ARTICLE pubs.acs.org/JPCA
Spectroscopic StructureProperty Relationships of a Series of Polyaromatic Platinum Acetylides Thomas M Cooper,*,† Douglas M. Krein,†,‡ Aaron R. Burke,†,‡ Daniel G. McLean,†,§ Joy E. Haley,† Jonathan Slagle,†,§ Jennifer Monahan,†,^ and Albert Fratini# †
Materials and Manufacturing Directorate, Air Force Research Laboratory, WrightPatterson Air Force Base, Ohio 45433, United States General Dynamics Information Technology, Dayton, Ohio 45431, United States § Science Applications International Corporation, Dayton, Ohio 45434, United States ^ SOCHE Student Research Program, Dayton, Ohio 45420, United States # Department of Chemistry, University of Dayton, Dayton, Ohio 45469, United States ‡
bS Supporting Information ABSTRACT:
To develop a structurespectroscopic property relationship in platinum acetylides having poly(aromatic hydrocarbon) ligands, we synthesized a series of chromophores with systematic variation in the number of fused aromatic rings (nFAR) and ligand topology (polyacene (L), polyphenanthrene (Z), or compact(C)). We measured ground-state absorption, fluorescence, and phosphorescence spectra. We also performed nanosecond and femtosecond transient absorption experiments. To extend the range of compounds in the structureproperty relationship, we did DFT calculations on an expanded series of chromophores. Both the DFT results and experiments show that the S1 and T1 state energies are a function of both nFAR and the ligand topology. In the L chromophores, the S1 and T1 state energies decrease linearly with nFAR. In contrast, the S1 and T1 state energies of the Z chromophores oscillate around a fixed value with increasing nFAR. The C chromophores have behavior intermediate between the L and Z chromophores. A parallel series of calculations on the ligands shows the same behavior. The S1Sn energy obtained from ultrafast time-resolved spectra has a linear variation in nFAR. The rate constant for nonradiative decay, knr, was calculated from the S1 state lifetime and decreases with an increasing number of π electrons in the aromatic ring. The result is consistent with the spinorbit coupling caused by the central platinum heavy atom decreasing with larger nFAR. The present work shows that the framework developed for the analysis of poly(aromatic hydrocarbon) properties is useful for the understanding of the corresponding platinum acetylide complexes.
’ INTRODUCTION Platinum acetylides are exceptional systems for investigating triplet excited-state phenomena like ground-state absorption to the triplet state, intersystem crossing, triplet-state absorption to higher triplet states, and phosphorescence.1 In our laboratory, we have been investigating the relation between chemical structure and spectroscopic properties of platinum acetylide complexes having the molecular formula trans-Pt(PBu3)2L2. Platinum complexes containing the ligand L = H(C6H4CtC)nH, n = 13, named PE1, PE2, and PE3, have been studied in considerable r 2011 American Chemical Society
detail. Analysis of the dependence of singlet- and triplet-state energies on chromophore length gives evidence that the singlet exciton is delocalized through the central platinum, while the triplet exciton is confined to one ligand. A less-investigated class of platinum acetylides has ligands containing polyaromatic hydrocarbons (PAH).2 PAHs vary in Received: April 18, 2011 Revised: November 7, 2011 Published: November 11, 2011 139
dx.doi.org/10.1021/jp203616c | J. Phys. Chem. A 2012, 116, 139–149
The Journal of Physical Chemistry A
ARTICLE
(m, 12H, CH2), 7.518.79 (m, 18H, ArH) ppm; 13C NMR (CDCl3): δ 14.09 (s, CH3), 23.96 (t, J(CP) = 17 Hz, CH2), 24.19 (t, J(CP) = 7 Hz, CH2), 24.68 (t, J(CP) = 7 Hz, CH2), 26.64, (s, CH3), 123.3 (t, PtCtC), 106.3 (PtCtC), 122.9, 124.9, 125.5, 128.6, 132.0, 132.1 (Ar) ppm; 31P NMR (CDCl3): δ (s and d centered at δ4.25, J(PPt) = 2351 Hz, PBu3) ppm. Z3: EA: found C: 66.48; H: 7.29%. C56H72P2Pt requires C: 67.11; H: 7.24%; MW = 1002; EIMS: (m/z) 1002; IR: (KBr, thin film) 2176.0 cm1 ν(PtCtC); 1H NMR (CDCl3): δ 0.98 (m, 18H, CH3), 1.52 (m, 12H, CH2), 1.81 (m, 12H, CH2), 2.24 (m, 12H, CH2), 7.508.90 (m, 18H, ArH) ppm; 13C NMR (CDCl3): δ 14.1 (s, CH3), 24.3 (t, CH2), 26.87 (t, CH3), 114.6 (t, Pt—CtC), 107.5 (PtCtC), 122.7, 125.6, 125.8, 126.4, 126.5, 126.8, 127.9, 128.3, 128.4, 129.1, 130.4, 132.7, 132.9 (Ar); 31P NMR (CDCl3): δ (s and d centered at δ5.52, J(PPt) = 2352 Hz, PBu3) ppm. C5: EA: found C: 70.72; H: 6.63%. C68H76P2Pt requires C: 71.00; H: 6.66%; MW = 1150; EIMS: (m/z) 1150; IR: (KBr, thin film) 2081.4 cm1 ν(PtCtC); 1H NMR (CDCl3): δ 0.98 (m, 18H, CH3), 1.52 (m, 12H, CH2), 1.74 (m, 12H, CH2), 2.24 (m, 12H, CH2), 7.468.5 (m, 22H, ArH) ppm; 13C NMR (CDCl3): δ 14.2 (s, CH3), 24.7 (t, CH2), 26.87 (t, CH3), 117.3 (t, PtCtC), 108.4 (PtCtC), 119.9, 120.2, 120.5, 120.6, 126.2, 126.8, 126.9, 127.0, 127.3, 127.8, 128.2, 128.9, 129.0, 129.2, 131.5, 131.9, 132.0, 135.1, 135.3 (Ar); 31P NMR (CDCl3): δ (s and d centered at δ5.45, J(PPt) = 2347 Hz, PBu3) ppm. X-ray Diffraction. We were able to grow X-ray quality crystals of L3 and Z3 by slow evaporation of a DCM/MeOH solution. For X-ray examination and data collection, a suitable crystal measuring approximately 0.40 0.30 0.20 mm was mounted on a glass fiber and transferred immediately to the goniostat bathed in a cold nitrogen stream. Intensity data were collected at 140 K on a Oxford Diffraction Xcalibur3 system equipped with a graphite monochromator and an Enhance (Cu) X-ray