J . Phys. Chem. 1994,98, 1888-1894
1888
Luminescence and Configurations of Perylene Dimers in a Langmuir-Blodgett Film J. Mahrt, F. Willig,' W. Storck, D. Weiss, R. Kietzmann, K. Schwarzburg, B. Tufts, and B. Triisken Fritz-Haber-Institut der Max-Planck-Gesellschaft. Faradayweg 4-6, 141 95 Berlin, Germany Received: August 17, 1993; In Final Form: November 30, 1993'
Luminescence and excitation spectra of a Langmuir-Blodgett multilayer with a high concentration of densely packed perylene chromophores are measured at 2 K. They are compared with corresponding spectra of the dimeric a-perylene crystal and of perylene dimers formed in a glass. The majority of the perylene chromophores are arrested in the Langmuir-Blodgett film in the form of open metastable dimer configurations with monomerlike spectra. They dominate the absorption spectrum and excitation spectrum of luminescence. From these open dimers the excitation energy is rapidly transferred to the lower excited state of a stable dimer species functioning as a trap. Luminescence from this excited g-state leads to the peak at 550 nm in the luminescence spectrum. A second peak at 630 nm and the red side of the luminescence spectrum are due to E-excimer luminescence. The wide spectral range of the luminescence, extending from below 500 nm up to 800 nm, reflects sub-angstrom differences in the configurations of the dimers.
Introduction Preferential orientation of certain molecules can be achieved in a monomolecular layer by applying the Langmuir-Blodgett preparation technique. It is also fairly easy to achieve specific spatial arrangements and ordering of suitable molecules in Langmuir-Blodgett multilayer assemblies. One of the best examples for the order in such a molecular multilayer system can be found in the celebrated experiment of Drexhage and Kuhn, where the distance dependence of energy transfer was measured by placing a monolayer with dye molecules at different distances from a metallic support.' The distance between dye layer and metal surface was varied by inserting different numbers of a monomolecular fatty acid layer. The smallest change in distance was of the order of 25 A, corresponding to the thickness of the fatty acid monolayer. Requirementsfor molecular order become much more stringent when electron or energy transfer is to be confined to a specific group of molecules that is part of a densely packed molecular system. Control of the time scale and of the energetics of the reaction requires molecular order on a sub-angstrom scale. Such a high degree of order appears to be realized in the complex molecular system of the reaction center of bacterial photosynthesis.* Undesired reactions of neighboring chromophore groups can be prevented via insertion of a sheath of inert spacer molecules. This could be one of the functions of the protein environment in the reaction center of bacterial photosynthesis. Molecular order on a sub-angstromscale is also necessary when one wants to create a coherent exciton with giant oscillatorstrength in a molecular aggregate of identical chromophores.' This type of optical transition achieves efficient light absorption in a narrow spectral range, as is desired for photographic applications. Molecular order on a sub-angstrom scale is also required for the realization of the other extreme, i.e., rapid localization of the electronic excitation energy via excimer formation in a suitable dimer.4 Excimer formation at theentranceof a molecular reaction chain embedded in a molecular antenna system could function as sink and harvest the excitation energy from the antenna system to feed it into the reaction chain. Rapid localization of the excitation energy with a subsequent reversibleor irreversible lightdriven reaction is also required when one wants to write a nearfield optical imageS with about 200-A spatial resolution into a molecular system. The change in the configuration of the excited dimer when forming the excimer brings about an activation energy
* Abstract published in Aduance
ACS Abstracts, January 15, 1994.
