Article pubs.acs.org/JPCB
Intramolecular Triplet Energy Transfer in Anthracene-Based Platinum Acetylide Oligomers Yongjun Li,*,†,§ Muhammet E. Köse,‡ and Kirk S. Schanze† †
Department of Chemistry, University of Florida, Gainesville, Florida 32611, United States TUBITAK National Metrology Institute, Gebze, Kocaeli, 41470 Turkey
‡
S Supporting Information *
ABSTRACT: Platinum acetylide oligomers that contain an anthracene moiety have been synthesized and subjected to photophysical characterization. Spectroscopic measurement and DFT calculations reveal that both the singlet and triplet energy levels of the anthracene segment are lower than those of the platinum acetylide segment. Thus, the platinum acetylide segment acts as a sensitizer to populate the triplet state of the anthrancene segment via intramolecular triplet−triplet energy transfer. The objective of this work is to understand the mechanisms of energytransfer dynamics in these systems. Fluorescence quenching and the dominant triplet absorption that arises from the anthracene segment in the transient absorption spectrum of Pt4An give clear evidence that energy transfer adopts an indirect mechanism, which begins with singlet−triplet energy transfer from the anthracene segment to the platinum acetylide segment followed by triplet− triplet energy transfer to the anthracene segment.
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INTRODUCTION Triplet−triplet energy transfer is an important type of energy transfer in chemical and biochemical processes.1 The mechanism for triplet energy transfer is usually described by Dexter electron-exchange interaction and may be visualized in terms of a two-electron transfer process. Triplet−triplet energy transfer has been studied in many donor−acceptor systems.2−15 Eng and co-workers12 designed a series of porphyrin-based donor−bridge−acceptor systems with varying bridge length to investigate triplet excitation energy transfer. They found that triplet−triplet energy-transfer rates decrease exponentially with increasing the distance between the donor and the acceptor. Both their experimental and theoretical results indicate that energy transfer adopts an electron superexchange mechanism. In another study, Danilov and co-workers8 reported ultrafast energy migration in platinum(II) diimine complexes bearing pyrenylacetylide chrompophores. They found that triplet− triplet energy transfer (from 3MLCT [metal to ligand charge transfer] to ligand 3(π,π*)) occurs with a few hundred femtoseconds because of highly electronically coupled structures. We recently reported intrachain triplet energy transfer in platinum acetylide copolymers.9 These copolymers consist of major repeat units of the type [trans-Pt(PBu3)2(−CCPhCC−)], where Ph = 1,4-phenylene, with randomly incorporated repeat units of the type [transPt(PBu3)2(−CCTCC−)], where T = 2,5-thienylene. Photoluminescence and transient absorption spectroscopy indicate that at room temperature intrachain triplet transfer from 1,4-phenylene to the 2,5-thienlyene repeats occurs rapidly © 2013 American Chemical Society
and efficiently in the copolymer with a low content of 2,5thienylene repeats. The rate constant for energy transfer is greater than 108 s−1. At low temperature, triplet energy transfer is much less efficient and a fraction of the triplet excitations is trapped on the high-energy 1,4-phenylene units. In a continuation of our investigation of intrachain triplet energy transfer in π-conjugated platinum acetylide systems, we designed and synthesized donor−acceptor systems that incorporate an anthracene moiety into the platinum acetylide backbone. The anthracene-based conjugated systems have been extensively studied due to their high fluorescence yield.16−27 Kashiwagi and co-workers25 recently reported fluorine-containing biethynylanthracene derivatives, which they utilized to fabricate organic field-effect transistors. Zhao and co-workers27 reported synthesis and characterization of a series of conjugated anthracene/fluorene oligomers. They found that these oligomers emit yellow fluorescence with a high quantum yield due to the 9,10-biethynylanthracene incorporated into the oligomer backbone. However, little work has been done to explore phosphorescence and triplet excited states of anthracene-based derivatives due to low intersystem crossing yield in these systems. Triplet−triplet energy transfer provides an effective means to sensitize a triplet excited state that is not efficiently populated by intersystem crossing. It requires that the lowest triplet Received: April 1, 2013 Revised: June 14, 2013 Published: July 2, 2013 9025
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Scheme 1. Chemical Structures of Anthracene-Based Platinum Acetylide Oligomers and the Reference Compounds