source (λ = 1.5418 Å) operated at 2 kW power (50 kV and 40 mÅ). The detector was set at a distance of 45.0 mm from the crystal. A series of data frames measured at 1.0 increments in ω were collected. Final cell constants were obtained through a global refinement of all reflections. Intensity data were collected and processed by employing the CrysAlisPro program.8 Absorption corrections were based on the analytical numerical method applied to a multifaceted crystal. The structure was solved and refined using Bruker SHELXTL (v6.10).9 Spectroscopy. Ground-state UV/vis absorption spectra of samples dissolved in benzene were measured in 1 cm quartz cuvettes on a Cary 500. Corrected steady-state emission spectra and quantum yields were measured using a PerkinElmer model LS 50B fluorometer. Samples were placed in 1 cm quartz cuvettes, and the optical density was adjusted to approximately 0.1 at the excitation wavelength. Fluorescence quantum yields were determined using the method of relative actinometry. Quinine sulfate was used as an actinometer with a known fluorescence quantum yield of 0.55 in 1.0 N H2SO4.10 For the phosphorescence experiments, samples were deoxygenated by bubbling with nitrogen. Time-correlated single-photon counting (TCSPC) (Edinburgh Instruments OB 920 spectrometer) was utilized to determine fluorescence lifetimes. The samples were pumped using a 70 ps laser diode at either 401 or 375 nm. Emission was detected on a cooled microchannel plate PMT. Data were analyzed using a reconvolution software package provided by Edinburgh Instruments. Femtosecond transient absorption (FTA) measurements were performed using a commercially available femtosecond
structure from chromophores to graphite, carbon nanotubes and graphene.3 As depicted below, the edges of PAHs can have either an armchair or zigzag configuration, as well as complex edge combinations.
El-Sayed’s rule predicts forbidden intersystem crossing of a PAH singlet ππ* state to the triplet state.4 The heavy atom effect promotes intersystem crossing from singlet to triplet excited states. For example, a hypothetical graphene nanoribbon containing heavy atoms will have enhanced spinorbit coupling, promoting photoinduced triplet-state formation and possibly modifying magnetic behavior.5 We have previously shown that platinum acetylides, upon excitation, convert to the triplet state with nearunit quantum yield.1a Platinum acetylides containing PAH ligands allow study of their triplet properties. For example, a recently synthesized mixed-ligand PE1hexabenzocoronene shows a phosphorescence band at 2.14 eV from the hexabenzocoronene ligand and no emission from the PE1 ligand.6 A platinum acetylide having a pyrene ligand has also been described.7 In the current work, we describe structureproperty relationships in platinum acetylides that have PAH ligands. We describe three series of platinum acetylides, designated L, Z, and C (Figure 1). The L series has zigzag-edged polyacene ligands, the Z series has armchair-edged polyphenanthrene ligands, and the C series has condensed ligands. These complexes serve as an initial point for studying larger PAHs bonded to heavy atoms. We have synthesized L1 (benzene), L2 (naphthalene), L3 (anthracene), Z3 (phenanthrene), C4 (pyrene), and C5 (perylene). The number in the label gives the number of fused aromatic rings (nFAR) in the parent PAH. A structureproperty relationship for these compounds plus additional compounds shown in Figure 1 has been investigated though computational chemistry. By combining results from both measurements on synthesized compounds and data from computational chemistry, we show that the spectroscopic behavior is a function of both nFAR and the PAH topology. The trends in singlet- and triplet-state energies in the platinumcontaining chromophores are similar to trends seen in the analogous PAH compounds, showing that structureproperty relationships developed for PAH can be used as a framework for predicting the behavior of corresponding platinum acetylide complexes.
’ EXPERIMENTAL SECTION Synthesis. The compounds L1 and C4 were synthesized by published methods.1a,4 L2: EA: found C: 63.97; H: 7.11%. C48H68P2Pt requires C: 63.91; H: 7.60%; MW = 902; EIMS: (m/z) 902; IR: (KBr, thin film) 2174.9 cm1 ν(PtCtC); 1H NMR (CDCl3): δ 0.92 (m, 18H, CH3), 1.46 (m, 12H, CH2), 1.81 (m, 12H, CH2), 2.24 (m, 12H, CH2), 7.378.63 (m, 14H, ArH) ppm; 13C NMR(CDCl3): δ 14.1 (s, CH3), 24.4 (t, CH2), 26.84 (t, CH3), 114.6 (t, PtCtC), 107.3 (t, PtCtC), 121.1, 124.0, 125.5, 125.8, 125.9, 127.2, 127.7, 128.2, 133.7, 134.1 (Ar); 31P NMR (CDCl3): δ (s and d centered at δ5.38, J(PPt) = 2357 Hz, PBu3) ppm. L3: EA: found C: 67.12; H: 7.15%. C56H72P2Pt requires C: 67.11; H: 7.24%; MW = 1002; EIMS: (m/z) 1002; IR: (KBr, thin film) 2167.6 cm1 ν(PtCtC); 1H NMR (CDCl3): δ 0.92 (m, 18H, CH3), 1.42 (m, 12H, CH2), 1.81 (m, 12H, CH2), 2.24 140
dx.doi.org/10.1021/jp203616c |J. Phys. Chem. A 2012, 116, 139–149
The Journal of Physical Chemistry A
ARTICLE
Figure 1. Chemical formulas of the platinum acetylide complexes investigated in this paper. Platinum complexes L2L5 have polyacene ligands with zigzag-edge structure, while Z3Z5 have polyphenanthrene ligands with armchair edge structure. We synthesized and characterized L1, L2, L3, Z3, C4, and C5. Properties of the remaining complexes were determined by ab initio calculations. The red-colored phenyl acetylene region refers to calculation of the spin density of the triplet state (see Table 2).
