Article pubs.acs.org/journal/apchd5
Antenna-Enhanced Triplet-State Emission of Individual Mononuclear Ruthenium(II)-Bis-terpyridine Complexes Reveals Their Heterogeneous Photophysical Properties in the Solid State Janning F. Herrmann,† Paul S. Popp,† Andreas Winter,‡,§ Ulrich S. Schubert,‡,§ and Christiane Höppener*,† †
Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany Laboratory for Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Humboldtstraße 10, 07743 Jena, Germany § Center for Energy and Environmental Chemistry Jena (CEEC Jena) Jena, Philosophenweg 7a, 07743 Jena, Germany ‡
S Supporting Information *
ABSTRACT: The ability of supramolecular transition metal coordination complexes to form stabilized, long-living, radiative charge-separated states has drawn interest to employ these triplet-state emitters for the design of photonic devices. Their applicability as photosensitizers of electron transfer in molecular photonic systems is directly coupled to fundamental studies of their rich and highly versatile photochemical and photophysical properties. Here, we demonstrate that the properties of individual dual-luminescent Ru2+-bis-terpyridine complexes can be addressed with excellent sensitivity in singlecomplex antenna-enhanced phosphorescence investigations. This sensitivity enables studying environmentally imposed alterations of their photophysical properties, e.g., in thin film applications. In contrast to ensemble averaging investigations in solution, single-complex antenna-enhanced phosphorescence investigations corroborate the existence of Ru2+-bis-terpyridine complexes with spectrally shifted emission peaks and diverging intrinsic quantum yields in the solid state. Across the sample of investigated individual complexes the observed emission spectra resemble the expected unique features of this specific Ru2+-bisterpyridine complex. The origin of the shifted emission is discussed in terms of the existence of different molecular conformers of this specific Ru2+-bis-terpyridine complex facilitated by the embedment into a rigid solid matrix. KEYWORDS: optical antennas, quantum yield enhancement, single molecules, triplet-state emitter, metal-to-ligand charge transfer states, coordination complexes.
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synthetic routes, i.e., by the insertion of electron-donating or -accepting functional groups, in order to control the electronic coupling of the ππ* excited state and the simultaneously present MLCT states.5,11,13−16 Alternatively, the insertion of heterocyclic groups can change the torsion angle between the terpyridine cage and the ligand, leading to a less distorted planar ground-state configuration.15,17 Key for the improvement of their photophysical properties with respect to an application in photovoltaics or light-emitting devices is an extension of the electron delocalization and the minimization of nonradiative relaxation channels. In order to improve the applicability of these coordination compounds in optoelectronic devices, a detailed knowledge of their homogeneity and environmentally influenced alteration of their luminescent properties is indispensable. It is well accepted that this information can be precisely addressed only in single-molecule
ransition metal coordination complexes have gained increasing interest as materials in photovoltaic, optoelectronic, and photonic devices, in sensors, and in photocatalytic processes.1−7 This versatile applicability is related to their widely tunable electro-optical properties and their function as an ideal substituent for the commonly employed π-conjugated organic polymers such as poly(p-phenylenevinylene) (PPV) and poly(p-phenylene ether) (PPE) in optoelectronic devices. From this point of view, polypyridyl complexes, such as terpyridines, linked via transition metal ions are promising entities. However, this class of macromolecular complexes often suffers from low intrinsic quantum yields (Qoi ) due to an effective coupling between the radiative metal-to-ligand charge transfer (MLCT) states and metal-centered states caused by their distorted octahedral geometry.8−10 Nonetheless, it has been already shown that their emission properties can be improved in terms of their room-temperature (RT) emission, their intrinsic quantum yields, extended lifetimes of their MLCT states, and their spectral properties.2,11,12 In general, tailored photophysical properties can be accomplished by © XXXX American Chemical Society
Received: June 20, 2016
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ensemble averaging studies.29−31 Primarily, our investigations focus on the use of an antenna as a tool to accomplish singlecomplex sensitivity in spectrally and temporal-dependent investigations. Furthermore, this approach also provides information that is crucial for the design optimization of hybrid optoelectronic devices. As such, the antenna interaction range as a function of the initial quantum efficiency is a valuable measure in terms of an optimization of the structure of hybrid devices utilizing plasmonic antenna effects. Applications of plasmonic nanostructures demonstrated the improvement of the efficiency of LEDs and photodiodes32,33 and have been widely employed to improve the light harvesting in photovoltaic cells.34,35 In the present study, a well-defined optical antenna in the form of a dielectric tip-supported spherical gold nanoparticle (AuNP) is employed to locally probe the photophysical properties of individual ruthenium coordination complexes with subnanometer precision (schematic outline, Figure 1).25,26 This approach enables relating deviations in their quantum efficiencies to their spectral properties. The homoleptic Ru-(4′(4-((2,5-bis(octyloxy)-4-styrylphenyl)ethynyl)phenyl)2,2′:6′,2″-terpyridine) complex of the 2,2′:6′:2″-terpyridine class (further referred to as Ru2+[L]2(PF6−)2) is particularly the focus of this study.36 This specific mononuclear complex bears a chromophore in the form of a poly[phenylene-vinylene], poly[phenylene-ethinylene] derivative, and ruthenium acts as a coordination ion (chemical structure, Figure 2A). Complexation of the ligands (L) via a Ru2+ ion leads to the formation of two energetically well-separated, highly stabilized excited states, i.e., the ligand-centered S1 state and an energetically lower state with metal-to-ligand charge transfer character leading to exceptional dual-luminescent properties (Jablonski diagram, Figure 2A).12,37 Therefore, its luminescence properties in solution have been extensively investigated and compared with other chemically closely related complexes of the same type to explore the fundamental mechanisms leading to dual luminescence.12,37 Here, we focus on the electronic transition related to a direct excitation of the 1MLCT state of the Ru2+[L]2(PF6−)2 complex, which falls in the visible part of the absorption spectrum and is centered at ∼500 nm (absorption spectrum, Figure 2B). Photoexcitation of these complexes in the visible spectral region induces at first a transfer of charges from the Ru2+ ion to the