Synthesis, Ensemble, and Single Molecule Characterization of a

Article pubs.acs.org/JPCB

Synthesis, Ensemble, and Single Molecule Characterization of a Diphenyl-Acetylene Linked Terrylenediimide Dimer Koen Kennes,† Yannick Baeten,† Sebastian Stappert,‡ Klaus Müllen,‡ Johan Hofkens,† Mark Van der Auweraer,† Tom Vosch,§ and Eduard Fron*,† †

Molecular Imaging and Photonics, Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium ‡ Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany § Nano-Science Center/Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark S Supporting Information *

ABSTRACT: The synthesis and the photophysical characterization at the ensemble and single molecule level of a terrylenediimide (TDI) dimer are reported. The spectroscopic experimental data are compared with those obtained for the corresponding model compound TDI. Steady-state and ps time-correlated single photon counting have shown that both chromophores in the TDI dimer are in the weak coupling regime allowing their interaction by Förster resonance energy transfer. Femtosecond transient absorption experiments showed an excitation power dependence of the fluorescence decay, which could indicate the occurrence of singlet−singlet annihilation. Single molecule intensity traces were investigated for the TDI dimer and suggested two intensity levels. For both intensity levels several parameters among which emission maximum, fluorescence decay times, antibunching, blinking off-times and rate of dark state formation were compared. The blinking analysis revealed that the yield of dark state formation is an order of magnitude higher when the two chromophores are still active compared to the case where one is photobleached. The off-times remain however similar.

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INTRODUCTION The synthesis and photophysical characterization of new rylene-based molecular architectures have attracted much attention in recent years due to their potential applications as active materials in OLEDs,1 photovoltaic devices2 or optoelectronics.3 Terrylenediimide (TDI), a member of this class of rylene fluorophores, is known for its high fluorescence quantum yield of emission in the red part of the spectrum, its large molar extinction coefficient, and its outstanding photostability.4 As a fluorescent label, TDI and its derivatives have been used in a large number of fundamental single molecule studies.5−15 Upon investigation of a series of molecules where two rylene groups (perylenemonoimide, PI) were separated by a number of fluorene moieties, our group reported that the two systems are efficient single photon (SP) sources at room temperature16 due to efficient singlet-to-singlet annihilation. Due to their quite rigid and linear structure pentaphenylene moieties have been used as spacers for separating two PI units.17 However, it has been shown that such moieties play more than a “bridge” role by being involved in the photophysics via the generation of states with charge transfer character (where the pentaphenylene moiety acts as electron donor to the excited PI chromophore).18 The participation of a charge transfer state is compatible with singlet−singlet annihilation by Förster type energy transfer given that no change occurs in the electron © XXXX American Chemical Society

multiplicity during the acceptor transition. When the charge transfer state is formed in polar solvents, the excited-state wave function significantly spreads over the neighboring pentaphenylene skeleton.18 As a consequence, the center-to-center separation distance diminishes by about 2 nm leading to considerable enhancement in the annihilation efficiency. Hence if one wants to study the interaction between the rylenediimide chomphores as such it is important to have a spacer with a high lying LUMO, a low lying HOMO and a high lying S1-state which does not alter the properties of the terrylenediimides. These requirements can be met by a diphenylacetylene spacer (cfr. infra). For fundamental research, it is also important to characterize these features and to understand the excited state processes that are related to energy transfer in multichromophoric systems like, e.g., light harvesting complexes19 or conjugated polymers.20,21 Additionally, the high interest in super resolution fluorescence microscopy techniques22 will require the need of increased label densities in order to visualize structures with sub diffraction limits. In this view, it is expected that more and more experiments will use common dyes with a separation distance smaller than the FRET radius. Detailed Received: October 30, 2015 Revised: February 8, 2016

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DOI: 10.1021/acs.jpcb.5b10651 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Figure 1. Chemical structure of TDI model (A) and TDI dimer (B); stationary absorption and emission spectra (λexcitation = 580 nm) of TDI model (C) in toluene (black) and chloroform (red); stationary absorption and emission spectra of TDI dimer (D) in toluene (black) and chloroform (red).