pumpprobe UVvis spectrometer (HELIOS) purchased from Ultrafast Systems LLC. Briefly, 1 mJ, 105 fs pulses at 800 nm at a 1 kHz repetition rate were obtained from a Ti:sapphire laser (Spectra Physics Hurricane). The output laser beam was split into pump and probe by a beam splitter. The pump beam was directed into a frequency doubler (400 nm) and then focused into the sample. The probe beam was delayed in a computercontrolled optical delay (Newport) and then focused into a sapphire plate to generate white light continuum. The white light was then overlapped with the pump beam in a 2 mm quartz cuvette and then coupled into a CCD detector (Ocean Optics). Data acquisition was controlled by software (Surface Explorer Pro)
developed by Ultrafast Systems LLC. The chirp effects on the spectra were within experimental error; therefore, no chirp corrections were made. Nanosecond transient absorption (NTA) measurements were carried out by exciting the sample at the third harmonic (355 nm) of a Q-switched Nd:YAG laser (Quantel Brilliant, pulse width of ca. 5 ns) and probing the sample from 330 to 800 nm. Data were collected from 50 ns to 1 ms. All samples were deoxygenated by freezepumpthaw. Pulse fluences of up to 1 mJ cm2 at the excitation wavelength were typically used. Linear excitation into the S1 state and use of low pulse fluence minimized the effect of multiphoton processes. A detailed description of the laser flash photolysis apparatus has been published earlier.1a,c 141
dx.doi.org/10.1021/jp203616c |J. Phys. Chem. A 2012, 116, 139–149
The Journal of Physical Chemistry A
ARTICLE
Table 1. Absorbance and Emission Properties of PE1 Derivatives in Benzene Absmaxa
εb
fc
Δνe
Flmaxd
ESf
ϕFg
Phmaxh
ETi
L1
324
24 100 (900)
0.28
364
3390
3.60
0.00130
435
2.85
L2
347
41 900 (400)
0.74
382
2640
3.40
0.00265
551
2.25
L3
435
53 000 (700)
0.51
449
717
2.81
0.132
Z3
343
58 800 (600)
1.12
364
1680
3.51
0.00340
546
C4
402
98 900 (1900)
1.24
421
1120
3.01
0.00630
663
C5
484
126 500 (2600)
1.09
499
621
2.52
0.115
1.80 2.27 1.87 1.44
a Absorption maximum (nm) from UV/vis spectra of samples dissolved in benzene. b Extinction coefficient (M1 cm1) with the standard deviation in parentheses. c Oscillator strength calculated from fitting the UV/vis spectra to Gaussian functions. d Emission maximum (nm) from fluorescence spectra of samples dissolved in benzene e Stokes shift (cm1). f Energy of the singlet state in eV. g Fluorescence quantum yield. h Phosphorescence maximum (nm) from emission spectra at room temperature of samples deoxygenated by the freezepumpthaw method. The spectrum of L1 was collected at 77 K. i Energy of the triplet state in eV. No phosphorescence for L3 and C5 was observed. The values for L3 and C5 were calculated using DFT (Table 3) and are included for reference.
Computational Chemistry. Calculations were done using Gaussian 03W, version 6.1.11 The presence of the heavy platinum center required a basis set that includes relativistic effects through an effective core potential. We used density functional theory (DFT) with the B3LYP functional and the relativistic LANL2DZ basis set.12 To save computer time, the phosphine portion of the molecule was converted from tributyl phosphine to trimethyl phosphine. We performed geometry optimizations for the ground and T1 states. The ground-state energy minimizations were performed with the ligand plane perpendicular to the PPtP axis and the symmetry constrained to Ci. A frequency calculation was used to verify that there were no imaginary frequencies in the minimized ground-state structure. The FranckCondon Sn excited states resulting from the optical transition S0 f Sn were calculated by density functional response theory (TDDFT) on the ground-state relaxed geometry, where the three lowest singlet roots were obtained.13 To calculate the T1 state geometry, we utilized the fact that DFT is known as a ground-state theory only rigorously valid for the ground state of a given symmetry (including spin symmetry).14 In this instance, the T1 state is the “ground state”. Our starting geometry for these minimizations was that previously found for PE1,15 where the ligand plane was parallel to the PPtP axis and the symmetry was no more than C1. A frequency calculation was used to verify that there were no imaginary frequencies in the minimized T1 state structure. The T1 state energy, ET, was estimated by the expression
compounds have 0% compactness (PC = 0). The L and Z ligands are built from naphthalene. The Ln series is analogous to polyacenes consisting of parallel trans-(CH)x chains. For nFAR = 3, the L ligand is generated from naphthalene by adding a butadiene unit to the C2C3 carbon atoms. The Zn series is analogous to polyphenanthrene cis-(CH)x chains.18 The Z ligand is generated from naphthalene by adding a butadiene unit to the C1C2 carbon atoms. The compounds with smaller HA have a higher PC, designated as a compact (C) conformation. The compounds with the smallest HA for a given nFAR are the pericondensed PAH. In this work, we investigated the C compounds having nFAR = 4 and 5 and PC = 100. There are different structural features in PAH described in ref 16, including cove, bay, and fjord regions as well as hexagonal holes. For example, the Z3 parent PAH has one bay region, while the C5 parent PAH has one hexagonal hole and two bay regions. We obtained X-ray crystallography data on L3 and Z3 (Figure 2). In the crystal, L3 packs in a linear fashion with the unit cell consisting of one L3 molecule. The chromophore in the crystal environment retains centrosymmetry, with the two acety. In contrast, lenic CtC bonds having the same length of 1.214 Å Z3 packs in a herringbone arrangement. The unit cell contains two Z3 chromophores oriented perpendicular to one another. Although Z3 belongs to the Ci point group, the crystal packing causes symmetry breaking. The “outer” bonds of both the Z3s . The perpendicular interaction have a bond length of 1.218 Å between the two Z3s causes changes in the “inner” acetylenic bond lengths. The inner CtC bond length of the Z3 on the left , while the inner CtC bond of Figure 2 shortens to 1.203 Å length of the Z3 on the right of Figure 2 lengthens to 1.246 Å. We performed DFT calculations on the dimer and found the dipole moment of the dimer to be 0.421 D, demonstrating charge-transfer interactions between the subunits. The differences between L3 and Z3 crystal structures can be attributed to polarity and symmetry differences between the L3 and Z3 parent PAH, where anthracene has D2h symmetry and phenanthene has C2v symmetry, resulting in a nonzero ligand dipole moment. Table 1 lists linear spectroscopic properties of these compounds. We expected to observe a red shift as nFAR increased. Instead, the ground-state absorption (Figure 3) and emission (Figure 4) spectra of the chromophores dissolved in benzene show a complex trend. The maximum absorption energies follow the trend ES(L1) > ES(L2) ≈ ES(Z3) > ES(C4) > ES(L3) > ES(C5). Even though nFAR(L2) = 2 and nFAR(Z3) = 3, the two chromophores have nearly the same S1 state energy, although the increased