coordinated ligands. Delocalization of the MLCT extends over the terpyridine moiety and the phenyl ring. The extent of delocalization is largely independent of the size of the conjugated ligand system.38 The relaxation of this excited state involves intersystem crossing (ISC) to a 3MLCT state with a lifetime of ∼36 ns.12,37 The corresponding phosphorescence emission is centered at ∼640 nm. Therefore, optical antennas in the form of a spherical AuNP are ideally suited to enhance the phosphorescence emission from these complexes since their absorption peak coincides well with the antenna’s plasmon resonance and the emission is detuned to longer wavelengths, leading to a reduced quenching of the emitted phosphorescence signal by the metal nanostructure.39,40 Furthermore, direct excitation of the 1MLCT transition decouples fluorescence and phosphorescence emission, such that an antenna can be employed to boost the phosphorescence emission in regard to the more intense fluorescence emission. In terms of an identification of environmentally conditioned alterations of their photophysical properties the phosphorescence emission is a sensitive indicator due to the prolonged
studies, which in particular avoid ensemble averaging effects.18,19 In general, such investigations for polypyridyl complexes are hindered by the extremely low quantum efficiencies of these materials under ambient conditions. As a consequence, so far, knowledge on their luminescent properties stems only from averaging far-field studies of solutions or films, often carried out at low temperatures. In order to investigate the luminescence of the individual building blocks under ambient, technically relevant conditions, more sophisticated methods have to be employed, which rely on the improvement of the signal-to-noise ratio. An elegant way to approach this requirement is to utilize highly confined optical fields of resonators and plasmonic nanostructures to control the light−matter interaction.20,21 Plasmon-induced modifications of the spontaneous emission rate of individual complexes aim at an enhancement of the optical contrast without altering their chemical structure.22−24 Noble metal nanostructures are well known to affect the spontaneous emission rate of a coupled quantum emitter twice (see Figure 1).24−28 First, the spontaneous emission rate of a
Figure 1. Schematic representation of the metal-enhanced luminescence investigations of individual bis-terpyridine complexes by means of a spherical AuNP antenna. Inset: FTDT simulation of the electric field distribution resulting from irradiation of an 80 nm AuNP with a vertically polarized plane wave. The AuNP antenna accomplishes an efficient coupling of the incident light to the antenna−complex region and also helps to couple out the emitted phosphorescence. γexc/γoexc denotes the excitation rate enhancement factor (fexc), and the quantum yield enhancement ( f Qi) is defined as Qi/Qoi .
quantum emitter can be increased by utilizing strongly enhanced and localized fields associated with an irradiated metal nanostructure (known as excitation rate (γexc) enhancement (fexc)) and, second, by the stimulation of its radiative decay rate (γrad). In principle two regimes have to be distinguished since the enhancement of the spontaneous emission rate of a quantum emitter is a function of its intrinsic properties, its environment, and the antenna properties. While for quantum emitters with a Qoi yield close to unity the luminescence enhancement is largely determined by the excitation rate enhancement, for low Qio yield emitters additionally the radiative rate enhancement has to be taken into account. The perspective to accomplish a Qi yield enhancement is of particular interest for investigations of inorganic and organic materials integrated into optoelectronic devices due to their Qoi yields. The latter can be extremely low depending on their internal structure and can also be strongly affected by their density and their degree of order in the solid state due to aggregation-induced quenching effects. Currently, the efficiency of plasmon-coupled optoelectronic devices as a function of their plasmonic properties is subject to numerous B
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setup and specific filter configurations is given in the Materials and Methods section. At first the emitted phosphorescence emission signal is accumulated across a spectral range from ∼540 to 800 nm. Due to the high density of the Ru2+[L]2(PF6−)2 complexes in the film and their low phosphorescence quantum yield, it is assumed that the confocal phosphorescence images show an inhomogeneous, faint phosphorescence signal across the imaged sample area. In contrast to this expectation, the confocal image reveals only a small number of phosphorescence spots with characteristic emission patterns. These resemble the ones typically obtained for randomly oriented individual, highquantum-yield dye molecules excited with a radially polarized laser beam (confocal image, Figure 2C). The observed singlespot and double-lobe patterns commonly correspond to an outof plane and in-plane alignment of the transition dipole of these molecules.41 Partially observed blinking and bleaching also indicates that the found signatures most likely correspond to individual molecules. Due to these unexpected findings, antenna-enhanced phosphorescence microscopy is employed to investigate the samples with improved sensitivity and spatial resolution. For this an optical antenna in the form of an 80 nm spherical AuNP is centered in the laser focus at a distance of ∼5 nm from the sample surface. The antenna−sample separation is maintained with subnanometer precision by means of a force feedback loop based on a piezoelectric quartz tuning fork acting as a force sensor, while the sample is scanned relative to the fixed position of the antenna and the objective (for further details refer to the Materials and Methods section). Figure 3 displays a confocal
Figure 2. Dual-luminescent Ru-(4′-(4-((2,5-bis(octyloxy)-4styrylphenyl)ethynyl)phenyl)-2,2′:6′,2″-terpyridine) complex. (A) Chemical structure and a Jablonski-diagram highlighting the relevant optical transitions for a direct excitation of the phosphorescence emission. So and S1 correspond to the ground and first excited singlet state of the ligand. T1 denotes its lowest energy triplet state, 1MLCT is the singlet and 3MLCT/3MLCT′ are triplet MLCT states of the coordinated complex. Relaxation from the excited 1MLCT state involves ultrafast intersystem crossing (ISC) to the 3MLCT state and either radiative decay to the ground state or nonradiative coupling due to internal conversion to a high-spin metal centered state (HSdd-state). (B) Typical absorption spectrum and ensemble phosphorescence spectrum (λexc = 532 nm) acquired on a thin film. (C) Confocal phosphorescence overview image of the ruthenium bis-terpyridine complex acquired by scanning the sample through a tightly focused radially polarized laser beam with an excitation wavelength of λexc = 532 nm and an excitation power (Pexc) of ∼1.1 μW.
