7928
`
`J. Phys. Chem. B 2006, 110, 7928-7937
`
`Photophysical Properties of Dioxolane-Substituted Pentacene Derivatives Dispersed in
`Tris(quinolin-8-olato)aluminum(III)
`
`Mason A. Wolak,† Joseph S. Melinger,† Paul A. Lane,† Leonidas C. Palilis,† Chad A. Landis,‡
`Jared Delcamp,‡ John E. Anthony,‡ and Zakya H. Kafafi*,†
`U. S. NaVal Research Laboratory, Washington, DC 20375, and Department of Chemistry, UniVersity of
`Kentucky, Lexington, Kentucky 40506
`ReceiVed: March 3, 2005; In Final Form: July 14, 2005
`
`Two novel dioxolane-substituted pentacene derivatives, namely, 6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-
`tetraoxa-dicyclopenta[b,m]pentacene (TP-5) and 2,2,10,10-tetraethyl-6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-
`tetraoxa-dicyclopenta[b,m]pentacene (EtTP-5), have been synthesized and spectroscopically characterized.
`Here, we examine the steady-state and time-resolved photoluminescence (PL) of solid-state composite films
`containing these pentacene derivatives dispersed in tris(quinolin-8-olato)aluminum(III) (Alq3). The films show
`narrow red emission and high absolute photoluminescence quantum yields ((cid:30)PL ) 59% and 76% for films
`containing (cid:24)0.25 mol % TP-5 and EtTP-5, respectively). The Fo¨rster transfer radius for both guest-host
`systems is estimated to be (cid:24)33 Å. The TP-5/Alq3 thin films show a marked decrease in (cid:30)PL with increasing
`guest molecule concentrations, accompanied by dramatic changes in the PL spectra, suggesting that
`intermolecular interactions between pentacene molecules result in the formation of weakly radiative aggregates.
`In contrast, a lesser degree of fluorescence quenching is observed for EtTP-5/Alq3 films. The measured
`fluorescence lifetimes of TP-5 and EtTP-5 are similar ((cid:24)18 ns) at low concentrations but deviate at higher
`concentrations as aggregation begins to play a role in the TP-5/Alq3 films. The onset of aggregation in EtTP-
`5/Alq3 films occurs at higher guest molecule concentrations (>1.00 mol %). The addition of ethyl groups on
`the terminal dioxolane rings leads to an increase in the intermolecular spacing in the solid, thereby reducing
`the tendency for (cid:240)-(cid:240) molecular stacking and aggregation.
`
`Introduction
`
`Pentacene derivatives comprise a particularly versatile class
`of materials that have shown great promise when utilized as
`the electroactive component of thin film transistors (TFTs),1-3
`photovoltaic cells (PVs),4-6 and organic light-emitting diodes
`(OLEDs).7,8 The electronic and optical properties of pentacene
`can be easily tuned by altering the substitution pattern on the
`acene backbone. For example, the introduction of phenyl groups
`at the 6 and 13 positions yields a bathochromic shift of the
`fluorescence maximum and a saturated red emission. 6,13-
`Diphenylpentacene was used as the red-emitting center in an
`OLED structure,7 and electroluminescence quantum efficiencies
`close to the theoretical limit were achieved.9 When triisopro-
`pylsilylethynyl groups are substituted at the 6 and 13 positions,
`improved solid-state packing is observed,10 and thin film
`transistors based on this material exhibited field effect hole
`mobilities on the order of 0.4 cm2/(V s).3 Pentacene can also
`be converted to an n-type semiconductor by replacing the
`hydrogen atoms with fluorine atoms; n-channel TFTs based on
`perfluoropentacene displayed electron mobilities of approxi-
`mately 0.1 cm2/(V s).11 Indeed, subtle modification of the
`molecular structure of pentacene can lead to large changes in
`the bulk electronic and optical properties of the molecular solid.
`Recently, we reported on several diaryl-substituted pentacene
`derivatives and their performance as red-emitters when dispersed
`
`* Author to whom correspondence should be addressed. Phone: (202)
`767-9529. Fax: (202) 404-8114. E-mail: Kafafi@nrl.navy.mil.
`† U. S. Naval Research Laboratory.
`‡ University of Kentucky.
