9522
J. Phys. Chem. C 2007, 111, 9522-9529
Simultaneous External and Internal Heavy-Atom Effects in Binary Adducts of 1-Halonaphthalenes with Trinuclear Perfluoro-ortho-phenylene Mercury(II): A Structural and Photophysical Study Oussama Elbjeirami,† Charlotte N. Burress,‡ Franc¸ ois P. Gabbaı1,*,‡ and Mohammad A. Omary*,† Department of Chemistry, UniVersity of North Texas, P.O. Box 305070 Denton, Texas 76203-5070, and Department of Chemistry, Texas A&M UniVersity, 3255 TAMU, College Station, Texas 77843-3255 ReceiVed: January 6, 2007; In Final Form: March 16, 2007
The 1-halonaphthalenes series has often been used to demonstrate the internal heavy-atom effect provided by the halide. In a continuation of our work on the phosphorescence of arenes induced by π-complexation to trimeric perfluoro-ortho-phenylene mercury (1), we now present a structural and photophysical study of the halonaphthalene adducts [1‚1-chloronaphthalene] (2), [1‚1-bromonaphthalene] (3), and [1‚1-iodonaphthalene] (4). The triplet lifetimes in these adducts are considerably shorter than those for the free 1-halonaphthalenes. Analysis of lifetime data versus temperature affords room-temperature phosphorescence quantum yields of 70%, 64%, and 7% for the solid adducts 2, 3, and 4, respectively, compared to 54% for [1‚naphthalene]. The photophysical data suggest that the synergy of the internal and external heavy-atom effects has a sensitizing effect for adducts 2 and 3 but a quenching effect for adduct 4 compared to [1‚naphthalene]. The luminescence excitation spectra of the solid binary adducts show intense bands that are significantly red-shifted from the absorptions of the individual molecular components, and thus assigned to charge transfer (CT) states. Excitation bands corresponding to the S0 f T1 direct absorption of the 1-halonaphthalene are also detected, albeit much less intense than the CT absorption. The spectral analyses suggest that CT is the major excitation route that leads to the green phosphorescence of adducts 2-4.
Introduction Since the report of Tang and VanSlyke in the 1980s regarding efficient electroluminescence from an organic light-emitting diode (OLED),1 much research has been dedicated to the development of organic and metal-organic luminescent materials as emitters for OLEDs with superior quantum efficiency, brightness, chromaticity, and/or long-term stability.2 In an OLED, light is emitted upon radiative relaxation of an electronhole recombination event, which occurs in the organic emitting layer of the diode and is equivalent to the generation of an excited molecule with light.2 Statistically, this recombination event generates 3 times the number of triplet excitons as singlet excitons in molecular species that have a closed-shell ground state.2,3 In hydrocarbon materials, the excited triplet states are typically nonemissive because of the spin-forbidden nature of the T1 T S0 transitions, thereby limiting the internal electroluminescence efficiency to a maximum of 25%.2,3 In principle, this major drawback can be overcome by significant spin-orbit coupling due to a heavy atom, so it should be possible to harness triplet excitons via electroluminescence of triplet emitters. Indeed, the utilization of phosphorescent metal-organic luminophores as emitting materials has led to a quantum leap in the performance of solid-state OLED devices owing to the pioneering work of Thompson and Forrest,4 which has stimulated significant research by numerous groups since the turn of the 21st century to improve the performance of phosphorescent emitters and devices.5 * Corresponding authors. E-mail:
[email protected];
[email protected]. † University of North Texas. ‡ Texas A&M University.
