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Synthesis, Ensemble, and Single Molecule Characterization of a Diphenyl-Acetylene Linked Perylenediimide Trimer† Tom Vosch,‡ Eduard Fron,‡ Jun-ichi Hotta,‡ Ania Deres,‡ Hiroshi Uji-i,‡ Abdenacer Idrissi,| Jaesung Yang,⊥ Dongho Kim,⊥ Larissa Puhl,§ Andreas Haeuseler,§ Klaus Mu¨llen,*,§ Frans C. De Schryver,‡ Michel Sliwa,| and Johan Hofkens*,‡ Department of Chemistry, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200F, HeVerlee 3001, Belgium, Max-Planck-Institut fu¨r Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany, Laboratoire de Spectrochimie Infrarouge et Raman (UMR 8516 du CNRS), Centre d’Etudes et de Recherches Lasers et Applications (FR 2416 du CNRS), UniVersite´ des Sciences et Technologies de Lille, Baˆt C5, 59655 VilleneuVe d’Ascq Cedex, France, Spectroscopy Laboratory for Functional π-electronic Systems and Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea ReceiVed: February 27, 2009; ReVised Manuscript ReceiVed: April 6, 2009
Perylenediimide (PDI) dyes have attracted a great deal of attention as they possess excellent photochemical stability, high extinction coefficients, and fluorescence quantum yields. The use of multiple PDI chromophores in one synthetic architecture increases their versatile use and functionality even more. However, bringing multiple chromophores in close proximity also leads to interactions among the chromophores and opens up new photophysical pathways. Here, the synthesis and photophysical characterization, both at the ensemble and single molecule level, of a diphenyl-acetylene linked perylenediimide trimer (3PDIAc) is presented. Fo¨rster type energy transfer processes like energy hopping and singlet-singlet annihilation among the chromophores are investigated. Despite the lower singlet-singlet annihilation rate of the phenoxy substituted perylenediimide chromophores (356 ps) versus for example perylenemonoimide (10 ps), the system still behaves as a single photon emitter. Sequential fitting of the dipole emission pattern recorded with defocused wide field imaging of single 3PDIAc, immobilized in a PMMA polymer film, demonstrated that emission can switch between sequential emission of all of the chromophores or emission from one chromophore that likely is the lowest in energy. 1. Introduction Perylenediimide (PDI) dyes have attracted a great deal of attention since they possess exceptional chemical, thermal, photochemical, and photophysical stability in combination with high extinction coefficients.1-11 The use of multiple chromophores in one synthetic architecture increases the versatility and functionality even more. However, bringing multiple chromophores in close proximity introduces interactions between the chromophores and opens up new photophysical pathways. The study of photophysical properties of these synthesized multichromophoric model structures has led to a better understanding of for example processes in biological energy transfer systems (LH2, allophycocyanines, DsRed, etc.)12-14 and contributed in material sciences to the development and understanding of new photonic devices like organic OLED15,16 and single photon sources.17-19 In order to fully understand and characterize the photophysical properties, a well-defined structure is necessary. The latter was previously achieved by using a rigid polyphenylene backbone.18,20-26 The orientation of the chromophores is however still variable due to the presence of structural isomers.24 In this contribution we have synthesized and studied a rigid and linear structure existing out of three †
Part of the “Hiroshi Masuhara Festschrift”. * To whom correspondence should be addressed. E-mail: muellen@ mpip-mainz.mpg.de;
[email protected]. ‡ Katholieke Universiteit Leuven. | UMR 8516 du CNRS. ⊥ Yonsei University. § Max-Planck-Institut fu¨r Polymerforschung.
