PHYSICAL REVIEW B 86, 195203 (2012)

Phosphorescence superradiance in a Pt-containing π-conjugated polymer

Bagrat Khachatryan,1 Tho D. Nguyen,2,* Z. Valy Vardeny,2 and Eitan Ehrenfreund1,†1Physics Department and Solid State Institute, Technion-Israel Institute of Technology, Haifa 32000, Israel

2Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah 84112, USA(Received 20 September 2012; published 5 November 2012)

We used a variety of optical probes to investigate the triplet excitons of a platinum containing π -conjugatedpolymer (Pt1) having large spin-orbit interaction, in the form of films and solid state solutions. The optical probesinclude absorption, photoinduced absorption, and emission spectroscopy and their corresponding dynamics, aswell as their excitation dependencies. We found that the intensity of the 0-0 line in the vibrationally resolvedphosphorescence emission spectrum anomalously increases at low temperatures in Pt1 films, but less so in Pt1solid state solutions, or when using below-gap excitation. Making use of the measured Raman scattering spectrumwe analyze the vibration progression in the phosphorescence spectrum, and conclude that the increase in the 0-0line intensity is due to triplet excitons that interact coherently at low temperatures, giving rise to phosphorescencesuperradiance.

DOI: 10.1103/PhysRevB.86.195203 PACS number(s): 78.55.Kz

I. INTRODUCTION

Phosphorescence (PH) is the main form of photopumpedradiation emission in heavy-metal-containing π -conjugatedpolymers and oligomers due to the significant spin-orbit (SO)interaction provided by the large atomic number elements. Inparticular, the triplet excitons (TE) turn emissive because ofthe SO interaction mixing of the triplet and singlet exciton (SE)states, which substantially increases the TE optical transitiondipole to the singlet ground state.1 In many cases the heavymetal rich organic chromophore may show both PH andfluorescence (FL) emission bands. Also, because of the largerprobability of forming TE from injected polarons in organiclight emitting diodes due to the spin quantum mechanics rules,electro-PH emission has been extensively used for displays.

We note, however, that the emission spectra in mostpolymers are not composed of a single sharp line. Owingto the relatively small extent of the TE (SE) wave functionthe PH (FL) emission spectrum in these materials oftencontains a vibronic side band structure due to the interactionof the exciton with local molecular vibrations.2–4 The vibronicspectral structure is composed of the fundamental 0-0 lineand a series of 0-n replicas that are sensitive to both electron-vibration coupling as well as to local molecular arrangement.5

The electron-vibration coupling strength is measured by theHuang-Rhys (HR) parameter S which is proportional to thesquare of excited state displacement � relative to the groundstate.6 Typically S < 3 for the strongest coupled vibrationalmodes in π -conjugated organic molecules and polymers,3,7–9

and this makes it possible to discern the vibronic structurefor n up to n � 2 vibronic replicas. In addition, the emissionspectrum of π -conjugated polymer films is also influencedby interchain interaction and aggregate formation.8,10,11 Inmany π -conjugated aggregates the emission intensity thatcorresponds to the purely electronic transition (the 0-0 tran-sition) is substantially weaker than predicted by theory,12–16

because the transition dipole moments of the aggregateensemble interfere destructively.17 It is noteworthy that thelocal symmetry reduces only the 0-0 intensity in the emissionspectrum, but leaves the intensity of the 0-n (n > 0) transitionsunaltered.18

By and large, aggregates have been identified as low-energyquenching sites leading to redshifted and longer-lived emissionradiation. In contrast, the intriguing superradiant aggregateemission in organic films, where the radiative rate exceedsthat of a single chromophore, has been established only influorescence emission. Few examples of FL superradianceinclude molecular J aggregates of pseudoisocyanine dye,19

films of PPV derivatives,20 and quarterthiophene.21 Spano hasshown that superradiance arises in aggregates with long-rangeorientational order, in which the off-axis molecular transitiondipole moments constructively interfere.5

In this work we report the occurrence of superradiance inthe PH emission spectrum, in particular that of a π -conjugatedPt-containing polymer. We show that at low temperatures the0-0 electronic transition in the PH emission spectrum of aPt-rich polymer is substantially enhanced with respect to theobserved 0-1 and 0-2 vibronic side bands. This cooperativeTE emission is caused by coherent triplet-triplet coupling inthe film, due to an unusually large TE wave-function extentcaused by the large SO coupling in this polymer.