Table 1. Steady-State Spectroscopic Data Obtained for Model TDI and Dimer TDIa absorption max. (nm)

emission max. (nm)

Stokes shift (cm−1)

ext. coef. (L mol−1 cm−1)

fluorescence quantum yield (%)

fwhm abs (cm−1)

chloroform (4.8; 1.44) toluene (2.4; 1.5)

652

668

370

103000

64 ± 0.06

610

651

664

300

/

69 ± 0.07

590

chloroform (4.8; 1.44) toluene (2.4; 1.5)

657

673

360

194000

62 ± 0.06

600

656

669

300

/

66 ± 0.07

570

solvent (ε0, n) model TDI* model TDI* dimer TDI dimer TDI

λexcitation = 580 nm; the molar extinction coefficient was measured at the maximum of the absorption wavelength. The full width at the halfmaximum (FWHM) was calculated considering only the 0−0 absorption peak from the S0−S1 transition subtracted from the absorption spectrum which was plotted on a wavenumber scale using a multiple Gaussian fitting procedure. ε0: dielectric constant, n: refractive index of the solvent. *Data taken from Kennes et al.35 a

model studies of well-defined multichromophoric systems can also help to address unexpected changes in photophysical behavior, e.g., changes in the blinking dynamics and state switching.23 In this paper we investigate the photophysical properties of a newly synthesized TDI dimer. As mentioned above, the bridge moiety can affect the photophysics of the chromophore, especially in high polar solvents. Here, the two TDI chromophores are chemically linked by a diphenyl-acetylene unit which is characterized by a higher lying LUMO and a lower lying HOMO than e.g. a pentaphenylene moiety.24,25 In this way, we expect to avoid the complexities such as the formation of a state with charge transfer character and/or the delocalization of HOMO and/or LUMO over the bridge, induced by long bridges. Such a dimer can serve as a welldefined model system of two chromophores connected by a, further inert, rigid linker.17,18 The diphenyl-acetylene linker will keep the TDI subsystems at a well-defined distance and leads to a nearly collinear orientation of the transition dipole moments. However, the close proximity of two TDIs can have an impact on the photophysical properties of such dimers which prompted us to investigate in detail their properties in bulk and at single molecule level. The results are compared to those obtained from a TDI monomer to estimate the coupling and determine which photophysical properties change. Single molecule experiments have been performed before and after bleaching one of the two chromophores (as assumed by

analyzing the high and low intensity levels of TDI dimers displaying two intensity levels).22

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RESULTS AND DISCUSSION Synthesis. The synthesis of the TDI dimer and its precursors are described in detail in the Supporting Information. Briefly, the synthesis involved a palladium catalyzed stille-coupling of the corresponding N-(4-bromo2,6-diisopropylphenyl)-substituted TDI monomer. N-(4Bromo-2,6-diisopropylphenyl)-N′-(1-heptyloctyl)terrylene3,4:11,12-tetracarboxdiimide (25 mg, 0.026 mmol) was dissolved in dry toluene and the solution was flushed with argon for 1 h. 1,2-Bis(tributylstannyl)acetylene (7 mg, 0.013 mmol) and tetrakis(triphenylphosphine)palladium (3 mg, 0.03 mmol) were added before the flask was sealed with a rubber septum. The solution was stirred at 110 °C under argon atmosphere for 21 h. After cooling to room temperature, the crude product was precipitated by adding methanol to the solution. After filtration, the crude mixture was subjected to preparative size exclusion chromatography (BioBeads S-X1, THF), yielding 1,2-bis(4-(2,6-diisopropylphenyl-N′-(1heptyloctyl)terrylene-3,4:11,12-tetracarboxdiimide))-acetylene (TDI dimer) as a blue solid (4 mg, 17%). Steady-state Absorption and Emission Properties. Figure 1A,B displays the chemical structure of the TDI model and TDI dimer, respectively, while Figure 1C,D shows their normalized room temperature absorption and fluoresB


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dynamics on a shorter time range fs transient absorption has been used. Time-Correlated Single Photon Counting. Using timecorrelated single photon counting (TC-SPC) the fluorescence decays of both the TDI model and the TDI dimer were determined in chloroform and toluene. All decays could be fitted to a single exponential decay. This analysis allowed us to recover the fluorescence decay times, τ, shown in Table 2. The fluorescence decay times of the dimer are about 10% smaller than those of the monomer which is larger than the experimental error.