against further energy transfer in the molecular film. Thus, excimer formation can prevent energy transfer at a sufficiently low temperature. Mixing the monomolecular antenna system with a high concentration of molecules with a lower excitation energy that will function as traps could be an alternative approach to realizing rapid localization of the excitation energy. However, island formation in densely packed molecular systems with two different molecular components presents a difficult problem for this approach. Measurements of the dynamic process, e.g., electron or energy transfer, with the necessary temporal and spectral resolution over a wide temperature range down to He temperatures can show whether the required order or disorder on a sub-angstrom scale has been achievedin the molecular system. X-ray data and optical spectra measured at room temperature are generally not sufficient for obtaining definite information on sub-angstrom variations in a molecular system. This is well-known for molecular single crystals6and has been found to hold true for Langmuir-Blodgett films7and Langmuir-Blodgett type single crystals8of a modified anthracene chrom~phore.~ In this paper we report on stationary and time-resolved measurements between room temperature and 2 K of the luminescence of densely packed perylene chromophores in a Langmuir-Blodgett film. Perylenechromophoresarechosen since we aim at the above-mentioned case of rapid localization of blue excitation energy via excimer formation in the Langmuir-Blodgett film. Perylene chromophores are known to assemble as dimers in the stable a-perylene single crystal.'O We reported already in a previous paper" that excimers are formed by perylene dimers in the Langmuir-Blodgett system at 1.5 K. The perylene chromophores were modified for alignment in the LangmuirBlodgett film by covalently attaching a fatty acid chain at the 3-position. There is a considerable difference in the spatial requirement of the perylene chromophore compared to that of the fatty acid chain. Such a spatial mismatch is usually encountered when an alkane chain is attached to a bulky dye molecule for incorporation into a Langmuir-Blodgett film. The mismatch is expected to present serious difficultiesfor the ordered packing of the chromophores in the Langmuir-Blodgett film. In the case of the perylene chromophore, one has excellent X-ray and spectroscopic reference data for the single crystal10 of the naked perylene chromophore without fatty acid chain attached. In this paper we present optical spectra obtained at 2 K for the modified perylene chromophores that were mixed with arachidic acid to form a Langmuir-Blodgett film. The synthesis of the
0022-3654/94/2098-1888$04.50/0 0 1994 American Chemical Society
Luminescence of Perylene Dimers in a LB Film modified perylene compound 3Pe12 (defined below) is reported. We give assignments of the optical spectra and thereby revise those given in our previous paper]' on this topic. The new more straightforward assignments are made from a comparison with the luminescence and excitation spectra of the stable perylene dimer and of various metastable dimer configurations that were arrested in a glass. The dimers were prepared by exposing a 10-5 M concentration of a covalently bound bichromophoric perylene compound to different cooling rates. The peak of the so-called Y-emission in the yellow spectral range is identified as emission from the lower excited g-state of stable perylene dimers. It is not necessary to invoke luminescence and excimer formation of the upper u-state of a parallel perylene dimer, as was considered in our previous paper." As in the previous paper," we assign the peak at the red side of the luminescence spectrum to E-excimer emission. The wide spectral distribution of the luminescence at 2 K is explainedwithsubangstromvariationsin theconfigurations of the light-emitting dimer centers in the Langmuir-Blodgett film.
Experimental Section The 3Pe12 compound was synthesized as follows. 11-(fPerylenony1)undecanoic Acid Methyl Ester. Dodecanedioic acid monomethyl esterI2 (8 mmol; 1.96 g) was reacted with thionyl chloride (16 mmol; 1.90 g) for 3 h at 45-55 OC. The excess of thionyl chloride was eliminated under reduced pressure at 40 OC,last traces at 70 OC, yielding a colorless syrup of the ester chloride. Pulverized perylene (7.8 mmol; 1.97 g) was suspended in dry chlorobenzene (250 mL) with magnetic stirring for 10 min. Anhydrous aluminium chloride (1 1 mmol; 1.48 g) was added at once followed by more chlorobenzene (125 mL). The blue-green suspension was cooled (0 "C)and the ester chloride dissolved in chlorobenzene (50 mL) added drop by drop (1.25 h). After stirring for a further 1.25 h at 0 OC,the reaction was completed at rt (12 h). At 0 O C , dry methanol (20 mL) was carefully added, along with a mixture of ice water (100 mL), and concentrated hydrochloric acid (1.5 mL) followed after 0.5 h. The two phases separated immediately. The organic layer was washed with icecold 0.1 N hydrochloricacid and then water and sodium hydrogen carbonate solution. Afterward, it was dried (sodium sulfate). The chlorobenzene was distilled off under reduced pressure and the residue dissolved in methylene chloride (400 mL) and chromatographed in two portions at SiOz (50 g of Kieselgel60, 63-125 pm; Merck Germany) with the same solvent. Crude perylene (1.44 g) and crude product were obtained, which after repeated crystallization from cyclohexane yielded orange-colored leaflets(0.69g) withmp 138.5-139.5OC. IR(KBrpel1et): 1737 vs (-CO-0-), 1671 s (-CO-), 832 m (sh;ar), 811 vs (ar), 791 vw (ar), 770 vs (ar), 761 m (sh, ar) cm-l. Anal. Found: C, 82.50; H, 7.04. C33H3403 calcd: C, 82.81; H, 7.16. 12-Perylen-3-yldodecanoicAcid (3Pe12). The above keto acid ester (1 mmol; 479 mg) was dissolved with stirring in triethylene glycol (20 mL) at 110 OC in a nitrogen atmosphere. Hydrazine hydrate (100%; 0.5 mL) was added carefully, followed by pulverized potassium hydroxide (0.5 9). After 30 min, more hydrazine hydrate (0.4 mL) was added and the temperature of the heating bath was raised to 190 OC within 1 h and left there for 2 h; some water distilled off. After cooling, the solution was poured into water (100 mL), acidified with hydrochloric acid. The product was extracted with chloroform (3 X 8 mL), and the combined organic phases were washed with water and dried with sodium sulfate. After the chloroform was distilledoff, the residue was recrystallized twice from chloroform yielding light-brown crystals (254 mg; 56.3%) melting at 148.5-149.5 OC (turbid); the melt clarifies at 154-154.5 OC. IR (KBr pellet): 1708 vs (-CO-0-), 828 m (ar), 81 1 vs (ar), 788 w (ar), 764 vs (ar), 756 m (sh, ar) cm-l. Anal. Found: C,85.22; H, 7.66. C3&3&2 calcd: C,85.29; H, 7.61.