autotuning temperature controller. Samples were degassed by three consecutive freeze−pump−thaw cycles on a high-vacuum (10−5 Torr) line. Photoluminescence quantum yields were determined by relative actinometry. BPEA (ΦF = 1.0 in THF) and Ru(bpy)32+ (ΦF = 0.0379 in air-saturated H2O) were used as actinometers for Pt2An and Pt4An, respectively. Time-resolved emission measurements were conducted on a previously described home-built apparatus.30 Optically dilute solutions were used. Transient absorption measurements were carried out on a home-built apparatus consisting of a Continuum Surelite series Nd:YAG laser as the excitation source (λ = 355 nm, 10 ns fwhm). Typical excitation energies were 5 mJ/pulse. The source for monitoring optical transients was a Hamamatsu Super-Quiet series xenon flashlamp, and the monitoring light was detected by a Princeton Instruments PI-MAX intensified CCD camera detector coupled to an Acton SpectroPro 150 spectrograph. Samples were contained in a cell with a total volume of 10 mL, and the contents were continuously circulated through the pump−probe region of the cell. Solutions were degassed by argon purging for 30 min. Sample concentrations were adjusted so that A355 ≈ 0.8. Calculations. Density functional theory (DFT) calculations were carried out using the Gaussian 03 program. Geometry optimization and time-dependent DFT calculations were performed with B3LYP functional along with SDD basis set. SDD basis set incorporates relativistic effective core potential with explicit treatment of platinum and the Dunning−Huzinaga valence double-ζ basis set for the other atoms. This method has been successfully used for similar systems in our previous study.31
energy level of the donor lies above that of the acceptor. According to literature reports,28 the first singlet excited-state energy of 9,10-bis(phenylethynyl)anthracene (BPEA) lies at ∼2.4 eV, which is less than that of the platinum acetylide chromophore (∼3.0 eV). Our studies of platinum acetylide oligomers containing the phenyl spacer indicate that the first triplet excited-state energy of the platinum acetylide chromophore is ∼2.38 eV.29 The first triplet excited-state energy of BPEA or its derivatives has not been reported to the best of our knowledge. However, according to the DFT calculations in our newly designed compounds (refer to the Results and Discussion section), the first triplet excited-state energy of the anthracene moiety lies at ∼1.31 eV, which is much lower than that of the platinum acetylide chromophore. Because of the difference in energy levels between the anthracene moiety and the platinum acetylide moiety, it allows us to study the influence of the low-energy trap on the triplet exciton in the platinum acetylide systems. The structures of the focus of the current study are shown in Scheme 1. Pt2An and Pt4An consist of the core segment of anthracene and one and two repeat units of the type [−CC-trans-Pt(PBu3)2C CPh], respectively. Our previously studied platinum complexes Pt2 and Pt4 are also shown as references (Scheme 1).
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EXPERIMENTAL SECTION Synthesis. The synthesis of the reference compounds Pt2 and Pt4 was reported previously.29 The synthesis and characterization of Pt2An and Pt4An are described in detail in the Supporting Information. Photophysical Measurements. Steady-state absorption spectra were recorded on a Varian Cary 100 dual-beam spectrophotometer. Corrected steady-state emission measurements were conducted on a SPEX F-112 fluorescence spectrometer. Samples were degassed by argon purging for 30 min, and concentrations were adjusted such that the solutions were optically dilute (Amax < 0.20). Low-temperature emission measurements were conducted in 1 cm diameter borosilicate glass tubes in a liquid-nitrogen-cooled Oxford Instruments DN1704 optical cryostat connected to an Omega CYC3200
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RESULTS AND DISCUSSION UV−Visible Absorption Spectroscopy. The absorption spectra of Pt2An and Pt4An were recorded in dilute THF solutions (Figure 1). Both spectra feature two primary transitions: the low-energy bands with two absorption maxima (λmax = 450 and 480 nm), which are attributed to π,π* transition of the anthracene segment. This is consistent with the literature report on an anthracene-based platinum9026
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CC−] for Pt2An and the unit of the type [−PtCC PhCCPt−] for Pt4An. Steady-State Photoluminescence Spectroscopy. The photoluminescence spectra of Pt2An and Pt4An were recorded in deoxygenated THF solutions with several excitation wavelengths (Figure 2). The photoluminescence spectrum of
Figure 1. Absorption spectra in THF solution. The spectra were normalized at λ ≈ 480 nm for Pt2An and Pt4An, and λ ≈ 450 nm for BPEA.