cence spectra obtained in chloroform and toluene. Table 1 compiles the steady state spectroscopic values obtained for the TDI model compound and TDI dimer. The absorption spectra of both compounds consist of a main band that peaks at 651−657 nm, two less pronounced maxima, of which the first is situated at 598 and 600 nm for respectively the monomer and the dimer, and a shoulder between 600 and 500 nm. These secondary maxima and the shoulder correspond to a vibronic progression of the S0-S1 transition of the TDI-core whose transition dipole moment is oriented parallel to the long molecular axis.38 The spacing between the vibrational levels of the fist excited state, determined from this vibronic progression, amounts to 1360 ± 20 and 1420 ± 20 cm−1 for respectively the monomer and the dimer. The maximum of the dimer is shifted 116 cm−1 (0.0145 eV) to lower energy compared to the monomer. A second and less intense absorption band is observed in the 400−450 nm region and attributed to a transition to a higher excited state (S0−S2) with a transition dipole oriented parallel to the short molecular axis.38 The fluorescence spectra are characterized by a maximum between 664 and 676 nm. There are furthermore two less pronounced maxima between 700 and 800 nm of which the first is situated at 726 and 733 nm for respectively the monomer and the dimer, corresponding to a vibronic progression of the S1−S0 transition of the TDI-core. The spacing between the vibrational levels of the ground state, determined from this vibronic progression amounts to 1290 ± 20 and 1360 ± 20 cm−1 for respectively the monomer and the dimer. The maximum of the dimer is shifted 112 cm−1 (1.39 × 10−2 eV) to lower energy compared to the monomer. This shift of both the absorption and fluorescence maximum of the dimer is of the order of magnitude of what can be expected for the exciton interaction26−30 between two collinear transition dipoles of 3.84 × 10−29 Cm at 2.17 nm (cfr. infra). This suggests that interaction with the diphenyl-acetylene linker contributes little to the red shift. This interaction will indeed be limited as the two ortho methyl groups rotate the phenyl moiety out of the plane of the TDI and hence break the conjugation. Within experimental error the molar extinction coefficient of the dimer is twice that of the monomer. Furthermore, the features and the vibronic progression are nearly identical for the monomer and the dimer. The Stokes shift amounts for both monomer and dimer to 365 cm−1 in chloroform and 300 cm−1 in the less polar toluene. This indicates a minor increase of the dipole moment upon excitation which is similar for the monomer and the dimer.31−33 The latter is also reflected in a small increase of the fwhm of both absorption and emission spectra when going from toluene to chloroform.34 The absence of a systematic change of spectral properties of monomer and dimer is due to the fact that the exciton interaction is much smaller than the exciton phonon coupling.26 The relative difference of the fluorescence quantum yield between monomer and dimer amounts to 5%, which is close to the experimental error. Hence there is no evidence for any strong electronic interaction between the two chromophores in the dimer. Time-Resolved Absorption and Emission Experiments. In order to further investigate relevant chromophore−chromophore interactions in the ground and excited states, several types of time-resolved spectroscopic methods have been applied. On a ps-ns time scale the fluorescence decays have been monitored using the TC-SPC technique while for the

Table 2. Fluorescence Decay Times Obtained by TC-SPC for the TDI Model and TDI Dimer in Chloroform and Toluene upon Excitation at 560 nma solvent (ε0, n) TDI model* TDI model* TDI dimer TDI dimer TDI dimer

chloroform (4.8; 1.44) toluene (2.4; 1.5) chloroform (4.8; 1.44) toluene (2.4; 1.5) PMMA film (2.8 ; 1.49)36

τ (ps)

kr (×108 s−1)

knr (×108 s−1)

3000 3300 2686 3044 3060

2.1 2.2 2.3 2.2 n.d.