The Journal of Physical Chemistry, Vol. 98, No. 7, 1994 1889
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[ A2I mole c Iel Figure 1. Pressure-area diagram for the mixed monolayerof the modified perylene chromophore 3Pe12 and arachidic acid, mixed at molar ratio 0.5. The subphasc was triply distilled water of pH = 7 with 5.7 X l e M CdCI2. The inset shows the electron density across a corresponding bilayer in a Langmuir-Blodgett multilayer system as determined from small-angle X-ray scattering.14 Mixed multilayer systems with a high concentration of the 3Pe12 perylenechromophoreswere assembledwith the LangmuirBlodgett technique on the hydrophobized surface of a quartz slide. The preparation of the multilayer system was described in detail before." The multilayer system consisted typically of 30-40 bilayers of the compound 3Pe12 mixed at molar ratio 0.5 with arachidic acid molecules. The Langmuir-Blodgett film of the pure compound 3Pel2 cannot be transferred without damage onto a solid substrate. The multilayer system for determining the electron density distribution (compare the inset of Figure 1) consisted of identical bilayers. For optical spectroscopy a slightly different multilayer system was used. Layers of pure arachidic acid were inserted such that the monolayers containing 3Pe12 molecules were isolated from each other in the multilayer system. We found that the spectroscopicproperties of both these samples were very similar. The interactions of the perylene chromophores in the same monomolecular layer determine the spectroscopic behavior of the layer system. The latter remained the same after the samples has been stored for 6 months in the dark. Thin a-perylene crystals, 10-pm thickness and O.l-cmz area, were grown via plate sublimation in a temperature gradient from zone-refined material kindly supplied by Prof. N. Karl. Stable and metastable dimers of perylene chromophores were formed by slowly cooling a 10-5 M solution of a bichromophoric compound, two perylene chromophores covalently linked by a five- or seven-membered alkane chain, in a glass forming solvent mixture, isopentane and methylcyclohexane. To obtain stable dimers, the solution was kept for 20 h at 190 K before the glass was formed. The experimental procedure and the spectroscopic signals of the dimers between room temperature and 1.5 K will be described in detail elsewhere.
Mahrt et al.
1890 The Journal of Physical Chemistry, Vol. 98, No. 7, 1994
Stationary and time-resolved luminescence spectra were measured in a He cryostat (Cryovac Konti IT) with quartz windows. Temperature-controlled measurements were carried out between room temperature and 2 K. Stationary luminescence andexcitation spectra weregenerated with a 150-Whigh-pressure Xenon arc or a 100-W tungsten halogen lamp combined with an ISA H 25 monochromator. Stationary fluorescence spectra were measured with an ISA triple spectrometer (Mole S 3000), where the detector was either a Peltier-cooled linear diode array or a Peltier-cooled Hamamatsu photomultiplier (R943-02). Timeresolved fluorescencespectra were measured with the well-known time-correlatedsinglephoton counting technique.13 We employed a Hamamatsu microchannel plate photomultiplier (R2809-01) as detector, the H10 ISA monochromator, and excitation pulses of 6-1 2-ps half-width from a cavity-dumped frequency-doubled rhodamine 6G dye laser (295 nm) driven by a mode-lockedArgon ion laser (Spectra physics).
