Interactions of Perylene Bisimide in the One-Dimensional Channels of

ARTICLE pubs.acs.org/JPCC

Interactions of Perylene Bisimide in the One-Dimensional Channels of Zeolite L Michael Busby,†,^,z Andr
e Devaux,†,z Christian Blum,‡ Vinod Subramaniam,‡ Gion Calzaferri,*,§ and Luisa De Cola*,† †

Physikalisches Institut, Westf€alische Wilhelms-Universit€at M€unster, Mendelstrasse 7, 48149 M€unster, Germany Nanobiophysics, MESAþ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands § Department of Chemistry and Biochemistry, University of Berne, Freiestrasse 3, 3012 Bern, Switzerland ‡

bS Supporting Information ABSTRACT: Supramolecularly organized host guest systems have been prepared by inserting three perylene dyes with differing end substituents into the nanosized channels of zeolite L (ZL) by gas-phase adsorption under vacuum conditions. The end substituents allowed controlling the core-to-core distances of the molecules in the channels. The three perylene dyes investigated have very similar absorption and fluorescence spectra in diluted solutions, as well as fluorescence lifetimes ∼ 4 ns. Large ZL crystals in the size range of 1500-3000 nm in length and about 1000 nm in diameter as well as nanosized NZL crystals of about 30 nm in length and diameter were used as hosts. Different loadings have been investigated. The photophysical properties of the materials were analyzed as suspensions in refractive index matching solvents, such as toluene or ethyl benzoic acid ester; as bulk materials in glass ampules; and by means of time-, space-, and spectrally resolved single-crystal fluorescence microspectrocopy techniques. The inserted dyes can form J-aggregates if the structure of the perylene derivative allows for short distances between the electronic transition moments in an in-line arrangement. J-coupling was not seen for the molecules with substituents that keep them further apart. Aligned and stabilized J-aggregates in one-dimensional channels provide new options for preparing optical devices, where coherent exciton delocalization over nanometer-to-micrometer scales may result in efficient photonic wires. The exciton coupling can be controlled by varying the molecular tail groups.

’ INTRODUCTION The most fascinating topic of modern photochemistry is the design of structurally organized and functionally integrated artificial systems, capable of elaborating the energy and information input of photons to perform functions, such as processing information, sensing microscopic environments on a nanoscale level, or transforming and storing solar energy.1-6 One-dimensional channel materials are attractive hosts for preparing and investigating hierarchically organized structures, which present a successive ordering from the molecular up to the macroscopic scale.7-10 Artificial photonic antenna systems have been built by incorporating dyes into one-dimensional nanochannel materials, such as Zeolite L (ZL), which has proven to be an excellent choice for host material.11,12 ZL crystals feature strictly parallel, one-dimensional nanosized channels arranged in hexagonal symmetry that can be filled with suitable guests small enough to pass the pore openings. Geometrical constraints imposed by the host structure lead to supramolecular organization of the guests in the channels. A special twist is added to these systems by r 2011 American Chemical Society

plugging the channel openings with a stopcock molecule. Such a molecule consists of a “tail” part that fits into the channel and a “head” group that is too bulky to pass the pore opening. The spectral properties of different molecule types can be precisely tuned to each other so that the stopcocks are, for example, able to accept excitation energy via F€orster resonance energy transfer (FRET) from the dyes inside the channel, but cannot pass it back.13,14 Stopcock molecules have also been used for plugging the channel entrances in order to prevent inserted molecules from escaping, to prevent other molecules from entering the channels, or to achieve other functionalities.15,16 The increasing complexity of these host-guest systems highlights the importance of understanding and controlling not only the interactions of guests with the host but also interactions between guests and the influence of coadsorbed solvents that might be present. Received: November 14, 2010 Revised: January 26, 2011 Published: March 04, 2011 5974

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The Journal of Physical Chemistry C

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Figure 1. Zeolite L (ZL) and dye-loaded ZL. (A) Schematic view of the hexagonal channel arrangement. The center-to-center distance between two channels is 1.84 nm. (B) View of the hexagonal framework of ZL showing a top view of 9 channels. (C) Side view of a channel that consists of 0.75 nm long unit cells with a van der Waals opening of 0.71 nm at the narrowest and 1.26 nm at the widest sections. The double arrow indicates the orientation of the electronic transition dipole moment (ETDM) of an inserted molecule that has a length corresponding to about three unit cells (u.c.). (D) SEM image of ZL crystals with a length between 1.5 and 3 μm. (E) Comparison of the length of a PR149 molecule and the length lμ* of its ETDM and a single channel densely packed with PR149. The core-to-core distance is expressed in units of the length lμ*. (F) Single channel densely packed with DXP.

Table 1. Chemical Formulas of Dyes, Abbreviations, and Molar Extinction Coefficienta

a

For details, see the Experimental Section.

Fluorescent dyes are excellent probes for the study of host-guest interactions and molecular dynamics within nanometer or subnanometer confined environments. Examples exist for zeolites, clays, polymers, dendrimers, mesoporous silicates, nanotubes, and proteins.3,17,18 Their spectral sensitivity toward nanoenvironments gives insight into the geometric organization and intermolecular interactions that result from the host-guest system. Of particular interest to us are host-guest systems based on ZL nanochannels.11,19 ZL crystals can be tuned in aspect ratio and size between micro- and nanometer scale and contain virtually defect-free channels with atomically defined minimum and maximum diameters of 7.1 and 12.6 Å with a unit cell length

of 7.5 Å that run along the crystals' c axis (Figure 1A-D).20-22 Because of the presence of the (AlO2) unit, the framework has overall negative charges that are compensated by cations. The presence of exchangeable cations makes them ideal for the intercalation of cationic molecules, such as methylviologen, pyronine, oxonine, thionine, and many others. Alternatively, neutral molecules may be inserted by means of gas-phase intercalation methods.23 Small neutral aromatic molecules, such as naphthalene, anthracene, and perylene, have been inserted into the cavities of zeolites X, Y, and L.3,24,25 In these and similar cases, the small size of the molecules and/or the high dimensionality of the cavity system allows for the molecule to overlap and 5975

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The Journal of Physical Chemistry C stack in a “face-to-face” arrangement, resulting in excimer emission similar to that seen in solution. This behavior is characterized by the extended lifetimes of the excimers. Larger molecules in the linear ZL channels have been shown to behave electronically isolated from each other with no evidence of excimer formation or π stacking. This is evident from spectroscopic and lifetime measurements and is due to the geometric inability of molecules to slide over each to form “face-to-face” interactions.11,24 Recently novel “end-to-end” organized J-aggregates have been observed.12,26,27 Host-guest, guest-guest, and guest-cosolvent interactions may not only considerably influence the optical and electronic properties of the resulting material but also determine, at a fundamental level, their stability. In view of the well-known affinity of zeolites for water, stability of the hybrid material upon hydration is particularly critical. It was observed that, for example, exposure of dry p-terphenyl-ZL samples to air of 22% relative humidity at room temperature leads to displacement of the organic dye from the channels. Upon heating and hence drying, the molecule can be inserted again. Similar observations were also made for 1,6-diphenylhexatriene (DPH), 1,2-bis-(5-methylbenzoxazole-2-yl)ethane (MBOXE), and other molecules of comparable structure.23 In contrast, incorporated fluorenone is not displaced by water molecules under ambient conditions but remains inside the channels.28 The interaction of the fluorenone carbonyl group with the zeolite extra-framework potassium cations has been identified recently as the leitmotiv for both the stability of the dye-ZL composite and the anisotropy of such a fluorescent dye in the nanochannels of ZL. Water molecules do not displace fluorenone from ZL because the Kþ 3 3 3 OdC interaction is dominant; rather, they share the channel space with the dye and fine tune its observable electronic and optical properties through hydrogen-bond interactions.29 To further understand the interactions between guests or of the guests with the one-dimensional nanochannels of ZL, we have chosen three perylene bisimide derivatives with differing end substituents, as shown in Table 1. Because of geometric restrictions, these dyes are forced to align parallel to the channel axis in a linear “end-to-end” arrangement, as illustrated in Figure 1E,F. The rigid van der Waals width of the molecules is about 0.76 nm, making it difficult to pass through the 0.71 nm channel opening. They can be inserted only at elevated temperature, above ca. 450 K in vacuum, where the channel ring breathing vibrations of ZL are at least partially activated.30 The chosen perylene dyes have a similar oscillator strength, f, and S0 f S1 transition energy, ΔE, of about 0.76 and 19 000 cm-1, respectively, which results in a similar length lμ* for the electronic transition dipole moment (ETDM) of 0.19 nm, as can be calculated from eq 1.5,31 The dyes differ, however, in the shortest core-to-core distance that they can achieve inside of the channels. This makes them an ideal choice for a comparative study of the dependence of the ETDM interaction on structural details that do not affect the properties of the “optical electrons”. rffiffiffiffiffiffiffi f ð1Þ lμ ¼ 3:036  10-6 cm0:5 ΔE The Davydov splitting, βC, depends on the third inverse power of distance (eq 2), whereas the rate constant for F€orster resonance energy transfer (FRET), kEnT, depends on the sixth