lifetime of the corresponding triplet states. Alterations of the complex structure are most likely associated with a structural reconfiguration in the region of the terpyridine sphere, which is also the most sensitive interlock to tune the optical properties of this class of complexes.12,37
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Figure 3. Phosphorescence images of a thin layer of the specific mononuclear Ru2+[L]2(PF6−)2 complex shown in Figure 2A. The phosphorescence emission is detected across a spectral range of Δλem ≈ 540 to 800 nm. (A) Confocal phosphorescence images acquired with an excitation power of ∼500 nW. (B) Antenna-enhanced phosphorescence image of the region marked in (A) clearly identifies individual phosphorescence spots. The antenna-enhanced image reveals phosphorescence signals even in regions that appear dark in the confocal image (red circle). For the antenna-enhanced image the excitation power was reduced to 50 nW.
RESULTS AND DISCUSSION According to their potential application in photonic devices, the presented investigations of Ru2+-bis-terpyridine complexes were addressed in the solid state. Therefore, the Ru2+[L]2(PF6−)2 complexes were dissolved in chloroform and then spin coated on a clean glass substrate. Subsequently, a ∼2 nm poly(methyl methacrylate) (PMMA) layer is deposited on top of the Ru2+bis-terpyridine complex layer to increase their adhesion to the substrate (a detailed description is given in the Materials and Methods section). At first these samples were characterized by means of diffraction-limited confocal luminescence microscopy. For far-field illumination of the sample a radially polarized laser beam with an excitation wavelength (λexc) of 532 nm and a laser power (Pexc) of 500 nW to 1.1 μW is tightly focused with a high NA oil-immersion objective onto the sample. The samples are raster-scanned through the focus, and for each image pixel the collected luminescence signal is detected with a single-photoncounting avalanche photodiode (SPAD). Separation of the luminescence signal from the excitation light is accomplished by a combination of a suitable dichroic mirror or a beamsplitter and a long-pass or bandpass filter. A detailed outline of the
image along with an antenna-enhanced measurement of the region marked in the confocal image. In the confocal image some residual phosphorescence is observed in the upper part of the marked area, but defined emission patterns are not visible due to the small intermolecular distances. Regions of phosphorescence emission identified in the confocal image display bright spots in the corresponding antenna-enhanced image with clear emission patterns (see Figure 3B). The improved image contrast enables observing more image details. Peculiarly, dark regions of the confocal image displayed in Figure 3A reveal also phosphorescence spots in the antennaenhanced image (red circle, Figure 3B). The observed emission C
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measurements acquired in weakly emitting areas of these thin films reveal emission spectra similar to the phosphorescence emission of the dissolved Ru2+[L]2(PF6−)2 complexes (see Figure 4C and Figure SI-3). Individual spectra show a high similarity over a large number of measurements. The emission peak is found to be centered at 660 nm (±25 nm). The image of the marked region in Figure 4A, acquired with an 80 nm AuNP antenna, reveals a high density of individual phosphorescence spots in accordance with the observations from confocal time-dependent ensemble measurements. Evidently the antenna approach leads to a significant enhancement of the phosphorescence emission from these complexes. A sensitivity down to the single-complex level is accomplished, which is verified by the characteristic single-step bleaching observed in phosphorescence time trajectories acquired on these spots (phosphorescence trajectory, Figure 4B). The obtained sensitivity even allows for a spectral characterization of individual complexes and, thus, enables their unique assignment to the Ru2+[L]2(PF6−)2 complex. Figure 4C compares a corresponding antenna-enhanced emission spectrum with a confocal ensemble spectrum. Apparently the line width decreases for the single-complex spectrum, and the signal noise is reduced. Due to this sensitivity, clearly an asymmetry in the spectral profile is observed, which is largely hidden in the RT ensemble measurements, but which is well known from ensemble averaging emission spectra of diluted Ru2+[L]2(PF6−)2 complexes acquired at low temperatures. Optical antennas are known to be ideally suited for probing light−matter interactions in materials with low quantum efficiencies.21−23 This ability has been demonstrated for example by probing the emission of NIR dyes and rare-earthdoped fullerenes and also by investigations of the fluorescence emission of Ru2+-bis-terpyridine complexes coupled to AuNP ensembles.31,42−44 For low-Qi emitters the signal enhancement stems from the excitation rate enhancement and the modification of the radiative decay rate. Hence, the antenna interaction range is altered compared to that of high-Qi emitters. Figure 4D depicts the Ru2+[L]2(PF6−)2 complex− antenna interaction range and that of the spectrally akin, highQoi dye molecule Alexa Fluor 633 for comparison. In the case of the high-Qio Alexa Fluor dye molecule the fluorescence enhancement stems largely from the provided excitation