`
`in the common electron-transporting host tris(quinolin-8-olato)-
`aluminum(III) (Alq3).8 Organic light-emitting diodes based on
`this guest-host system feature reasonably high external elec-
`troluminescence (EL) quantum efficiencies (ŁEL (cid:25) 1.5%), low
`turn-on voltages, and excellent color purity. Guest-host emitting
`layers are often used in OLEDs to hinder crystallization of the
`emitting material, thereby ensuring that the electroluminescence
`arises from strong molecular emission rather than weak ag-
`gregate emission. In addition, guest-host layers have been used
`to balance charge injection and transport, optimize device EL
`quantum efficiency, and improve operational longevity.12-15
`Although numerous doped polymer systems have been thor-
`oughly investigated for their energy transfer characteristics,16-20
`less work has been done to understand the complex nature of
`the energy transfer process in doped small molecule thin films.21
`We have recently examined the steady-state and time-resolved
`fluorescence of thin films featuring increasing concentrations
`of 6,13-bis(2,6-dimethylphenyl)pentacene dispersed in Alq3.22
`Using these techniques, we have demonstrated efficient Fo¨rster
`resonance energy transfer from the host Alq3 molecules to the
`guest diarylpentacene molecules with energy transfer rates in
`the range of 0.5-3.0 ns-1.22 There was evidence that the
`estimated Fo¨rster radius23 may be extended possibly due to
`exciton migration within the Alq3 host, consistent with the large
`exciton diffusion length of Alq3 ((cid:24)100 ( 40 Å).24 We further
`noted that strong fluorescence quenching and fast decay
`dynamics at high guest molecule concentrations (g1.5 mol %)
`are the result of the formation of nonemissive aggregates,
`leading to additional nonradiative decay pathways. Developing
`new pentacene
`derivatives with modified
`electronic
`
`10.1021/jp0511045 CCC: $30.25 © 2006 American Chemical Society
`Published on Web 03/31/2006
`
`Idemitsu Ex. 2004 (pg. 1)
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`

`
`Dioxolane-Substituted Pentacene Derivatives
`
`J. Phys. Chem. B, Vol. 110, No. 15, 2006 7929
`
`CHART 1: Chemical Structures of Guest (TP-5 and
`EtTP-5) and Host Alq3 Moleculesa
`
`molecules to the pentacene guest molecules as a function of
`guest concentration.
`
`Experimental Methods
`
`Materials. Alq3 was obtained from H. W. Sands. TP-5 was
`synthesized as previously reported,25 and EtTP-5 was prepared
`in an analogous manner beginning with condensation of 2,2-
`diethyl-1,3-benzodioxolane-6,7-dicarboxaldehyde with 1,4-cy-
`clohexanedione.26 Each compound was purified via duplicate
`train sublimation (270-290 (cid:176)C, 5.0 (cid:2) 10-6 Torr) prior to use.8
`Crystals of TP-5 were grown from toluene solution at -20 (cid:176)C.
`Crystals of EtTP-5 were grown by the slow evaporation of a
`1,2-dichloroethane solution at room temperature.
`Preparation of Thin Films. Fused silica substrates were
`cleaned with organic solvents and subjected to UV-O3 treat-
`ment prior to use (30 min, 4:1 N2/O2, flow rate ) 1.5 SCFH).
`Neat and composite films ((cid:24)750-nm-thick) were prepared by
`vacuum sublimation (3.0 (cid:2) 10-7 Torr) from resistive heating
`furnaces. Quartz crystal microbalances were used to monitor
`the rate of deposition and determine the film composition and
`thickness. The guest and host materials were codeposited at a
`combined rate of 2-4 Å/s to yield films with varying guest
`molecule concentrations (0.2-2.5 mol %).
`Photoluminescence Measurements. The PL spectra were
`measured using a Cary Eclipse fluorimeter ((cid:236)ex ) 350 nm). The
`PL quantum yields of dilute toluene solutions of TP-5 and
`EtTP-5 were measured relative to a rubrene standard ((cid:30)PL (cid:25)
`100% in benzene) using our previously reported method.8
`Absorption spectra were measured using a Hewlett-Packard
`8423 spectrophotometer and used to confirm guest molecule
`concentrations in the composite films. All spectra were collected
`under ambient conditions. Absolute PL quantum yields of the
`solid films were measured using an integrating sphere and a
`HeCd laser ((cid:236)ex ) 325 nm) in dry N2.21 The UV laser line was
`filtered at the silicon photodiode with a Kodak Wratton 2B filter
`((cid:236) < 385 nm cutoff).