Experimental Results Without the addition of fatty acid molecules, we were not able to build a multilayer system via transfer of only the compound 3Pe12 from the water surface to a solid substrate. We attribute this poor stability of the pure film to the significant mismatch in the spatial requirementsof the perylene chromophore compared to that of the covalently attached fatty acid chain. The compression diagram of a transferable mixed monolayer, Le., with molar ratio 0.5 of 3Pe12 to arachidic acid, is shown in Figure 1. At 30 mN m-1, the average area per molecule is about 40.5 A2. This average area requirement is obviously dominated by the spatial requirement of the perylene chromophore. The area of 40.5 A2 is much larger than the about 25 A2 average area required by the anthracene chromophore tilted by about 30° to the surface normal and the about 18.5 AZof a straight fatty acid chaine7 Transfer of the above mixed monolayer onto a hydrophobized quartz slide was carried out at 30 mN m-'. The average electron density distribution across a bilayer in the mixed 3Pe12 multilayer system (molar ratio 0.5 of 3Pe12 to arachidic acid) is shown as the inset in Figure 1. It was derived from small-angle X-ray diffraction.14 The electron density distribution suggests that the perylene chromophoresdid not penetrate noticeably into the adjacent monolayer. The total width of the bilayer and the electron distribution across the diameter suggest that the fatty acid chains were stretched out. This is illustrated by the two 3Pe12 structures shown in the inset of Figure 1. Details of the molecular order in the plane cannot be derived from such data. From the X-ray data of the much better ordered single crystal of the anthracene compound 2A4,8 we know that the fatty acid tails have to be arranged already in a complicated criss-cross pattern to compensate for a much smaller mismatch between chromophore and fatty acid tail than is occurring in the present mixed 3Pel2 Langmuir-Blodgett layer. In contrast to the behavior of pure Langmuir-Blodgett multilayer systems built with the anthracene chromophore substituted at the 2-position, Le., 2Al7 or 2A4,Bthe mixed 3Pe12 multilayer systems showed no signs of long-range crystal-like packing of the perylene chromophores. There were neither wellknown contrast patterns that can be observed with the respective a-and 8-perylene single crystals of the perylene chromophores upon rotation of the analyzer in the polarization microscope nor the well-known Davydov splittingknown for the absorption spectra of perylene single crystals. The polarization microscope did not reveal any noticeable structures in the 3Pel2 Langmuir-Blodgett multilayer system except for a few crystalline spots covering less than one per mill of a given area. The absence of long-range crystal-like order in the case of the 3Pel2 monolayers is in strong contrast to the X-ray738 and spectroscopic data9 for the correspondinganthracenecompounds,Le., the 2A7 Langmuir-Blodgett film and the 2A4 crystal.
xpe+ XA
=1
h Inml
Figure 2. Stationary luminescence spectra (uncorrected) of two mixed
Langmuir-Blodgett multilayer systems with small (xp, = 0.01) and large (xp, = 0.25) molar ratio of 3Pe12 molccules in the arachidic acid matrix at room temperature. At thelow concentrationof 3Pe12 thelumincscence spectrum is similar to that of perylene monomers. At the high concentration(x =0.25) the luminescence is similarto the E-luminescence of the dimeric a-perylene crystal at room temperature. Both spectra were recorded for excitation at 337 nm. The following spectroscopic data refer to the alternating multilayer system, where each monolayer containingthe perylene chromophoreswas separated from the next equivalent monolayer by insertion of monolayers containing only pure arachidic acid." The room temperature absorption spectrum of the mixed x = 0.5 3Pe12 Langmuir-Blodgett multilayer system for light of perpendicular incidence was shown in ref 11. Vibrational peaks in this spectrum resemble the absorption spectrum of the perylene monomer; however, the peaks in the Langmuir-Blodgett system are red shifted by about 200 cm-l with respect to the perylene monomer in cyclohexane. In addition, there is a prominent shoulder at the red absorption edge. We show experimental evidence below that strongly points to a dimeric nature of the perylene moieties that give rise to the absorption spectrum of the Langmuir-Blodgett system. The two stationary room temperature luminescence spectra in Figure 2 illustrate the influence of the perylene chromophore concentration on the luminescence spectrum. For the low concentration (xp, = 0.01), Figure 2 shows a vibrational progression with monomer-likepeaks. We