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Scheme 1. Angles Describing the Relative Orientation of the ETDM μ1 and μ2 of Two Moleculesa

a

Left: representation of the ETDM as oscillators. Right: vector representation of the ETDM.

inverse power of distance (eq 4).13,34,35 Both equations are based on dipole-dipole interaction theory. They are valid for describing the interaction between two ETDM only if the core-core separation, R, is large with respect to the length lμ* of the ETDM. We, therefore, compare in Figure 1E,F the length of the ETDM with the length of the molecules and the shortest possible corecore distance inside a channel for DXP and PR149. This comparison shows that the distance requirement for the validity of these dipole-dipole interaction based theories is sufficiently well-fulfilled. It remains, however, to be determined if, and in which cases, Davydov splitting becomes so important that the F€orster equation is no longer appropriate for describing resonance energy transfer. βC ¼ AD

f k 1 ΔE R 3 n2

k ¼ sin θ1 sin θ2 cos φ12 - 2 cos θ1 cos θ2

ð2Þ ð3Þ

The value of the constant AD is equal to 1.615  10-18 m2 cmif we express βC in cm-1, which is convenient. The meaning of the angles used in the equation for κ is explained in Scheme 1. F€orster’s explicit formula for the electronic excitation energy transfer rate constant, kEnT, can be expressed as follows 1

k2 φ  1 KEnT ¼ TF 3 6 3 D JDA 3 4 n R τD

ð4Þ

where R is the distance between the donor and the acceptor, τD* the decay time and φD* the luminescence quantum yield the of the donor, both in the absence of energy transfer, and n the refractive index of the environment. JD*A is the spectral overlap between the luminescence spectrum of the donor and the absorption spectrum of the acceptor. It is convenient to define the Theodor F€orster constant, TF, which is equal to 8.785  10-25 mol, if the spectral overlap integral JD*A is expressed in cm3 M-1.31 Fundamental and applied studies exist based on the intercalation, self-assembly, and coassembly of perylene bisimide dyes in liquid crystals,36,37 hydrogen-bonded arrays,38 nanoporous materials,12,39,40 mesoporous materials,41 Langmuir-Blodgett and electroluminescent thin films,42,43 and in polymers.44 Various types of interactions ranging from π stacking and excimer formation to J- and H-aggregates have been proposed, though these are not easily controllable based on the soft nature of the systems. Aligned and stabilized J-aggregates in one-dimensional channels will provide new options for preparing optical devices, where coherent exciton delocalization over nanometer-to-micrometer scales may result in efficient photonic wires.45 We have employed the differing end substituents of DXP, PR149, and PDI in order to control the core-to-core distances of the molecules in 5976

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The Journal of Physical Chemistry C the channels of ZL. DXP contains a short 2,6-dimethyl phenyl ring, whereas PDI, on the other hand, has a hexylheptyl alkyl chain bound to the bisimide, lengthening the molecule. It is, therefore, predicted that DXP will allow for closer intermolecular interactions in the nanochannels (Figure 1E,F). PDI and PR149 have an end substituent that will hinder the approach and subsequent close contact between neighboring molecules. The spectral and electronic insensitivity of the perylene core toward the end substitution allows for direct comparison between intermolecular interactions and molecular structure within the zeolite nanochannels that result in tunable spectral, temporal, and microscopic properties linked to the macroscopic appearance of the material.

’ EXPERIMENTAL SECTION Zeolite L Synthesis. Long cylindrically shaped ZL crystals were synthesized as published previously.22 Potassium hydroxide (Fluka, pellets g 86%), sodium hydroxide (Merck, pellets g 99%), and aluminum hydroxide (Riedel-de Ha€en, powder purum) were diluted in bidistilled water. To this solution was added a silica suspension (Evonik Degussa AeroDisp W 1226) under vigorous stirring in order to obtain a white gel with the following composition: 3.0 K2O/9.7 SiO2/1.0 Al2O3/161.3 H2O. The gel was transferred into a Teflon vessel that was sealed and put into an oven for 144 h at 160 C. The obtained white solid was exchanged with 0.1 M KNO3 and washed several times with bidistilled water and dried. The product was characterized with XRD, SEM, and EDX. The crystals had a regular morphology (see Figure 1D) with a length in the range of 1500-3000 nm and a diameter of 800-1000 nm. Nanosized ZLs with a length and diameter of around 30 nm were also prepared following a previously published procedure.22 In a first step, a silica suspension was prepared by adding 28.04 g of doubly distilled water to 12.02 g of silica powder (Aerosil OX-50). The silica was dispersed by means of an Ultra Turrax mixer (IKA T18 Basic) for 8 min at 18 000 rpm and left standing at room temperature for 1 h. The suspension was then dispersed again for 8 min before being added to a solution of 7.23 g of potassium hydroxide in 21.68 g of doubly distilled water. A potassium aluminate solution was prepared by adding 4.84 g of potassium hydroxide and 1.56 g of aluminum hydroxide to 20.00 g of doubly distilled water. Both solutions were refluxed for 15 h at 120 C, resulting in clear solutions. After letting the solutions cool to room temperature, the potassium aluminate solution was added under vigorous stirring to the potassium silicate, leading to a gel with a composition of 9.34 K2O/1.00 Al2O3/20.20 SiO2/ 412.84 H2O. After 3-6 min of further stirring, the cloudy gel was transferred to PTFE pressure vessels. Crystallization took place at 170 C for 6 h under dynamic conditions in a rotating oven (16 rpm). The workup procedure and characterizations were carried out as described for the cylindrical zeolites. Loading of ZL with Dyes. DXP and PDI were purchased from Fluka chemicals and used without further purification, whereas pure PR149 was obtained from Dr. Metz, Clariant GmbH. In a typical experiment, 80 mg (2.8  10-5 u.c.) of 3 μm long ZL crystals was mixed with the desired amount of dye in a glass ampule and dried on a vacuum pump at about 6  10-5 mbar. The ampules were then sealed at this pressure and placed in a rotating oven for 48 h at 300 C for DXP and PR149 and at 180 C for PDI. Crystals were washed with and centrifuged from acetone and dichloromethane repeatedly until the supernatant

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was clear of emission. Samples were also inspected under the microscope to ensure that no free dye crystals were present. This procedure was slightly altered for the loading of DXP into the nanosized ZL. In this case, 40 mg of the 30 nm long crystals was weighed into an ampule and a solution consisting of 3 mg of DXP in 2 mL of DCM was added. The mixture was treated in an ultrasonic bath for 15 min before the solvent was evaporated at 50 C. The DXP impregnated zeolites were then dried under dynamic vacuum conditions for 8 h at 6  10-5 mbar before being sealed. The insertion took place in a rotating oven for 4 days at 270 C. The crystals were washed with dichloromethane and toluene until the supernatant showed no DXP luminescence. Loading assays were performed by decomposing the crystals by adding two drops of 1% HF solution to a suspension of the dye-loaded zeolite crystals in 2 mL of doubly distilled water. The dye molecules were extracted with 2 mL of DCM, and the aqueous phase was neutralized with NaCO3. Absorption spectra of the DCM phase was used to calculate the concentration of the molecules in solution, and hence the loading in the zeolite as percentage of filled sites. The length of a site was estimated to correspond to three unit cells for all three dyes (DXP, PDI, and PR149). Bulk Spectroscopy. The value of the extinction coefficient of PR149 at 526 nm in DCM is 84 817 M-1 cm-1. It was determined at different concentrations in the range of 1  10-7 to 5  10-7 M, where Beer-Lambert’s law was valid. Some care is needed for accurate determination of extinction coefficients and other photophysical parameters of the perylene dyes because they easily adsorb on surfaces or form aggregates, depending on the surface and the solvent, respectively. The suspension measurements were carried out in solutions or suspensions of spectroscopic grade methanol in 10 mm quartz cuvettes at low concentrations (less than 0.5 mg/mL). Toluene was used for absorption spectra as its refractive index matching to the zeolites reduces scattering. Toluene was deemed to be an inappropriate solvent for the rest of the experiments due to its ability to wash out the dyes. Measurements with powder materials were carried out in glass ampules. To obtain the dried state, the material was treated for ca. 3 h under high-vacuum conditions (5  10-5 mbar) at room temperature, and the sealed ampule was directly used for measurements. To record the spectra in the humid state, the ampule was opened and the powder was exposed to humid air (22% RH) for 30 min. UV/vis spectra were recorded on a double-beam UV/vis spectrometer (Varian Cary 5000) with baseline correction. Emission and excitation spectra were measured with a slit width of 1 nm. Steady-state emission spectra (HORIBA Jobin-Yvon IBH FL-322 Fluorolog 3 spectrometer) were recorded on a spectrometer equipped with a 450 W xenon arc lamp, double grating excitation and emission monochromators (2.1 nm/mm dispersion, 1200 grooves/mm), and a single-photon-counting detector (TBX-4-X). Emission spectra were corrected for source intensity (lamp and grating) and emission spectral response (detector and grating) by standard correction curves. Time-resolved measurements were performed using the timecorrelated single-photon-counting (TCSPC) option on the Fluorolog 3. A light-emitting diode (NanoLED, 431 nm, fwhm < 750 ps) with a repetition rate of 1 MHz was used to excite the sample. The excitation sources were mounted directly on the sample chamber at 90 to the monochromator. Signals were processed using an IBH DataStation Hub photon-counting module, and data analysis was performed using the commercially 5977