II. EXPERIMENT

The Pt-rich polymer, namely trans-[–Pt(PBu3)2–C≡C–R–C≡C–]n (where R = phenylene) (dubbed hereafter “Pt1”), wassynthesized as described elsewhere;22 its chemical structureis shown in Fig. 1(a) inset. For our optical measurementsPt1 films of ∼100 nm thick were spin cast or drop castfrom a toluene solution on quartz substrates. For the radi-ation emission studies, the films were photoexcited usinga continuous-wave (cw) Ar+ laser (Coherent, Innova 90C)operating at the UV lines (∼355 nm) or blue line (488 nm)with intensities of ∼20 mW/cm2. The excitation pump beamintensity was kept low enough to practically eliminate sampleheating and photodegradation. The emitted light from the filmwas collected using a small F-number lens and dispersed by a1/4 met. monochromator (Oriel 77700). It was subsequentlymonitored using a photomultiplier (Hamamatsu R666) cou-pled to a lock-in amplifier (EG&G PAR 5210) operating atfrequencies f up to 120 kHz which enabled recombinationtime dynamic measurements as short as ∼10 μs. The samples

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KHACHATRYAN, NGUYEN, VARDENY, AND EHRENFREUND PHYSICAL REVIEW B 86, 195203 (2012)

FIG. 1. (Color online) Absorption and photoinduced absorption(PA) spectra of Pt1 film. (a) and (b) Room temperature optical density(OD) spectrum and its respective first derivative; the triplet excitonenergy level is assigned. The inset in (a) shows the molecular structureof Pt1. (c) The PA spectrum measured at ∼60 K using below gapexcitation of an Ar+ cw laser at 488 nm with intensity of ∼0.1 W/cm2;the LE and HE bands are assigned. (d) The PA excitation (PAE)spectrum at ∼60 K probed at the LE band (808 nm) using as pump axenon lamp dispersed by a monochromator. The TE energy level andthe SE transition energy are marked. Note the logarithmic scales in(a) and (d).

were mounted in a continuous flow helium cryostat in thetemperature range 4–300 K. To obtain the photoinducedabsorption (PA) spectrum, defined as PA = − �T /T , wemeasured the sample transmission T and its photoinducedmodulation �T using various gratings and solid state detectorsthat span the spectral range from 0.2 to 2.4 eV. For measuringthe excitation spectra of the PH emission and PA bands weused a xenon lamp followed by a monochromator as a pump.In addition, we also limited the excitation spectrum by along (short) pass filter at 350 nm (520 nm). Subsequently,the excitation yield of the PA or PH emission per impingingexcitation photon was calculated taking into account themeasured sample absorbance.

III. EXPERIMENTAL RESULTS

Figures 1(a) and 1(b) show the Pt1 film absorbancespectrum and its respective derivative in the spectral range1.6–3 eV. The broad absorption spectrum shown in Fig. 1(a) isthe tail of the SE absorption band that peaks at 3.2 eV, which isbroadened by the existing disorder in the film. Nevertheless, aclear optical structure is seen at ∼2.44 eV, which is accentuatedin the derivative spectrum of Fig. 1(b). This band is assigned tothe direct absorption into the TE lowest excited state since it isvery close to the 0-0 transition in the PH emission spectrum4

[Fig. 2(a)].Figure 1(c) shows the PA spectrum excited by the Ar+ line

at 488 nm (photon energy hωL = 2.54 eV), that is, belowthe optical gap at 3.2 eV. In this case, only PA from the TEcan be generated since the excitation photon energy is slightly

FIG. 2. (Color online) (a) Pt1 phosphorescence (PH) emissionspectra excited at 355 nm measured at various temperatures. The0-0 line and the 0-1 and 0-2 side bands are assigned. (b) The PHexcitation (PHEX) spectrum monitored at ∼1.53 eV. Note the PHEXonset at ∼2.44 eV that indicates direct excitation into the TE energylevel. (c) Temperature dependence of the recombination rate (1/τ ,blue symbols) and inverse 0-0 line intensity (1/I0−0, red symbols);the solid lines are fits to an activation energy behavior with activationenergies �E = 60 and 70 meV for 1/τ and I0−0, respectively. Theinset shows the modulation frequency (f ) dependence of the in-phase(In, red symbols) and quadrature (Qu, blue symbols) componentsof the 0-0 PH intensity measured at 4.8 K. The solid lines are fitsusing a standard model (see text). (d) The temperature dependence ofthe 0-0 to 0-1 integrated intensity ratio R01 for Pt1 film (100%, redsymbols), and diluted Pt1 in PMMA at weight concentration c = 1%(green symbols) and c = 0.1% (blue symbols).