`Time-resolved photoluminescence spectra were measured
`under ambient conditions using the time-correlated single-
`photon-counting technique.27 Samples were irradiated at (cid:236) )
`300 nm from a frequency-doubled, synchronously pumped
`rhodamine dye laser at an energy fluence of 10-20 nJ/cm2 per
`pulse and a repetition rate of 1 MHz. The pump intensity per
`pulse ((cid:24)1015 photons/cm3) was well below the number density
`of the host (1021 molecules/cm3) and guest (1018-1019 molecules/
`cm3). The resulting emission was dispersed in a monochromator
`and detected using a cooled microchannel plate Hamamatsu
`photomultiplier tube. The monochromator was set at or near
`the peak fluorescence wavelength, and the transient signals were
`collected at the magic angle of 54.7(cid:176). An instrument response
`function (full width at half-maximum) of (cid:24)50 ps was measured
`by collecting scattered laser light off a ground glass surface.
`Transient fluorescence decays were collected in 75 or 100 ns
`time windows. In a typical experiment, approximately 104
`photons were collected at the peak channel of the fluorescence
`transient.
`Fluorescence decay data were deconvoluted with the instru-
`ment response function then normalized by setting the maximum
`photon count equal to 1. The fluorescence decays of guest
`molecules were fit to a sum of exponential decay functions:
`I(t) ) “i(Ai exp(-t/(cid:244)i)) where Ai is the preexponential factor
`and (cid:244)i is the time constant. Weighting factors were determined
`for each decay component relative to the total decay as wi )
`Ai(cid:244)i/“i(Ai(cid:244)i). The average time constant was calculated from the
`individual decay components as Æ (cid:244)æ ) “i(Ai(cid:244)i)/“iAi. The
`
`a The nondioxolane-substituted TIPS pentacene derivative is included
`for purposes of comparison.
`
`and chemical structures leading to improved photoluminescence
`(PL) quantum yields (while suppressing strong intermolecular
`interactions in the solid state) remains a challenge.
`To this end, we have recently designed and synthesized two
`new highly fluorescent pentacene derivatives, namely, 6,14-bis-
`(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]-
`pentacene (TP-5) and 2,2,10,10-tetraethyl-6,14-bis-(triisopro-
`pylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]pentacene
`(EtTP-5) (Chart 1). Both compounds feature 1,3-dioxolane
`moieties fused to the terminal benzenoid rings of the pentacene
`core, in addition to triisopropylsilylethynyl substituents at the
`6,13-position. Chart 1 also shows the chemical structure of 6,13-
`bis(triisopropylsilylethynyl)pentacene (TIPS). The dioxolane
`units on TP-5 were initially added to lower the oxidation
`potential of the materials, thereby improving charge injection
`from gold electrodes in TFTs.25 Spectroscopic characterization
`of TP-5 showed strong red emission ((cid:30)PL ) 60% in toluene)
`relative to that previously reported for diaryl-substituted pen-
`tacenes ((cid:30)PL (cid:25) 15% in toluene),8 suggesting that TP-5 is a good
`candidate as a red-emitting center in OLEDs. TP-5 has also
`been shown to form well-ordered crystals that exhibit significant
`(cid:240)-overlap, as shown by the tight stacking between (cid:240)-faces that
`are separated by only 3.35 Å.25 Newly synthesized EtTP-5
`features two ethyl groups on the sp3-hybridized carbon of the
`dioxolane rings; this modification was undertaken to disrupt the
`solid-state packing and hinder interaction between neighboring
`aromatic rings.
`Herein, we report on the solid-state photophysical properties
`of these two new red-emitting dioxolane-substituted pentacene
`derivatives. We have measured the steady-state PL spectra and
`the absolute PL quantum yields of guest-host films containing
`various concentrations of TP-5 or EtTP-5 in Alq3 ((cid:24)0.25-2.25
`mol %). In addition, we have investigated the dynamics of these
`films using time-resolved photoluminescence to determine the
`rate and efficiency of energy transfer from the Alq3 host
`
`Idemitsu Ex. 2004 (pg. 2)
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`7930 J. Phys. Chem. B, Vol. 110, No. 15, 2006
`
`Wolak et al.
`
`Figure 1. Normalized emission of Alq3 host (bold line) and extinction
`spectra of pentacene guests (TP-5, solid line; EtTP-5, dotted line).