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The Journal of Physical Chemistry C available DAS6 software (HORIBA Jobin Yvon IBH). Fits were analyzed by omitting the cross-correlation, which, in the case of the zeolite samples, were affected by the highly scattering medium. Polarization Microscopy. Polarization microscopy (BX 41, Olympus) was performed illuminating with a tungsten lamp, through a filter cube (WB, Olympus), and excitation 450-480 nm emission > 515 nm. Pictures were taken with an Olympus Soft Imaging camera. Epi-fluorescence, Spatially, Spectrally, and Time-Resolved Microscopy. The emission from individual loaded fluorescent zeolite crystals was characterized using a custom-built setup capable of wide-field fluorescence imaging as well as scanning stage confocal microscopy for fluorescence lifetime and spectral imaging. Light sources used were a mercury lamp for wide-field fluorescence imaging and a pulsed laser diode emitting at 469 nm (BDL475, Becker & Hickl, Germany) for local excitation when recording lifetimes and emission spectra. The sample was illuminated using a 100 objective (100, 1.4 NA oil, UPlanSapo, Olympus), and the emission from the sample was collected by the same objective. Wide-field images were recorded with a color camera (AxioCam HRc, Zeiss). For the emission images, a standard blue filter cube (U-MWB2, Olympus) was used. White balance was optimized for a halogen light temperature of 3200 K in accordance to the manufacturer’s recommendation for fluorescence imaging. We verified the consistency between the color camera image and the coloring visible via the eyepiece of the microscope. Contrast, brightness, and gamma were globally optimized for the whole images, and no digital color-changing filters were applied. When a laser for confocal imaging was used, the filter cube was replaced by a glass plate acting as a beam splitter and a long-pass filter (RazorEdge 473.0 nm, Semrock, USA) to block reflected and scattered excitation light. For fluorescence lifetime imaging, a time-correlated single-photoncounting (TCSPC) module (SPC-830, Becker & Hickl, Germany) attached to a single-photon avalanche diode detector (PDM Series, MPD, Italy) was used. The lifetime data were analyzed using the Becker & Hickl SPCImage software package. Binning values of 0 were used in all cases to maximize resolution. To record local emission spectra, emitted light was imaged via a prism spectrometer onto a cooled CCD camera (Newton EMCCD DU970N-BV, Andor). Wavelength calibration was achieved using a calibrated light source (Cal-2000 Mercury Argon Calibration source, Ocean Optics, USA).

’ RESULTS The structure and morphology of the ZL host is illustrated in Figure 1. The primary building unit of the framework consists of TO4 tetrahedrons where T represents either Al or Si. The channel system exhibits hexagonal symmetry. The molar composition of ZL is (Mþ)9[(AlO2)9(SiO2)27]  nH2O, where Mþ are monovalent cations compensating the negative charge resulting from the (AlO2) unit. n is equal to 21 in fully hydrated materials and amounts to 16 for crystals equilibrated at 22% relative humidity.23 It is useful to imagine ZL as consisting of a bunch of strictly parallel channels, as shown in Figure 1A. The channels have a smallest free van der Waals diameter of about 0.71 nm, and the largest diameter inside is 1.26 nm. The breathing vibrations of the T12O24 ring marking the smallest free opening are in the frequency range of 400 cm-1 and below,30 which means that they can be activated by moderate heating, thus leading to a larger dynamic radius than estimated based on a rigid

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Table 2. Dye-Loaded ZL Materials Used in This Study loading p

averaged density c(p) in mol/L

DXP-ZL.01

0.01

0.0025

DXP-ZL.09

0.09

0.023

DXP-ZL.17

0.17

0.043

DXP-ZL.25

0.25

0.063

DXP-ZL.30

0.30

0.075

DXP-NZL.29

0.29

0.073

PDI-ZL.025

0.025

0.0063

PR149-ZL.10

0.10

0.025

label

van der Waals model. The distance between the centers of two neighboring channels is 1.84 nm. Each ZL crystal consists of a large number of channels (nch) that can be estimated as follows nch ¼ 0:267ðdz Þ2

ð5Þ

where dZ is the diameter of the crystal in nanometers. For example, a crystal with a diameter of 600 nm features nearly 100 000 strictly parallel channels. The ratio of void space available in the channels with respect to the total volume of a crystal is about 26%. An important consequence is that ZL allows, through geometrical constraints, the realization of extremely high concentrations of well-oriented molecules that behave essentially as monomers. A 30 nm by 30 nm crystal can accept up to about 3000 molecules that occupy three unit cells, whereas a 60 nm by 60 nm crystal can host up to nearly 26 000 such dyes. Many different molecules, complexes, and clusters have been inserted into the channels of ZL.5,12 It is convenient to introduce a parameter bearing the information on dye concentration that is based on purely geometrical (spacefilling) properties of the host, that is, showing to what extent the ZL channels are filled with dye molecules. The loading, or occupation probability, p, of a dye-ZL material is defined in eq 6. p¼

number of occupied sites total amount of sites

ð6Þ

The sites, ns, represent the number of unit cells occupied by one dye molecule. It can, for example, be equal to 1, 2, 3 or higher values. ns must not necessarily be an integer number. The loading ranges from 0 for an empty ZL to 1 for a fully loaded one. The three perylene bisimide molecules in Table 1 all occupy about three unit cells. At highest theoretical packing, DXP could fit into 2.2 u.c., whereas 3 are needed for PR149. An estimate for PDI based on van der Waals arguments is more difficult as we do not know how densely the aliphatic tails can be packed, because they will probably coil to some extent. The most convenient way to express the loading of the three dyes in ZL is to express it in terms of the occupation probability p or loading level by assuming ns = 3. The dye concentration of a loaded ZL material c(p) in units of mol/L can be expressed as a function of the loading as follows:   p mol cðpÞ ¼ 0:752 ð7Þ ns L A list of the dye-loaded zeolite materials along with loading levels and labels is given in Table 2. The lengths of the ZL crystals were in the range of 1500-3000 nm, with the exception of the NZL, which had a size of about 30 nm. The averaged density has been calculated by using eq 7. All samples used in this study were prepared by means of gas-phase insertion. The molecular integrity and the exact loading level of the guests were checked by first 5978