above the lowest TE energy, but much below the SE transition.The triplet PA spectrum consists of two bands: A lower energyrelatively narrow band (LE) that peaks at ∼1.49 eV, and abroader higher energy band (HE) centered at ∼2.0 eV. Thenarrow LE band is assigned to the T1→Tn transition, whereTn is an excited TE level at ∼3.9 eV, which was previouslyidentified in Pt polymers.23,24 We conjecture that the higherand broader HE PA band corresponds to transitions into aband that is composed of a group of closely packed levels(possibly both singlet and triplet states). Figure 1(d) showsthe PA excitation (PAEX) spectrum of the LE band in the PAspectrum; the onset at T1 ≈ 2.4 eV (namely, close to the TElowest excited state) can be clearly identified.

Figure 2(a) shows the PH emission spectrum excited“above the gap” at ∼355 nm (hωL ∼ 3.5 eV) for severalselected temperatures in the range 4.4–280 K. A clear vibronicprogression structure is observed, which includes the 0-0 lineat 2.38 eV and the vibration assisted 0-1 and 0-2 side bands.The 0-0 line is very close to the TE absorption band [Fig. 1(b)]and the PAE threshold [Fig. 1(d)], and therefore is assigned toPH from the TE lowest excited level into the lowest groundstate level, in agreement with earlier studies.4 The slightlyredshifted PH 0-0 line from the TE onset in the PAEX spectrumindicates that the TE are mobile in the Pt1 film, and thereforeemit from the lowest energy chromophore. We note that each

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PHOSPHORESCENCE SUPERRADIANCE IN A Pt- . . . PHYSICAL REVIEW B 86, 195203 (2012)

of the 0-n (n > 0) vibronic side bands resolved in Fig. 2(a) iscomposed of several molecular vibrations.

Figure 2(b) shows the PH excitation (PHEX) spectrummonitored at the emission photon energy of ∼1.53 eV. Similarto the PAEX spectrum [Fig. 1(d)], the onset at ∼2.4 eV isclearly visible; this is in agreement with the interpretationof the PH emission and the TE absorption feature in Pt1.Figure 2(c) (inset) shows the modulation frequency depen-dence of the 0-0 line at 4.8 K for both the in-phase (In)and quadrature (Qu) PH emission components. From themaximum in Qu(f ) at f ∼ 4.7 kHz and the fit to the standardequations,25 namely In ∝ 1/(1 + ω2τ 2) and Qu = ωτIn,where ω = 2π f [solid lines in Fig. 2(b) inset], we obtain arecombination time τ ∼ 34 μs, as expected for long-livedPH emission radiation. Similarly we measured the frequencydependence of the PH components at various temperatures,and fit them with the standard model; the obtained temperaturedependence of the recombination rate 1/τ is shown in Fig. 2(c).1/τ (T ) response can be well fit by an activation energy de-pendence formula 1/τ = A exp(−�E/kBT ) + B with �E =60 meV [Fig. 2(c), solid blue line]. The activated temperaturedependence implies that the nonradiative recombination rateis larger than the radiative PH rate, and thus controls therecombination rate; and in turn also the temperature depen-dence of the PH emission intensity. Consequently, the inverse0-0 intensity 1/I (T ) can be similarly fitted with an activationenergy dependence [Fig. 2(c), red solid line].