ET ¼ Eðtriplet-state-relaxed-geometryÞ Eðground-state-relaxed-geometryÞ
This method has been used successfully in several previous publications and accounts for the experimental evidence that the T1 state forms through intersystem crossing from the delocalized S1 state to the T1 state that is confined to one ligand.1b
’ RESULTS AND DISCUSSION Tables 1 and 2 summarize the linear and time-resolved spectroscopic properties of these compounds. Table 3 summarizes the DFT calculation results. The compounds investigated in this study (Figure 1) are based on a theoretical study of PAHs.16,17 PAHs are classified in terms of several descriptors, including the hydrogen content (HA), the number of fused aromatic rings (nFAR), and the carbon ratio (CR), defined as the ratio between isolated double bonds and the number of double bonds in a resonant sextet. For a given nFAR, the compounds with the largest HA are the cata-condensed PAH. The cata-condensed PAHs are designated as either the L or Z conformation. The cata-condensed 142
dx.doi.org/10.1021/jp203616c |J. Phys. Chem. A 2012, 116, 139–149
The Journal of Physical Chemistry A
ARTICLE
Table 2. Photophysical Properties of PE1 Derivatives in Benzene (S1fSn)max
τ1 70 ( 40 fs
L1a L2
τ2
τisc(PAH)c
4.1 ( 0.4 ps
0.44 ( 0.15 ps
470 nm
τ3
70 ns
7.9 ( 0.7 ps
(T1fTn)max
τT
630 nm
590 ps
0.3 μs
550 nm
19 μs; 92 μsd
7 ns
480 nm
1 μs; 25 μse
0.4 μs
450 nm
94 μs
0.4 μs
440 nm
16 μs; 97 μsd
7 ns
570 nm
10.3 μs
ET(C4). The behavior of both the ground-state
conjugation of Z3 results in a larger extinction coefficient. In contrast, nFAR(Z3) = nFAR(L3), but there is a very significant red shift in L3. Finally, nFAR(C4) > nFAR(L3), but L3 is redshifted. The CR,16 defined as the ratio of isolated double bonds relative to resonant sextets, shows a simple relation with the absorption and emission maxima, as listed in Table 3. For a given PAH class, as the CR increases, the absorption maximum red shifts, for example CR(L1) < CR(L2) < CR(L3) and CR(C4) < CR(C5). Similarly, CR(Z3) < CR(L3), which agrees with the red shift going from Z3 to L3. The fluorescence quantum yields tend to increase with ligand size, ϕF(L1) < ϕF(L2) < ϕF(L3) and ϕF(C4) < ϕF(C5); however, ϕF(Z3) < ϕF(L3), which parallels the CR trend. The fluorescence Stokes shift, Δν, follows the trend Δν(L1) > Δν(L2) > Δν(L3); however, Δν(L2) >Δν(Z3) > Δν(L3) and Δν(C4) > Δν(C5). The Stokes shift is described by the LippertMataga equation19 Δν ¼
2Δf ðΔμ2 Þ þ Const hca3 143
dx.doi.org/10.1021/jp203616c |J. Phys. Chem. A 2012, 116, 139–149
The Journal of Physical Chemistry A
ARTICLE
Figure 4. Emission spectra of the dyes dissolved in air-saturated benzene.
Figure 2. Structures of the unit cells obtained from X-ray diffraction measurements for L3 (above) and Z3 (below). The numbers are the acetylenic CtC bond lengths in Å. The uncertainty in the bond length is . (0.01 Å
Figure 5. Phosphorescence spectra of materials dissolved in deoxygenated benzene. The spectrum of L1 was taken at 77 K.
were done under room-temperature and air-saturated conditions. The transient absorption spectra taken immediately after the laser pulse that results from relaxation of upper vibrational and electronic states (IVR) are shown in Figure 6. The excited-state lifetimes are given in Table 2, and the time-resolved spectra are given in Figures S1S5 (Supporting Information). Also included in Table 2 are S1 state lifetimes measured by TCSPC. The timeresolved absorption spectra decay with more than one time constant. To distinguish which species is associated with intersystem crossing, we assume that the dominant decay measured by FTA associated with emission decay measured by TSCPC and whose associated excited-state absorption spectrum at longer times converts to the T1Tn spectrum measured by NTA are the criteria to measure the S1 excited-state decay time. The excited-state spectrum of L2 immediately after excitation shows an absorption band at 470 nm (Figure S1, Supporting Information). This spectrum decays rapidly (τ1 = 0.44 ps) to a broad band having a slight peak at 725 nm. The broad band absorption then decays to the triplet state (τ2 = 7.9 ps) accompanied by decay of the emission. Excitation of L3 produces an excited-state absorption at 550 nm, with biexponential decay having a minor decay (τ1 = 30.8 ps, 3%) and major decay (τ2 = 369 ps, 78%) (Figure S2, Supporting Information), both components having associated emission decay. There is also a longer decay (τ3 = 3368 ps, 19%) not associated with emission. There is an isosbestic point at 475 nm, showing that there are only two species contributing to the spectrum. Comparison of absorption spectra of L3 before and after excitation shows
Figure 3. Ground-state absorption spectra of the dyes dissolved in benzene.
absorption spectra and the phosphorescence spectra is very different from that of previously investigated complexes PE1, PE2, and PE3, having ligands H(C6H4CtC)nH, n = 13.1a,b In these complexes, there is a linear increase in the number of phenyl acetylene units. The S1 and T1 state energies decrease in a linear fashion with 1/n, n being the number of phenyl acetylene groups per ligand.1a The singlet-state lifetime of L1 has been measured to be 70 fs by the fluorescence upconversion technique and is reported in a previous publication.1 FTA was utilized to measure the shorttime kinetics of the other complexes. In this experiment, we excited the sample at 400 nm and then probed transient absorption with a white light continuum from 450 to 800 nm. All experiments 144
dx.doi.org/10.1021/jp203616c |J. Phys. Chem. A 2012, 116, 139–149
The Journal of Physical Chemistry A
ARTICLE
Figure 6. Ultrafast time-resolved absorption spectra measured immediately following the 400 nm laser pulse.