`
`goodness of the fits was assessed using visual inspection and
`the ł2 parameter. Satisfactory fits were obtained when ł2 was
`1.3 or less. Time constants were determined as an average of 3
`or 4 measurements per sample. The standard deviation of the
`average was approximately (4% for the low concentration films
`(e1.00 mol %) and (9% for the high-concentration films
`(>1.00 mol %). Fluorescence decay of the Alq3 host is analyzed
`in terms of Fo¨rster theory with a random distribution of dopants.
`This analysis is described in the Discussion section.
`X-ray Crystallography. The crystal structure of TP-5 was
`determined as previously reported.25 The crystal structure of
`EtTP-5 was determined using a Bru¨ker-Nonius X8 Proteum
`diffractometer at 90 K, solved by direct methods using
`SHELXS-97, and refined using SHELXL-97.
`
`Experimental Results
`
`Absorption and Photoluminescence of Pentacene Solu-
`tions. Both TP-5 and EtTP-5 were designed for use in guest-
`host composite films with Alq3 serving as the host matrix. Alq3
`was chosen as the host due to the large overlap between its
`emission spectrum and the absorption of the pentacene guests,
`a necessary prerequisite to achieve efficient Fo¨rster resonance
`energy transfer from the host to the guest molecules. Figure 1
`shows both the emission spectrum of a neat Alq3 film and the
`extinction spectra of TP-5 and EtTP-5 in dilute toluene solutions.
`The broad, featureless emission from Alq3 is centered at (cid:24)535
`nm and extends beyond 700 nm, making good overlap with the
`(cid:240)-(cid:240)* absorption bands of the pentacene derivatives. The two
`pentacene compounds display very similar absorption spectra
`in the visible region ((cid:236)max (cid:25) 620 nm); the only appreciable
`difference occurs at the additional absorption peak centered at
`(cid:24)460 nm, where EtTP-5 has a somewhat stronger absorption
`than TP-5. This transition is barely noticeable in the absorption
`spectrum of TIPS28 and is thought to arise from the introduction
`of the terminal dioxolane rings of TP-5 and EtTP-5.
`The fluorescence spectra of dilute toluene solutions (1 (cid:237)M)
`of TP-5 and EtTP-5 are shown in Figure 2. The spectral features
`of the two molecules are quite similar with (cid:236)max (cid:25) 625-630
`nm, yielding very small Stokes shifts of (cid:24)5-6 nm. The emission
`maximum of EtTP-5 is slightly red-shifted relative to that of
`TP-5 as a result of the minor electron-donating nature of the
`ethyl substituents on the terminal dioxolane rings. A photolu-
`minescence quantum yield of 72% was measured for EtTP-5
`in toluene, a nearly 5-fold enhancement over previously studied
`diarylpentacenes8,22 and a moderate improvement over TP-5 ((cid:30)PL
`) 60%).
`
`Figure 2. Normalized fluorescence spectra of dilute toluene solutions
`of TP-5 (solid line) and EtTP-5 (dashed line).
`
`Photoluminescence of Composite Films. The PL spectra of
`TP-5 and EtTP-5 dispersed in Alq3 as a function of the guest
`molecule concentration are illustrated in Figure 3. Red emission
`dominates the spectra of the composite films, indicating efficient
`energy transfer from the host Alq3 molecules to the guest
`pentacene molecules. The strongest emission peak is centered
`at (cid:236)max ) 651 nm for TP-5/Alq3 composite films and slightly
`red-shifted relative to those featuring the alkylated analogue
`EtTP-5 ((cid:236)max ) 647 nm). Note that the photoluminescence from
`the composite films is red-shifted relative to that of the solutions
`by approximately 15-20 nm. Low concentration films (<1.00
`mol %) of the two compounds exhibit a vibronic side band with
`a maxima at (cid:236)max = 710 nm. The PL spectra for both sets of
`films also show emission from the Alq3 host molecules centered
`at (cid:24)530 nm, which continually decreases as the guest molecule
`concentration is increased. A distinctive feature of the PL spectra
`of the TP-5/Alq3 films is the marked increase of the relative
`magnitude of the low-energy side band with increasing guest
`molecule concentration (Figure 3a). At concentrations of 1.45
`and 2.25 mol %, this long-wavelength emission peak becomes
`increasingly prominent and nearly dominates the PL spectra.
`In contrast, the relative increase in low-energy emission in the
`EtTP-5/Alq3 films is barely noticeable as the guest molecule
`concentration is increased (Figure 3b).