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The Journal of Physical Chemistry C decomposing the zeolite host with hydrofluoric acid, followed by organic extraction.46 The absorption spectra of the freed molecules were in all cases identical to those of the free molecule, which confirms the lack of chemical change that occurs during the insertion. Loading was also attempted from a refluxing solution of toluene, although, after washing, the crystals showed no visible sign of intercalation, confirming that gas-phase insertions carried out at elevated temperature is appropriate for inserting these molecules. Bulk Absorption and Luminescence Spectroscopy. The absorption spectra of the pure dyes were measured as diluted solutions in dichloromethane. The tendency to form aggregates in solution is very pronounced for PR149 for which features due to aggregates are visible even at 5  10-6 M so that the spectra had to be measured in a 5  10-7 M solution. The absorption spectrum of all dyes shows a vibronic progression with bands for DXP at 458 and 488 nm and a maximum absorption at 525 nm. PDI and PR149 have an almost identical spectral shape as DXP, with band maxima slightly blue and red shifted, respectively; see Figure 2A. Absorption measurements of dye-loaded crystals were carried out on suspensions in toluene, whose refractive index is close to that of ZL.47 Absorption spectra measured, including empty ZL as a reference, showed a strong background due to Rayleigh scattering. On the basis of the high oscillator strength of the molecules, the scattering background could be subtracted by means of a baseline function, and good spectra could be obtained (see Figure 2B). DXP-ZL.01, DXP-ZL.09, and PDI-ZL.10 show absorption bands similar to that of the molecules in solution, with maxima at 535 nm for the DXP and 525 nm for PDI. The spectrum of DXP-ZL.30 is different. The vibronic progression still resembles that of the free molecules, although the maximum is shifted to 530 nm. A distinct shoulder is clearly visible between 550 and 615 nm, a spectral feature not present in the samples with lower loading. Emission spectra of diluted solutions were performed upon excitation into the main absorption band at 500 nm. All molecules gave emission spectra mirroring the absorption spectrum. The three vibronic maxima are seen to decrease in intensity from 535 to 575 nm and 625 nm for DXP, and similarly for PDI and PR149. The samples show a concentration dependence of emission, resulting in a significant reduction in intensity along with a red shift of the first band even at low concentration. This phenomenon has been previously reported for perylene and is due to self-absorption caused by the large overlap between the absorption and the fluorescence spectra, as can be seen in Figure 2 for DXP, PDI, and PR149.12,48 The quantum yields of DXP and PDI in various solvents were reported to be 0.94 and 0.99, respectively.33,49,50 Intermolecular interactions, such as excimer formation or π stacking, typically seen as new shifted bands,34 were not observed in the concentration range used here for DXP and PDI for which the substituents hinder aggregation, but these bands are present for PR149 already at a concentration as low as 5  10-6 M. This indicates that the free molecules of the latter have significant affinity to each other in the solution phases studied here in contrast to DXP and PDI. The emission spectra of the dyes inserted into ZL were measured upon excitation at 500 nm; see Figure 2D. DXPZL.01 and DXP-ZL.09 show similar spectra with maxima at 555 nm and a second, less intense, vibronic band at 600 nm and a weak shoulder between 630 and 750 nm. The PDI-ZL.10 emission maximum was blue shifted to 545 nm. The difference in intensities between the first two vibronic bands varied between

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Figure 2. Absorption and luminescence spectra of pure dyes in solution and of dye-ZL dispersions. (A) Absorption (dashed) and emission (solid) spectra of DXP (red, c = 5  10-6 mol/L) and PDI (black, c = 5  10-6 mol/L) in CH3Cl, as well as PR149 (blue, c = 5  10-7 mol/L) in CH2Cl2. The emission spectra were excited at 500 nm. (B-E) Absorption spectra of dye-ZL measured as dispersions in refractive index matching toluene, and luminescence spectra measured from methanol suspensions. Spectra have been scaled for display purposes. (B) Base line subtracted absorption spectra. (C) Excitation spectra at 650 nm emission. (D, E) Emission spectra at 500 nm and at 550 nm excitation, respectively. The corresponding colors are DXP-ZL.01, black; DXP-ZL.09, green; DXP-ZL.30, red; and PDI-ZL.025, blue. (F) Comparison of the excitation (dashed) and fluorescence (solid) spectra of a 5  10-7 M PR149 in DCM solution (black) and of PR149-ZL.10 suspended in toluene (red). The baseline is shifted for better comparison.

the samples as a result of reabsorption. The weak shoulder between 630 and 750 nm may correspond to the third vibronic band seen in the free molecules. The spectrum of DXP-ZL.30 again is different. We observe a blue shift of the first emission band to 547 nm and a second band at equal intensity at 590 nm and a third intense very broad band at 650 nm. A second set of emission spectra was recorded by exciting the samples at 550 nm, corresponding to the shoulder seen in the absorption spectra of DXP-ZL.30; see Figure 2E. The samples DXP-ZL.01, DXPZL.09, and PDI-ZL.10 show the second and third vibronic bands, 5979

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Figure 3. Absorption and luminescence spectra of DXP-NZL.29 in ethyl benzoate (c = 0.5 mg/mL). (A) Absorption spectrum. (B) Excitation (dashed) and emission (solid) spectra. The emission was recorded after excitation at 480 nm (black) or 550 nm (red), whereas excitation spectra were detected at 580 nm (blue) and 620 nm (green), respectively.

Figure 4. Epi-fluorescence micrographs of (A) DXP-ZL.01, (B) DXP-ZL.09, (C) DXP-ZL.30, and (D) PDI-ZL.10. Bottom insets show orthogonally orientated crystals and the corresponding images taken through a polarizer at 0 and 90, indicating the alignment of the ETDM along the crystals' channel axis.

at similar positions and ratios as observed for excitation at 500 nm. The emission spectrum of DXP-ZL.30, however, consists almost entirely of a broad band centered at 650 nm and does not resemble the spectra at lower loadings. Excitation spectra were measured, upon monitoring the emission at 650 nm; see Figure 2C. In all cases, the excitation spectra resembled the absorption spectra, confirming the presence of the

new transition in the case of DXP-ZL.30. The latter shows the maximum at 525 nm and a red shifted shoulder between 550 and 615 nm. DXP-ZL.01 and DXP-ZL.09 have their maxima at 535 nm. The excitation spectrum of PDI-ZL.10 was again similar in shape, but with a slight red shift with respect to the corresponding absorption spectrum. Furthermore, we compare in Figure 2F the excitation and emission spectra of a 5  10-7 M PR149 5980

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The Journal of Physical Chemistry C solution in dichloromethane with PR149-ZL.10 dispersed in toluene. It is interesting that all vibronic features are present in both samples, just red-shifted by about 6 nm in the second case, and that no features as seen for DXP-ZL.30 are present. In light of the results obtained with the DXP-ZL.30 sample, the high-loading experiment was repeated by using much shorter ZL crystals of about 30 nm, which we name nano ZL (NZL). A 30 nm long channel consists of 40 u.c. or about 13 sites of 3 u.c. in length. This means that, at a loading of p = 0.25, each channel contains on average between three and four DXP molecules. The absorption and luminescence spectra of the DXP-NZL.29 material suspended in ethyl benzoate are given in Figure 3. Ethyl benzoate was chosen over toluene as a solvent in this case because it allows for better index matching.49 The absorption spectrum in Figure 3A shows the usual vibronic progression, with maxima at 525, 488, and 465 nm. The new band around 550 nm is seen only as a prolonged tail. The emission spectra upon excitation at 480 and 550 nm are shown in Figure 3B along with the corresponding excitation spectra detected at 580 and 620 nm, respectively. When excited at 480 nm, the DXP-NZL.29 shows the expected vibronic progression with maxima at 535, 580, and 630 nm. The spectral shape changes significantly when the material is excited at 550 nm: it resembles that of DXP-ZL.30 with a first shoulder at 585 nm and a broad band centered at 630 nm. The excitation spectra detected at 580 and 620 nm exhibit the three known peaks at 525, 488, and 460 nm along with a new shoulder at about 550 nm. This shoulder is better visible in the excitation spectrum recorded at 620 nm. Fluorescence Microscopy Studies. Samples were inspected under an epi-fluorescence microscope, in order to study loading patterns and emission colors. A representative population of crystals can be seen in Figure 4; the emission colors appear greener, which means blue shifted than they are in reality, for better visibility. All crystals are seen to be fluorescent. For the DXP samples, a distinct loading-dependent emission was observed. DXP-ZL.01 was seen to emit green light from the middle of the crystal, whereas the edges show yellow luminescence. Most crystals display a shallow intensity gradient decreasing from the ends to the center. All zeolites in the DXP-ZL.09 sample showed yellow emission throughout the crystal with a green and red tinge in the middle and on the ends, respectively. The crystals all displayed homogeneous fillings with no distinct gradient. Interestingly, the DXP-ZL.30 material exhibited a severe filling gradient. Crystals were virtually empty in the middle with intense red luminescence coming from the outer quarters of the zeolites. Regions of yellow emission were seen on the inner edges of the red bands. The ZL-PDI.10 samples prepared here showed similar inhomogeneous loading patterns, where a green band of emission was localized on the ends of the crystals and nearly all of the crystals were devoid of emission from the center. The observed green emission is typical for PDI. Homogeneous filling with PDI can be reached and was observed for smaller crystals of about 1400 nm in length by using longer infiltration times. However, such a homogeneous loading was not of interest here and will not interfere with the aim of this study. The emission of the all DXP and PDI samples was polarized along the axis of the crystals, indicating that the molecules' ETDMs were aligned parallel to the crystal axis; see Figure 4 (insets). If we assume that the ETDM of the perylene bisimide is along the molecule’s long axis, we can conclude that they are aligned parallel to the crystals' channels. This is in agreement with several polarization studies performed on different