Figure 7. Plot of absorption maximum of S1 f Sn and S0 f S1 spectra as a function of nFAR.
a decrease of the absorbance from 1.3 to 0.4, giving evidence of photodegradation (Figure S2A, Supporting Information). Photodegradation is not observed in any of the other complexes. Immediately after excitation, Z3 shows a peak at 525 nm and another peak at 625 nm (Figure S3, Supporting Information). The 525 nm band rapidly decays (τ1 = 0.65 ps) followed by slower decay of the 625 nm band (τ2 = 20 ps). Accompanied by excited-state emission, the remaining broad-band spectrum decays to the triplet state (τ3 = 102 ps). The ultrafast spectra and kinetics of C4 agree with published results (Figure S4, Supporting Information).4 Excitation of C5 initially decays to produce a band at 730 nm, which decays to the triplet state (τ = 525 ps) (Figure S5, Supporting Information). Figure 7 shows a plot of the S1 f Sn transition absorption maximum as a function of nFAR obtained from Figure 6. The data could be fitted to a regression equation EðS1 f Sn ÞðeVÞ ¼ 3:25 0:31ðnFARÞ
Figure 8. Triplet-state absorption spectra. The data are normalized to the bleaching peak for comparison.
r ¼ 0:993
From this relation, we estimate the energy for the L1 S1Sn state to be 2.94 eV. The corresponding S1 state energies are also plotted and fit to a regression equation EðS0 f S1 ÞðeVÞ ¼ 4:16 0:30ðnFARÞ
absorption maxima follow the trend Emax(L1) < Emax(C5) < Emax (L2) < Emax(L3) < Emax(Z3) < Emax(C4). The trends are difficult to interpret as calculation of open-shell systems by unrestricted KohnSham-based TDDFT methods is often heavily spincontaminated and therefore meaningless.20 The series L1L3 is of interest in that the absorption maximum blue shifts with increasing ligand conjugation. This trend is opposite from that seen from phosphorescence spectra where the T1 state energy red shifts with increased conjugation. Previous investigation of the relation between ligand length and triplet-state properties has shown the platinum influence is very strong for smaller ligands, while complexes with larger ligands behave like the free ligands.1a,b The platinum influence is largest in L1 and decreases going from L2 to L3. For L1, the triplet lifetime is very short relative to the other chromophores.1 The triplet spectra of L2 (Figure S6, Supporting Information) show an isosbestic point resulting from depletion of the ground state and formation of the triplet state. The triplet state of L2 shows decay that is fitted to a biexponential
r ¼ 0:860
The slopes of the two lines are equal, but there is more scatter in the S0 f S1 energies as E(Z3) < E(L2) and E(Z3) = E(L3) in the excited-state spectra but E(L2) = E(Z3) in the ground-state spectra. Also, E(C4) < E(L3) in the excited-state spectra, but E(L3) < E(C4) in the ground-state spectra. The results suggest that higher excited states are more delocalized than the S1 state and are therefore predominantly a function of nFAR. On the basis of the behavior of the linear spectra, there will most likely be a continuing red shift for L4 and L5 but not for Z4 and Z5. The triplet-state spectra are shown in Figure 8, and the triplet lifetimes are given in Table 2. Also included are the data collected for L1 in benzene on a picosecond time scale.1 The T1 f Tn 145
dx.doi.org/10.1021/jp203616c |J. Phys. Chem. A 2012, 116, 139–149
The Journal of Physical Chemistry A
ARTICLE
Figure 9. Plot of experimental S1 and T1 state energies versus calculated energies.
Figure 10. Plot of HOMO1, HOMO, LUMO, and LUMO+1 energy levels for L3, L2, and Z3.
curve resulting from direct decay to the ground state and possible triplettriplet annihilation. The time-resolved triplet state spectrum of L3 (Figure S8, Supporting Information) has a poor signal-to-noise ratio. As shown in Figure S9 (Supporting Information), the transient absorption of L3 shows a growth and decay at 470 nm, possibly consistent with triplet excimer formation, as has been previously observed in PE2-Pt.21 The tendency to aggregate is suggested from poor solvent interaction, indicated from the small fluorescence Stokes shift seen in L3 compared to L2 and Z3 (Table 3), promoting excimer formation and photoinstability (Figure S2A, Supporting Information). In contrast, the triplet-state spectrum of Z3 has good signal-to-noise ratio and a single exponential decay consistent with good solubilization, also indicated from the larger Stokes shift compared to that of L3. Its time-resolved spectrum (Figure S7, Supporting Information) shows an isosbestic point at 375 nm, showing the contribution of singlet-state depletion and formation of the triplet state. The triplet-state decay of C4 also fits to a biexponential curve, and its spectrum (Figure S10, Supporting Information) shows an isosbestic point at 425 nm. The triplet state of C5 has a single exponential decay, and its spectrum (Figure S11, Supporting Information) has isosbestic points at 425 and 515 nm. Future work will focus on the photodegration, excimer formation, and triplettriplet annihilation behavior of these compounds in more detail. Table 3 lists the results of various DFT calculations performed on these compounds. For all compounds except L1, the lowestlying singlet state results from a HOMO f LUMO transition. The ground state has Ag symmetry and can be described as a linear combination of dxz orbitals on the central platinum and π orbitals on the two ligands.