A major objective in our recent studies is to apply fundamental principles in coordination chemistry and molecular spectroscopy to design arene-sensitized phosphors as new emitting materials for OLEDs. With this in mind, we have investigated the potential of trimeric perfluoro-ortho-phenylene mercury (1) as a heavy-atom inducer. This trinuclear polydentate Lewis-acidic mercury complex interacts with various electronrich substrates, including anions and Lewis-basic organic derivatives. As part of our contribution to this area, we have found that 1 interacts with aromatic substrates to form extended supramolecular binary stacks where 1 and the arene alternate.6 The short Hg-Carene contacts, ranging from 3.2 to 3.5 Å, reflect the presence of secondary polyhapto-π interactions occurring between the electron-rich aromatic molecules and the π-acidic mercury centers. As a result of a mercury heavy-atom effect, such solid adducts exhibit intense room-temperature phosphorescence characteristic of the arene T1 state; hence, the choice of the arene controls the triplet emission energy and the color of the binary adduct. For example, we have communicated that facial complexation of the polycyclic arenes pyrene, naphthalene, and biphenyl to 1 results in crystalline adducts that exhibit bright red, green, and blue (RGB) phosphorescence bands at room temperature,7 making these binary materials attractive for multicolor electronic displays and/or white light sources (both of which need RGB emitters). Time-resolved measurements indicate that the triplet lifetimes in such materials fall in the 0.1-1 ms range and are thus considerably shorter than the lifetimes observed for the phosphorescence of the free arenes; the latter are detectable only in frozen glass with lifetimes of seconds. Despite such lifetime reductions, the phosphorescence
10.1021/jp070110t CCC: $37.00 © 2007 American Chemical Society Published on Web 06/12/2007
Heavy-Atom Effects in 1-Halonaphthalene:[Hg3] Complexes lifetimes of those binary adducts remain too long to allow rapid on/off switching of the emission required in displays because it has been suggested that brightness saturation due to groundstate depletion is a factor that limits the luminance of cathodoluminescent phosphors.8 To make our binary materials more attractive for OLEDs, we have been investigating various strategies that would allow us to further shorten the long phosphorescence lifetime. One such a strategy relies on the combination of both external and internal heavy-atom effects, which should allow for a further reduction of the triplet lifetimes. In one approach toward this goal, we have reported in a recent communication that complexation of the N-heterocycles, Nmethylindole and N-methylcarbazole, to 1 results in bright roomtemperature phosphors with a 5-order-of-magnitude reduction of the triplet lifetime relative to the free N-heterocycle!9 The resulting lifetimes of the solid adducts were below 100 µs at room temperature, making these adducts suitable for OLEDs. In another attempt to harness and combine external and internal heavy-atom effects, we now report on the photophysical properties of a series of adducts formed by complexation of 1-halonaphthalenes to 1. The 1-halonaphthalene series has often been used to demonstrate the internal heavy-atom effect provided by the halide in frozen glass. Given the strong π acidity of 1, we have undertaken this investigation on the premise that the 1-halonaphthalenes will act as π bases despite their electronwithdrawing halide substituents. Experimental Section General. Atlantic Microlab performed the elemental analyses. All commercially available starting materials were purchased from Aldrich and VWR and used as received with no further purification. Freshly distilled dry solvents were used in all syntheses. Compound 1 was prepared according to a published procedure.10 CAUTION! Organomercury complexes such as 1 are toxic. Therefore, extra care should be taken to aVoid contact with solid, solution, and airborne particulate mercury compounds. Physical Measurements. Steady-state luminescence spectra were acquired with a PTI QuantaMaster Model QM-4 scanning spectrofluorometer equipped with a 75-watt xenon lamp, emission and excitation monochromators, an excitation correction unit, and a PMT detector. The emission spectra were corrected for the detector wavelength-dependent response. The excitation spectra were also corrected for the wavelength-dependent lamp intensity, but the correction was done only at λ > 240 nm because of the unreliability of the correction at shorter wavelengths at which the samples here absorb and the xenon lamp output is rather low. Long-pass filters were used to exclude light scattering due to the excitation source from reaching the detector. Temperature-dependent studies were acquired with an Oxford optical cryostat using liquid helium as a coolant. Lifetime data were acquired using a nitrogen laser interfaced with a tunable dye laser and a frequency doubler, as part of fluorescence and phosphorescence subsystem add-ons to the PTI instrument. The 337.1 nm line of the N2 laser was used to pump a freshly prepared 1 × 10-2 M solution of the organic continuum laser dye Coumarin-540A in ethanol, the output of which was tuned and frequency doubled to attain the 280 nm excitation used to generate the time-resolved data. Luminescence and lifetime studies for frozen solutions were conducted for selected samples by placing a 5-mm Suprasil quartz cylindrical tube containing the appropriate solution in a liquid-nitrogen-filled Dewar flask with a Suprasil quartz cold finger and then inserting this setup in the sample compartment of the PTI instrument.