rate enhancement, leading at best to a signal enhancement factor of ∼10.25 Evidently the interaction range is much more confined for the Ru2+-bis-terpyridine complex (dint < 20 nm), which is in accordance with its expected low Qoi . The found spectral properties of single complexes largely coincide with the wide-field ensemble spectrum acquired over a large area of the Ru2+[L]2(PF6−)2 film shown in Figure 2B. However, these results cannot account for the peak observed at ∼580 nm. Therefore, the spectral detection band for the confocal and antenna-enhanced imaging was tuned to overlap with this region by implementation of a bandpass filter with a transmission band from 550 to 610 nm in front of the detector. Hence, the previously identified Ru2+[L]2(PF6−)2 complexes with the phosphorescence emission centered at ∼650 nm do not contribute to the detected signal. Corresponding confocal and the antenna-enhanced images of the same sample area, shown in Figure 4A and B, are characterized again by bright spots with orientation-sensitive patterns (see Figure 5A and 5B). Time-dependent measurements acquired on these spots show that these spots largely correspond to individual molecules (see phosphorescence trajectories in Figure 5A and
count rate for these spots is significantly lower than for the bright spots (green circle, Figure 3B). Apparently the antenna-enhanced images reveal two subsets of molecules with significantly altered phosphorescence emission rates. According to the ensemble emission spectrum shown in Figure 2B the Ru2+[L]2(PF6−)2 complexes show a broad emission band. In order to assign possible spectral deviations, the samples were again imaged with the detection band being adjusted by means of bandpass filtering to a spectral range that corresponds to the long-wavelength tail of the expected phosphorescence emission spectrum of these complexes. Figure 4 displays corresponding confocal and
Figure 4. Phosphorescence emission of Ru2+[L]2(PF6−)2 complexes integrated over a spectral range from 645 to 710 nm. (A) Confocal phosphorescence overview image reveals only a weak residual background phosphorescence for an excitation power Pexc of ∼500 nW. With increasing time the phosphorescence signal bleaches exponentially. (B) Antenna-enhanced phosphorescence image of the marked area in (A) acquired with a reduced excitation power of ∼100 nW. The improved image contrast reveals a large number of individual phosphorescence spots with a resolution of 33 nm. Largely these spots are characterized by single-step bleaching, as exemplified by the shown phosphorescence time trajectory. (C) Ensemble averaging confocal and single-complex antenna-enhanced phosphorescence spectra show an emission peak at ∼650 nm. (D) Phosphorescence emission as a function of the antenna−complex separation of an individual Ru2+[L]2(PF6−)2 complex shown along with the one of the high-Qoi dye molecule Alexa Fluor 633.
antenna-enhanced images restricted to a spectral range of 645 to 715 nm. For comparable excitation conditions to the measurements shown in Figure 3, the confocal image lacks any single-molecule-like pattern. Instead, a faint, inhomogeneous distribution of the emitted phosphorescence at a level close to the typically observed background contributions is observed. Time-dependent investigations of the confocal signal reveal a slow exponential decrease of the signal intensity, indicating that several weakly emitting molecules are excited at the same time in the confocal excitation area. Spectrally resolved confocal D
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Figure 6. Antenna-enhanced spectroscopy of red- (i) and greenemitting (ii) Ru2+[L]2(PF6−)2 complexes. (A) Single-complex spectra of the subsets taken with an 80 nm AuNP antenna with an excitation power of 100 nW. The experimental data (dots) are fitted with a theoretical two-state model (solid line). Each dashed line accounts for the (vo*−vo) transitions of the corresponding excited state and its associated vibronic progression. (B) Antenna-enhanced phosphorescence emission as a function of the antenna−complex separation. Dotted lines display the experimental data, and the solid lines correspond to the theoretical fits of the spontaneous emission mediated by the antenna taking into account analytical solutions of the excitation rate enhancement, the radiative rate enhancement, and the rate of energy dissipation due to nonradiative electromagnetic coupling to the AuNP.
Figure 5. Phosphorescence emission of Ru2+[L]2(PF6−)2 complexes integrated over a spectral range from 545 to 620 nm. (A) Confocal phosphorescence overview image acquired with an excitation power of 500 nW reveals typical orientation-sensitive luminescence patterns. The total count rate is clearly increased compared to the red-emitting Ru2+[L]2(PF6−)2 complexes identified in Figure 4B. (B) Antennaenhanced phosphorescence image of the marked area in (A) acquired with a reduced excitation power of ∼100 nW. For a majority of spots single-step bleaching is observed. (C) Confocal and antenna-enhanced phosphorescence spectra reveal several emission peaks. In the case of a confocal approach these peaks can be assigned to multiple excited molecules with distinct spectral properties, while the antennaenhanced spectrum stems from a single complex. (D) Luminescence emission as a function of the antenna−complex separation in comparison to the high-Qoi dye Alexa Fluor 532.