`The PL quantum yields of the composite films measured as
`a function of guest molecule concentration are displayed in
`Figure 4. Both dioxolane-substituted pentacenes are considerably
`more efficient emitters when dispersed in Alq3 than the
`nonsubstituted analogue (a maximum (cid:30)PL ) 34% ( 3% was
`reported for a 0.26 mol % TIPS/Alq3 film).28 Maximum PL
`quantum yields for TP-5 and EtTP-5 in Alq3 were measured at
`a guest molecule concentration of (cid:24)0.25 mol % ((cid:30)PL ) 59% (
`6% and 76% ( 8%, respectively). Significant quenching of the
`(cid:30)PL is observed upon increasing the guest molecule concentra-
`tion, particularly in the TIPS/Alq3 and TP-5/Alq3 films. The
`quantum yield of the composite film containing 0.93 mol %
`TP-5 falls to approximately 30% ( 3%, then plummets to
`(cid:24)5.0% ( 0.5% for films with concentrations of 1.45 mol % or
`higher. Concentration quenching of the TIPS/Alq3 films is even
`more severe, with the PL quantum yield falling to one-fifth of
`its original value at a TIPS concentration of 1.0 mol % ((cid:30)PL )
`7.0% ( 0.7%). In the TP-5/Alq3 films, concentration quenching
`is accompanied by concurrent growth of the long-wavelength
`peak at (cid:24)725 nm in the photoluminescence spectra (Figure 3a).
`In comparison to films containing TP-5, the EtTP-5/Alq3 films
`display a more gradual dropoff in PL quantum yield with
`increasing guest molecule concentration. The largest discrepancy
`
`Idemitsu Ex. 2004 (pg. 3)
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`Dioxolane-Substituted Pentacene Derivatives
`
`J. Phys. Chem. B, Vol. 110, No. 15, 2006 7931
`
`Figure 4. Absolute PL quantum yields of EtTP-5/Alq3 films (triangles),
`TP-5/Alq3 films (circles), and TIPS/Alq3 films (squares) as a function
`of the guest molecule concentration. The experimental error is estimated
`to be (10% of the measured values.
`
`and efficiencies of our guest-host system can be obtained by
`analysis of the PL decay dynamics of this emission. The
`temporal profiles of Alq3 emission (probed at 530 nm) of films
`doped with TP-5 and EtTP-5 are illustrated in Figure 5. The
`figures also show the single-exponential time-resolved PL decay
`of a pristine Alq3 film for comparative purposes. The temporal
`profiles quickly change upon the introduction of a small amount
`of pentacene derivative to the films; the Alq3 decay becomes
`more and more rapid as the guest molecule concentration
`increases. The decay in the EtTP-5-doped films appears slightly
`faster.
`Guest (TP-5 and EtTP-5) PL Decay Dynamics. Figure 6
`depicts the transient radiative decay profiles of TP-5/Alq3 and
`EtTP-5/Alq3 composite films (probed at 650 and 645 nm,
`respectively) as a function of guest concentration. The films
`were excited at (cid:236) ) 300 nm so as to minimize direct excitation
`of the guest pentacene derivatives, which have very strong
`absorption bands at (cid:24)325 nm ((cid:15) ) 1.8-2.0 (cid:2) 105 M-1 cm-1
`in toluene). Although the extinction coefficients of TP-5 and
`EtTP-5 at 300 nm are approximately a factor of 6 lower than
`the values measured at 325 nm, a small amount of directly
`excited pentacene molecules may exist (particularly at high
`concentrations).
`The transient signal shows an initial rise followed by a short
`plateau at the maximum PL intensity, whereupon the pentacene
`excited state starts its natural decay. The initial rise is attributed
`to the
`to the energy transfer process from the Alq3 host
`pentacene guest, resulting in an increasing population of guest
`molecules in the excited state. After the initial population
`buildup, the guest molecules undergo natural radiative and
`nonradiative decay. The length of time required for buildup of
`guest molecules in the excited state (referred to as the rise time)
`decreases as the guest concentration increases. In general, the
`rise times of EtTP-5/Alq3 films are slightly shorter than the rise
`times of TP-5/Alq3 films at comparable concentrations. Sig-
`nificant differences are noted between the decay profiles of TP-5
`and EtTP-5 for high-concentration films. At guest molecule
`concentrations g1.00 mol %, the TP-5/Alq3 films display much
`more rapid decay than the EtTP-5/Alq3 films. Furthermore, the
`TP-5 decay begins to show multiexponential character at (cid:24)1.00
`mol % while the EtTP-5 decay retains a single-exponential
`character (after the initial rise time). Note that at low concentra-
`tions (e0.50 mol %) the TP-5 and EtTP-5 decay profiles are
`nearly identical.