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9523 Synthesis of [1‚1-Chloronaphthalene] (2). Compound 1 (0.020 g, 0.019 mmol) in CH2Cl2 (5 mL) was combined with 1-chloronaphthalene (0.003 g, 0.019 mmol) in CH2Cl2 (2 mL). Slow evaporation of the solvent resulted in the crystallization of 2 in a 99% yield (0.023 g, 0.020 mmol), mp 292-294 °C (decomp). Anal. Calcd for C28H7ClF12Hg3: C, 27.72; H, 0.58. Found: C, 27.72; H, 0.44. Synthesis of [1‚1-Bromonaphthalene] (3). Compound 1 (0.020 g, 0.019 mmol) in CH2Cl2 (5 mL) was combined with 1-bromonaphthalene (0.004 g, 0.020 mmol) in CH2Cl2 (2 mL). Slow evaporation of the solvent resulted in the crystallization of 3 in a 96% yield (0.023 g, 0.018 mmol), mp 302-304 °C (decomp.). Anal. Calcd for C28H7BrF12Hg3: C, 26.75; H, 0.56. Found: C, 26.94; H, 0.42. Synthesis of [1‚1-Iodonaphthalene] (4). Compound 1 (0.020 g, 0.019 mmol) in CH2Cl2 (5 mL) was combined with 1-iodonaphthalene (0.005 g, 0.019 mmol) in CH2Cl2 (2 mL). Slow evaporation of the solvent resulted in the crystallization of 4 in a 95% yield (0.024 g, 0.018 mmol), mp 290 °C (decomp.). Anal. Calcd for C28H7IF12Hg3: C, 25.77; H, 0.54. Found: C, 26.59; H, 0.67. Crystal Structure Determinations. X-ray structures for 2-4 were collected on a Bruker SMART-CCD diffractometer using graphite-monochromated Mo KR radiation (0.71073 Å). Specimens of suitable size and quality were selected and glued onto a glass fiber with freshly prepared epoxy resin. The structure was solved by direct methods, which successfully located most of the nonhydrogen atoms. Subsequent refinement on F 2 using the SHELXTL/PC package (version 5.1) allowed us to locate the remaining nonhydrogen atoms. Further crystallographic details can be found in Table 1 and in the Supporting Information. Results and Discussion Synthesis and Structures of 2-4. When a CH2Cl2 solution of 1 is mixed with a solution of 1-chloro-, 1-bromo-, or 1-iodonaphthalene in the same solvent, slow evaporation of the solvent leads to the formation of the corresponding 1:1 adducts ([1‚1-chloronaphthalene] (2), ([1‚1-bromonaphthalene] (3), ([1‚ 1-iodonaphthalene] (4)), which have been isolated in almost quantitative yields. The colorless, air-stable solid adducts obtained have been characterized by elemental analysis and single-crystal X-ray analysis. They dissolve only in polar solvents such as DMSO or acetone. 1H and 19F NMR spectra recorded in either d6-DMSO or d6-acetone indicate complete dissociation of the adducts in solution. The crystal structures of 2-4 have been determined (Table 1). The solid-state structure of these three adducts consists of extended binary stacks where molecules of 1 alternate with the halogenated naphthalene. In all three cases, there are no unusual intramolecular bond distances and angles in the structure of the individual components. Compounds 2-4 crystallize in the space groups P21/n, P-1, and P21/c, respectively (Table 1). Examination of the cellpacking diagram for all three adducts confirms the formation of extended stacks, as observed previously in [1‚naphthalene].6 All three adducts display short Hg-Caromatic distances ranging from 3.28 to 3.43 Å, indicating secondary Hg-π interactions. These distances are all within the summed van der Waals radii of mercury (rvdw ) 1.73-2.00 Å) and carbon (rvdw ) 1.7 Å).11 In 2, the molecule of 1-chloronaphthalene in the asymmetric unit is disordered over two equally occupied positions, with the 8 position of the naphthalene ring acting as a pivot point between the two molecules. In the first orientation, the chlorine atom
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TABLE 1: Crystal Data, Data Collection, and Structure Refinement for 2-4 crystal data formula Mr crystal size (mm3) crystal system space group A (Å) B (Å) C (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalc (gcm-3) µ(Mo)(mm-1) F(000) (e)
2
3
C28H7ClF12Hg3 1208.56 0.26 × 0.11 × 0.08 monoclinic P2(1)/n 20.069(4) 6.8937(14) 20.918(4)
2622.9(9) 4 3.060 17.724 2160