containing compound that is structurally similar to Pt2An.32 According to the polarized electronic spectroscopy study of BPEA by Levitus and co-workers,28 the lower energy absorption band (λmax = 480 nm) is attributed to the longaxis π,π* transition (the short axis of anthracene) and the higher energy absorption band (λmax = 450 nm) arises from the overlap of the long- and short-axis π,π* transition of the anthracene segment. Compared with the absorption spectrum of BPEA, two features can be seen with respect to the anthracene absorption in Pt2An and Pt4An. First, both anthracene absorption bands in Pt2An and Pt4An are redshifted ca. 30 nm, which are caused by conjugation of the diethynylanthracene unit with the platinum centers in Pt2An and Pt4An. However, the red shift from Pt2An to Pt4An is small (∼3 nm), indicating that the effective oligomer chain length is confined within one repeat unit. Second, the two absorption bands (λ = 450 and 480 nm) are better resolved in Pt2An and Pt4An likely due to the conformational restriction in these molecules caused by the steric hindrance between the P(n-Bu)3 ligands and the anthracene segment. The absorption in the near-UV region is very different for the two complexes. For Pt2An a narrow absorption band centered at λ = 317 nm likely arises from the anthracene segment since a similar absorption band also appears in BPEA at λ = 300 nm. Due to the conjugation of the platinum centers with the diethynylanthrancene, this band is red-shifted 17 nm from BPEA to Pt2An. For Pt4An a broad and intense band concentrated at λ = 355 nm is assigned to the π,π* transition of the platinum acetylide segment [−PtCCPh−CC−Pt−]. Note that a shoulder appears at about 350 nm in the Pt2An absorption spectrum likely due to the π,π* transition of the platinum acetylide segment that features the [PhCCPtCC−] chromophore. To explore influences of the conjugation length of the platinum acetylide segment on the absorption spectra, we used our previously studied platinum acetylide oligomers Pt2 and Pt4 as references for Pt2An and Pt4An, respectively. The absorption spectra of the complexes Pt2 and Pt4 exhibit the dominant bands at UV region (355 nm for Pt2 and 367 nm for Pt4) according to our previous report.29 The absorption bands of the platinum acetylide (phenylene chromophores) in both Pt2An and Pt4An are slightly blue-shifted compared with their corresponding references, indicating that the effective conjugation length is confined within the platinum acetylide segment, in particular, the unit of the type [PhCCPt
Figure 2. Photoluminescence spectra in deoxygenated THF solution of (a) Pt2An and (b) Pt4An (legends indicate the excitation wavelengths).
Pt2An exhibits an intense emission band centered at λmax = 505 nm along with a vibronic band at λ = 550 nm. The emission intensity increases with excitation wavelength and reaches a maximum with λex = 480 nm. We assigned these bands to the fluorescence emission that arises from the anthracene segment. Compared with BPEA, the emission band is red-shifted ca. 40 nm due to the conjugation with the platinum centers in Pt2An (Figure 3). The fluorescence lifetime for Pt2An is ∼10 ns. A small Stokes shift was also observed with respect to the absorption. The fluorescence quantum yield measured with BPEA as the standard is ∼0.96 for Pt2An. Phosphorescence emission originating from the platinum acetylide segment was not observed. For Pt4An the fluorescence bands originating from the anthracene segment are also shown when the anthracene segment was directly excited (λex = 450 and 480 nm). With direct excitation of the platinum acetylide segment at λ = 355 nm, the spectrum features the fluorescence arising from the anthracene segment, as well as a weak band centered at λ ≈ 400 nm that is assigned to fluorescence originating from the platinum acetylide segment. The fluorescence quantum yield of Pt4An measured with Ru(bpy)32+ in air-saturated water as the standard is ∼0.014. Compared with Pt2An, the fluorescence in Pt4An is significantly less efficient. Photophysical data for both complexes are shown in Table 1. 9027
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Figure 3. Normalized photoluminescence spectra of Pt2An and Pt4An in deoxygenated THF solution with λex = 450 nm. For BPEA λex = 440 nm.
Table 1. Photophysical Data of Pt2An and Pt4An in THF Solutions λmax/nm Pt2An Pt4An a
absorption
emission
quantum yield (F)a (ΦF)
317, 450, 480 355, 453, 483
504 504
0.96 0.014
F = fluorescence.
Figure 4. Excitation spectra in deoxygenated THF solutions of (a) Pt2An and (b) Pt4An monitored at 536 nm. Absorption spectra are also shown for comparison purposes.