1.2 0.86 1.4 1.1 n.d.

a

Radiative (kr) and non-radiative (knr) rate constants calculated on the basis of fluorescence decay time constants and quantum yields. *Data taken from Kennes et al;35 (n.d.) not determined.25

As τ =

1 k r + k nr

and φf =

kr k r + k nr

where kr and knr are

respectively the rate constants for radiative and nonradiative decay while φf is the fluorescence quantum yield. Hence the rate constants kr and knr could be calculated from the fluorescence decay time and fluorescence quantum which yields the results displayed in Table 2. While kr increases by 1 to 7%, which is within the experimental error, in the dimer, knr is increased by 20 to 25%. This indicates that the decreased fluorescence decay time of the dimer is mainly due to an increase in knr. The steady state data, corroborated with fluorescence time-resolved measurements, clearly indicate that the two chromophores are in the weak coupling regime. Starting from the rate constant of fluorescence, kr, the transition dipole a value of 3.86 × 10−29 Cm was obtained for the transition dipole of the monomer in toluene (see details in the SI). This allows us to calculate the exciton coupling,27−30 ΔE for two collinear monomers separated by a diphenyacetylene spacer using ΔE = 2

⟨ψ00|xe + ye + ze|ψn0⟩2 4πε0R3

where R is the distance between (in meter) the center of the two chromophores (2.17 nm). In this way an exciton coupling of 1.64 × 10−2 eV or 132 cm−1 is obtained. Femtosecond Transient Absorption experiments. Figure 2 displays the femtosecond transient absorption spectra obtained for the TDI dimer in toluene upon 600 nm excitation, recorded in two time windows of respectively 6 and 45 ps. A broad and negative band which appears instantaneously after excitation can be observed. Based on stationary spectra the signal in the 500−665 nm range can be attributed to groundstate depletion whereas the signal in the 665−700 nm range is due to stimulated emission. Although no positive bands are found in the spectral region investigated here37 we cannot C


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after excitation (Figure 2A). This shift is an artifact due to dispersion of the white light probe beam.39 Figure 2C shows the transients signals at 670 nm obtained upon excitation at 600 nm at intensities of 200 μW (1), 400 μW (2), 750 μW (3), and 1000 μW (4) corresponding to upper limits of 3.42 × 1015, 6.84 × 1015, 1.28 × 1016, and 1.71 × 1016 photons pulse−1 cm−2, respectively. The transient absorption traces could be analyzed by a multiexponential decay model. Besides the fluorescence related long component of which the decay time was accurately determined in the TCSPC experiments, additional time constants were retrieved: an ultrafast component of 100 fs and a fast component of 1.9 ps. These time constants can probably be attributed to relaxation processes within the excited state like intramolecular vibrational redistribution (IVR) and/or vibrational relaxation (VR). Earlier work on a monomeric terrylenediimide yielded, depending on the emission wavelength 4 to 6.8 ps for the time constant of the VR while a subpicosecond component was attributed to IVR.23,36,40 An important point to note is that an additional decay component (300 ± 50 ps) can be clearly observed in the 1000-μW experiments when this is compared with the decays recorded with 200, 400, and 750-μW. This can be seen in Figure 2C. By increasing the number of photons impinging on the sample within a single femtosecond pulse the probability of exciting both chromophores increases and the potential interactions between the excited states in the TDI dimer can be investigated.36 It was shown previously for other multichromophoric rylene systems that singlet−singlet annihilation can occur when chromophores are in close proximity and such a characteristic confers to the system a single photon emitter performance.8,41−44 If we assume that this process occurs by Förster type excitation transfer its occurrence requires: a good overlap between the fluorescence spectrum and the first excited state absorption spectrum, a short distance between both chromophores and a suitable orientation factor between the transition dipole moment of the two chromophores. Although we could not clearly detect the excited state absorption in the femtosecond transient absorption measurements, Bullock et al. reported the presence of the excited state absorption spectrum for a similar terrylenediimide chromophore above 765 nm, allowing for some overlap with the emission spectrum.38 Based on the paper from Bullock et al. and the power dependence of this 300 ps component, we suggested singlet−singlet annihilation as a possible explanation. However, our calculations of the number of photons absorbed per excitation pulse indicate that even at 750 microWatt and lower, we should still be in a multiple photon absorption regime. Hence, our suggestion of singlet−singlet annihilation should be taken with care. Single Molecule Confocal Fluorescence Microscopy. Single molecules of the TDI dimer compound were embedded in a PMMA polymer film and their fluorescence was recorded with a scanning confocal fluorescence microscope (cfr. infra). The time trajectories of the fluorescence intensity of individual molecules can be divided in 3 categories and the following distribution was found: 16% of the trajectories showed only a single intensity level (with a duration of a few seconds), 68% (77 molecules) of the TDI dimers showed two intensity levels, and 16% showed more than 2 intensity levels. Considering the presence of two chromophores in a dimer, we focused our interest in the 68% (77 molecules) of the trajectories that displayed two clear intensity levels in the fluorescence intensity