C28H7BrF12Hg3 1253.02 0.27 × 0.10 × 0.07 triclinic P-1 7.5525(15) 15.968(3) 22.542(5) 88.35(3) 85.00(3) 86.94(3) 2703.5(9) 4 3.079 18.575 2232
2723.3(9) 4 3.171 18.107 2304
110(2) ω -25 f 20, -2 f 9, -25 f 23 14488 5366 [0.0325] 5366 SADABS 0.358426
273(2) ω -8 f 8, -18 f 18, -25 f 24 17952 8462 [0.0340] 8462 SADABS 0.295691
110(2) ω -21 f 21, -10 f 9, -30 f 30 22349 6517 [0.0643] 6517 SADABS 0.323027
357 0.0434, 0.1013 4.119, -2.697
781 0.0433, 0.0993 2.264, -1.310
397 0.0577, 0.1080 3.821, -2.308
115.00(3)
4 C28H7IF12Hg3 1300.01 0.38 × 0.19 × 0.08 monoclinic P2(1)/c 15.993(3) 7.5544(15) 22.787(5) 98.43(3)
data collection T/K scan mode hkl range measured refl. unique refl., [Rint] refl. used for refinement absorption correction Tmin/Tmax refinement refined parameters R1, wR2 [I > 2σ(I)] Ffin (max/min) (eÅ-3)
Cl(1) points outward from the stack and does not form any short contacts with the mercury centers (Figure 1). In the second orientation, the chlorine atom Cl(2) interacts simultaneously with two mercury centers (Hg(1A)-Cl(2) ) 3.419 Å and Hg(3)Cl(2) ) 3.501 Å) as these distances are within the sum of the van der Waals radii for Hg and Cl (rvdw ) 1.58-1.78 Å)13 (Figure 1). It is also important to note that these distances fall in the range typically observed for secondary Hg-Cl interactions.12 Although the crystal structure of 2 is affected by positional disorder, compound 3 has two distinguishable 1-bromonaphthalene molecules located between molecules of 1. Each molecule of 1 exhibits a different type of interaction, with one molecule displaying mostly Hg-Carene interactions and the other displaying Hg-Br interactions. In the unit where the two components interact only via Hg-Carene interactions, the bromine atom Br(2) is positioned at the periphery of the stacks and does not interact with any of the mercury centers (Figure 2). Yet, in the other unit, the bromine atom is coordinated to all three Hg atoms with distances ranging from 3.57 to 3.84 Å, which are within the summed van der Waals radii of mercury and bromine
(rvdw ) 1.54-1.84 Å)13 (Figure 2). Finally, compound 4 has only one molecule of 1-iodonaphthalene in the asymmetric unit. Its structure is similar to that observed for one of the two independent molecules in the structure of 3. The sandwiched 1-iodonaphathalene interacts with the two neighboring molecules of 1 by secondary Hg-Carene and Hg-I interactions (Figure 3). The three distances between the three mercury centers Hg(1), Hg(2), and Hg(3) and the iodine atom I(1) are 3.814, 3.836, and 3.626 Å, respectively, which fall within the range of the van der Waals radii of the two elements (rvdw (I) ) 1.98-2.13 Å).13 These distances are close to those found in complexes involving tetranuclear mercuracarborand hosts and iodocarborane guests (3.6 Å).14 The coexistence of two distinct orientations of the halonaphthalene molecule in 2 and 3 suggests that the interactions responsible for the formation of the adducts are not directional but dispersive and electrostatic. It also indicates that the HgCl and Hg-Br interactions are not sufficiently strong to dictate the supramolecular structures of these adducts. This conclusion cannot be extended to the case of 4 in which the Hg-I
Figure 1. Molecular structure of 2. Thermal ellipsoids are at 50%. Hydrogen atoms are omitted for clarity. Significant contacts (Å) in I: Cl(2)Hg(1A) 3.500(12), Cl(2)-Hg(3) 3.419(14), C(37)-Hg(2A) 3.250(20), C(38)-Hg(2) 3.400(20), C(39)-Hg(2A) 3.359(19). Significant contacts (Å) in II: C(24)-Hg(3) 3.374(12), C(26)-Hg(1) 3.358(14), C(27)-Hg(2A) 3.277(14), C(28)-Hg(2A) 3.402(14), C(29)-Hg(2) 3.406(19).
Heavy-Atom Effects in 1-Halonaphthalene:[Hg3] Complexes
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9525
Figure 2. Molecular structure of 3. Thermal ellipsoids are at 30%. Hydrogen atoms are omitted for clarity. Significant contacts (Å) in I: Br(1)Hg(1) 3.843(2), Br(1)-Hg(2) 3.565(2), Br(1)-Hg(3) 3.809(2), C(23)-Hg(1A) 3.439(17), C(28)-Hg(1) 3.421(13). Significant contacts (Å) in II: C(52)-Hg(4) 3.388(11), C(49)-Hg(5A) 3.426(20).