To further confirm the origin of the bands in the photoluminescence spectra, the emission−excitation spectra of Pt2An and Pt4An were measured in deoxygenated THF solutions (Figure 4). The absorption spectra are also shown for comparison purposes. The fluorescence band at λ = 536 nm was monitored when scanning the excitation. The resulting excitation spectra exhibit the same shape compared to the corresponding absorption spectra. The observation of the anthracene-based bands at 450 and 480 nm in the excitation spectrum confirms that this chromophore is responsible for the fluorescence emission. To explore the effects of temperature on the spectroscopy of Pt4An, variable-temperature photoluminescence was carried out in deoxygenated 2-methyltetrahydrofuran (Me-THF) solution with the excitation wavelength at λ = 450 nm (Figure 5). Below the solvent glass temperature, the vibronic band is more resolved and a slightly narrower line shape was also observed. Low-temperature emission spectra were also recorded with λex = 355 nm. The spectra are similar to the one that was obtained with λex = 450 nm. Note that the weak peak at ca. 610 nm is a scattering peak. According to our DFT calculations, the energy level of the first triplet state of the anthracene segment in Pt2An and Pt4An is ∼1.31 eV (946 nm). In an attempt to detect the corresponding phosphorescence emission, the near-IR photoluminescence was carried out in deoxygenated THF solution. We monitored the spectra ranging from 850 to 1400 nm. The phosphorescence emission was not observed with excitation at λ = 355, 450, and 480 nm. Because of the low triplet energy of the anthracene segment, we expect that the nonradiative decay should be very rapid, which makes the detection of the phosphorescence difficult.33,34 Time-Resolved Photoluminescence Spectroscopy. To further gain insight into the dynamics and decays of singlet and triplet excited states, time-resolved emission measurements
Figure 5. Low-temperature emission spectra of Pt4An in deoxygenated Me-THF solution (the band at 610 nm is a scattering peak).
were carried out in deoxygenated THF solutions for Pt2An and Pt4An. The spectra were obtained following 355 nm laser excitation (Figure 6, Figure S1 in the Supporting Information for a comparison of the two emissions). For Pt2An the fluorescence decays quickly and the lifetime recovered from the time-resolved spectra is less than 10 ns. For Pt4An, in the early time (less than 20 ns, Figure S2 in the Supporting Information) following laser pulses, the fluorescence emission of the anthracene segment appears and it decays rapidly. A slowly decaying band centered at λ = 517 nm along with a vibronic band is observed for times longer than ∼60 ns after the laser pulse; this emission is assigned to the phosphorescence decay of the platinum acetylide segment consistent with our previous study.29 The lifetime of this emission is ∼1.6 μs, which is much shorter compared with the lifetime of its corresponding reference compound Pt4 (τ = 18.6 μs), indicating that the 9028
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Figure 6. Time-resolved emission spectra in deoxygenated THF solution of (a) Pt4An, delay increment 1 μs, initial delay 100 ns, and (b) Pt2An, delay increment 10 ns, following 355 nm laser excitation.
phosphorescence emission of the platinum acetylide segment in Pt4An is reduced by a nonradiative decay pathway that is not available in the Pt4 model. Transient Absorption Spectroscopy. In order to gain insight into the dynamics of the triplet excited states, transient absorption spectra of Pt2An and Pt4An were recorded in deoxygenated THF solutions following 355 nm laser excitation (Figure 7). Both compounds exhibit intense excited-state absorption bands centered at λmax = 380 and 530 nm along with the ground-state bleaching bands (450−500 nm). In order to assign the origin of these bands, we compare the spectra with the triplet−triplet absorption spectra of the reference compound Pt4. The triplet absorption band of Pt4 is centered at λ = 660 nm, which arises from the platinum acetylide segment according to our previous study.31 A ca. 130 nm blue shift and the completely different band shape in Pt4An transient absorption spectra indicate that the excited-state absorption band of Pt4An does not arise from the platinum acetylide segment. It is therefore reasonable to assign this band to the triplet−triplet absorption originating from the anthracene segment. Similar triplet−triplet absorption spectra for anthracene derivatives have been reported.35 Note that the long absorption tail between 600 and 700 nm may arise from remaining triplet−triplet absorption of the platinum acetylide segment (note that there is clear enhancement in the transient absorption of Pt4An in the 600−700 nm region). The triplet lifetimes recovered from transient absorption spectra at λ = 530 nm are 3.7 μs for Pt2An and 4.2 μs for Pt4An, respectively. Compared with their reference compounds, the lifetimes are much shorter in Pt2An and Pt4An, suggesting that the triplet decay is faster likely due to the low triplet energy levels in Pt2An and Pt4An that result in rapid nonradiative decay. This is consistent with the energy gap law which states that nonradiative decay rates decrease exponentially with decreasing triplet energy.