Figure 2. 2D femtosecond transient absorption spectra obtained for TDI dimer in toluene, λexcitation = 600 nm, λdetection = 500−700 nm. The spectra are recorded in a 6 ps (A) and 45 ps (B) time window. (C) Femtosecond transient absorption traces obtained for TDI dimer in toluene recorded in 45 ps time window (λexcitation = 600 nm, λdetection = 670 nm) with 200 μW (1), 400 μW (2), 750 μW (3), and 1000 μW (4) excitation intensities.

exclude that the excited state absorption is located at longer wavelengths as suggested by Bullock et al. in a related compound.35,38 While in the 45 ps time windows no significant band shifts with respect to the spectra at time 0 are observed (see Figure 2A,B and Figure SI1B) in the 6 ps range a red shift as a function of time is observed from 0 to 1 ps (3rd spectrum) D


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intensity level can be found in Figure 4 A and B. Both histograms are centered around 3 ns, which is, within

trajectories. An example of such a trajectory can be seen in Figure 3. The presence of two intensity levels can be ascribed to

Figure 3. A. Fluorescence intensity trajectory (number of detected photons/200 ms), fluorescence decay time (1 s bins), and emission maximum (10 s bin) for a single TDI dimer. B. Sum of the fluorescence intensity trajectories of 79 single molecules, fitted with to an exponential decay. The single TDI dimer molecules were excited at 635 nm (5.4 kW/cm2).

Figure 4. Single molecule fluorescence decay time histograms of 77 molecules of (A) the high intensity level and (B) the low intensity level. Histograms of the position of the second emission maximum of 70 single molecules of (C) high level and (D) low level. Scatter plot of the position of the second emission maximum (vide supra) versus decay time for 45 molecules of (E) high intensity level and (F) low intensity level.

the stepwise bleaching of the two chromophores, although it is well established that other processes like spectral diffusion, quenching or reorientation can cause changes in the fluorescence intensity levels as well. In the latter cases one expects the presence of more than two intensity levels, which is actually the case for 16% of the molecules. Therefore, we will further analyze the data assuming that stepwise photobleaching is the cause of the two intensity levels. In this way we are able to compare the different photophysical behavior of the TDI dimer when two or one TDI chromophores are active.45 The first analysis we performed was to determine the survival time of the TDI dimer at 5.4 kW/cm2 excitation power. In this analysis the fluorescence intensity of the traces from all the molecules were added together and the sum can be seen in Figure 3B. Using an exponential fit,38 the fluorescence intensity trajectory of the sum revealed a decay time of 62 s at a laser intensities of 5.4 kW/cm2, highlighting the well establish excellent photostability of the TDI chromophore.18,35 Although the fitting could be improved with a two-exponential function, the represented value, obtained for a single exponential fit is a good representative for the stability of this TDI dimer.46 Simultaneously, the emission spectra were recorded and the fluorescence decay times measured as a function of time. Histograms of the decay times of the highest and lowest

experimental error, the value found for bulk sample in toluene solution. The histogram of the low intensity level is broader than for the higher intensity level, which could be due to the lower statistical accuracy on determining the fluorescence decay time (lower number of photons) or an effect of the photobleaching of one of the chromophores. When comparing the trajectory of the intensity and the fluorescence decay time in Figure 4 A, one can observe that in this particular example, the fluorescence decay time is slightly lower for the higher intensity level than for the lower intensity level. The absence of very large changes in the average fluorescence decay time excludes stronger interchomophore interactions, especially in level 1, which is in agreement with ensemble solution data where similar fluorescence decay times and fluorescent maxima were observed (cfr. supra) for the monomer and the dimer of TDI. A similar analysis of the emission spectra reveals also no significant differences between the highest and lowest intensity level (see Figure 4C,D). For the analysis we used the maximum of the 0−1 vibrational transition since for some molecules the 0−0 transition was too close to the spectral margins of the dichroic and long-pass filter to accurately determine the exact E