either 1La or 1Lb orbital characteristics.22 In L-shaped PAHs, the S1 state is the 1La band, with HOMO f LUMO character, while in Z-shaped PAHs, the S1 state is the 1Lb band with mixed HOMO1 f LUMO and HOMO f LUMO+1 character. In general, f(1Lb) < f(1La).23 Table 3 gives calculated S1 state energies and oscillator strengths for the ligands containing the parent PAH and a terminal acetylene group, designated as Xn-l. In L1-l to L5-l, the S1 state has1La character. In Z3-l and Z5-l, the S1 state is the 1Lb band, and the S2 state is the 1La band. The S1 state of Z4-l is the 1La band. The S1 states of both C4-l and C5-l have 1La character. With the exception of L1, the oscillator strength of the platinum complexes is larger than that of the ligand. The oscillator strengths follow the trend f(Z) ≈ f(C) > f(L), which parallels the experimental values given in Table 1. A plot of the measured energies (Figure 9) of the S1 and T1 states versus calculated energies shows a good correlation. The relation between calculated and experimental S1 and T1 state energies in eV is EðS1 , expt, eVÞ ¼ 1:12 þ 0:666EðS1 , calcÞ EðT1 , expt, eVÞ ¼ 0:02 þ 0:994EðT1 , calcÞ
3
The S1 state has MLCT character and Au symmetry Ψ1
¼
21=2 ð1 π1
r ¼ 0:995
The T1 state energy was calculated from the ΔSCF method, which has been shown to be accurate in other platinum acetylides.1b In the previously published calculations of triplet-state properties of platinum acetylides, the spin density of the energyminimized T1 state structure shows that the triplet state is confined to one ligand, while the other ligand is in the ground state.1a,b The relaxed T1 state can be described according to the expression
ψ0 ¼ c1 π1 þ c2 dxz ðPtÞ þ c1 π2
1
r ¼ 0:959
ψ ¼ c1 ð3 π1 π2 þ 3 π2 π1 Þ þ c2 dxz ðPtÞ
The triplet exciton resides on one ligand, while the other ligand is in the ground state. Table 3 lists values for SD1, the total spin density on the phenyl acetylene portion of the ligand containing the triplet exciton, as shown in red (Figure 1), and SD2, the spin density in the rest of the ligand. The sum SD1 + SD2 ≈ 2 verifies
1 π2 Þ
where there is charge transfer from the dxz orbital of the central platinum atom to ligand π* orbitals. In PAHs, the S1 state has 146
dx.doi.org/10.1021/jp203616c |J. Phys. Chem. A 2012, 116, 139–149
The Journal of Physical Chemistry A
ARTICLE
Figure 11. (A) Plot of calculated S1 and T1 state energies versus the number of fused aromatic rings (nFAR) in the ligand. There is a separate curve for the L, Z, and C series of chromophores. (B) Plot of the calculated dominant low-lying singlet-state energy for the ligands and their corresponding platinum complexes.
that the lowest-energy T1 exciton is confined to one ligand in these systems. The difference between the calculated and experimental energies of the S1 state has been attributed to problems with TDDFT calculations of ionic excited states.24 The 1La S1 state of PAH has significant ionic character, while the T1 state has less ionic biradical character.25 There is a strong correlation between calculated and experimental S1 and T1 energies with the calculated HOMOLUMO gap (Figure S12, Supporting Information). EðS1 state, eVÞ ¼ 0:028 þ 0:913EHL
r ¼ 0:999
EðT1 state, eVÞ ¼ 0:522 þ 0:742EHL
r ¼ 0:987
decreases 0.19 eV and the LUMO increases by 0.43 eV. C5 can be described as L2 where naphthalene is added to each ligand, increasing the HOMO of C5 by 0.34 eV and decreasing the LUMO by 0.89 eV. Figure 11A shows a plot of calculated S1 and T1 state energies for the entire L, Z, and C series compounds as a function of nFAR. We were interested in the effect of ligand shape on the lowest-lying singlet-state and triplet-state energies. For a given nFAR, the singlet-state energies follow the order ES(Z) > ES(C) > ES(L). Similarly, ET(Z) > ET(C) > ET(L). Comparing the ligand types, the state energies show different behavior with increasing nFAR. The L series shows linear decrease in the S1 and T1 state energy with increasing nFAR. Indeed, the T1 energy of L5 is predicted to be less than 1 eV. Published UB3LYP/6-31G(d) calculations on linear polyacenes show that they behave like polyacetylene diradicals when nFAR approaches 10,27 with each of the singly occupied orbitals occupying one ribbon and behaving like a disjointed diradical. We performed a similar unrestricted calculation on the L5 ground state and saw no evidence of biradical formation in this compound, although biradicaloid behavior is expected to appear with larger ligands. The Z series shows completely different behavior. Both the S1 and T1 energies oscillate with increasing nFAR. This band gap oscillation has been shown to occur in phenanthrene oligomers of varying length.13 The C series has behavior similar to that of the L series, with the S1 and T1 energies decreasing with increasing nFAR. Figure 11B shows calculated S1 state energies for both the ligands and their corresponding platinum complexes. Aside from a lowering in state energy due to conjugation across the central platinum atom, the state energy is a function of ligand shape (L, Z, or C) for both the ligands and platinum complexes. We calculated the nonradiative decay constant, knr, from the S1 state lifetime measured from FTA experiments (Table 2)
The result illustrates that the S1 and T1 states are dominated by HOMO f LUMO transitions. As nFAR increases, the calculated singlettriplet splitting converges to 0.91.0 eV for all three types of chromophores. Figure 10 compares energy levels for L2, L3, and Z3 and explains why the band gap of the Z compounds is larger than that of the L compounds.26 L series PAHs are built by adding a butadiene unit onto the growing chain. Because of destabilizing antibonding interactions between the HOMO of the linear unit and the HOMO of the butadiene unit, E(HOMO) increases 0.27 eV upon conversion of L2 to L3. There are stabilizing bonding interactions between the LUMO of the L2 unit and the butadiene LUMO, causing E(LUMO) to decrease 0.60 eV. The combined effect is to cause a decrease in the band gap upon conversion from L2 to L3. In contrast, addition of the butadiene unit to build Z3 causes E(HOMO) to decrease 0.03 eV and E(LUMO) to also decrease 0.03 eV, causing the HOMOLUMO gap of the Z3 compound to be larger than that of the L3 but nearly equal to that of L2. The other complexes can be analyzed in a similar way. C4 can be described as L3 with an ethylene fragment added to the ligand’s bay region where the E(HOMO)
τS ¼ 147
1 kr þ knr dx.doi.org/10.1021/jp203616c |J. Phys. Chem. A 2012, 116, 139–149
The Journal of Physical Chemistry A
ARTICLE
The dependence that we observed can be analyzed in terms of the basic expression for the intersystem crossing rate constant34 kisc ¼
2π 2 β FF p e
where βe is the spinorbit coupling energy, F is the Franck Condon factor between the S1 state and the various Tn vibronic levels, and F is the density of states in the triplet manifold. The spinorbit coupling energy is35 βe ¼ ÆS1 jHSO jT1 æ The quantity HSO is the spinorbit coupling Hamiltonian HSO ≈
where μS is the magnetic moment due to electron spin and μL is the magnetic moment due to electron orbit. The fourth power of Z is rigorously true for one-electron atoms but is an effective exponent in polyatomic molecules reflecting shielding of the nucleus by core electrons,36 although an empirical measurement of βe as a function of metal atomic number in a series of organometallics does give a fourth power.37 The only significant contribution to HSO comes from the central platinum atom (Z = 78); therefore, βe is a sum of matrix elements involving short-range interactions between orbitals on the platinum atom and orbitals on nearby atoms. As Nπ increases, orbitals on distal atoms have negligible spinorbit coupling. As a result, βe decreases with increasing nFAR, leading to smaller knr values. A secondary influence on knr is the CR (Table 3). For the L chromophores and the C chromophores, increasing CR is associated with a lower knr. This is also true when comparing Z3 and L3, where CR(Z3) = 0.17 while CR(L3) = 1.33.