measured this sample only at room temperature. Insufficient spectral resolution of these inhomogeneously broadened peaks does not allow further analysis. With x p e 1 0.25 the spectrum was the red-shifted structureless E-type lumine~cence'~ that is generally attributed to the configuration with the strongest interaction in the excited dimer.l6 Excimer-type luminescence spectra were reported also for Langmuir-Blodgett films, where the unsubstituted perylene moiety was simply incorporated into a matrix of fatty acid molecules.17 It will become evident from the low-temperature data shown below that the room temperature excimer-type luminescence(Figure 2) cannot reveal any details of the molecular packing in the Langmuir-Blodgett film. The stationary 2 K luminescence excitation spectrum of the 3Pe12 multilayer luminescence is shown in Figure 3 with the emission wavelength fixed at 650 nm. The energy difference between the peaks in the excitation spectrum is similar to that in the excitation spectrum of perylene monomer luminescenceor in the absorption spectrum of the perylene monomer. The peak to valley ratios are much weaker in the excitation spectrum of the x = 0.5 3Pe12 Langmuir-Blodgett film than in the excitation or absorption spectrum of the perylene monomer. In addition, the peak positions are red-shifted by about 200 cm-1 with respect
The Journal of Physical Chemistry, Vol. 98, No. 7, 1994 1891
Luminescence of Perylene Dimers in a LB Film
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to those of the perylene monomer absorption spectrum in cyclohexane or CH2C12. There is a pronounced shoulder visible a t 470 nm in Figure 3. The stationary 2 K luminescence spectrum of the 3Pe12 multilayer system is shown in Figure 3 for the excitation wavelength at 470 nm. The luminescence spectrum has a pronounced peak at 550 nm and a less distinct peak at about 630 nm. We show below that the 550 nm peak position is identical to that of the Y-emission from the lower excited g-state of the stable parallel perylene dimer. The peak at 630 nm occurs at the peak position of the E-excimer emission in the a-perylene crystal.15 Note that, in contrast to the Langmuir-Blodgett system, E-excimer emission does not occur in the a-perylene crystal at temperatures < 30 K.15 Comparison of the luminescence spectrum in Figure 3 with that of the a-perylene crystal, e.g., at 50 K where Y- and E-emission occur with similar peak height,l5 suggests that the spectrum of the 3Pe12 Langmuir-Blodgett film at 2 K does not consist only of the superposition of a broadened Y-type and E-type spectrum but that it contains additional luminescence signals in the in-between wavelengths range. This is confirmed by the time-resolved luminescence spectra of the mixed 3Pe12 Langmuir-Blodgett multilayer system shown in Figure 4. The spectrum in the early time window, 0-300 ps, is similar in its shape, though much more broadened, and similar in its peak position to the Y-spectrum of the stable perylene dimer shown below. The luminescence decay measured at the peak position of 550 nm was nonexponential with a fast initial time constant in the range of 5 ns." There is a much smaller peak at 455 nm in the range of either monomers, metastable crystalline &perylene,Is or dimers with an open configuration and monomerlike spectra. The spectral range of monomer-like luminescence from the 3Pe12 Langmuir-Blodgett system was not investigated in more detail, becauseof considerableoverlap with the absorption spectrum complicating the analysis. The peak around 575 nm in the time window 70-1 50 ns (Figure 4) cannot be explained by the superposition of a Y-type and E-type spectrum. In still later time windows the spectra became more similar to the E-type emission of the a-perylene crystal. Note that the latter disappears at temperatures < 30 K,I5 whereas the spectra of the LangmuirBlodgett system in Figure 4 were recorded a t 2 K. The dominant decay timeof luminescenceobservedat 636 nm, Le., in the spectral range of the E-type emission, was about 68 nm at 2 K." This value is of a magnitude comparable to the E-excimer lifetime in the a-perylene ~rysta1.l~ Figure 5 repeats the 2 K luminescence and excitation spectra of the 3Pe12 Langmuir-Blodgett system from Figure 3 (thick,
'
I
~
650
~
~
~
70q ,
750 ,
Xe,=295nm (33898cm")
wavenumber [ l o 3 cm-'I Figure 3. Stationary luminescencespectrum and excitation spectrum of the 3Pe12 multilayer system at 2 K. The luminescence spectrum was excited at 470 nm, and the excitation spectrum was recorded at 630 nm. Note the two peaks at 550 and 630 nm in the otherwise structureless luminescencespectrum. Note the shoulder at 470 nm and the monomerlike peaks in the excitation spectrum.