a characteristic emission profile, which comprises the (v*o −vo) transition along with a long wavelength tail red-shifted by ∼60 nm from the center peak position caused by vibronic progressions.45,46 Evidently the spectrum of the red-emitting Ru2+[L]2(PF6−)2 complex does not obey a single-Gaussian profile. The corresponding tail is red-shifted by ∼40 nm from the peak maximum, which is less than expected for the Ru2+[L]2(PF6−)2 complex. A typical spectrum of the bright spots found in the sample is shown in Figure 6A(ii). At first, distinct differences compared to the spectra of red-emitting Ru2+-bis-terpyridine complexes seem to appear. Clearly the spectrum shows a double-peak structure. Time-dependent spectrally resolved measurements have shown that both peaks concomitantly bleach. Therefore, the spectra can be uniquely assigned to a single complex. The shape of this emission spectrum cannot be explained by means of a simple vibronic progression model. Even taking into account second- and thirdorder contributions to the vibronic progression in the model, the experimentally measured spectral profile cannot be reproduced. In prevoius studies, ensemble phosphorescence emission spectra of these Ru2+[L]2(PF6−)2 complexes diluted in acetonitrile were acquired at 77 K in order to prevent thermal activation of nonradiative decay channels. These investigations revealed a deviant spectral profile for this specific ruthenium polypyridyl complex.47 Compared to other complexes of this type, e.g., without a chromophoric ligand or with a decreased conjugated chromophore attached to the 4′-position of the
B). Spectrally resolved measurements reveal a distinct double/ multi-peak structure (see Figure 5C). While for the confocal spectra a broad contribution from Ru2+-bis-terpyridine complexes with an emission peak at ∼660 nm is visible, the reduced excitation volume of the optical antenna selectively excites molecules, which emit a blue-shifted peak from the common phosphorescence emission peak of the Ru2+[L]2(PF6−)2 complexes. Additional confocal spectra taken on these spots have shown that the peak position of the blueshifted emission varies across the short wavelength range from 540 to 620 nm (single-molecule spectra, Figure SI-4). Further comparative measurements of the antenna interaction range for these molecules and spectrally akin Alexa Fluor 532 dye molecules show a similar behavior (see Figure 5D), which indicates that these spots stem most likely from molecules with Qoi comparable to that of the reference dye. In order to assign the origin of these molecules, their spectral properties are further analyzed and compared to the spectral profile observed for the red-emitting Ru2+[L]2(PF6−)2 complexes. Figure 6A(i) and (ii) show typically observed antennaenhanced spectra for the two subsets identified in the Ru2+[L]2(PF6−)2 films. According to previous investigations of this complex, the emission results from a radiative decay of the energetically lowest 3MLCT state. Furthermore, it is well known that particularly ruthenium polypyridyl complexes show E
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terpyridine unit,47 the phosphorescence spectra of the Ru2+[L]2(PF6−)2 complex are characterized by a clear doublepeak structure of the main emission peak, which is accompanied by contributions reflecting vibronic progressions. This spectral behavior was assigned to two MLCT states associated with the extended chromophoric ligand of the molecular structure. Distinct decay mechanisms lead to different excited-state lifetimes. Taking into account such a two-state model and the common emission properties, which cover the (v*o −vo) transition along with a tail accounting for vibronic progressions, the spectrum was fitted with two Gaussian double peaks (dashed lines in Figure 6A). This leads to an excellent agreement with the experimental data. While the first peak in the spectrum accounts for two MLCT states, the second peak is the sum of the state-associated vibronic progression. For the RT spectra shown in Figure 6 thermal activation of nonradiative decay channels is not suppressed, and therefore, in contrast to the low-temperature spectra, the antenna-enhanced spectra do not reflect a pronounced splitting of the peak into two peaks of equivalent height. However, the main peak is characterized by a significant asymmetry, which is well reproduced by the applied model. Hence, the antenna-enhanced spectrum reveals the (v*o −vo) transitions of two states, which are accompanied by their corresponding vibronic progressions. These occur ∼60 nm redshifted from the center of the corresponding (vo*−vo) transitions, which is in excellent agreement with previous low-temperature ensemble measurements. In contrast, the distance of the peaks reflecting the two (vo*−vo) transitions is found to be shorter than expected. Values determined from low-temperature ensemble measurements yield a distance of ∼30 nm.47 An application of the two-state model to the spectrum shown in Figure 6A(i) also leads to a good agreement with the experimental data. Apparently, contributions of the vibronic progression largely vanish due to the overall low photon emission rates for these red-emitting Ru2+[L]2(PF6−)2 complexes. Hence, in accordance with the spectra shown in Figure 6A(ii), the observed asymmetry of the spectral profile might be rather correlated to two MLCT states. The peak distance of 30 nm is again in excellent agreement with previously reported values.47 Due to the observed spectral similarities in the antenna-enhanced spectra, the identified phosphorescence spots in the confocal and near-field optical images can be unambiguously assigned to this specific Ru2+[L]2(PF6−)2 complex. One might consider that the blue-shifted emission may arise from other Ru2+-bis-terpyrindes, e.g., with partially dissociated ligands, or molecules with different chemical composition. In particular it has been shown that the photophysical properties of complexes with shorter ligands, i.e., coordination complexes not bearing a chromophore, and also Fe2+-coordinated chromophoric bis-terpyridines are nonluminescent at RT due to an increased internal conversion to the high-spin metal centered (HSdd) states.37 Therefore, it is unlikely that the observed subset of complexes with blue-shifted emission corresponds to dissociated species. Also molecular contaminations are likely to be ruled out. These would be already detectable in Ru2+[L]2(PF6−)2 solutions, and emission profiles with fewer similarities to the Ru 2+ [L] 2 (PF 6 − ) 2 complexes would be expected. Due to the single-molecule sensitivity provided by the antenna-enhanced investigations, aggregation of the complexes, i.e., the formation of dimers or small oligomers, can be excluded. Just as unlikely is the fact that