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Figure 5. Normalized confocal volume emission spectra of individual crystals, taken at 488 nm (A) DXP-ZL.01, (B) DXP-ZL.09, (C) DXPZL.30, and (D) PDI-ZL.10. Red and black lines correspond to spectra taken at the crystals' ends; green lines are spectra taken in the crystals' center.

molecules.12,51 It also proves that the molecules were intercalated in the channel systems of the crystals, and not merely absorbed on the external surface, which may be the case with other systems.25 To further understand the loading process and to see if the samples displaying emission gradients could be homogenized, the same samples were reloaded under the same conditions, (i.e., same molar amount of dye per site) for 48 h. DXP-ZL.01, which originally had a green/yellow appearance, became significantly more orange and resembled sample DXPZL.09. For DXP-ZL.30 and PDI-ZL.10, no significant change could be recognized, and the crystals still had empty centers. From these studies, it is evident that the homogenization of the severe loading gradients as seen in samples DXP-ZL.30 and PDIZL.10 may take a longer time or other reaction conditions. Diffusion inside of the zeolite channels can be quite slow, as has, for example, been reported for oxonine and pyronine in ZL.52 Armbruster et al. reported that a reaction time of 12 weeks at 90 was necessary to insert methylene blue homogeneously into the channels of modernite.53 In other cases, complete filling of the zeolite is easy.5 Transport kinetics under such conditions is not yet well-understood, and more systematic studies are needed before general conclusions can be made. The anionic nature of the ZL framework is expected to show different transport properties than, for example, ALPO4-5, which has very similar, but neutral, channels.54 Spectral Imaging in Fluorescence Microscopy. The variations in loading patterns and colors seen under the epi-fluorescence microscope are interesting and challenging for understanding the photophysics on the submicrometer scale. Single crystals of the DXP samples were analyzed with a confocal microscope capable of spectral imaging. Emission spectra from selected points of the crystal were measured within the confocal volume. Three representative normalized spectra were taken across a central cross section. In all cases, the signal intensity corresponded to the loading pattern seen in the crystals. Both DXP-ZL.01 and DXP-ZL.09 have spectral bands and structures similar to that measured in the bulk, with maxima at about 555 and 600 nm; see Figure 5A,B. The DXP-ZL.30 material, which shows red and yellow stripes at the crystal ends with an empty 5981

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The Journal of Physical Chemistry C center, exhibit significant spectral variations across the crystal. Spectra taken in the middle of the crystal are much weaker than those taken at the ends and have bands at about 555 and 600 nm with a similar shape to those seen for DXP-ZL.01 and DXPZL.09. The ends from where the red emission originates features a new broad band at 660 nm; see Figure 5C. Additional small peaks at 560 and 600 nm can be made out. The emission spectra recorded from all parts of the PDI-ZL.10 crystal resemble the bulk spectra with bands at 550 and 590 nm (Figure 5D). The emission intensity in the center of the zeolite is significantly lower than that observed on the edges, but the spectral shape does not vary over the crystal length. From the absorption, emission, and excitation spectra, it is clear that increasing the DXP loading results in both reabsorption and the presence of a new broad red shifted band in absorption and emission spectra. Both factors can be related to the increase in the observed red luminescence of the crystals upon inspection under the epi-fluorescence microscope. Such a behavior is typical for the formation of J-aggregates or π-stacked perylene bisimides in both solution and solid state. On the basis of the presence of the new transition in the absorbance, diffusion-based excimers can be categorically ruled out as they cannot be populated by Franck-Condon transitions.55 Furthermore, it is highly unlikely that the perylene cores can stack on top of each other to form columnar structures. Also, both red and green emission components have the same polarization. The observations are in accordance with the interpretation that the new excited state corresponds to “in line” excitonic coupling of perylene units. The regions of orange and yellow seen under the microscope are thought to result from mixtures of two defined states, corresponding to the green-emitting free molecule and the redemitting aggregate. The spectra of PDI-ZL.025 strongly resemble those of the free molecules, albeit the emission maximum is shifted to the red by 10 nm. The lack of a strong emission beyond 600 nm is testament to the green color seen under the epifluorescence microscope. It is important to note that the spectra were taken from a sample with a maximum obtainable loading so that this situation is comparable to that seen in DXP-ZL.30. The difference between the samples is thought to result from the long aliphatic chain in PDI acting like a spacer and thus preventing the transition dipole moments in adjacent molecules from significantly coupling. No evidence of a red emission line can be found in the spectral imaging. This is a further indication that, even if a certain amount of PDI molecules would interact like DXP, their concentration is in the submonolayer regime. A full monolayer would consist of a few hundred thousand molecules on each crystal end and would, therefore, be spectrally easily detectable. Luminescence Lifetime Studies. The fluorescence decay of the free molecules DXP and PDI in solution have been reported to be monoexponential with lifetimes and fluorescence quantum yields of 3.45 ns and 0.96 for DXP in ethanol,32 and 4.0 ns and 1.0 for PDI in DCM.33 The fluorescence decay of a PR149 solution in DCM was determined as being monoexponential with a lifetime of 3.6 ns. A more complex picture is presented for the dye-loaded zeolite systems. Bulk emission lifetimes of dye-ZL dispersions in methanol were measured at 550 nm. In all cases, no traces showed monoexponential decay and a biexponential model gave good fits. The data are summarized in Table 3. The fluorescence decays of DXP-ZL.01 and PDI-ZL.025 are similar, whereas those of DXP-ZL.09, DXP-ZL.17, and DXPZL.25 all decay faster than that of DXP-ZL.01. In the case of DXP-ZL.30 and DXP-NZL.29, the trend is not followed, as their

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Table 3. Bulk Lifetime Measurements sample

τ1 [ns] a1 [%] τ2 [ns] a2 [%] [ns]

DXP, EtOH sol.

3.4532

DXP-ZL.01

3.8

60

1.7

40

3.0

DXP-ZL.09

3.1

45

1.0

55

1.9

DXP-ZL.17

3.1

38

0.7

62

1.6

DXP-ZL.25

3.3

35

0.7

65

1.6

DXP-ZL.30

4.5

60

1.6

40

3.3

DXP-NZL.29

4.8

95

1.4

5

3.7

PDI DCM sol. PDI-ZL.025

4.033 3.7

100 85

1.2

15

3.3

0.8

54

2.5

PR149, 5  10-7 M DCM sol. 3.6 PR149-ZL.10

4.5

100

100 46

decay times are longer by about 1 ns, an observation that is surprising. Interestingly, the PR149-ZL.10 material also showed a biexponential decay with a long component around 4.5 ns and a shorter one of 0.8 ns. It is not clear if this behavior is due to the heterogeneity of the samples or if it is caused by specific photophysical events. Therefore, single crystal measurements have been carried out. Lifetime imaging was performed in order to correlate the spectral and microscopic data with the inconclusive bulk lifetimes. Intensity and lifetime images of single crystals were measured with a step size of 50 nm upon excitation with a pulsed 468 nm laser (80 ps pulse width), and emission was measured between 550 and 700 nm. For all samples, multiple crystals were measured and analyzed. Representative data sets of the natural distribution in crystal loadings are displayed in Figure 6. DXPZL.01 and DXP-ZL.09 showed a slow gradient between the ends and the middle, which corresponds to typical diffusion patterns seen in the literature, and which is well-known to depend on the loading procedure, reaction time, size of crystals, and on the type of dye.11,12,23,52,56 The DXP-ZL.09 samples were filled more homogeneously with a flatter gradient than DXP-ZL.01. In contrast, samples DXP-ZL.30 and PDI-ZL.025 had a steep gradient between the seemingly empty crystal centers and the highly loaded ends. In the case of the DXP-ZL.30 sample, channel filling was observed only between the crystal ends and the first quarters. The loading method used in this study for PDI leads to a strong localization of the dyes at the crystal entrances. Obtaining more complete and homogeneous filling would require a different loading technique, such as the double ampule or impregnation method.23 The technique used here and the size of the large crystals are actually preferable for our study. The uneven distribution leads to much shorter interdye distances so that interactions between the chromophores are more probable. Cases where crystals show outlying lifetime behavior generally result from their higher or lower loading that is visually evident upon inspection under the epi-fluorescence microscope. All crystals showed lifetimes that were shorter or comparable to that of the free molecule in solution and that fell between the limits seen for the biexponential decays of the bulk measurements. A monoexponential model was deemed satisfactory for all fits; a biexponential model did not give meaningful values with amplitudes greater than 10%. The crystal shown for DXP-ZL.01 shows a slight (50%) gradient in intensity across the crystal; see Figure 6a. The crystal displays a homogeneous lifetime distribution at all points with an average of 3.5 ns. This indicates that the molecules are neither in an aggregated state nor quenched by 5982

