquired in a front-face configuration; this technique renders a high spatial overlap, maximizing the TSLIE signal. Similar to that in the solution phase, the high-voltage gate of the ICCD is synchronously opened with the probe pulse to ensure the acquired signal is free from any prompt emission excited by the pump pulse. This microscopic TSLIE setup, however, is not recommended for study of the solution phase because of the diffusion of the transient species, which has a relatively long life span, out of the small focused region (∼500 μm) in solution. As a result, Figure 5 uncovers the temporal evolution of the TSLIE signal for A-3 in solid film. The TSLIE spectrum is essentially identical to the steady-state fluorescence of A-3 in solid film. The decay of the TSLIE is fitted to be 120 μs, which correlates very well with the phosphorescence decay time (120 μs) in solid film. Evidently, the mechanism of efficient Tk → Sm (k, m > 1) reverse ISC is fully operative in both solution and solid. Using A-3 as a prototype, an overall pump−probe mechanism is thus depicted in Scheme 2. Upon the T1 → Tk (k > 1) excitation, the Tk → Sm ISC, in which either Tk or Sm possesses substantial MLCT%, is fast enough to compete with the Tk → T1 IC, which is followed by Sm → S1 IC, giving back S1 → S0 fluorescence.

fluorescence of A-3 in CH2Cl2. Note that the obscurity of the vibronic resolution is due to the relatively large slit used for the TSLIE experiment. Because the absorptivity of ground-state A3 at 532 nm is virtually zero, this fluorescence signal could be solely from 532 nm T1 → Tk (k > 1, and is very likely to be 5) excitation followed by fast Tk → Sm (m > 1) ISC and Sm → S1 IC, giving rise to the S1 → S0 fluorescence. Note that the phosphorescence appears as well in the TSLIE experiment, which has been mostly eliminated by opening the gate window of the ICCD as narrow as 50 ns. Also, Figure 4 depicts only the emission at 1) ISC, which gives direct population at the triplet state and hence the D

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the ratiometric (fluorescence:phosphorescence) emission may be altered at different applied voltages16by applying the ππ* emissive transition-metal complexes in organic light-emitting diodes (OLEDs), eventually attaining a white light OLED in a single active layer. Harnessing the singlet and triplet branching via efficient forward/backward intersystem crossing in the higher-lying excited states may pave a way for development of new optoelectronics.

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ASSOCIATED CONTENT

S Supporting Information *

Additional data and complete listing of ref 10. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 5. Time-dependent TSLIE spectra of A-3 in solid film. The spectra were obtained at various delay times between pump (355 nm) and probe (532 nm) pulses. Inset: integrated TSLIE intensity (in logarithmic scale) versus delay time and the single exponential fit, which reveals good linearity due to negligible contribution of triplet− triplet annihilation in the solid film. Also shown is the scaled steadystate (SS) fluorescence spectrum (black solid) acquired by the same system. The loss of the vibronic progression of fluorescence (see Figure 2) is due to the relatively low resolution (large slit width) used for the TSLIE experiment.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: igor.koshevoy@uef.fi (I.O.K). *E-mail: [email protected] (P.-T.C.). Author Contributions ∥

Y.-C.C. and K.-C.T. contributed equally.

Notes

The authors declare no competing financial interests.

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a

Scheme 2

ACKNOWLEDGMENTS This paper is dedicated to Michael Kasha in honor of his great contributions to condensed phase spectroscopy. This research has been supported by the National Science Council, Taiwan (NSC-100-2923-M-002-005-MY3), the strategic funding of the University of Eastern Finland (Spearhead project), and the Academy of Finland (Grant 138560/2010, A.J.K.; 268993/ 2013, I.O.K.).

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Illustration of the Tk → Sm reverse intersystem crossing (RISC) via pump−probe two-step laser induced emission (TSLIE) measurement. In this illustration, complex A-3 is used as a prototype. a

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