Figure 12. The upper line is a plot of log(knr) versus the number of π electrons in the ligand aromatic ring. The lower line is a plot of log(knr(platinum complex)) log(kisc(parent PAH)). The kisc values of the parent PAHs are published and also listed in Table 2.
and the fluorescence quantum yield ϕfl ¼
kr kr þ knr
The nonradiative rate constant knr = kic + kisc includes internal conversion and intersystem crossing. The internal conversion rate constant is given as kic ≈ 1013 e
γΔE pω
where γ is inversely proportional to the displacement of the S1 surface relative to the S0 surface, pω is the “acceptor” vibrational frequency of the S0 surface, considered to be the 3000 cm1 CH stretch vibration in aromatic chromophores, and ΔE is the S0S1 energy gap.28,29 Values for γ/pω range from 3.8 in platinum acetylide complexes30 to 7.54 in osmium complexes.31 Significant internal conversion may occur in C5 as ES has been measured to be 2.5 eV and kic is estimated to be 7 104 to 7 108 s1, suggesting that kic may be a few percent of kisc in C5 but knr ≈ kisc for the other complexes. Also listed in Table 2 are literature τisc values for the parent PAHs.32 Figure 12 is a plot of log(knr) versus Nπ, the number of π electrons in the PAH aromatic ring. There is a linear relation. logðknr ðsec1 ÞÞ ¼ 14:7 0:32Nπ
ðr ¼ 0:920Þ
Also shown is the ratio log(knr(platinum complex)/kisc(parent PAH)). In comparison with published kisc of the parent PAH, the platinum effect on knr is very large (106) in L1 and L2, intermediate with Z3 and C4, and relatively small (10) for L3 and C5. The enhancement seen with C4 is similar to that reported for organophosphine gold- and nitropyrene, which is attributed to intersystem crossing from S1 to higher triplet levels followed by internal conversion to the T1 state.33
Z4 μμ r3 S L
’ CONCLUSION We have developed a structurespectroscopic property relationship in platinum acetylides having poly(aromatic hydrocarbon) ligands. The synthesis of a series of chromophores with systematic variation in the number of fused aromatic rings (nFAR) and ligand topology (polyacene (L), polyphenanthrene (Z), or compact (C)) has been combined with results from ab initio calculations. In both the DFT results and experiment, the S1 and T1 state energies are a function of both nFAR and ligand topology. In the L chromophores, the energies decrease linearly with nFAR, while the energies of the Z chromophores oscillate around a fixed value with increasing nFAR. The C chromophores have behavior intermediate between that of the L and Z chromophores. The present work shows that the framework described in ref 16 developed for the analysis of PAH properties is useful for the understanding of the corresponding platinum acetylide complexes. This approach makes possible the analysis of platinum acetylides containing complex PAH ligands. For example, ref 6 describes a platinum complex containing a benzocoronene ligand having nFAR = 13, CR = 0, six hexagonal holes, and six bay regions that can be analyzed by the approach described in this paper. Future work will focus on systematic variation of these structural descriptors in a variety of complexes to give more insight into the structureproperty relationship. 148
dx.doi.org/10.1021/jp203616c |J. Phys. Chem. A 2012, 116, 139–149
The Journal of Physical Chemistry A
ARTICLE
’ ASSOCIATED CONTENT
(23) Clar, E. Polycyclic Hydrocarbons; Academic Press: London, 1964. (24) (a) Kadantsev, E. S.; Stott, M. J.; Rubio, A. J. Chem. Phys. 2006, 124, 134901. (b) Parac, M.; Grimme, S. Chem. Phys. 2003, 292, 11. (25) (a) Wiberg, K. B. J. Org. Chem. 1997, 62, 5720. (b) Wataru, S.; Hiroyuki, S.; Azuma, M.; Norio, S. J. Mol. Struct: THEOCHEM 2006, 759, 165. (c) Kadantsev, E. S.; Stott, M. J.; Rubio, A. J. Chem. Phys. 2006, 124, 134901. (26) Yoshizawa, K.; Yahara, K.; Tanaka, K.; Yamabe, T. J. Phys. Chem. B 1998, 102, 498. (27) Bendikov, M.; Duong, H. M.; Starkey, K.; Houk, K. N.; Carter, E. A.; Wudl, F. J. Am. Chem. Soc. 2004, 126, 7416. (28) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145. (29) Turro, N. J. Modern Molecular Photochemistry: University Science Books: Sausalito, CA, 2006; p 183. (30) Wilson, J. W.; Chawdhury, N.; Al-Mandharym, M. R. A.; Younus, M.; Khan, M. S.; Raithby, P. R.; Kohler, A.; Friend, R. H. J. Am. Chem. Soc. 2001, 123, 9412. (31) Caspar, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952. (32) (a) Nijegorodov, N. I.; Downey, W. S. J. Phys. Chem. 1994, 98, 5639. (b) Nijegorodov, N.; Winkoun, D. P. Spectrochim. Acta, Part A 1997, 53, 2013. (c) Nijegorodov, N.; Vasilenko, V.; Monowe, P.; Masale, M. Spectrochim. Acta, Part A 2009, 74, 188. (33) (a) Vogt, R. A.; Peay, M. A.; Gray, T. G.; Crespo-Hernandez, C. E. J. Phys. Chem. Lett. 2010, 1, 1205. (b) Crespo-Hernandez, C. E.; Burdzinski, G.; Arce, R. J. Phys. Chem. A 2008, 112, 6313. (34) Freed, K. F. Acc. Chem. Res. 1978, 11, 74. (35) McGlynn, S. P.; Azumi, T.; Kinoshita, M. The Triplet State; Prentice-Hall: New York, 1969; p 190. (36) Su, M.-D. J. Phys. Chem. 1996, 100, 4339. (37) Jacko, A. C.; McKenzie, R. H.; Powell, B. J. J. Mater. Chem. 2010, 20, 10301.