Figure 7. Transient absorption spectra in deoxygenated THF solutions of (a) Pt2An, delay increment 1 μs, and (b) Pt4An, delay increment 0.7 μs; λex = 355 nm.
Density Functional Theory Calculations. Density functional theory (DFT) calculations were applied to provide insight concerning the energies of the singlet and triplet states of Pt2An and Pt4An. Both calculation and spectroscopic energies of the singlet and triplet states are shown in Table 2. It is important to note that the calculation results are consistent 9029
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Table 2. Energies of Pt2An and Pt4Ana S1(An)/eV Ca. Pt2An Pt4An
2.54 2.52
T1(Pt)/eV
Sp.
Ca.
2.45 2.45
2.42
T1(An)/eV
Sp. 2.84 2.40
to unity, the fluorescence quantum yield of Pt4An is low (ΦF = 0.014); (3) Pt4An shows fluorescence decay in the early time stage followed by slower phosphorescence decay, and Pt2An only shows fast fluorescence decay arising from the anthracene segment; (4) the phosphorescence emission of the anthracene segment is not present in near-IR photoluminescence spectra; (5) DFT calculations indicate that the triplet energy gap between the platinum acetylide segment and the anthracene segment is large (∼1.1 eV); (6) triplet−triplet absorption of the anthracene segment dominates the transient absorption spectra along with small contributions of triplet−triplet absorption of the platinum acetylide segment for Pt4An at long wavelength (compare parts a and b of Figure 7). In order to explain the above observations, we propose energy-transfer mechanisms as shown in Figure 9 for Pt4An when excited at wavelengths corresponding to the different chromophoric units in the oligomer. Two important energytransfer processes are involved: (1) singlet to triplet energy transfer from the anthracene segment to the platinum acetylide segment; (2) triplet to triplet energy transfer from the platinum acetylide segment to the anthracene segment. Singlet to triplet energy transfer is rare because it is a spin-forbidden process, but it can occur in certain systems if a number of conditions are fulfilled.37 First, the singlet state of the donor must lie higher than the triplet state of the acceptor. For Pt4An, the triplet excited state localized on the platinum acetylide segment (2.40 eV) is lower than the singlet state energy of the donor (2.45 eV). Second, the singlet excited state of the acceptor must lie above the singlet state of the donor to avoid singlet−singlet energy transfer. In Pt4An, the singlet excited state of the platinum acetylide segment (3.1 eV) is ∼0.65 eV higher than that of the anthracene segment. Third, the donor triplet level must be considerably lower than the acceptor triplet level in order to rapidly depopulate the triplet state of the acceptor via immediate triplet−triplet energy back transfer to the donor. In Pt4An, the triplet state of the anthracene segment lies ∼1.1 eV lower than the triplet state of platinum acetylide segment, and the energy gap is large enough to make the triplet−triplet energy-transfer process efficient. The dramatic fluorescence
b
Ca.
Sp.
1.31 1.31
a
An = anthracene segment; Pt = platinum acetylide segment; Ca. = calculation; Sp. = spectroscopy (calculated from the emission spectra); S1 = first singlet excited state; T1 = first triplet excited state. bFrom ref 36.