Figure 3. (I) Space-filling model of the binary stacks observed in the extended structure of 4. (II) Molecular structure of 4. Thermal ellipsoids are at 50%. Hydrogen atoms are omitted for clarity. Significant contacts (Å): I(1)-Hg(1) 3.814(16), I(1)-Hg(2) 3.836(14), I(1)-Hg(3) 3.626(13), C(23)-Hg(1A) 3.325(17), C(24)-Hg(1A) 3.324(17), C(26)-Hg(2A) 3.421 (13).
interaction largely dominates the stacking motif. These structural differences accentuate the preference that the soft mercury Lewis acidic sites render for the softer halogen.15 Luminescence of Crystals and Frozen Solutions. Crystals of 2-4 exhibit green emissions whose energies and vibronic progressions are similar to the emissions reported for the T1 states of the corresponding 1-halonaphthalene.16 In all three cases, the intensity of the bands is enhanced by lowering the temperature, which results from the inhibition of nonradiative depopulation of the emitting state upon cooling. Frozen CH2Cl2 solutions containing equimolar amounts of 1 and any of the halonaphthalenes also exhibit similar green structured emissions corresponding to the T1 state of the 1-halonaphthalene. The steady-state luminescence spectra for solids and frozen solutions are shown in Figures 4 and 5. The observed enhanced phosphorescence results from substantial spin-orbit coupling provided by the mercury centers of 1 to the arene. To assess the extent of the heavy-atom effect induced by 1, we analyzed the kinetics of the radiative decay in both the solid state and frozen CH2Cl2 solutions (Tables 2 and 3). The phosphorescence lifetimes for solids 2-4 are below 2 ms at room temperature (RT) and 77 K and are in the same range as that reported for [1‚naphthalene] (Table 2).7 These lifetimes are longer than the fluorescence lifetimes (ns scale), thus confirming the triplet nature of the emissions of solids 2-4. Interestingly, CH2Cl2 frozen solutions of 2-4 have triplet lifetimes in the range of 0.06-0.60 ms, which are much shorter than the lifetimes for the monomer phosphorescence bands of the free halonaphthalene in EPA glass16,17 or in CH2Cl2 frozen solutions (Table 2). Altogether, these lifetime measurements underscore the difference that exists in the spin-orbit perturbation provided by an innocent matrix such as EPA and the heavy-atom
Figure 4. Photoluminescence excitation (thin lines) and emission (thick lines) spectra of 1‚naphthalene (traces I) and 2 (traces II) in the following environments: CH2Cl2 frozen solutions of 1‚naphthalene (a), crystals of 1‚naphthalene at 77 K (b), CH2Cl2 frozen solutions of 2 (c), crystals of 2 at 77 K (d), and crystals of 2 at room temperature (e). The spectra are normalized and offset for clairty purposes.
environment provided by the mercury atoms of 1.9 Indeed, the data herein suggest that the external mercury heavy-atom effect induced by 1 leads to a more drastic reduction in the triplet lifetime than the reduction due to the internal halogen heavyatom effect (e.g., the frozen CH2Cl2 data in Table 2 show that
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Figure 5. Photoluminescence excitation (thin lines) and emission (thick lines) of 3 (traces I) and 4 (traces II) in the following environments: CH2Cl2 frozen solutions of 3 (a), crystals of 3 at 77 K (b), crystals of 3 at room temperature (c), CH2Cl2 frozen solutions of 4 (d), crystals of 4 at 77 K (e), and crystals of 4 at room temperature (f). The spectra are normalized and offset for clarity purposes.
the τP value for naphthalene decreases from 2300 to 0.60 ms in solutions containing dissolved [1‚naphthalene] but only to 1.7 ms for the free 1-iodonaphthalene)! The very short triplet lifetimes observed for 2-4 most likely result from the synergy of both the external mercury and internal halogen heavy-atom effects. A rare situation similar to the one herein in which both effects exist simultaneously has been reported by Fackler, Omary, and co-workers for a sandwich adduct consisting of perfluoronaphthalene and an electron-rich trinuclear Au(I) complex.18 This has resulted in yellow phosphorescence attributed to the T1 state of C10F8 with a lifetime of 3.5 ms in the solid state. Despite being external, the large heavy-atom effects of mercury and gold in this work and in ref 18, respectively, are related to the high spin-orbit coupling constant (ξ) of 4270 and 5104 and cm-1 for the 5d orbital of these two heavy atoms, respectively.19 Although the halogen provides an internal heavyatom environment (ξ ) 587, 2460, and 5069 for Cl, Br, and I, respectively),19 the cooperative effect due to the presence of six Hg atoms in close proximity to the chromophore in the stacked solid-state arrangement makes the external mercury
Figure 6. Photoluminescence excitation and emission spectra for crystals of 1‚naphthalene (a), 2 (b), 3 (c), and 4 (d) at 4 K. The spectra are normalized and offset for clarity purposes.