perylenediimide chromophores (substituted in the bay area with phenoxy arms, PDI) linked together by diphenyl-acetylene units (3PDIAc, see compound 9 in Scheme 1). Together with this a reference compound containing one perylene chromophore and a diphenyl-acetylene unit was investigated (PDIAc, see Figure 1B). This system relates to a synthetic rigid trichromophoric perylenediimide (3PDI, see the Supporting Information for structure) that was studied by Hoogenboom et al. and Hernando et al.27-30 In the 3PDI system however, the chromophores are directly linked together via the nitrogen atoms of the imide sections of the chromophore. As a result, a strong interaction and bathochromic shift is observed when comparing the absorbance and fluorescence spectra of one PDI versus 3PDI. 3PDIAc only has weak interactions, due to the increased distance between the perylenediimide chromophores. In this contribution, a combination of ensemble and single molecule techniques is presented resulting in a detailed picture of the photophysical properties of 3PDIAc. At the same time, some results of the weakly coupled 3PDIAc will be compared to the strongly coupled 3PDI system. 2. Experimental Section Steady-State Spectroscopy. Steady-state absorption and fluorescence spectra were recorded on a Lambda 40 spectrophotometer (Perkin-Elmer) and Fluorolog-3 fluorimeter (ISA instruments Jobin Yvon), respectively. The compounds were dissolved in toluene or THF and had an optical density of about 0.1 in a 1 cm cuvette at the absorption maximum. Fluorescence
10.1021/jp901820y CCC: $40.75 2009 American Chemical Society Published on Web 04/21/2009
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SCHEME 1: (i) Propionic Acid, 160 °C, 16 h; (ii) Phenol, K2CO3, DMF, 90 °C, 16 h; (iii) TMS-Acetylene, Pd(PPh3)2Cl2, CuI, PPh3, Triethylamine, THF, 30 min Microwave-Irradiation; (iv) Bu4NF, THF, 5 min, rt.; (v) TMS-Acetylene, Pd(PPh3)2Cl2, CuI, PPh3, Triethylamine, THF, 30 min Microwave-Irradiation, 120 °C
quantum yields (Φf) were measured using cresyl violet in methanol as reference (Φf ) 0.55), with λex ) 543 nm.31 Limiting anisotropy measurements were performed with a PTI Quantamaster UV/vis (Photon Technology International) equipped with polarizers in the excitation and the emission path. The compounds, dissolved in methyl-THF, were frozen with liquid nitrogen in a quartz tube and placed in the fluorimeter. Four spectra with horizontal/horizontal, horizontal/vertical, vertical/horizontal, and vertical/vertical excitation and emission polarization were recorded from which the limiting anisotropy was determined.32 Picosecond Time Correlated Single Photon Counting Measurements. Fluorescence decay times were recorded by the time-correlated single photon counting (TC-SPC) method using a setup described previously.33 Briefly, the 543 nm pulsed excitation light (8.18 MHz, 1.2 ps full width at half-maximum) was obtained from the frequency doubled output of an optical parametric oscillator (GWU) pumped by a Ti:sapphire laser (Tsunami, Spectra Physics). The detection system consists of a subtractive double monochromator (9030DS, Sciencetech) and a microchannel plate photomultiplier (R3809U, Hamamatsu). A time-correlated single photon counting PC module (SPC 830, Becker & Hickl), which has two constant fraction discriminators (CFD), a time-to-amplitude converter (TAC), and an analog-to
digital converter (ADC) on the PC board, was used to obtain the fluorescence decay histograms with time increments of 5 ps. The decays, recorded at magic angle, were analyzed globally with time-resolved fluorescence analysis (TRFA) software.34,35 The quality of the fits has been judged by the fit parameters χ2 (10 ms) in the fluorescence intensity to the background level. These jumps to the background level, typical for single molecules are also observed for other multichromophoric PDI sytems.17,28,44,54 Those long off events are not due to excursions to the triplet state, since the decay time of the triplet state in a polymer matrix is on the order of hundreds of microseconds.54 Recently it was found that in some cases (depending on the environment), these off events on a several millisecond or longer time scale (up to several seconds) follow a power-law behavior.28,55,56 The most likely explanation is that this long off process originates from an electron transfer from or to trap sites close to the molecule, creating a long-lived charge separated state. This possibility was already ascribed as the cause for the long off blinking in other PDI systems, studied at the
Figure 3. (A and B) Two examples of 3PDIAc fluorescence intensity trajectories. (C) Sum of 46 fluorescence intensity trajectories of 3PDIAc molecules measured at 0.5 kW/cm2. The data was fitted with an exponential decay yielding a survival time value of 112 s. (D) Histogram of the number of long off (>10 ms) events (P(τlongoff)) that were observed versus the long off time (τlongoff) for 46 3PDIAc molecules, fitted with a powerlaw function, yielding a moff value of 1.24.