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In a last step, we analyzed the blinking behavior of the intensity trajectories for the two intensity levels of the TDI dimer. The autocorrelation function of the time trajectories was calculated and fitted to a sum of exponentials. From these fits the on- and off-times could be determined for both intensity levels.42 The on- and off-times are determined from the fit of the autocorrelation function according to Yip et al.49 Figure 6

position of the emission maximum. Both histograms are centered close to 727 nm, which matches well with the emission spectrum in bulk (for examples of single molecule spectra, see Figure 2 SI). Finally, we made a scatter plot correlating the average emission maximum for each molecule with the average fluorescence decay time, see Figure 4E,F. There seems to be no obvious correlation between the emission maximum and the fluorescence decay time. In this respect the TDI dimer differs from previously investigated PDI and TDI chromophores with substituents in the bay position where specific conformations result in correlations between the emission maxima and the decay times.25,47,48 However, here it appears that the unsubstituted TDI chromophores do not show this behavior. In a next step, antibunching measurements 41 were conducted. The latter can be used to make sure one is studying individual dimers and not small aggregates. It was previously shown by Kennes et al. that some TDI derivatives have a tendency to aggregate.35 An example of interphoton arrival times histogram for the low and high intensity levels of a TDI dimer, of which the time trajectory is shown in Figure 5A can

Figure 6. (A) Single molecule intensity trace (1000 counts/100 ms) for TDI a dimer excited at 635 nm, with an intensity of I: 5.4 kW/cm2. (B) Logarithmic−linear plot of the corresponding autocorrelation functions (red) and fits (black) for the highest (I) and lowest intensity level (II).

shows an example of a single TDI dimer molecule together with the autocorrelation of the two intensity levels. In this example, it is clear (Figure 6A) that more on-off blinking is present in the high intensity level. In this study, we only focused on fastest autocorrelation component in the 10−1000 μs range for further analysis. For the specific single TDI dimer molecule in Figure 6, the on-time increases approximately by a factor of 24 going from the high intensity level (τon: 859 μs) to the low intensity level (τon: 20 700 μs) while the off-time changes little (τoff: 77 μs for the high level, 66 μs for the low level). A histogram of the on- and off-times of 77 TDI dimers is shown in Figure 7. Going from the higher level to the lower level, the off-times and their distribution do not differ significantly between “higher” and the “lower” level. This means that the duration of the off process is not influenced significantly by the number of chromophores. To check the origin of the off-times, i.e., to know whether this off state is due to triplet formation or an electron transfer process, samples were measured under normal atmospheric conditions and under nitrogen atmosphere. From Figure 7C it is clear that the off-times increase slightly under N2-atmosphere and the maximum of distribution shifts from 42 to 66 μs, while the on-times stay fairly constant (Figure 7B). While the lengthening of the off-time under nitrogen atmosphere would be in line with a triplet related process, it is worth noticing that the off-times of this TDI dimer (∼42 μs) are significantly longer than that of the off-times found in single molecule experiments on the TDI monomer (∼4 μs).25,35 Ns transient absorption revealed a triplet−triplet absorption band around 700 nm with a decay constant of ∼2 μs (Figure 3 SI) for the TDI dimer in toluene, which is similar to the value

Figure 5. (A) Fluorescence intensity trajectory (1000 detected photons/200 ms). (B) Histogram of the NC/NL ratio for the highest intensity level for 53 molecules, where NC is the number of coincidence events at the zero delay peak and NL the number of coincidence events at the lateral peaks. (C) Interphoton arrival times histogram of the higher intensity level (I). (D) Interphoton arrival times histogram of the low intensity level (II).

be seen in Figure 5C,D, respectively, for an excitation intensity of 5.4 kW/cm2. Under these excitation conditions (we estimate a 0.31 probability to excite both chromophores), both the high and low intensity levels in Figure 5C,D show the absence of the central peak at zero delay. Figure 5B shows a histogram of the ratio of the number of coincidence events at the zero delay peak (NC) to the number of coincidence events at the lateral peaks (NL). The histogram was constructed from the highest intensity level of 53 single TDI dimer molecules. The profile was fitted to a Gaussian distribution and a central value of 0.067 ± 0.001 was found for the NC/NL ratio showing that the TDI dimer emits one photon under these excitation conditions.24 A more detailed power dependence antibunching study could confirm the presence of singlet−singlet annihilation, suggested from the femtosecond transient absorption experiments. F


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used), If = detected count rates). Histograms of YDS for the high and low intensity are given in Figure 8. It is clear that the yield