heavy-atom effect overwhelm the internal halogen heavy-atom effect. As established recently by our groups, the cooperativity occurring between the three mercury atoms of 1 also contributes to the intensity of these external heavy-atom effects.20 Although the mercury heavy-atom effect dominates, the differences that exist in the photophysical parameters of 2-4 are in line with the ξ values of the different hologens. The (τ P)-1 values represent the sum of the radiative (kr) and nonradiative (knr) decay rate constants. Temperature-dependent lifetime measurements allow the separation of the two components due to the exponential annihilation of knr at temperatures approaching 0 K.21 We have carried out such measurements for all solid adducts by obtaining their lifetimes at approximately 10 temperatures for each sample between room temperature and 4 K. Cooling to 4 K leads to an enhancement of the luminescence intensity and increased resolution of the vibronic bands without significant shifts in the band energies, as shown in Figure 6. The lifetimes of 2-4 and [1‚naphthalene] increase at 4 K (Table 2), owing to a lower knr . Assuming unity intersystem crossing yield and a simple two-level system for the radiative and nonradiative decay from the phosphorescent T1 state to the S0 ground state, the experimental values of the lifetimes versus temperature follow eq 1:21
TABLE 2: Triplet Lifetimes (ms Units) for Polycyclic Hydrocarbons and Their Adducts with 1 free arene
EPA glass16,17
frozen CH2Cl2a
adduct
frozen CH2Cl2b
solid 4K
solid 77 K
solid RT
naphthalenec 1-chloronaphthalene 1-bromonapthalene 1-iodonaphthalene
2700 330 15.8 2.43
2300 320 20.5 1.7
1‚naphthalene 2 3 4
0.600 0.248 0.212 0.066
1.423 1.901 1.448 1.249
0.723 1.557 1.018 0.991
0.568 1.254 0.831 0.087
a Dry CH Cl frozen glass matrix for polycyclic hydrocarbons. b Equimolar amounts of 1 and polycyclic hydrocarbons in frozen glassy solution 2 2 of CH2Cl2. c Data taken from ref 7.
TABLE 3: Photophysical Parameters for 2, 3, 4, and 1‚Naphthalene Solids adduct
kr (s-1)
φRT (%)
φ77K (%)
φ4K (%)
knr (s-1), RT
knr (s-1), 77 K
knr (s-1), 4 K
2 3 4 1‚naphthalene
547 657 799 755
70.0 64.0 6.96 53.5
86.4 66.1 79.3 73.6
100. 95.1 100. 100.
235 370. 1.07 × 104 656
86.1 338 209 271
1.13 × 10-9 34.0 0.0403 6.89 × 10-8
Heavy-Atom Effects in 1-Halonaphthalene:[Hg3] Complexes
1/τ P ) kr + knr ) kr + ko exp(-Ea/RT)
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9527
(1)
Figure 7 shows a representation of a least-squares analysis carried out to fit the experimental data of 1/τ versus 1/T to eq 1 to obtain the best values for the parameters kr , ko, and Ea for the solid adduct 2. This approach affords the following optimized phosphorescence radiative decay rate constants: kr (2) ) 547 s-1, kr (3) ) 657 s-1, and kr (4) ) 799 s-1. Combining these temperature-independent values with the lifetimes at different temperatures allows the determination of knr and thus phosphorescence quantum yield values; the latter are 70%, 64%, and 7% at room temperature for solids of 2-4, respectively. A similar treatment for crystals of [1‚naphthalene] gives rise to kr ) 755 s-1 and a room-temperature phosphorescence quantum yield of 54%. Table 3 summarizes the photophysical parameters extracted via this analysis. Because 1-iodonaphthalene has the heaviest atom among the three halonaphthalenes, it is not surprising that 4 exhibits the shortest lifetime at all temperatures. Table 3 illustrates that an increase in kr is found with an increase in halogen size. Although the higher kr for 4 must be a contributing factor to the much shorter lifetime observed when compared to 2 and 3, the relative increase in kr with the halogen size is modest (Table 3) compared to the relative decrease in τP with the halogen size at room temperature (Table 2). The reduction in the triplet lifetimes only implies that the summed (kr + knr) value is higher due to a faster T1 f S0 process and thus cannot indicate whether the acceleration is due to a sensitization effect (higher kr) or quenching effect (higher knr). Using the temperature-dependent lifetime data discussed above, we are able to separate the two components. Thus, the quantum yield values suggest that the synergy of the internal and external heavy-atom effects has a sensitizing consequence in adducts 2 and 3 at RT (mercury heavy-atom effect increases kr with respect to [1‚naphthalene]) but a quenching consequence in the case of 4 (halogen internal heavyatom effect