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Figure 6. Normalized confocal intensity images (top) and corresponding fitted monoexponential lifetime images (bottom): (a) DXP-ZL.01, (b) DXP-ZL.09, (c) ZL-PDI.025, (d) DXP-ZL.30, (e) DXP-ZL.30 (green band-pass filter), and (f) DXP-ZL.30 (red band-pass filter).

energy transfer to the aggregates. The DXP-ZL.09 crystals appear more homogenously filled; see Figure 6b. Their color is quite homogeneously yellow, with a slight green and red tinge in the center and on the ends, respectively. The intensity image displays an intensity gradient that decreased by about 25% from the ends to the middle, again corresponding to the confocal microscope images of multiple crystals. The measured lifetime in the lower concentrated middle of the crystal was about 2.5 ns, whereas in the more concentrated ends, a drop to 1.0 ns could be seen. The average lifetime over the whole crystal turns out to be 1.75 ns. A significant drop in lifetimes is seen here compared with the free molecule. This trend is also observed in other regions of higher loading where the corresponding lifetimes decay faster. The representative DXP-ZL.30 crystal was chosen with a visibly empty crystal center, and distinct red bands, with inner yellow edges. The intensity image showed intense emission in the first and final quarters of the crystal, corresponding to red and yellow regions; see Figure 6d. The crystal centers displayed low emission with typically less than 10% of the intensity observed at the crystal entrances. The lifetime distribution over the entire crystal is quite broad. Fast decays up to 1 ns can be seen on the ends, corresponding to the regions of highest intensity. The crystal centers, where the dye concentration is lower, exhibited longer lifetimes that reached up to 3.75 ns. On the basis of the observed red and yellow regions of the DXP-ZL.30 crystals, lifetime imaging was repeated using a green (523-550 nm, Figure 6e) and red (640-700 nm, Figure 6f) band-pass filter in order to assign specific lifetimes to the spectral regions. The intensity image taken with the green filter displayed the region of highest emission centered at the 1/4 and 3/4 marks of the crystals, where the yellow emission could be visualized by eye. The lifetime image shows the fastest decay of about 1.25 ns

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in the region of highest intensity, at the interface of the red region. This increases to about 3.75 ns toward the comparatively empty middle. An average value of 2.5 ns can be assigned. It is clear that the lifetimes in the regions closest to the red ends decay much faster than those in the low concentration middle. On the basis of the premise that lifetimes measured using the green filter represent emission obtained mostly from the isolated molecule, an explanation for this phenomenon may be the occurrence of energy transfer from the isolated molecule to the aggregate in regions where both are mixed. Lifetime imaging performed with the red filter shows the regions of highest emission intensity on the crystal ends, as seen by eye. Under these experimental conditions, the crystal centers showed no luminescence intensity. Shorter lifetimes down to 1 ns were seen on the ends, which increased slightly toward the 1/4 and 3/4 regions. An average value of about 1.75 ns could be assigned. Measurements with the red filter are assumed to show emission specifically from the aggregate. This is supported by the lack of slow lifetime components, which would be expected in the case of the isolated molecule and which are seen with the green filter. The emission from the ZL-PDI.025 sample is sharply localized at the crystal ends, with the centers being dark. The average lifetime over the whole crystal was 2.75 ns with a spread between 2 and 3.5 ns. The fast component was seen entirely at the crystal edges. These lifetimes deviate somewhat from those obtained in the bulk measurement (3.7 and 1.2 ns). The longer component can, however, still be attributed to noninteracting monomeric PDI, whereas the shorter lifetime might result from a low concentration of aggregated molecules adsorbed at or close to the external surface. Macroscopic Appearance of the Material and Influence of Coadsorbed Water. The color and luminescence properties of DXP-ZL materials are sensitive to the presence of coadsorbed water. We expect loss of the cosolvent, if the washed dye-loaded material is subjected to dynamic vacuum conditions (5  10-5 mbar). The presence of coadsorbed water, even in the presence of larger organic molecules, and the possibility to remove it have been previously demonstrated by thermogravimetric analysis and IR spectroscopy. The TGA desorption curve of a dye-loaded ZL sample typically shows a first peak around 100 C, corresponding to water loss from the main channel system.23,28 We observed that the color of DXP-ZL samples with high loading changes from pink to orange upon drying in vacuum, as can be seen from the photographic images in Figure 7. In the case of lower dye loading (i.e., p = 0.01), this change is much less pronounced. This color switch is fully reversible. Exposure to humid air brings the sample back to its pink color, whereas simply drying the material for 3 h at r.t. under dynamic vacuum conditions restores the orange coloring. Emission and excitation spectra of hydrated and dry DXPZL.01, DXP-ZL.17 (red), and DXP-ZL.25 are shown in Figure 7B-E. All spectra were measured as powders in sealed glass ampules, leading to some broadening and enhanced selfabsorption. The consequences of self-absorption for DXP have been discussed and illustrated in detail in Figure 9 of ref 12. After drying, the emission spectrum of the DXP changes to a single broad band with a shoulder at about 650 nm, which is less clear for the DXP-ZL.01. Self-absorption leading to a remarkable red shift in all cases appears to be more pronounced in the dry samples, perhaps because of more intense scattering within the powder. The emission spectra of hydrated samples exhibit the expected bands with a deformation of the vibronic progression with increasing dye concentration. The intensity decrease of the 0 r 0 band is due to self-absorption, which becomes more 5983

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Figure 7. (A) Photographic images of dry and hydrated DXP-ZL materials with different dye loadings. Top: 0.01, 0.17, and 0.25 dry DXP-ZL (from left to right). Bottom: hydrated DXP-ZL with loadings of 0.01, 0.17, and 0.25 (from left to right). (B-E) Emission and excitation spectra of DXP-ZL.01 (blue), DXP-ZL.17 (red), and DXP-ZL.25 (green). Panels B and C show emission and excitation spectra recorded from dried samples, whereas panels D and E show the corresponding spectra for hydrated materials. All emission spectra were excited at 480 nm and the excitation spectra recorded at 600 nm.

significant at higher dye loadings. The shoulder at about 650 nm —which is hardly seen for the DXP-ZL.01—resembles the features that we assign to J-aggregate coupling. The excitation spectra of dried DXP-ZL show clear saturation phenomena. This is well supported from the fact that the distortion is least pronounced for the sample with the lowest loading and by the fact that the otherwise weak band at 360 nm is prominent in all samples and shows little distortion. Saturation is enhanced by stronger scattering of the dry samples with respect to the hydrated samples. We, nevertheless, report these spectra in order to illustrate macroscopic aspects of the samples. These observations lead us to perform similar experiments with a DXP analogue called PR149, whose structure differs from DXP only in the methyl substitution of the terminal phenyl rings. In PR149, the methyl groups are located in the meta instead of ortho positions, thus hindering the formation of J-aggregates. A PR149-ZL.10 sample does not show any significant color changes after the drying procedure was carried out; see photographic images and luminescence spectra in the Supporting Information (SI1). This material shows a similar increase in self-absorption upon drying as DXP-ZL.

’ DISCUSSION Supramolecularly organized host-guest systems have been prepared by inserting the three perylene dyes reported in Table 1 with differing end substituents into the nanosized channels of ZL by sublimation under vacuum conditions. The three perylene dyes have very similar absorption and fluorescence spectra in diluted solutions (Figure 2A,F) and very similar fluorescence lifetimes in the range of 4-3.45 ns (Table 3). Large ZL crystals in the size range of 1500 up to 3000 nm in length and about 1000 nm in diameter were used as hosts. Experiments carried out with nanosized crystals NZL of about 30 nm in length and diameter are indicated accordingly. Assuming that one molecule occupies 3

u.c., we can calculate that one channel of a 2250 nm long crystal contains 1000 dye molecules for p = 1 and 10 on average for p = 0.01. The samples were analyzed as suspensions in refractive index matching solutions, toluene or ethyl benzoic acid ester; as bulk materials in glass ampules; and by means of single-crystal fluorescence microscopy techniques. The van der Waals diameter of the perylene core is about 0.76 nm, whereas the van der Waals diameter of the narrowest part of the ZL channel is about 0.71 nm, as illustrated in Figure 1. Insertion of the dyes was, therefore, carried out at elevated temperatures, between 180 and 300 C, depending on the dye. At these temperatures, the ring breathing vibrations of ZL are activated30 so that the dynamic radius becomes sufficiently large to allow the dyes to slip through. DXP-ZL samples with different loadings were investigated because they showed properties we found especially interesting. We, therefore, start by summarizing essential observations made with them. For the low-loading regime, that is, DXP-ZL.01, we observe the same spectral and temporal characteristics as for the free molecule. The DXP molecules appear to be homogeneously distributed across the crystal, with the highest concentration at the entrances, resulting in a typical diffusion pattern. Some crystals in this sample exhibit higher loading caused by the applied loading procedure. The lifetime measured on individual crystals is the same as that observed in diluted solution, and no change along the crystal can be seen. This means that the two components seen in the bulk lifetime measurements are due to inhomogeneities in the sample. The middle-loading regime, that is, DXP-ZL.09, shows most of the crystals to be quite homogenously loaded. A slight orange color is seen throughout the crystals. The centers appear greener and the ends redder. Both features are clearly reflected in the spectra reported in Figures 2B-E and 5B. This indicates the onset of intermolecular interaction between the DXP molecules. The effect appears to be larger at the crystal ends and is also 5984