bS
Supporting Information. Transient absorption spectra of L2, L3, Z3, C4, and C5, images of molecular orbitals, and X-ray crystallographic files (CIF) of L3 and Z3. This material is available free of charge via the Internet at http://pubs.acs.org.
’ ACKNOWLEDGMENT The authors thank Yosadara Ruiz-Morales (Mexican Petroleum Institute) for teaching us the framework for analyzing poly(aromatic hydrocarbon) structureproperty relationships and the referees for useful feedback. We thank the support of this work by AFRL/RX Contracts F33615-99-C-5415 for D.G.M., F33615-03-D-5408 for D.M.K. and A.R.B., and F33615-03-D5421 for J.E.S. ’ REFERENCES (1) (a) Rogers, J. E.; Cooper, T. M.; Fleitz, P. A.; Glass, D. J.; McLean, D. G. J. Phys. Chem. A 2002, 106, 10108. (b) Cooper, T. M.; Krein, D. M.; Burke, A. R.; McLean, D. G.; Rogers, J. E.; Slagle, J. E. J. Phys. Chem. A 2006, 110, 13370. (c) Rogers, J. E.; Slagle, J. E.; Krein, D. M.; Burke, A. R.; Hall, B. C.; Fratini, A.; McLean, D. G.; Fleitz, P. A.; Cooper, T. M.; Drobizhev, M.; Makarov, N. S.; Rebane, A.; Kim, K.-Y.; Farley, R.; Schanze, K. S. Inorg. Chem. 2007, 46, 6483. (d) Ramakrishna, G.; Goodson, T.; Rogers-Haley, J. E.; Cooper, T. M.; McLean, D. G.; Urbas, A. M. J. Phys. Chem. C 2009, 113, 1060. (e) Haley, J. E.; Krein, D. M.; Monahan, J. L.; Burke, A. R.; McLean, D. G.; Slagle, J. E.; Fratini, A.; Cooper, T. M. J. Phys. Chem. A 2011, 115, 265. (2) (a) Anthony, J. E. Chem. Rev. 2006, 106, 5028. (b) Anthony, J. E. Angew. Chem., Int. Ed. 2008, 47, 452. (3) Radovic, L. R.; Bockrath, B. J. Am. Chem. Soc. 2005, 127, 5917. (4) Baba, M. J. Phys. Chem. A 2011, 115, 9514. (5) Wang, Y.; Huang, Y.; Song, Y.; Zhang, X.; Ma.; Liang, J.; Chen, Y. Nano Lett. 2009, 9, 220. (6) Kim, K.-Y.; Liu, S.; Kose, M. E.; Schanze, K. S. Inorg. Chem. 2006, 49, 2509. (7) Danilov, E. O.; Pomestchenko, I. E.; Kinayyigit, S.; Gentili, P. L.; Hissler, M.; Ziessel, R.; Castellano, F. N. J. Phys. Chem. A 2005, 109, 2465. (8) Oxford Diffraction. CrysAlisPro CCD and CrysAlisPro RED; Oxford Diffraction Ltd.: Yarnton, Oxfordshire, U.K., 2008. (9) Sheldrick, G. M. SHELXTL, version 6.10; Bruker AXS Inc.: Madison, WI, 2000. (10) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991. (11) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, revision B.05; Gaussian, Inc.: Pittsburgh, PA, 2003. (12) (a) Norman, P.; Crostrand, P.; Ericsson J. Chem. Phys. 2002, 285, 207. (b) Baev, A.; Rubio-Pons, O.; Gel’Mukanov, F.; Agren, H. J. Phys. Chem. A 2004, 108, 7406. (13) Furche, F.; Ahrlichs, R. J. Chem. Phys. 2002, 117, 7433. (14) Koch, W.; Holthausen, M. C. A Chemist’s Guide to Density Functional Theory; Wiley-VCH: Weinheim, Germany, 2001. (15) Batista, E. R.; Martin, R. L. J. Phys. Chem. A 2005, 109, 9856. (16) (a) Ruiz-Morales, Y. J. Phys. Chem. A 2002, 106, 11283. (b) Ruiz-Morales, Y.; Mullins, O. C. Energy Fuels 2007, 21, 256. (17) Diaz, J. R. Acc. Chem. Res. 1985, 18, 241. (18) Roncali, J. Chem. Rev. 1997, 97, 173. (19) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 1983. (20) Li, Z.; Liu, W. J. Chem. Phys. 2010, 133, 064106. (21) Slagle, J. E.; Cooper, T. M.; Krein, D. M.; Rogers, Joy E.; McLean, D. G.; Urbas, A. M. Chem. Phys. Lett. 2007, 447, 65. (22) Platt, J. R. J. Chem. Phys. 1949, 17, 484. 149
dx.doi.org/10.1021/jp203616c |J. Phys. Chem. A 2012, 116, 139–149