impurities may act as annihilators in triplet−triplet annihilation (TTA) of the investigated chromophoric Ru2+[L]2(PF6−)2 complex, leading to upconversion of the emission.48 Although TTA has been shown for chromophoric Ru2+-bis-terpyridine complexes with long-lived T1 lifetimes of the ligand, the required lifetimes for an efficient TTA are rather on the order of microseconds. Luminescence decay analysis of both subsets of Ru2+[L]2(PF6−)2 complexes in the solid state demonstrated that the excited-state lifetime is on the order of a few nanoseconds to tens of nanoseconds and is not shifted to the microsecond regime (lifetime measurements, Figure SI-5). Furthermore, for these lifetimes a saturation-limited emission rate from the excited states can be excluded for the used excitation powers. Therefore, the observed alterations are suspected to be caused by different intrinsic quantum yields. In order to approach the intrinsic quantum yield of these complexes, we take advantage of the large homogeneity of the employed AuNP antennas in terms of their electromagnetic field enhancement factor and their simple geometry, which enables straightforward analysis of the excitation and decay rates of a quantum emitter in the presence of the antenna.49 Following the theoretical description outlined in refs 22 and 49 and taking into account intrinsic losses and coupling to nonradiative electromagnetic modes, the spontaneous emission rate is given by γem = γexc(ωi)Qi with Qi = [γrad/γorad]/[γrad/γorad + γabs/γorad + 1/Qi(1 − Qi)]. Here, γexc denotes the excitation rate, and γrad the radiative rate, and γabs accounts for nonradiative transition to the metal nanostructure leading to energy dissipation. Qi is the intrinsic quantum yield of the complex. The superscript “o” denotes the corresponding transition rates without coupling to the antenna. This implies that in the strong coupling regime the spontaneous emission rate of a quantum emitter is affected by the electric field enhancement of the antenna, the radiative decay enhancement, and the intrinsic properties of the complex. Analytical solutions of the excitation rate enhancement (γexc/γoexc) of a quantum emitter mediated by the near-field interaction of the spherical NP antenna, of the corresponding radiative rate enhancement (γrad/γorad), and of the intrinsic losses (γabs) show characteristic distance dependencies (the corresponding expressions are given in the SI). This enables a quantitative comparison of the experimental data with the theoretical treatment of the spherical antenna in the dipolar approximation limit. Therefore, the phosphorescence emission as a function of the antenna−molecule separation can be rendered by matching the intrinsic quantum yield Qoi of the molecule. As a reference, approach curves of high-Q-yield dye molecules with a vertical orientation of their transition dipole were acquired. Their fluorescence emission as a function of the antenna−molecule separation could be best reconciled assuming an initial quantum efficiency of QoiAlexa532 = 65% and QoiAlexa633 = 55%, respectively. These values are in good agreement with the expected Q yields for Alexa Fluor dyes. The obtained enhancement factors of 4 to 5 for the Alexa 532 dye and 8 to 10 for the Alexa 633 are consistent with several previous investigations on high quantum yield emitters. The deviating fluorescence enhancement factor is ascribed to the wavelength dependence of the modification of the excitation and radiative and nonradiative electronic transitions.39,40 Equivalently, the distance-dependent progression of the phosphorescence enhancement of the Ru2+[L]2(PF6−)2 complexes can be compared with the theoretical model. The exemplified curves shown in Figure 6B(i) and (ii) correspond to the two subsets of F
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Ru2+[L]2(PF6−)2 complex identified in the solid state. In the case of the weakly emitting complexes, with the phosphorescence centered at 650 nm, the intrinsic Q-yield ranges from ∼10−3 to 10−4. This is in agreement with the typically low quantum efficiencies observed for Ru2+-bis-terpyridine complexes. For the brightly emitting complexes with emission peaks blue-shifted from 650 nm, the determined intrinsic quantum yields vary over a range of 0.4 to 0.05. As a consequence, the found phosphorescence enhancement factors for both subsets are often much higher than for the fluorescence emission of the Alexa Fluor dyes, which finally facilitates the sensitivity for investigations on a single-complex level. Considering the density of complexes and the image contrast in Figure 4B, the determined enhancement factors of >100 are conceivable. The observed fluctuations of the Qi within the drawn sample of red-emitting complexes and complexes with blue-shifted emission spectra, respectively, can be explained, at least partly, by environmental changes, e.g., surface charges and oxygen exclusion. In contrast, an alteration of the intrinsic quantum yield by several orders of magnitude between the two subsets in conjunction with significant spectral changes of the emission spectra is more likely attributed to molecular changes of the Ru2+[L]2(PF6−)2 complexes. The strong alterations of their photophysical properties in the solid state implies that these are imposed by an environmental heterogeneity of the embedding polymer matrix, e.g., its rigidity. Heterogeneous environmental conditions are known to affect the state energies and/or the character of the radiative state in different molecules. For instance, homoleptic [Ru(bpy)3]2+ complexes integrated into metal−organic frameworks have been shown to possess dual emission and different emission spectra.50 Taking into account the emission centered at ∼580 nm, the energy difference of the 1 MLCT state would first of all point to a direct emission from the 1MLCT state. For the Ru2+[L]2(PF6−)2 complex ultrafast ISC from the excited 1MLCT to 3MLCT is known to take place with an efficiency close to unity, which is in accordance with the general observation for ruthenium-polypyridyl complexes.51,52 Therefore, a direct emission from the 1MLCT state must involve a significant distortion of the complex geometry. Often the function of chromophoric ligands with long-lived triplet states is discussed in terms of an energy storage element.53−55 In these complexes the ligand T1 state matches energetically the 3MLCT states and, thus, enables a reversible population of the T1 state. Although the considered Ru2+[L]2(PF6−) 2 complex has a T1 state lifetime that is on the order of 20 to 100 μs, its energetic position of 1.63 eV, as determined from oxygen perturbation spectroscopy, results in a large energy gap to the phosphorescent state of 0.33 eV.37 This prevents energy back transfer to the 3MLCT state (Jablonski diagram, Figure 2A). Possibly, alterations of the ligand structure could lead to a reduced energy gap between these states. However, assuming a reversible population of this state, the triplet-state lifetime of the phosphorescent state should be rather on the order of μs than 40 ns. The remarkable ability for RT phosphorescence emission of these Ru2+[L]2(PF6−)2 complexes is in contrast to the related [Ru(ter)2]2+ complexes. Partly, this ability is ascribed to an excited-state planarization of the terpyridine ligand in these classes of coordination complexes. However, for the here-studied Ru2+[L]2(PF6−)2 complex further stabilization mechanisms of the 3MLCT state must be present. In this context, a secondary triplet state (3MLCT′) with ligandcentered charge-transfer character (LCCT) was postulated in addition to the lowest energy T1 state of the ligand (Jablonski