with the experimental results of the spectroscopic study. The calculations indicate that one of the phenylene groups of Pt4An adopts planar conformation (in which the planes defined by the square-planar trans-Pt(PBu3)2(C)2 and phenylene units are coplanar) signaling the region of the oligomer where the triplet excited state is localized (Figure 8a). The triplet excited state of the platinum acetylide segment [PhCCPtCC−] in Pt2An has been reported36 as 2.84 eV, which is much higher than that of the [−PtCCPh−CCPt−] segment in Pt4An. Furthermore, the energy level of the first triplet excited state of the anthracene segment from the DFT calculations is ∼1.31 eV. Khan and co-workers 32 observed a weak phosphorescence shoulder at the energy of ∼1.50 eV in a platinum(II) diyne complex that consists of an anthracene spacer inserted into a platinum acetylide backbone that is structurally similar to Pt2An. Our DFT results are in good agreement with the literature report.32 A summary of the various state energies recovered from the DFT calculations is provided in Figure 8b. Energy-Transfer Dynamics. Prior to further discussion of the energy-transfer dynamics in the anthracene-based platinum acetylide systems, we first summarize the photophysical results of Pt4An and Pt2An based on the above spectroscopic and DFT studies: (1) both compounds show fluorescence emission originating from the anthracene segment, and the phosphorescence emission arising from the platinum acetylide segment is strongly quenched in the photoluminescence spectra; (2) in contrast with Pt2An whose fluorescence quantum yield is close
Figure 8. Energies of Pt2An and Pt4An by DFT calculations: (a) structure of Pt4An; (b) energy diagram of Pt2An and Pt4An. In the diagram, S1,An is the singlet excited state of the anthracene segment; T1,An is the triplet excited state of the anthracene segment; T1,Pt is the triplet excited state of the platinum acetylide segment. The triplet excited-state energy of Pt2An (T1,Pt = 2.84 eV) from ref 36. 9030
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Figure 9. Energy diagram of photophysical processes in Pt4An: (a) excited at λ = 355 nm; (b) excited at λ = 450 nm. An = the anthracene segment; Pt = the platinum acetylide segment.
Figure 10. Energy diagram of photophysical processes in Pt2An: (a) excited at λ = 355 nm; (b) excited at λ = 450 nm. An = the anthracene segment; Pt = the platinum acetylide segment.
evidence by the observation of the long-lived triplet−triplet absorption.
quenching in Pt4An (ΦF = 0.014) demonstrates that singlet to triplet energy transfer is efficient in this system. Following singlet−triplet energy transfer, triplet−triplet energy transfer from the platinum acetylide segment to the anthracene segment occurs. The significant phosphorescence quenching of the platinum acetylide segment provides experimental evidence for efficient triplet−triplet energy transfer. The phosphorescence emission originating from the anthracene segment was not observed in the near-IR spectroscopy, indicating that the decay of its triplet state occurs via a nonradiative pathway because of its low triplet energy level. When the platinum acetylide segment is directly excited (λex = 355 nm, Figure 9a), the singlet excited state of the platinum acetylide segment is created, followed by rapid intersystem crossing to give the corresponding platinum acetylide triplet state. Then triplet−triplet energy transfer to the anthracene segment occurs. The strong triplet absorption spectrum of the anthracene segement is the evidence for the efficient energytransfer process. The long-lived phosphorescence seen at 520 nm for Pt4An (Figure 6a) is due to thermal repopulation of the higher energy triplet from the lower energy anthracene localized triplet. For Pt2An, the singlet state on the Pt−phenylene units is at considerably higher energy due to the shorter length of the platinum acetylide segment (the phenylene is only conjugated with a single Pt center). Thus, its triplet energy level (E = 2.84 eV)36 is higher than the singlet energy of the anthracene segement. Hence, when the anthracene chromophore (λex = 450 nm) is excited (Figure 10b), singlet to triplet energy transfer does not occur as indicated by the high fluorescence quantum yield (ΦF = 0.96). When Pt2An is excited at lower wavelength (Figure 10a, λex = 355 nm, the dominant absorption of platinum acetylide chromophore), a similar triplet−triplet energy-transfer process as in the Pt4An occurs, which is confirmed by the observation of the strong triplet−triplet absorption of the anthracene segement. Some intersystem crossing occurs from the anthracene singlet to the triplet, as
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CONCLUSION In order to investigate how the chromphores interact within πconjugated, multichromophoric systems, we designed and synthesized two anthracene-based platinum acetylide oligomers in which the anthracene segment acts as an energy trap. Detailed triplet energy-transfer mechanisms of both compounds were proposed based on spectroscopic results and DFT calculations. We discovered a wavelength-dependent and indirect energy-transfer process. In particular, a rare singlet− triplet energy transfer followed by a triplet−triplet energytransfer process was proposed for the longer conjugated oligomer Pt4An. In addition, the system provides a unique platform for studying singlet−triplet energy transfer because of its fulfillment of energies required. Future studies of these systems with fast time scale spectroscopic techniques are in progress.
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ASSOCIATED CONTENT
S Supporting Information *
Synthesis and characterization of Pt2An and Pt4An, comparison of the emission spectra, and time-resolved emission spectra (two schemes, three figures). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: 212-8542179. Present Address §
Department of Chemistry, Columbia University, 3000 Broadway, New York, NY 10027. Notes
The authors declare no competing financial interest. 9031
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (grant no. CHE-115164).
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