increases knr with respect to [1‚naphthalene]). The drastic decrease in the triplet lifetime of 4 relative to 2 or 3 at RT, therefore, is mostly due to an extremely high knr, not kr; Table 3 lists the knr values extracted at three temperatures for all four solid adducts. Photophysical Pathways. To clarify the excitation mechanism leading to the observed phosphorescence of 2-4, we have analyzed the excitation spectra for the solid adduct 2 and a frozen solution of 1-chloronaphthalene (Figure 8). The luminescence excitation spectrum for 2 is also compared to the absorption spectra of pure 1 (2.40 × 10-6 M) in CH2Cl2 and pure 1-chloronaphthalene in CH2Cl2 (1.82 × 10-6 M). In the absorption spectrum of 1-chloronaphthalene, the low-intensity features between 310 and 320 nm (distinguished by the dashed line) correlate nicely with the low-energy band in the excitation spectrum of the frozen CH2Cl2 solution of 1-chloronaphthalene, characteristic of the lowest singlet-state absorption S0 f S1 assigned to a1(π,π*) state.22 It is somewhat surprising that the S0 f S1 transition gains intensity in the phosphorescence excitation spectrum of the frozen CH2Cl2 solution of 1-chloronaphthalene relative to the S0 f S2 transition, which is much stronger in the absorption spectrum of the fluid solution. But more importantly, Figure 8 shows that the solid binary adduct 2 exhibits not only absorption characteristics of its two constituents but also another major red-shifted band at 336 nm. We assign this excitation feature to a charge-transfer (CT) transition in the ground-state adduct 2, which is reasonable because 1 is electrophilic6 and thus able to accept electron density from the aromatic ring in 1-chloronaphthalene. Clearly,
Figure 7. Least-squares analysis of the experimental 1/τ vs 1/T data fit to eq 1 for the solid adduct 2.
Figure 8. Luminescence excitation spectra for crystals of 2 at RT (a) and a dilute frozen solution of 1-chloronaphthalene (b) while monitoring the phosphorescence peak maxima. For comparison, also plotted are the absorption spectra at RT for dilute solutions of 1-chloronaphthalene (c) and 1 (d) to represent the free monomeric components of adduct 2.
this CT band is a major excitation route for the phosphorescence observed in 2. Further analysis of the excitation spectra of the solid adduct 2 as well as the frozen CH2Cl2 solutions of 1-chloronaphthalene reveals features in the visible region (Figure 9). The vibronic profile and energy span (20-25 × 103 cm-1) of these weak features match extremely well with those reported for the direct S0 f T1 absorption in 1-chloronaphthalene.23,24 Figure 9 shows that, in the case of adduct 2, these features are more discernible and more intense than those seen in the frozen solution of pure 1-chloronaphthalene. This accentuates the stronger external heavy-atom effect provided by the three mercury atoms in 2 relative to the solvent-induced25 effect supplied by the CH2Cl2 matrix in the frozen solution of 1-chloronaphthalene. However, the very-low intensity of these features relative to the CT band at 336 nm suggests that the S0 f T1 transition has only a minor contribution to the excitation route in 2. We conclude that the sensitized phosphorescence in adducts 2-4 is caused primarily by direct absorption to the CT state of the adduct followed by nonradiative relaxation to the T1 state of the halonaphthalene, as summarized in Figure 10. The energy levels of the various states depicted in Figure 10 are based on the relative energies of the various bands observed in the
9528 J. Phys. Chem. C, Vol. 111, No. 26, 2007
Elbjeirami et al. corresponding to charge transfer due to an external heavy-atom effect in ground-state adducts is not known to cause phosphorescence in conventional luminescent organic molecules while such excitations represent the major photophysical route that leads to phosphorescence even at RT for the binary π acidbase adducts like those herein and the one reported in ref 18. Conclusions
Figure 9. Luminescence excitation spectra for crystals of adduct 2 (a) and 1-chloronaphthalene frozen solution in CH2Cl2 (b) monitoring the phosphorescence peak maximum in each. Dashed lines indicate the positions of the S0 f S1 and S0 f S2 absorption peaks known for the 1-chloronapthalene monomer. Note the weak signals corresponding to direct S0 f T1 excitation of 1-chloronapthalene, as illustrated by magnifying this region. The spectra are normalized and offset for clarity purposes.