`Figure 7 shows the time-resolved PL of TP-5/Alq3 films
`probed at 700 nm. This investigation was undertaken to gain
`
`Figure 3.
`(a) Normalized PL spectra of TP-5/Alq3 composite films:
`0.23 mol % (black), 0.48 mol % (red), 0.93 mol % (green), 1.45 mol
`% (blue), 2.25 mol % (orange). The PL spectrum of the TP-5 powder
`is depicted as a dotted gray line. The contribution from Alq3 ((cid:24)530
`nm) decreases and the peak centered at (cid:24)725 nm increases as the TP-5
`concentration increases. The arrows represent the trend in the spectral
`change as the guest molecule concentration is increased. (b) Normalized
`PL spectra of EtTP-5/Alq3 composite films: 0.26 mol % (black), 0.48
`mol % (red), 1.01 mol % (green), 1.51 mol % (blue), 1.97 mol %
`(orange). The PL spectrum of the EtTP-5 powder is depicted as a dotted
`gray line. The contribution from Alq3 ((cid:24)530 nm) also decreases as the
`EtTP-5 concentration increases.
`
`in concentration quenching between the dioxolane-substituted
`pentacenes is observed for films with guest molecule concentra-
`tions >1.0 mol %; a quantum yield of nearly 30% ( 3% is
`measured for the film containing (cid:24)2.0 mol % EtTP-5 while
`the corresponding TP-5/Alq3 film shows minimal emission ((cid:30)PL
`< 5.0% ( 0.5%).
`Surprisingly, vacuum-deposited neat films of TP-5 and
`EtTP-5 on fused silica substrates were nonemissive. However,
`we were able to detect emission from the purified powders of
`both pentacene derivatives (Figure 3), showing large batho-
`chromic shifts relative to their solution spectra. Emission from
`TP-5 powder is extremely weak ((cid:236)max ) 721 nm) whereas
`emission from EtTP-5 powder is readily visible ((cid:236)max ) 693
`nm).
`Host (Alq3) PL Decay Dynamics. Although the dominant
`feature of the PL spectra of the composite films is emission
`from the dioxolane-substituted pentacene guest, Alq3 host
`emission persists even at the highest guest molecule concentra-
`tions. Key information pertaining to the energy transfer rates
`
`Idemitsu Ex. 2004 (pg. 4)
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`7932 J. Phys. Chem. B, Vol. 110, No. 15, 2006
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`Wolak et al.
`
`Figure 5. (a) Time-resolved PL decays of Alq3 (probed at 530 nm) in
`neat and TP-5-doped composite films at various TP-5 concentrations
`(0.23-2.25 mol %). A continuous decrease in decay time is observed
`as the TP-5 concentration increases. The decays are normalized for
`clarity, and the solid lines represent the calculated fits as a monoex-
`ponential decay function (for neat Alq3) or using eq 8 as detailed in
`Table 1. (b) Time-resolved PL decays of Alq3 (probed at 530 nm) in
`EtTP-5-doped composite films at various EtTP-5 concentrations (0.26-
`1.97 mol %).
`
`further insight into the nature of the long-wavelength emission
`observed in high-concentration TP-5/Alq3 (not in EtTP-5/Alq3)
`films. The transient decay curves bear a great deal of similarity
`to the ones shown in Figure 6a, particularly at guest concentra-
`tions e0.93 mol %. However, there are notable differences
`between the temporal profiles for films with guest concentrations
`g1.45 mol %. The initial decline in PL intensity probed at 700
`nm appears considerably faster than the corresponding decline
`when probed at 650 nm. In addition, there is a more noticeable
`long-lived state that is observed in the decays probed at 700
`nm compared to those probed at 650 nm.
`
`Discussion
`Steady-State Fluorescence Spectra. Excitation of guest
`molecules may occur in more than one way:
`(1) direct
`excitation, (2) reabsorption of host emission (radiative transfer),
`(3) Fo¨rster resonance energy transfer (FRET), and (4) exciton
`diffusion within the host followed by energy transfer (diffusion-
`assisted FRET). The first process can be neglected as the dopant
`absorbs a negligible fraction (<1%) of the excitation light at
`
`Figure 6.