Figure 8. Distribution of the quantum yield (YDS) for formation of the dark state for the high (A) and low (B) intensity levels for 36 molecules under ambient air conditions.

of dark state formation is larger in the high intensity level compared to the low intensity level. The autocorrelation functions indicate a fast and efficient process occurring while a higher intensity level is observed whereas this is absent in the low intensity level. Singlet−singlet annihilation by Förster type energy transfer can form a higher excited singlet state (Sn) on one of the chromophores while the other chromophore goes to the ground state. Such a reactive higher excited singlet state could yield a triplet state (possibly via an excited state with nπ* character) and hence have an additional decay channels leading to the dark (triplet) state. Such a justification was previously put forward for a perylene monoimide multichromophoric system, where singlet−singlet annihilation was reported to increase the rate formation of a dark state (triplet state in that case).45,55 However, both the presence of singlet−singlet annihilation and the nature of the dark state in the TDI dimer are speculative at this point. Independent of the exact nature of the dark state, our results indicate that this state is being populated more efficiently in the case the two TDI chromophores are active (see Figure 8).

Figure 7. (A) Normalized distribution of off times for high (white rectangle) and low (black rectangle) intensity levels (ambient air: 51 molecules). (B) Normalized distribution of the on-times for the high intensity level under different atmospheric conditions). (Ambient air (full line): 51 molecules, N2 (dashed line): 37 molecules.) (C) Normalized distribution of the off times (highest level) under different atmospheric conditions (ambient air (white rectangle): 51 molecules, N2 (black rectangle): 37 molecules).

found for the TDI monomer in solution.35 Triplet decay times in liquid solutions have however to be regarded with some reserve as under these conditions the triplet decay can be due to (often diffusion controlled) quenching by residual impurities (including oxygen) or triplet−triplet annihilation rather than by intersystem crossing to the ground state. These processes will however be blocked to a major extent in a film where diffusion is orders of magnitude slower. The different off-times observed for the TDI monomer (in Zeonex35) and the TDI dimer (in PMMA) can be due to a better solubility of oxygen in Zeonex compared to PMMA50,51 leading to a larger pseudo-first order rate constant for the quenching of the TDI triplet in Zeonex. If the triplet exciton would be delocalized in the TDI dimer this will lead to a smaller change in the equilibrium position of the nuclear coordinates in the ground state and the T1 state. The consequent decreased electron phonon coupling will lead to a reduced Franck−Condon factor associated with the T1 → S0 intersystem crossing52 and hence a longer triplet decay time for the dimer. An alternative explanation for the observed blinking could be the occurrence of electron transfer between the polymer surrounding and the excited TDI chromophore, although usually such processes have been assigned to longer time scales for other rylene chromophores.53,54 Assuming a three level system49 (S0, S1 and a dark state), one can use the on-times to calculate the yield of the dark state formation YDS = ΦfΦdet/tonIf18,55 (where Φf = Quantum yield of fluorescence, Φdet = detection efficiency (an estimated value of 0.025 was

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CONCLUSIONS The synthesis and photophysical characterization of a new TDI dimer is presented. Stationary absorption and emission spectra together with time-correlated fluorescence decays are compared with a model TDI chromophore and show that both chromophores in the TDI dimer are in the weak coupling regime.56 Femtosecond transient absorption measurements detect a power dependent process in the ps range which could indicate a singlet−singlet annihilation process. This process is put forward in spite of the fact that in the spectral range 510− 700 nm, no clear excited singlet state absorption band could be observed. Literature results indicate thata weak excited state singlet absorption band could be present above 700 nm and overlaps with the emission spectrum.38 The majority of single molecule intensity traces show two intensity levels corresponding to the stepwise photobleaching of the two TDI chromophores. Single molecule emission spectra and fluoG