Importantly, the 0-0 line and the vibronic 0-1 side banddo not show the same temperature dependence; in fact the0-0 intensity has a stronger T dependence. This becomesclear in Fig. 2(d) (red squares, 100% data), where the 0-0to 0-1 integrated intensity ratio (R01 ≡ I0−0/I0−1) is displayedvs T . Since aggregation is known to affect differently the0-0 line and the vibronic side bands, then in order to reduceaggregates formation we studied the PH emission spectrumfrom a solid state solution film where the Pt1 polymer chainsare diluted in an inert matrix of poly(methyl methacrylate)(PMMA). Figure 2(d) shows the ratio R01 for films of weightconcentration c = 1% and 0.1% Pt1 in PMMA. It is seen that atT ∼ 5 K R01 decreases from ∼2.5 for c = 100% (i.e., undilutedfilm) to 1.5 at c = 1% and ∼1 at c = 0.1%. Furthermore, R01

decreases with T for all Pt1 concentrations reaching a valueof ∼0.7 at T = 250 K. We thus conclude that aggregationaffects differently the 0-0 line and vibronic 0-1 side band. Theconcentration and temperature dependencies of R01 indicatean aggregation enhanced 0-0 PH line at low temperaturesdue to a process that is temperature dependent.

We have also studied the relation between the 0-0 and 0-1line intensities by studying the PH spectrum when excitedjust above the TE band, namely “below-gap excitation” wherethe absorbance is relatively small. This is shown in Fig. 3(a)where the PH emission is generated using the 488 nm (hωL

= 2.54 eV) Ar+ laser line at various temperatures. It is clearthat the PH spectra with below-gap excitation are similar tothose obtained with above-gap excitation at 355 nm [hωL ∼3.5 eV, Fig. 2(a)], except for a broad emission band centeredat ∼1.9 eV (of which origin is not yet clear). In particular,with below-gap excitation the ratio R01 at T ∼ 6 K [Fig. 3(b)]is smaller, and its temperature dependence is milder than thatwith above-gap excitation [Fig. 2(d)]. We infer that above gap

FIG. 3. (Color online) (a) PH spectra pumped with below-gap excitation (i.e., at the TE energy level) measured at varioustemperatures. The PH 0-0 and 0-1 bands are assigned. (b) Thetemperature dependence of the ratio R01 obtained from (a).

excitation produces TE via intersystem crossing into a statehaving larger R01 ratio and stronger temperature dependence.We thus conclude that the “aggregate state” of the TE whenexcited “below gap” is not identical to its aggregate state whenexcited “above gap.”

IV. DISCUSSION

In order to quantify the 0-0 line emission intensity relativeto that of the vibronic side bands we analyzed the PH spectrumusing the correlation function approach6 outlined in ourprevious publications.3,26 In this model the emission spectrumis described as the Fourier transform of the autocorrelationfunction f (t) of the transition dipole moment P :

PH(E) =∫ ∞

0f (t) exp(iEt/h)dt, (1)

where f (t) is given by6

f (t) = |P |2e−iE0t/h−S+S++S−e−�t ,(2)

S± =∑

j

Sjw±j e∓iωj t ; S =

∑j

Sj (2nj + 1).

In Eq. (2) the summation is over all N vibrationalmodes j = 1, . . . ,N ; w+

j = w−j + 1 = nj + 1, where nj =

1/[exp(hωj/kBT ) − 1]; Sj = ωj�2j /2h; and �j is the equi-

librium normal-coordinate displacement of vibration mode j

in the excited electronic state relative to the ground state. Thedamping constant � in Eq. (2) is assumed to be the samefor all modes. The mode HR factor Sj in Eq. (2) is relatedto the measured Raman scattering intensity Ij and frequencyωj via the relation Sj ∝ Ij /ω

2j . Equation (1) then yields the

familiar vibronic structure in the emission spectrum, which iscomposed of the fundamental 0-0 line at E = E0, and a series

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FIG. 4. (Color online) (a) The PH emission spectrum (black line)measured at T = 6 K and f = 133 Hz, that is fit (blue line) usingthe vibronic model with HR factor S = 0.8 (see text). The PHspectrum below ∼2.1 eV and its fit are also shown enlarged (×10).(b) The room temperature Raman scattering spectrum of a Pt1 filmexcited at 515 nm that shows three vibrations at 1164, 1592, and 2100cm−1, respectively, which are used for the model fitting shown in (a).(c) The temperature dependence of the exciton coherent number Ncoh

extracted from the PH fit at various temperatures (see text).

of 0-n (n> 0) side bands each of which is composed of severallines broadened by the damping coefficient �. The relativeintensities of the vibronic structure are thus solely determinedby the mode intensities Ij obtained from the Raman scatteringspectrum [Fig. 4(b) ] via their individual HR parameter Sj . Theonly fitting parameters needed to calculate the PH emissionspectrum are therefore the damping factor � and the total HRfactor S [Eq. (2)].