`(a) PL decays of TP-5 (probed at 650 nm) in TP-5/Alq3
`composite films as a function of increasing guest molecule concentration
`(0.23-2.25 mol %). The monotonic decrease of the rise time as the
`TP-5 concentration increases is indicative of increasing energy transfer
`efficiency. The decays are normalized for clarity, and solid lines
`represent the calculated fits as the sum of exponential decays as detailed
`in Table 2. (b) PL decays of EtTP-5 (probed at 645 nm) in EtTP-5/
`Alq3 composite films as a function of increasing guest molecule
`concentration (0.26-1.97 mol %).
`
`these low concentrations. While radiative energy transfer is a
`slow and inefficient process, nonradiative energy transfer
`(FRET) is much faster and more efficient. The traditional model
`for excitation energy transfer as described by Fo¨rster involves
`a donor-acceptor system typically suspended in a liquid solvent
`or a polymer matrix.29,30 Here, we test the suitability of this
`model to doped, solid films in which the donor is also the host.
`The donor and acceptor molecules are in intimate contact with
`one another, thereby facilitating the energy transfer process.
`Exciton diffusion within the host may also play an important
`role in the energy transfer process, especially in dilute films
`where the typical donor-acceptor spacing is large.
`Fo¨rster transfer is a long-range resonant dipole-dipole
`interaction that
`is contingent upon strong spectral over-
`lap between the emission spectrum of the host molecules
`and the absorption spectrum of the guest molecules. The crit-
`ical Fo¨rster radius, R0, is defined as the distance between donor
`and acceptor molecules at which energy transfer to the acceptor
`or decay on the donor occurs with equal probability. R0 is
`
`Idemitsu Ex. 2004 (pg. 5)
`IPR2016-00148
`Duk-San v Idemitsu Kosan
`
`

`
`Dioxolane-Substituted Pentacene Derivatives
`
`J. Phys. Chem. B, Vol. 110, No. 15, 2006 7933
`
`slightly greater in the PL spectrum of the TP-5/Alq3 films than
`in the PL spectrum of the EtTP-5/Alq3 films. This observation
`is consistent with the EtTP-5/Alq3 films showing more efficient
`energy transfer. In addition, the Alq3 contributions in Figure
`3b appear smaller than those in Figure 3a partly because the
`absolute magnitude of emission from EtTP-5 is appreciably
`greater than that from TP-5. We note that the relative weight
`of the longer-wavelength peak of the pentacene PL spectrum
`increases with increasing concentration, especially for TP-5. This
`is correlated with a decrease in the PL quantum efficiency and
`will be discussed in terms of aggregation below.
`We first seek to explain these results in terms of Fo¨rster
`transfer from excited host molecules to nearby guest molecules.
`Assuming homogeneous dispersion of guest molecules and that
`each guest molecule is located at the center of a sphere, one
`may obtain a rough estimate of the average distance, Rda,
`between the donor (host) and acceptor (guest) molecules as
`follows
`
`)x3 { 3MW
`4(cid:240)NAcF}
`
`Rda
`
`(3)
`
`where NA is Avogadro’s number, MW is the molecular weight
`of the host (459.3 g/mol), c is the % mole fraction of the guest,
`and F is the host density (1.5 g/cm3).21a Rda is (cid:24)38 Å for the
`most dilute TP-5/Alq3 film (0.23 mol %), yielding a fraction of
`transferred excitons or energy transfer efficiency ÆŁ ETæ (cid:25) 43%.