600
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w a venu mber I: 1O3 cm-ll Figure 4. Luminescence spectra of the 3Pe12 Langmuir-Blodgett film in two different time windows at 2 K. The spectrumin the early (300-p) time window is Y-type and distinctly different from the spectrum in the later (70-150-ns) time window. The latter is neither Y- nor E-type. The excitation wavelength was 295 nm, and the half-width of the frequencydoubled laser pulse was about 8 ps.
wavelength Cnml 350
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wavenumber [ l o 3 c m - l l Figure 5. Excitation and luminescencespectra of the 3Pe12 multilayer system at 2 K as in Figure 3 (thick solid curves). Unpolarized absorption spectrumat 7 KI9 and Y-type and E-type luminescenceof the a-perylene crystal at 2 and 76 K, respectively (thin solid curves). Excitation and
luminescence spectra of the stable perylene dimer (dash-dot curve). Excitation spectrum of a metastable perylene dimer (solid curves). All emission spectra are for excitation at 470 nm. Excitation spectra of the dimers were recorded for luminescence at 550 nm (solid and dash-dot curve). Excitation spectrum of the 3Pe12 Langmuir-Blodgett system was recorded for emission at 630 nm (thick curve). solid curves). In addition, the unpolarized absorption spectrum and the Y- and E-type luminescence spectra of the dimeric a-perylene crystal are shown as thin solid curves. The unpolarized absorption spectrum of the a-perylene crystal was constructed from Matsui's data.19 Y-emission of the a-perylene crystal is clearly blue shifted by about 18 nm against the main peak in the stationary luminescence spectrum of the 3Pel2 LangmuirBlodgett system. On the red side, the luminescence spectrum of the 3Pel2 Langmuir-Blodgett system at 2 K is virtually identical with the E-emission from the a-perylene crystal. Note though that this signal disappears in the a-perylene crystal for temperatures < 30 K.15 Furthermore, Figure 5 shows the luminescence and excitation spectrum of the stable ground-state perylene dimer (dash-dot curve), formed from the bichromophoric perylene compound as described above. In this case, the spectral positions of the main peaks in the luminescence spectra are virtually identical for the 3Pe12 Langmuir-Blodgett system and for the stable dimer. The
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1892 The Journal of Physical Chemistry, Vol. 98, No. 7, I994
Figure 6. Configuration of the parallel perylene dimer in the a-perylene crystal with the centers shifted by 0.9 A in the direction of the long molecular axis and by 1.3 angstrom in the directionof the short molecular axis (after ref 16). main peak of the excitation spectrum of the stable dimer (dashdot curve) coincides with the shoulder around 470 nm in the excitation of the 3Pel2 Langmuir-Blodgett structure (thick solid curve). Applying the extremely slow coolingprocedure described above to a low concentration (10-5 to 10-6 M) of the bichromophoric perylene compound resulted always in the dash-dot curves shown in Figure 5 . Therefore, we ascribe these spectra to the stable dimer configuration. In contrast, with a faster approach to the glass transition temperature, i.e., with a faster cooling rate, we could arrest several different dimer species with monomer-like fluorescence and excitation spectra. The latter monomer-like spectra of the bichromophoric compound can be ascribed to metastable open dimer configurations. The excitation spectrum of the monomer-like fluorescence for one of these metastable open dimer configurations is shown in Figure 5 as the solid curve with the main peak at 450 nm. The same position of the main peak and also the monomer-like vibrational structure can be recognized in the excitation spectrum of the 3Pe12 Langmuir-Blodgett system (thick solid curve). The peak position is shifted by about 200 cm-' to the red with respect to that of the actual monomer in the same solvent (not shown).
Discussion The picture of the parallel dimer of perylene given in Figure 6 is helpful in the following discussion. In the ground-state configuration of the dimer in the a-perylene crystal, one of the molecular centers is shifted with respect to the other by about 0.9 A along the long molecular axis and by about 1.3 angstrom along the short molecular axis.10 It is generally assumed that the E-emitting excited state corresponds to a configuration where the distance between the two molecular centers and perhaps also the interplanar distance are shortened.16 Figure 5 shows that the absorption spectrum of the a-perylene crystal starts in the same spectral range as the excitation spectrum for the luminescence of the 3Pel2 Langmuir-Blodgett system, but these two spectra do not show any obvious resemblance. In accordancewith this result, the 3Pel2 Langmuir-Blodgett system showed only a negligible number of spotswith crystal-like behavior in the polarization microscope. The overwhelming part of the material showed no indication of crystallineorder. The absorption spectrum of the 3Pe12 Langmuir-Blodgett system also did not show any crystalline Davydov splitting effect,' in contrast to the behavior of the spatially better matched 2A7 Langmuir-Blodgett system of the anthracene compound.' The spectra of the
Mahrt et al.