Since aggregation affects the PH 0-0 line intensity, butthe intensities of the vibronic side bands in the spectrum areaffected to a lesser degree,18,27 we used a fitting procedure forthe PH spectrum in which the vibronic progression is modeledby an extra emission intensity �I superposed on the 0-0 line.It now becomes clear that both 0-1 and 0-2 vibronic side bandsplay a crucial role since when using these side bands the fittingprocess of the HR factor S does not rely on the measured 0-0line intensity (which changes with T ), but is solely determinedby the vibronic progression intensities. An example of a fitto the 0-1 and 0-2 PH side bands is shown in Fig. 4(a) forT = 6 K, where the blue line is the model. We note theexcellent fit quality for the 0-2 side band [shown enlargedin Fig. 4(a)]. From the fit to the entire PH spectrum at thistemperature we obtained for the extra emission intensity atthe 0-0 line: �I/I0−0 ≈ 33% at T = 6 K. Using this fittingprocedure we determined the HR factor at 6 K to be S ≈ 0.8,which is within the acceptable range values of PH emissionbands in the organics. We also found that S increases with the

temperature reaching S ≈ 1.4 at T = 250 K, possibly due todecreased conjugation length and increased disorder at hightemperatures.28–30

The enhancement of the 0-0 line intensity in the PHemission spectrum from aggregates in organics was recentlydiscussed by Spano and Yamagata (SY)27 in relation with theexciton coherence length. In the absence of exciton coherencein isolated chains the 0-0 to 0-1 integrated intensity ratio isexactly given by the inverse HR factor;6,31 that is, R01 =1/S. SY however showed that when neighboring excitonsare coherently coupled in aggregates formed in the films,then the 0-0 intensity is enhanced due to the coherence,whereas the 0-n (n > 0) intensities are unaffected. For anideal J aggregate SY showed that R01 = N/S, where N

is the number of chromophores comprising the aggregate.In disordered J aggregates, where the chain length may beshorter32 and excitons are more localized, SY obtained therelation R01 = Ncoh/S, where Ncoh < N is the number ofcoherently coupled excitons in the aggregate. Ncoh stronglydecreases with increasing disorder and temperature. Thisrelation holds also for more complex morphologies.27 Havingexperimentally determined the HR factor S from the measuredPH emission, in particular from the relative intensities of the0-1 and 0-2 side bands, we can readily obtain Ncoh from themeasured R01 ratio using the SY formula. For above gapexcitation, using R01 = 2.5 [Fig. 2(d)], S = 0.8 [Fig. 4(a)], andthe SY formula we find Ncoh = 2 for the number of coherenttriplet excitons at 6 K. The fact that with below gap excitationR01 is smaller with milder temperature dependence, shows thatin this disordered sample, various states of TE can be reacheddepending on the excitation conditions that lead to various TEgeneration processes.

Subsequently, we calculate Ncoh as a function of temper-ature using the same procedure for the PH emission spectraat various temperatures. We show in Fig. 4(c) the obtainedNcoh as a function of temperature for above gap excitation;Ncoh decreases substantially with T showing the hallmark ofsuperradiance. We therefore conclude that the PH emission inPt1 shows superradiance properties at low temperatures as aresult of coherent TE’s. This indicates that the wave functionextent of TE in Pt1 is unusually large, consistent with thestrong SO coupling in this material. We also envision similareffects in other organics that contain heavy atoms in the π

conjugation.

ACKNOWLEDGMENTS

We thank Leonard Wojcik for the Pt1 synthesis. Thiswork was supported in part by the Israel Science Foundation(ISF 472/11; E.E.; PH emission study), the DOE GrantNo. DE-FG02-04ER46109 (Z.V.V.; Pt1 synthesis and absorp-tion spectroscopy), and the MRSEC program at the UoU (NSFDMR 11-21252; T.D.N. and Z.V.V. PA and PH excitationspectra).

*Present address: Physics Department, University of Georgia, Athens,GA, USA.

†Corresponding author: eitane@physics.technion.ac.il

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