`A more realistic model must take into account a distribution of
`donor-acceptor distances generated by random doping. Alq3
`is an amorphous material and can be reasonably approximated
`by a random distribution of molecular positions. Assuming no
`aggregation, a fraction of the host molecules are then randomly
`replaced by guest molecules. For 0.23 mol % doping with these
`criteria, the average distance between a host molecule of the
`ensemble and the nearest guest molecule is 36 Å. Much more
`important than the slightly lower donor-acceptor spacing than
`predicted by eq 3 is the large distribution of nearest neighbor
`spacings; the standard deviation of Rda is 14 Å. As the transfer
`rate kET (cid:181) RDA
`-6, energy transfer will be dominated by the
`donor-acceptor pairs with the closest spacing. For 0.23 mol %
`doping, the predicted transfer efficiency Æ ŁETæ (cid:25) 55%. The actual
`measured energy transfer efficiency can be extracted from the
`contributions of Alq3 and TP-5 to the fluorescence spectra shown
`in Figure 3a. At 0.23 mol % doping, the ratio of TP-5 to Alq3
`emission is 2.4:1. Taking into account the fluorescence quantum
`yields of TP-5 (60%) and Alq3 (26%), Æ ŁETæ (cid:25) 51% is in general
`agreement with Fo¨rster transfer theory. Similar agreement
`between theory and observation is found for dilute EtTP-5/Alq3
`films (predicted transfer efficiency ÆŁ ETæ (cid:25) 60%, observed
`transfer efficiency ÆŁ ETæ (cid:25) 62% at 0.26 mol % doping).
`One interesting question is whether or not exciton diffusion
`on the Alq3 host contributes to energy transfer in these films.
`Time-resolved PL studies of pristine Alq3 films have yielded
`singlet exciton diffusion rates DS = 1.2 (cid:2) 10-5cm2/s with
`exciton diffusion lengths ld ) 100 ( 40 Å.24,34-36 Therefore, it
`is probable that exciton migration on Alq3 lengthens the effective
`Fo¨rster transfer radius, as the exciton diffusion length far exceeds
`the average donor-acceptor distance (Rda). Given that the
`donor-acceptor spacing in the most dilute films is comparable
`to the calculated Fo¨rster transfer radius, evidence for diffusion-
`assisted FRET may not be clear from the steady-state PL spectra.
`Once the typical donor-acceptor spacing is comparable to the
`Fo¨rster radius, energy transfer and subsequent guest emission
`competes favorably with Alq3 host emission. We examine the
`
`Figure 7. PL decays of TP-5 (probed at 700 nm) in TP-5/Alq3
`composite films as a function of increasing guest molecule concentration
`(0.23-2.25 mol %). The decays are normalized for clarity, and solid
`lines represent the calculated fits as the sum of exponential decays as
`detailed in Table 3.
`
`expressed as follows29,30
`
`R0
`
`2
`
`6 ) 9000 (ln 10)(cid:20)
`(cid:30)PL
`T
`128(cid:240)5n4NA
`
`(1)
`
`where (cid:30)PL is the PL quantum yield of the host, n is the refractive
`index of the film, NA is Avogadro’s number, (cid:20)2 is an orientation
`factor (2/3 for randomly oriented dipoles), and T is the overlap
`integral between the emission spectrum of the host and the
`extinction spectrum of the guest. The overlap integral is given
`by29,30
`
`Fm((cid:238))(cid:15)Q((cid:238))
`
`d(cid:238)
`(cid:238)4
`
`(2)
`
`T )s
`
`0¥
`
`where Fm((cid:238)) is the normalized fluorescence spectrum of the host
`and (cid:15)Q((cid:238)) is the molar decadic extinction coefficient of the guest,
`both of which are expressed as a function of wavenumber ((cid:238)).
`The overlap between the emission spectrum of Alq3 and the
`extinction spectra of the two dioxolane-substituted pentacenes
`is shown in Figure 1. From the spectra displayed in Figure 1,
`T ) 6.69 (cid:2) 10-14 cm3 M-1 for TP-5 and T ) 7.06 (cid:2) 10-14
`cm3 M-1 for EtTP-5. The similar absorption spectra of the two
`compounds result in essentially identical Fo¨rster radii. Assuming
`(cid:30)PL ) 26% and n ) 1.7 for Alq3, the Fo¨rster transfer radius is
`approximately 33 Å for both TP-5 and EtTP-5 and is comparable
`to that estimated for 6,13-bis(2,6-dimethylphenyl)pentacene
`doped in Alq3 ((cid:24)31 Å).22,23,31 We note that more elaborate
`analyses of resonance energy transfer in molecular assemblies
`through modification of the Fo¨rster theory have been recently
`reported.32,33
`The fluorescence spectra of the TP-5/Alq3 and EtTP-5/Alq3
`films (Figure 3) show considerable evidence of energy transfer
`from the Alq3 host to the pentacene guest molecules. Even at
`concentrations as low as (cid:24)0.25 mol %, the fluorescence spectra
`are dominated by TP-5 or EtTP-5. While emission from the
`guest molecules is the primary feature of these