diagram, Figure 2A). In general it is assumed that this 3MLCT′ state has a comparable energy to the 3MLCT state and, thus, forms a thermal equilibrium with this state.37 The relative position of the 3MLCT state, the T1(3LCCT) state, and the HS dd state sensitively alters the photophysical properties of this complex. Similar to the observation in multichromophoric ligand systems, the T1(3LCCT) state could also act as an energy-storage system and, thus, cause altered photophysical properties. Notably, such alterations have been observed previously at low temperatures, i.e., in a rigid solvent matrix. Time-dependent single-complex emission trajectories of the blue-emitting complexes frequently display emission intermittencies on different time scales (emission trajectory, Figure SI6). These intermittencies might result from coupling to the MLCT′ state. In addition, one might consider the planarity of the ground and excited state geometry. For Ru2+-bis-terpyridine complexes the phosphorescence emission is sensitively influenced by alterations in the terpyridine cage and the adjacent phenyl ring located at the 4′-position of the terpyridine unit.37 This part of the complex also shows the highest deviation from a planar geometry. For instance, conformational deviations due to viscosity of the external polymer matrix could lead to a reduced torsional angle and, thus, to a decreased energy difference between the ground and excited/emissive MLCT state. In the case of a solid environment interconversion of the two states is not likely to occur due to the rigidity of the matrix and was not observed in time-resolved phosphorescence measurements so far.
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CONCLUSION
In conclusion, antenna-enhanced spectral investigations of dualluminescent ruthenium polypyridyl complexes enabled for the first time their optical characterization on a single-molecule level. The antenna-enhanced spectra of individual complexes resemble well the spectral profile found in ensemble measurements and reflect the level structure of the radiative and nonradiative states found in this class of coordination complexes. Remarkably, two spectrally shifted subsets of this type of complexes were identified in the solid state, which most likely can be assigned to two conformers. Although these subsets show similar spectral profiles, distinct alterations are found for the spectral peak position and their quantum efficiencies. One may assign this to a viscosity-induced altered ground-state and/or excited-state geometry due to modification of the torsion angle of at least one ligand arm, which largely determines the planarity of the complex geometry. Such alterations are known to impart their spectral properties and may also be associated with promoting a greater extent of equilibration between radiative and stabilizing nonradiative triplet states. The presented results exemplify the extreme sensitivity of antenna-enhanced phosphorescence spectroscopy with respect to weakly emitting entities and, thus, enabled for the first time addressing the largely unknown heterogeneity and environmentally caused alterations in thin film applications of these coordination compounds. The latter is of major importance for the development of design strategies of organic optoelectronic devices, such as polymer solar cells and artificial light harvesting and organic light emitting devices. G
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filter (LP03-532RU, Semrock, USA) was used, and a signal accumulation corresponding to the long-wavelength tail of the emission peak (650 to 710 nm) was achieved by exchanging the long-pass filter with a bandpass filter (HQ 680, Chroma Technology Corp., USA). Similarly, the spectral region of the blue-shifted emission of the Ru2+[L]2(PF6−)2 complexes was covered by using a bandpass filter (HQ580, Chroma Technology Corp., USA) with a transmission window of ∼545 to 620 nm. Optical antennas were prepared from commercially available AuNPs (Britsh Biocell International, UK) with an average diameter of 80 nm. Glass tips were fabricated from quartz glass capillaries with an outer diameter of 1 mm by means of a micropipet puller (P-2000, Sutter Instruments, USA). These glass tips were chemically functionalized with 3-aminopropyltrimethoxysilane by vapor phase deposition.28 For the antenna-enhanced measurements an optical antenna in the form of a tip-supported 80 nm gold nanoparticle was positioned in the center of the focus at a distance of ∼2 nm from the sample surface. The antenna−sample distance was maintained with subnanometer precision by means of a sensitive force feedback loop (R9 SPM controller, RHK Technologies, USA). The normal forces acting on the antenna were detected with a piezoelectric quartz tuning fork with a resonance frequency of ∼32.6 kHz, which was mechanically excited near its resonance frequency. Interaction forces exerted on the antenna lead to a shift of the resonance frequency. The measured frequency shift depends on the antenna−sample distance and, thus, can be calibrated for distance variations. Hence, the antenna-enhanced measurements yield simultaneously information on the sample topology. The luminescence time trajectories, approach curves, and spectra are recorded in the form of a point-spectroscopy mode. For these measurements the antenna was precisely positioned on top of a previously identified complex. Antenna−sample approach curves measure the antenna-enhanced luminescence signal as a function of the antenna−complex separation. For this, the feedback loop is turned off and the antenna is continuously retracted from the sample surface to the confocal regime. At the same time, the frequency shift and the photon count rate are recorded as a function of the antenna distance. Typically the approach curves span a range from ∼1 to 50 nm. The approach curves are corrected for possible offsets, e.g., caused by the hysteresis of the piezoelements, and are corrected for the background luminescence of the antenna. The latter was determined from approach curves taken in a bleached region of the sample. Typical background count rates for an 80 nm AuNP antenna are ∼200−500 counts/ms. For the antennaenhanced phosphorescence time trajectories and spectrally resolved measurements the antenna was centered on a complex and was kept at a distance of ∼2 nm. Time stamping of the arrival times of photons and sequential binning to 5 to 50 ms recorded the temporal fluctuations of the emitted signal. For the acquisition of the confocal and antenna-enhanced spectra, the collected emission signal was sent to the spectrometer. Here, the dichroic mirror was replaced by a 50:50 beamsplitter (Chroma Technology Corp., USA), and a razor edge long-pass filter (LP03-532RU, Semrock, USA) was mounted in front of the spectrograph. All antenna-enhanced phosphorescence time trajectories, approach curves, and spectra were corrected for the background luminescence of the AuNP antenna and the substrate.