Figure 10. Energy-level diagram showing the interaction between the excited states of 1 and 1-chloronaphthalene (C10H7Cl) to form a lowerenergy charge transfer (CT) state in the 2 adduct. The energy levels of various states are based on the spectra in Figures 8 and 9. Solid and dashed arrows represent radiative and nonradiative processes, respectively, and the thickness of arrows denoting absorptions represent their relative contribution in the phosphorescence excitation route.
electronic spectra discussed above. Fortunately, the CT state in the binary adduct lies higher in energy than the T1 state of the organic component such that the latter state will not be depopulated as a result of the charge-transfer processes. As a result, the lowest-energy emitting state in 2 remains essentially unperturbed compared to the T1 state of the free halonaphthalene substrate. The spectral data suggest that the S1 state of 1-chloronaphthalene is not involved in the charge-transfer process because the absorption band corresponding to this state remains unperturbed in the electronic spectrum of the adduct 2. Figure 10 illustrates that the CT state forms as a result of molecular orbital interaction between the S2 state of 1-chloronaphthalene with a frontier orbital of 1, whereas both S1 and T1 states remain nonbonding in adduct 2. Numerous interpretations are available in the literature regarding the mechanism of the external heavy-atom effect in luminescent organic molecules, and there is lack of consensus on its origin.23,26 Nevertheless, charge transfer has been suggested to play a role in this effect.26 Excitation with wavelengths
The results presented herein illustrate the formation of binary supramolecular stacks by interaction of 1 with 1-halonaphthalenes. The room-temperature phosphorescence quantum yields for solid 2 (70%), 3 (64%), and 4 (7%), compared to 54% for the naphthalene solid adduct of 1, suggest that the synergy of the internal and external heavy-atom effects has a sensitization effect in the case of 2 and 3 but a quenching effect for 4. The spectral data for adducts 2-4 suggest that the major excitation route leading to phosphorescence entails absorption to a chargetransfer state followed by nonradiative relaxation to the T1 state of the halonaphthalene. The combination of the data in this work and the previous work we reported for adducts of 1 with N-heterocycles ([1‚N-methylindole]: φRT ) 44%, τ ) 29 µs; [1‚N-methylcarbazole]: φRT ) 14%, τ ) 49 µs)9 shows that the synergy of internal- and external-heavy atom effects can be used to synthesize materials that are potentially attractive for multiple OLED applications. The higher quantum efficiencies and longer lifetimes for 2 and 3 compared to both N-heterocyclic adducts make them suitable for general solid-state lighting applications, whereas the shorter lifetime and reasonable quantum efficiency for the N-methylindole9 adduct of 1 makes it more suitable for electronic displays. Acknowledgment. We thank Professors Nigel Shepherd and Martin Schwartz for helpful discussions. This work has been supported by the Texas Advanced Technology Program (Grant 010366-0039-2003 to F.P.G. and M.A.O.), the Robert A. Welch Foundation (Grant B-1542 to M.A.O.; Grant A-1423 to F.P.G.), the National Science Foundation (CAREER Award, Grant CHE0349313 to M.A.O.), the donors of the American Chemical Society Petroleum Research Fund (Grant 38143 -AC 3 to F.P.G.), and the Department of Energy (DE-FC26-06NT42856 to M.A.O.). Supporting Information Available: X-ray crystallographic data for 2-4 in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) VanSlyke, S. A.; Tang, C. W. Eur. Pat. Appl. 1984, 32. (b) Tang, C. W.; VanSlyke, S. A. Eur. Pat. Appl. 1988, 23. (c) Tang, C. W.; VanSlyke, S. A.; Chen. C. H. J. Appl. Phys. 1989, 65, 3610. (2) For reviews, see the following: (a) Sibley, S.; Thompson, M. E.; Burrows, P. E.; Forrest, S. R. Electroluminescence in Molecular Materials. In Optoelectronic Properties of Inorganic Compounds; Roundhill, D. M.; Fackler, J. P., Jr., Eds.; Plenum: New York, 1999; Chapter 5. (b) Yersin, H. Top. Curr. Chem. 2004, 24, 1. (3) Ko¨hler, A.; Wilson, J. S.; Friend, R. H. AdV. Mater. 2002, 14, 701. (4) (a) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151. (b) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4. (c) Baldo, M. A.; O’Brien, D. F.; Thompson, M. E.; Forrest, S. R. Phys. ReV. B 1999, 60, 14422. (d) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl. Phys. Lett. 2000, 77, 904. (e) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304. (f) Adamovich, V.; Brooks, J.; Tamayo, A.; Alexander, A. M.; Djurovich, P. I.; D’Andrade, B. W.; Adachi, C.; Forrest, S. R.; Thompson, M. E. New J. Chem. 2002, 26, 1171. (g) Sun, Y.; Giebink, N. C.; Kanno, H.; Ma, B.; Thompson, M. E.; Forrest, S. R. Nature 2006, 440, 908.
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