tl monomer dimer (metostable dimer)
tl E-excimer
Figure 7. Diagram illustrating light absorption, luminescence, energy transfer, electronic relaxation, and excimeric relaxation in the 3Pe12 Langmuir-Blodgett system of the perylenechromophores. Distributions of 0,O-energiesare illustrated by Gaussians. Details are explained in the text.
metastable @-perylenecrystall*Jgneed not be discussed here since they are toodifferent from thoseof the 3Pe12 Langmuir-Blodgett system. The main peak of the Y-emission of the a-perylene crystal is blue-shifted compared to the main peak at 550 nm of the 3Pe12 Langmuir-Blodgett system. This suggests a similar but slightly different configuration for the perylene dimer in the 3Pe12 Langmuir-Blodgett system than in the a-perylene crystal. It appears reasonable to conclude from Figure 5 that the stable ground-state dimer configuration of perylene, as formed in a glass, is also the dimer configuration from where most of the luminescenceisemitted in the 3Pe12 Langmuir-Blodgett system. In a parallel dimer configuration (Figure 6) this luminescence corresponds to the forbidden transition from the lower excited g-state to theg-groundstate (Y-statein Figure7), whichbecomes vibrationally allowed.20 Accordingly, the 0,O-transition to the lower excited g-state is missing in the luminescenceand absorption spectra of the stable dimer. The shoulder around 470 nm in the excitation spectrum of the 3Pe12 perylene Langmuir-Blodgett system (thickcurve) is at thesame position as the main absorption peakof the stable dimer (dash-dot curve). The latter is assigned to the allowed transition from the stable parallel ground-state dimer to its upper u-state (Y-state in Figure 7). Accordingly, the difference of about 1100 cm-l between this peak at 470 nm and the onset of the luminescence is ascribed to the splitting between the excited u- and g-states of the stable dimer. A sizable fraction of dimers with configurations similar to the stable dimer can lead in the Langmuir-Blodgett system to the shoulder in the absorption and excitation spectra at 470 nm. The excitation spectrum of the 3Pel2 Langmuir-Blodgett system shows a monomer-like vibrational structure (solid curve) with peak positions identical to those of the open metastable dimer (Figure 5). Indeed, one can simulate a spectrum similar to that of the 3Pel2 Langmuir-Blodgett system (thick solid curve) via superposition of a distribution of various metastable dimers and of stable perylene dimers with appropriate weight factors. This is not a direct proof but gives a strong suggestion that the excitation spectrum of the 3Pe12 Langmuir-Blodgett system (thick solid curve) represents a mixture of different dimer configurations. It
Luminescence of Perylene Dimers in a LB Film appears that these metastable ground-state dimer configurations
of the perylene chromophores are arrested during preparation of the Langmuir-Blodgett film, just as they can be arrested in the glass. We showed already in the preceding paper" that the excited dimer configuration with E-luminescence16 can be reached in the 3Pe12 Langmuir-Blodgett film a t 2 K, Le., without activation energy. Excitation of the luminescence in the 3Pe12 LangmuirBlodgett system a t 470 nm, Le., the transition from the ground g-state to the excited u-state of the stable parallel dimer configuration, enhances strongly the E-luminescence peak a t 630 nm (Figure 5 of ref 11). The long luminescence lifetime of 68 ns measured a t 630 nm for the 3Pe12 Langmuir-Blodgett system supports this assignment as E-excimer emission. E-emission is much weaker for excitation at 450 nm, i.e., of the transitions to the excited state of open metastable dimer configurations. Efficient excitation of E-luminescence at 470 nm indicates either that the ground-state configuration of the E-excimer-forming dimer is very similar to that of the stable parallel dimer or that the ground-state configurationsare neighborsconnected by energy transfer (Figure 7). It is remarkable that E-excimer formation occurs in the 3Pe12 Langmuir-Blodgett system a t 2 K. The E-excimerI6appearsin the a-perylenecrystal only at temperatures > 30 K.15 The wavelengths of the two peaks in the luminescencespectrum at 2 K (Figure 5, thick, solid curve) suggest that, first, the stable perylene dimer configuration and, second, configurations with better molecular overlap that convert without activation energy into the E-excimer are the two dominant luminescence centers in the 3Pe12 Langmuir-Blodgett system. In contrast, "open" perylene dimer configurations control the vibrational structure in the absorption and excitation spectrum of this system (Figure 5). The experimental observations reported in this and our previous1I paper are explained here employing standard models for molecular dimers2@22and with the further assumption that the E-excimer is formed in the excited g-state of certain parallel dimer configurations (Figure 7). Accordingly, we do not consider the assignments given in Figure 6 of our previous paper" any more as valid. At 2 K different dimer configurations dominate the excitation spectrum (Figure 5), Le., open monomer-like dimers, compared to the luminescence spectrum, which is controlledby stable parallel ground-state dimers and related configurations including E-excimer-forming dimers. This implies rapid energy transfer at 2 K from the excited states of the open dimer configurations to thoseof the parallel dimer configurations (Figure7). This appears possible since the lower excited states of the open monomer-like dimer configurationsare at higher energies than the about 20 000 cm-1 for the excited g-states of the stable parallel dimers (Figure 7). An attempt at measuring fluorescence depolarization with about 10-ps resolution in the 3Pel2 Langmuir-Blodgett system has not been successful. Measurements with better time resolution are required to follow the rapid energy trapping process in the 3Pel2 Langmuir-Blodgett film. Thedecay ofthe Y-type emission at 550 nm was found to be nonexponential with a leading time constant of 5 ns.11 This has to be investigated also in more detail, since the much longer lifetime in the range of 40 ns for the Y-emission of the a-perylene crystalls and of the stable dimer leaves the possibility that the Y-decay in the 3Pe12 system could be controlled by fairly slow energy transfer to neighboring dimer configurations with lower excited g-states. In agreement with the assumption of longer-lived surviving Y-states, the decay becomes much slower a t later times in Figure 3 of ref 11. A three-exponential fit gives some idea of the time scale for the energy-transfer process. In the disordered system, the influence of energy transfer on the decay is diminished in later time window^.^ The range of decay constants for Y-emission in the
The Journal of Physical Chemistry, Vol. 98, No. 7, 1994 1893 3Pe12 Langmuir-Blodgett system is indicated in Figure 7 by the shortest and longest fit parameter, Le., by 2.5 and 18 ns, respectively. The center-to-center distance of the two molecules in the a-perylene dimer (Figure 6) is about 1.6 A, parallel to the molecular planes. The correspondingspectral range and structure of the Y-emission from the a-perylene crystal (Figure 5, thin solid curve) and from the stable perylene dimer (Figure 5 , dashdot curve) suggest that the ground-state dimer configurations should be very similar in these two systems. Maximum overlap in the dimer configuration would involve a shift of the two molecular centers by about 1.6 A to reach the position on top of each other (Figure 6). This perfect sandwich configuration has been postulated by some authors, together with a sub-angstrom change in the distance between the molecular planes, as the configurationof the E-excimer.16 Thus, the observed great width of the luminescence spectrum, covering the range from yellowgreen to deep red (Figure 3), involves parallel ground-state dimer configurations in the 3Pe12 Langmuir-Blodgett film that differ by shifts on a sub-angstrom length scale. This represents already strong disorder with respect to the luminescence behavior of the 3Pel2 Langmuir-Blodgett film. The 'open" dimer configurations that control the absorption spectrum represent much stronger disorder in the 3Pel2 Langmuir-Blodgett film than thevariations in the configurations of the close to parallel dimer configurations that function as traps and luminescence centers. The packing of the perylene chromophores with covalently attached fatty acid tails in the 3Pel2 Langmuir-Blodgett film retains the preference for dimer formation known from the a-perylene single crystal but is otherwisestrongly disordered. Themixed 3Pel2 LangmuirBlodgett film with a high concentration of perylene chromophores represents the other extreme to the single crystal of the correspondingly modified anthracene chromophore 2A48 and even to the pure Langmuir-Blodgett film of 2A7.' The much stronger disorder in the case of the perylene chromophore is attributed to the much stronger mismatch between the space-filling requirements of the bulky perylene chromophore and its fatty acid tail than in the case of the anthracene chromophore with the tail attached at the 2-position. Stable parallel dimers and excimerforming dimer configurations form traps for the majority of the open metastable dimer configurations. The configurational disorder in the dimer configurations of the 3Pe12 LangmuirBlodgett film should achieve rapid localization of the excitation energy at low enough temperatures. Coupling of a reversible or irreversible reaction in an adjacent monomolecular layer that is driven by the excitation energy of the dimers in the 3Pe12 film could open the possibility of writing an optical near field images into such a molecular bilayer system.
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