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remaining excitation light. The APDs signals were send to a SPC-830 card from Becker & Hickl. One APD had an additional 780 nm short pass filter and was delayed 1 μs using a DG535 delay generator (Stanford Research System). Nanosecond Transient Absorption Experiments. A 580 nm laser pulse (8 ns, 10 Hz) generated by a pulsed Nd:YAG laser (Quanta-Ray INDI-40, Spectra Physics) was used to excite the sample. For the probe beam a Müller xenon lamp was used. Both beams were aligned in an almost-colinear geometry and focused on the sample. A SpectroPro-300i monochromator/spectrograph was used to disperse the probe light and select the desired wavelength. The optical signal was detected by a PMT (Hamamatsu, R928) and the transient electrical signal was amplified and sent to a computer controlled oscilloscope. The optical densities of the samples were around 0.4 at the excitation wavelength. All samples where degassed using a flash-freeze method. Femtosecond Transient Absorption Experiments. An amplified femtosecond double OPA laser system was used to generate the excitation and probe pulses. The assembly of a Tisapphire oscillator (Mai Tai SP, Spectra-Physics) coupled to a laser amplifier comprising a stretcher, Ti-sapphire regenerative cavity amplifier and compressor (Spitfire Pro 35F-XP, SpectraPhysics) is the source of 35 fs pulses (fwhm). The pulses have a bandwidth of 31 nm (fwhm), 4 mJ in the 800 nm range and have a repetition rate of 1 kHz. The regenerative amplifier is energized by a Q-switched, diode-pumped Nd:YLF pulsed laser (Empower-30, Spectra-Physics) capable of delivering 527 nm output beam of 30 W. The output of the amplifier is used to pump two identical two-stage optical parametric amplifiers (OPA) of white-light continuum (Topas-C, Light Conversion). Each (OPA) is pumped with 1 mJ pulse energy producing 60 fs (fwhm), 31 nm bandwidth (fwhm). The energy of the independently tunable excitation pulses is in the range of 100 μJ @ 500 nm and the output can be tuned over a spectral domain from 300 and 2600 nm. A small percentage of the regenerative amplifier output was used to generate a white light continuum that then served as probe light. This was done by focusing a small part of the 800 nm beam in a 3 mm sapphire plate to obtain white light in the 450−750 nm region. The monochromatic detection was performed using a PMT (Hamamatsu R928) whereas the spectra were recorded using a CCD camera (Princeton Instrument Pixis 100) mounted at the exit ports of a 300 mm focal length spectrograph (Acton Research 2300). The entire system provides pulses with duration of 100 fs (fwhm cross correlation between pump and probe) at a repetition rate of 1 kHz. After each experiment the integrity of the samples was checked by recording the steady state absorption and emission spectra.

rescence decay time histograms of both intensity levels are very similar suggesting that no strong interchromophore interactions take place.42 The autocorrelation functions of the intensity trajectories reveal that the average quantum yields of dark state formation is roughly an order of magnitude higher when two chromophores are active, compared to the situation when one is photobleached. The off-time of the dark state is similar for the high intensity and low intensity level of the TDI dimer and is at least ten times longer than that for the triplet off-time found for the TDI monomer and the TDI dimer triplet decay time in solution. Despite the increase of the off-time in the TDI dimer going from air to nitrogen atmosphere, we do not have straightforward evidence to ascribe the off-blinking process to intersystem crossing.

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EXPERIMENTAL SECTION Synthesis. The synthesis and characterization of the TDI dimer and intermediate products are described in the Supporting Information. Stationary Fluorescence Measurements. All stationary measurements have been recorded using a spectrophotometer (Lambda 40, PerkinElmer) and a fluorimeter (Fluorolog, PerkinElmer). The spectrometers were corrected for the wavelength dependence of the throughput of the emission monochromator and the sensitivity of the detector. The optical density at the absorption maximum of all solutions was kept below 0.1 in a 1 cm cuvette. The excitation wavelength was set to 580 nm. The fluorescence quantum yields was determined using cresyl violet in ethanol (ΦF = 0.54) as a reference. Picosecond Fluorescence Time-Resolved Experiments. The fluorescence decay times have been determined by TC-SPC measurements described in detail previously.57 A time-correlated single-photon-counting PC module (SPC 830, Becker & Hickl) was used to obtain the fluorescence decay histogram in 4096 channels. The decays were recorded with 10000 counts in the peak channel in two time windows of 20 ns corresponding to 4.9 ps per channel and analyzed globally with a time-resolved fluorescence analysis (TRFA) software. The full width at half-maximum (fwhm) of the IRF was typically in the order of 32 ps. The quality of the fits has been judged by the fit parameters χ2 (

Synthesis, Ensemble, and Single Molecule Characterization of a

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