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The Journal of Physical Chemistry C reflected by shortening of luminescence lifetimes toward the crystal ends. The latter explains the biexponential lifetime seen in the bulk. We note that the trend in the direction of shorter average lifetimes is followed in DXP-ZL.17, but not in DXPZL.25, where no change with respect to the latter is seen. Large changes in the spectral and temporal characteristics are observed for the DXP-ZL.30 samples. We first observe in Figure 4C that the middle part appears to be empty. Characteristic new bands are observed in the absorption, excitation, and emission spectra shown in Figure 2B-E and in the single-crystal luminescence spectra in Figure 5C. The coloring change observed in Figure 4 is well reflected by the spectral differences measured at the middle, intermediate position, and at the end. Parts of these remarkable spectral changes observed in the DXP-ZL.30 material are also present in the spectra of DXPNZL.29. We especially note the characteristic dependence of the emission spectrum on the excitation wavelength and the new band that appears as a shoulder at about 550 nm in the excitation spectrum when observed at 620 nm. The well-developed vibrational progression in the emission spectrum upon excitation at 480 nm and in the excitation spectrum when observed at 580 nm indicates that self-absorption and saturation do not cause important spectral changes. However, the long lifetime component seen in the luminescence decays of both DXP-NZL.29 and DXPZL.30, but not present in the lifetime measured on individual crystals of the latter, seems to be caused by self-absorption and reemission processes. We conclude that the reported observations can be understood by considering saturation phenomena, changes caused by self-absorption and reemission, new bands caused by Davydov splitting, and energy transfer from dyes at sufficiently large distances in the middle part of the crystals to lower lying states located in the region of the entrance. One also has to consider steric hindrance caused by dense packing in the region of the channel entrances during the preparation procedure. Distortion of bands due to saturation is obvious in the excitation spectra of the bulk material shown in Figures 7 and SI1 (Supporting Information). They cannot be completely ignored for individual DXP-ZL.30 crystals in the region of the channel entrance, where the packing density is high. Because of the pronounced anisotropy, the extinction coefficient for light falling parallel to the channel axis on the crystal has to be multiplied by a factor of 3, whereas no absorption is observed for light falling perpendicular to the channel axis. Using eq 7 and the extinction coefficient for DXP (Table 1), we find that the optical density of a crystal of 1000 nm in diameter at a loading of 0.5 is about 3, which means that a spectrum will clearly show saturation phenomena. A factor that severely influences the fluorescence spectra of the bulk samples, such as those shown in Figure 7, is self-absorption. It causes an apparent red shift of the spectrum that can be so severe that features of the original spectrum can be hardly recognized.12 This spectral change is illustrated in Figure 8. It seems that this effect is enhanced in the dry samples, most probably because light scattering and perhaps also wave guiding are more pronounced due to the larger refractive index change. Self-absorption effects are seen to increase with increasing loading in measurements carried out on dispersions (Figure 2D). It is interesting that they do not affect the spectra of the DXP-NZL.29, as can be seen in Figure 3, a fact that supports our interpretation. Self-absorption of individual crystals is not sufficiently important in these small particles to cause

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Figure 8. Calculated changes of the fluorescence spectrum of a DXP solution in a 1 cm cuvette caused by self-absorption. The spectra are scaled to the same peak height at their respective maxima. The respective concentrations are (1) 3.8  10-7 mol L-1 (measured in ethanol), (2) 5  10-7 mol L-1, (3) 5  10-5 mol L-1, and (4) 5  10-4 mol L-1.

important changes in the fluorescence spectrum but, followed by re-emission, seems to be responsible for the observed prolonged lifetime component. It is not clear if self-absorption plays an important role in the spectra of the individual DXP-ZL.30 crystals. We would expect to see this effect most prominently at the crystal entrances, but a comparison of the spectra measured at the crystal edges of DXP-ZL.30 and DXP-ZL.01 shows no spectral shifts. We also find no prolonged lifetime component in the luminescence decay of individual crystals. The DXP-ZL.30 spectra are, however, so much dominated by another new feature that we prefer to focus on this. This new feature is seen in all samples with high loading. We see it for the first time in Figure 2A-E in the absorption and excitation spectra of DXPZL.30, where a new shoulder appears at about 550 nm, and also in the fluorescence spectrum upon excitation at 500 nm (and more pronouncedly, upon excitation at 550 nm), which leads to a new emission band in the red. Some indications of this new feature are present in the DXP-ZL.09, while the feature is absent for DXP-ZL.01. These new bands are beautifully manifested in the spectra of DXP-NZL.29 (see Figure 3), as well as in the fluorescence spectrum of single DXP-ZL.30 crystals. It appears obvious that the most important new features are caused by intermolecular interactions of DXP inside of the channels, which we interpret as Davydov splitting. This is supported by the fact that they are not seen in the PR149-ZL and in the PDI-ZL samples we have investigated. We illustrate in Figure 9 the high packing situations expected for DXP and PR149. Using eq 2, we can estimate the expected spectral changes caused by Davydov splitting due to the interaction of two collinearly arranged perylene bisimidine molecules. The maximum splitting for many interacting molecules converges rapidly to a value that is twice as large as that for two (see Figure SI2 (Supporting Information).5,31,34 The shortest center-to-center distance we can imagine being possible for PR149 and for PDI is 2 nm. This distance is larger than the distance between the centers of two channels, which is 1.84 nm. At these distances, the interaction is so weak that we cannot expect to see it in absorption and fluorescence spectra at room temperature. Single-crystal spectroscopy at lower temperatures would be needed. This however, does not mean that such interactions will not influence fluorescence decay. The luminescence lifetime of two J-coupled molecules is calculated to be half of that of the individual molecules. The situation is different for DXP, where center-to5985

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Figure 9. Packing of molecules inside of the channels of ZL. Parts A and C illustrate two different situations for which intermolecular interaction apart from FRET is negligible and for which Davydov splitting is expected to occur, respectively. The schemes on the right were realized by packing van der Waals models of the dyes into a channel and illustrate that the situation (A) seems to apply for PR149, whereas the situation (C) is possible for DXP.

center distances as short as 1.5 nm seem to be possible. In this case, a bathochromic shift of more than 10 nm is predicted by calculation and can be correlated with the new shoulder observed in the absorption and excitation spectra especially of DXP-ZL.30 in Figure 2B,C as well as in the excitation spectra of DXP-NZL.29 (Figure 3). The bathochromic shift of the fluorescence spectra can be traced to this coupling along with the broadening and also effects on the whole vibronic pattern. The consequences of J-coupling is very well seen in the lifetime distribution of the single-crystal data of DXP-ZL.30 where a fast decay is expected at the crystal ends, based on the enhanced absorption cross section in this region of highest intensity. In this material, the crystal centers exhibit longer lifetimes corresponding to that of isolated molecules. The intermediate lifetimes observed in regions close to the J-aggregates are most probably caused by efficient energy transfer from the noninteracting DXP to the neighboring J-aggregates. We note that excimers and π-stacked molecules can be ruled out due to geometrical constraints as well as the absence of characteristic longer lifetimes, as reported in the literature.36,57 The last observation to be discussed is the apparently strange fact that the crystals seem to be homogeneously filled with DXP at low loading, such as in DXP-ZL.01 and DXP-ZL.09, whereas that the middle part of the DXP-ZL.30 material appears to be nearly empty. The initial high concentrations of molecules that are competing to diffuse into the channels induce a filling at the ends, which may approach the maximum theoretical limit, where molecules cannot approach each other any further. The pile up formed by the close-packed molecules can be understood as a “traffic jam”, slowing down further diffusion to the center of the zeolites. Nevertheless, molecules may still try entering the channels, hence inducing a region of very high loading and close packing at both ends of the crystal. This situation results in a high percentage of coupled J-aggregate molecules, giving rise to the spectral and lifetime changes we have discussed. The centers of the crystals have a comparatively low concentration and are seemingly empty. In reality, a low concentration of isolated molecules exists in the center, which are too far away for sensing the aggregates, giving rise to a region with longer fluorescence lifetime. The yellow region between the seemingly empty center and the ends defines the blockage boundary and comprises a mixture of isolated molecules as well as aggregates. The shorter lifetimes seen here are thought to be due to both efficient energy transfer quenching of the isolated molecules as well as fast decay