MATERIALS AND METHODS Preparation of the Ru-Bis-terpyridine Complexes in the Solid State. The synthesis of the investigated Ru2+[L]2(PF6−)2 complexes followed the protocols published in ref 56. The samples were prepared on glass coverslips (No. 1, Menzel, Germany), which were treated in a 1% aqueous Hellmanex solution at 70 °C for 30 min in a sonication bath in order to remove residual contaminations. Subsequently, the glass substrates were thoroughly rinsed and sonicated in ultrapure DI water (18 MΩ·cm). The cleaned coverslips were kept in the DI-H2O until use. Prior to use the substrates were dried with N2. Ru2+-bis-terpyridine complexes were dissolved in chloroform (Sigma-Aldrich, Germany) to a final concentration of ∼100 nM. Molecular layers of the Ru2+-bis-terpyridine complexes were prepared by spin-casting first 100 μL of this solution to a clean coverslip at 1500 rpm, followed by an overcoating step with PMMA. For the overcoating step PMMA was diluted in toluene to a final concentration of 1 vol %. The PMMA layer thickness was carefully adjusted to ∼2 nm by variation of the applied volume of the PMMA solution. Spin-casting 1 μL at 5000 rpm to a freshly cleaned glass substrate forms a smooth PMMA layer with a thickness of ∼2 nm. The layer thickness was determined by the razor edge knife scratching method. Atomic force microscopy (AFM) (NT-MDT, Russia) was used to measure the depth of the formed scratch (AFM image, Figure SI-1). The depth of the formed groove was determined to be ∼1.5−2 nm. Confocal and Antenna-Enhanced Microscopy. Confocal luminescence and antenna-enhanced investigations were carried out with an in-house-built microscope. Figure SI-2 displays a schematic outline of the setup. Briefly, the base was formed by an inverse microscope (Eclipse-Ti-U, Nikon, Japan), which was equipped with an oil immersion objective (PlanApo TIRF, 100×, NA 1.49, Nikon, Japan), a 100 × 100 μm2 piezoelectric xyz-scan stage (NanoH-100, Mad City Laboratories, USA), two equivalent Avalanche photodiodes (AQRH-14TR, Excelitas, Canada) with a typical dark count rate of 40−80 counts/s, and a 300 mm spectrograph (Acton SP2300, Princeton Instruments, USA) coupled to a N2-cooled CCD camera (Pylon, BR-400, Princeton Instruments, USA). For the time-resolved luminescence measurements a time-correlating single-photon-counting unit (TimeHarp 300, Picoquant, Germany) was used. For the excitation of the Ru 2+ [L] 2 (PF 6 − ) 2 complexes via their 1 MLCT band a frequency-doubled Nd:YAG laser (Compass 315, Coherent, Germany) was transferred to a radially polarized doughnut mode with a commercially available liquid crystal polarization converter (Arcoptix, Switzerland) and focused to the antenna− sample region with the microscope objective. For a tightly focused radially polarized laser beam the strength of the longitudinal field components was increased over the azimuthal ones. This leads to an increased coupling strength to the antenna. However, the spatial deviations of the field components cause differences in the excitation rate of a factor of ∼2 for molecules with out-of-plane and in-plane transition dipoles. Images were acquired by scanning the sample through the laser focus at a fixed z-position (height). Confocal and antenna-enhanced imaging of different spectral ranges was accomplished by insertion of suitable filter combinations. For a signal integration from ∼538 to 800 nm a combination of a dichroic mirror (Di02-R532, Semrock, USA) and a long-pass H
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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/acsphotonics.6b00419. Sample preparation, the spectral homogeneity, the luminescence lifetimes, and calculation of the enhancement rates (PDF)
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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 We thank the German Science Foundation (DFG) for financial support through the German-Chinese transregional project TRR61. C.H. acknowledges the support by the MIWF (NRW, Germany) in the framework of the “Rückkehrerprogramm: Nanotechnologie”.
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