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Figure 4. (A) Fluorescence intensity trajectory of a single 3PDIAc molecule embedded in PMMA, recorded with pulsed excitation. (B) Decay times (in bin times of 1 s) corresponding to the fluorescence intensity trajectory of panel A. (C) Histogram of the decay time values (2117 in total) obtained from 21 3PDIAc individual molecules. The histogram is fitted with a Gaussian distribution, centered at 5.2 ns. (D) Interphoton arrival time histogram of the first intensity level of panel A.
single molecule level.27,44,48,57 Additionally, it was demonstrated before that the presence of oxygen plays a role on the number of observed events.44 In the absence of oxygen, the number of long off events is drastically reduced, suggesting that an oxygenmediated pathway, for example by enhanced intersystem crossing to the triplet state induced by paramagnetic oxygen, is responsible for the formation of a long-lived dark chargeseparated state.48,58 Figure 3D shows an overview of all of the long offs of 46 molecules. This histogram was created by binning the fluorescence intensity trajectories in 10 ms and using a threshold to determine the difference between the on and off level.28 For the 46 measured molecules, 574 long off events where detected, displaying a large range of 0 to 83 long off events for each molecule. The histogram can be fitted by a -moff ) with a moff value of power-law function59 (P(τlongoff) ∼ τlongoff 1.24. This value is in good agreement with the moff value of 1.32 that was found for the 3PDI system by Hoogenboom et al. indicating that this long off blinking process is a property of the PDI chromophores and their environment, regardless whether the PDI chromophores are strongly coupled or not.28 Pulsed Excitation Single Molecule Experiments. Single 3PDIAc molecules embedded in a PMMA film were also
investigated with pulsed excitation in order to investigate the lifetime distribution and fluctuations. Figure 4A shows again a typical fluorescence intensity trajectory (constructed from the macro times). Three intensity levels with several long off periods of several seconds can be observed. From the recorded microtimes, histograms were constructed every second and these were fitted with a single exponential decay, using in house developed software program.34 Figure 4B shows the decay times of the corresponding intensity trajectory of Figure 4A. The decay times vary from about 4.8 ns in the first level to 4.4 in the second and third intensity level. Only in the beginning of the third intensity level (from 18 to 28 s) there is a drop in the decay times to 4 ns. Variations in the decay times can be due to slight differences in the environment of the chromophores or due to conformational differences between the chromophores.29,44,45 For the strongly coupled and directly linked 3PDI system, shorter decay times and decay time changes between the different chromophores were observed and attributed to a superradiance effect.29 This effect would lead to a general increase in the decay time upon consecutive bleaching of chromophores since bleaching results in a drop in the overall oscillator strength. For 3PDIAc there seems to be no systematic increase in the decay
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Figure 5. (A) Fluorescence intensity trajectory reconstructed from a movie (0.5 s bin time) of a single 3PDIAc molecule recorded with defocused wide field imaging. For each intensity level the observed dipole pattern is represented. For the second level, 2 types of dipole patterns are observed. (B) Overview of the observed dipole patterns in panel A, fitted with theoretical expected dipole patterns. The dipole moment pattern of the last intensity level (from 400-500 s, M3) could be fitted well with a theoretical expected dipole angle of 70° out of plane and 145° in plane. For the second and first intensity level, M3 and M3 plus M2 were subtracted, respectively.
time upon sequential bleaching of the chromophores. Figure 4C shows a histogram of 21 single 3PDIAc molecules containing 2117 decays times. The histogram of decay times is broad ranging roughly 4-6 ns with a maximum centered at 5.2 ns. This value corresponds well to the ensemble measured values of 5.2 ns for PDIAc and 4.7 ns for 3PDIAc in toluene (the polarity of PMMA is closer to that of toluene than to that of THF).60 By using a Hanbury-Brown and Twiss setup, coincidence measurements were done on individual 3PDIAc molecules. The idea of these experiments is to see if 3PDIAc acts as a single photon emitter (one photon is emitted per pulse) when excited with an intense laser pulse. Also this would demonstrate that the singlet-singlet annihilation process observed in the femtosecond time-resolved measurements can be observed at the single molecule level. This process makes sure that at high excitation powers when multiple excited chromophores are present, only one photon is emitted per excitation pulse. As discussed above, femtosecond transient absorption spectroscopy showed that at high excitation power singlet-singlet annihilation occurs with a time constant of 365 ps for 3PDIAc in toluene. This singlet-singlet annihilation rate is more than 10 times
slower than that of perylenemonoimide systems,24,25,61 lowering the efficiency of 3PDIAc for single photon source applications.62 Figure 4D shows the interphoton arrival time histogram of the first intensity level of the trace depicted in Figure 4A. The ratio (Nc/Nl) for the maximum the central peak at 1350 ns (time zero due to the added delay of 1350 ns) versus the lateral peaks is about 0.25. This value is similar to values found for other PDI containing multichromophoric systems indicating that this system behaves as a single photon source.17,54 Defocused Imaging. Defocused wide field movies (1000 frames with a 2 Hz imaging rate) of single 3PDIAc molecules embedded in a thin polymer film (