of the aggregates.44 These three spatially segregated environments throughout the crystal result in a broad lifetime distribution with fast and slow components (see Figure SI3 in the Supporting Information). The nonhomogeneous filling cannot be regarded as a thermodynamically stable situation because minimizing the entropy and hence the free enthalpy of the crystals would demand for homogeneous filling, but it seems to reflect a situation that is quite stable at room-temperature conditions. Filling at which the molecules had penetrated less than ca. 300 nm into the crystal was also realized for PDI-ZL.10, as seen in Figure 4D. The gradient between the loaded region and the empty centers of the crystals was very sharp. No clear indication for J-aggregate coupling was, however, seen. However, shorter crystals of about 1.4 nm in length were seen to be homogeneously filled under more favorable loading conditions. The spectral and microscopic evidence obtained from the PDI-ZL.10 material points to molecules behaving as in an isolated environment. This means that the hexylheptyl groups are preventing close contact between adjacent molecules, thus reducing excitonic coupling and aggregate formation. Homogeneity of the samples depends much on the insertion conditions and also on the length of the crystals. The double ampule or surface impregnation methods are often needed for realizing very homogeneous samples.23 Studying this parameter was the aim of this work in which the concentration gradient along the crystal axis was one of the most interesting factors that helped to understand observations. The effect of coadsorbed water on the luminescence behavior of DXP in ZL can be understood by considering a different interaction of the dye with the environment. When water is present in the channels, the DXP molecules will interact more strongly with each other through their hydrophobic phenyl groups, thus minimizing water contact and leading to the formation of J-aggregates. The emission spectra of dried DXP samples show no sign of spectral change with increasing loading, which could hint to a different interaction taking precedence over the formation of J-aggregates. The fact that the same behavior can be observed for PR149ZL.10—which cannot form J-aggregates—supports this view.

’ CONCLUSIONS Three perylene bisimide molecules with differing end groups have been inserted into the linear nanochannels of ZL. We have 5986

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The Journal of Physical Chemistry C employed the end substituent to control the core-to-core distances of the molecules in the channels of ZL. DXP contains a short 2,6-dimethyl phenyl ring, and PDI has a hexylheptyl alkyl chain bound to the bisimide, lengthening the molecule. The difference between DXP and PR149 lies in the substitution of the dimethyl phenyl rings. The PR149 has methyl substituents on the phenyl rings in an ortho position that point “outwards” from the molecule. DXP allows for closer intermolecular interactions than PDI and PR149. The spectral and electronic insensitivity of the perylene core toward the end substitution is reflected in the very similar absorption and fluorescence spectra as well as the fluorescence lifetimes of the three molecules in diluted solution. This allowed for direct comparison between intermolecular interactions and molecular structure within the zeolite nanochannels, which resulted in tunable spectral, temporal, and microscopic properties linked to the macroscopic appearance of the material. The most interesting behavior was observed for DXP-ZL samples where the spectroscopic properties depend very much on the loading. The properties of the different samples, which looked rather complex at the outset, could be well categorized and understood. J-aggregate coupling was observed to result from increasing loading, but it was absent for PDI and PR149 samples. For these two dyes, the end groups prevent the molecules from packing at a sufficiently short center-to-center distance needed for strong coupling. In conjunction with excitonic coupling, microscopic analysis shows that crystals with high loadings also exhibit strongly hindered diffusion of the dye molecules into the channel system. The excitonic coupling can be controlled upon varying the molecular tail groups. Aligned and stabilized J-aggregates in one-dimensional channel systems provide new options for preparing optical devices, where coherent exciton delocalization over nanometer-to-micrometer scales may result in efficient photonic wires. We conclude that the interaction between the chromophores can be controlled by careful design of the end groups of the molecules. Groups that act as spacers are needed if exciton coupling is to be avoided, and groups that allow close contact between the chromophores are favorable if strong exciton coupling is desired. Coherent lengths can be optimized by choosing end groups with favorable interacting forces.

’ ASSOCIATED CONTENT

bS

Supporting Information. Photographic images and luminescence spectra of the PR149-ZL material, calculated Davydov splitting for perylene bisimide dyes, and scheme of observed loading patterns. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (L.D.C.), gion.calzaferri@ iac.unibe.ch (G.C.). Present Addresses ^

Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, OX1, United Kingdom. Author Contributions z

These authors contributed equally to this work.

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’ ACKNOWLEDGMENT M.B. would like to thank the Alexander von Humboldt Foundation for funding. This work was supported by the European Commission through the Human Potential Program (Marie-Curie RTN NANOMATCH, Grant No. MRTN-CT2006-035884). We would like to thank Dr. H. Metz for the PR149 sample. ’ REFERENCES (1) (a) Armaroli, N.; Balzani, V. Angew. Chem., Int. Ed. 2007, 46, 52–66. (b) Balzani, V.; Credi, A.; Venturi, M. ChemSusChem 2008, 1, 26–58. (2) (a) Bard, A. J.; Fox, M. A. Acc. Chem. Res. 1995, 28, 141–145. (b) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40–48. (c) Benniston, A. C.; Harriman, A. Mater. Today 2008, 11, 26–34. (3) Ramamurthy, V. Photochemistry in Organized and Constrained Media; VCH: New York, 1991. (4) (a) Calzaferri, G.; Pauchard, M.; Maas, H.; Huber, S.; Khatyr, A.; Schaafsma, T. J. Mater. Chem. 2002, 12, 1–13. (b) Vohra, V.; Calzaferri, G.; Destri, S.; Pasini, M.; Porzio, W.; Botta, Ch. ACS Nano 2010, 4, 1409–1416. (5) Calzaferri, G.; Devaux, A. In Supramolecular Effects in Photochemical and Photophysical Processes; Ramamurthy, V., Inoue, Y., Eds.; John Wiley & Sons, Inc.: New York, US, 2011, in press. (6) (a) Inagaki, S.; Othani, O.; Goto, Y.; Okamoto, K.; Ikai, M.; Yamanaka, K.; Tani, T.; Okada, T. Angew. Chem., Int. Ed. 2009, 48, 4042–4046. (b) Gartmann, N.; Br€uhwiler, D. Angew. Chem., Int. Ed. 2009, 48, 6354–6356. (7) Tsotsalas, M.; Busby, M.; Gianolio, E.; Aime, S.; De Cola, L. Chem. Mater. 2008, 20, 5888–5893. (8) (a) Sen, T.; Jana, S.; Koner, S.; Patra, A. J. Phys. Chem. C 2010, 114, 707–714. (b) Johansson, E.; Choi, E.; Angelos, S.; Liong, M.; Zink, J. I. J. Sol-Gel Sci. Technol. 2008, 46, 313. (c) Minoofar, P. N.; Dunn, B. S.; Zink, J. I. J. Am. Chem. Soc. 2005, 127, 2656–2665. (9) (a) Seebacher, C.; Hellriegel, C.; Br€auchle, C.; Ganschow, M.; W€ohrle, D. J. Phys. Chem. B 2003, 107, 5445–5452. (b) Ganschow, M.; Hellriegel, C.; Kneuper, E.; Wark, M.; Thiel, C.; Schulz-Ekloff, G.; Br€auchle, C.; W€ ohrle, D. Adv. Funct. Mater. 2004, 14, 269–276. (10) Kim, H. S.; Lee, S. M.; Ha, K.; Jung, C.; Lee, Y. J.; Chun, Y. S.; Kim, D.; Rhee, B. K.; Yoon, K. B. J. Am. Chem. Soc. 2004, 126, 673–682. (11) (a) Calzaferri, G.; Huber, S.; Maas, H.; Minkowski, C. Angew. Chem., Int. Ed. 2003, 42, 3732–3758. (b) Br€uhwiler, D.; Calzaferri, G.; Torres, T.; Ramm, J. H.; Gartmann, N.; Dieu, L.-Q.; L
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