Influence of the Conjugation Length on the Optical Spectra of Single

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The Influence of the Conjugation Length on the Optical Spectra of Single Ladder-Type (para-Phenylene) Dimers and Polymers Sebastian Baderschneider, Ullrich Scherf, Jürgen Köhler, and Richard Hildner J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b10879 • Publication Date (Web): 22 Dec 2015 Downloaded from http://pubs.acs.org on December 24, 2015

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The Journal of Physical Chemistry

The Influence of the Conjugation Length on the Optical Spectra of Single Ladder-Type (para-Phenylene) Dimers and Polymers Sebastian Baderschneider,1 Uli Scherf,2 Jürgen Köhler,1 Richard Hildner1,*

1

Experimentalphysik IV and Bayreuth Institute for Macromolecular Research (BIMF), Universität Bayreuth, 95440 Bayreuth, Germany 2 Fachbereich C – Mathematik und Naturwissenschaften and Institut für Polymertechnologie, Universität Wuppertal, Gauss-Strasse 20, 42097 Wuppertal, Germany

*Corresponding Author: Phone.: +49 921 554040 Fax: +49 921 554002 E-Mail: [email protected]

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Abstract: We employ low-temperature single-molecule photoluminescence spectroscopy on a π-conjugated ladder-type (para-phenylene) dimer and the corresponding polymer methyl-substituted ladder-type poly(para-phenylene), MeLPPP, in order to study the impact of the conjugation length (π-electron delocalisation) on their optical properties on a molecular scale. Our data show that the linear electronphonon coupling to intra-molecular vibrational modes is very sensitive to the conjugation length, a well-known behaviour of organic (macro-) molecules. In particular, the photoluminescence spectra of single dimers feature a rather strong low-energy (150 cm-1) skeletal mode of the backbone, which does not appear in the spectra of individual chromophores on single MeLPPP chains. We attribute this finding to a strongly reduced electron-phonon coupling strength and/or vibrational energy of this mode for MeLPPP with its more delocalised π-electron system as compared to the dimer. In contrast, the line widths of the purely electronic zero-phonon lines (ZPL) in single-molecule spectra do not show differences between the dimer and MeLPPP; for both systems the ZPLs are apparently broadened by fast unresolved spectral diffusion. Finally, we demonstrate that the low-temperature ensemble photoluminescence spectrum of the dimer cannot be reproduced by the distribution of spectral positions of the ZPLs. The dimer's bulk spectrum is rather apparently broadened by electron-phonon coupling to the low-energy skeletal mode, whereas for MeLPPP the inhomogeneous bulk line shape resembles the distribution of spectral positions of the ZPLs of single chromophores.

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Introduction: Conjugated polymers and oligomers are successfully employed as active materials in e.g. organic solar cells, field-effect transistors, light-emitting diodes, and sensing devices.1–3 For such applications a crucial factor is the ability of these materials to efficiently transport excitation energy and charge carriers. These processes are generally governed by a subtle interplay between competing effects:4–7 On the one hand, transport processes are slowed down by localisation of electronic excitations, which is caused by electronic and structural disorder as well as by electron-phonon coupling to nuclear degrees of freedom. On the other hand, transport tends to be more efficient in materials with largely delocalised electronic excitations, which is achieved by strong electronic interactions between the building blocks (the chemical repeating units of the conjugated backbone) in combination with structural and electronic order. Additionally, the fundamental step of free charge carrier generation in solar cells has been shown to become more efficient using conjugated polymers with larger delocalisation of electronic excitations.8,9 The degree of delocalisation is usually quantified by the conjugation length, i.e. the length over which the π-electron system extends along the conjugated backbone without disruption, which also defines a chromophore on a chain.10,11 Hence, a clear molecular scale understanding of energy and charge transfer requires detailed knowledge about (de)localisation of electronic excitations and electron-phonon coupling, as well as of their mutual interplay. Optical spectroscopy generally allows to address these issues. The delocalisation of electronic excitations influences the transition energy between the singlet ground and lowest electronically excited singlet state. This absorption/emission gap decreases with increasing oligomer size and finally saturates at a certain length, which is typically of the order of 10 repeating units and defines the socalled effective conjugation length for the polymer.10 Moreover, for some materials a decrease of the inhomogeneous line widths of the electronic transitions was found when going from a short oligomer to the corresponding polymer.12,13 This observation was suggested to arise from motional narrowing, i.e. a more extended π-electron system is supposed to be less susceptible to a rough energy landscape in the local surrounding.14–16 The (linear) electron-phonon coupling is reflected in optical spectra by a 3 ACS Paragon Plus Environment


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vibrational progression and by a Stokes shift between absorption and photoluminescence (PL). The vibrational progression is usually dominated by high-energy (~ 1500 cm-1) intra-molecular carbonbond stretch modes and their overtones. The Stokes shift is largely due to coupling to low-energy (< 200 cm-1) vibrations,4 e.g. intra-molecular torsional degrees of freedom or stretching vibrations of the conjugated backbone as well as vibrational modes of the local surrounding. However, the situation is complicated because the linear electron-phonon coupling strength, quantified by the Huang-Rhys factor,17 is itself a function of the conjugation length.18–22 The Huang-Rhys factor typically decreases for increasing conjugation length, although for low-energy intra-molecular vibrations the opposite behaviour was predicted 20,21 (but this was not yet directly observed in experiments). Ensemble spectra of conjugated polymers usually exhibit strong inhomogeneous broadening, that is caused by a large degree of electronic and conformational disorder.11,23 Hence, it is very difficult, if not impossible, to retrieve the relevant parameters from bulk spectra, e.g. the energies and coupling strengths of (particularly low-energy < 200 cm-1) vibrational modes as well as possible line narrowing effects due to increasing π-electron delocalisation. In contrast, single-molecule spectroscopy allows to surpass these shortcomings by looking at one chain at a time, which has greatly advanced our knowledge about the photophysics of conjugated polymers and oligomers on a molecular scale.12,24–34 Systematic single-molecule studies as function of conjugation length, however, are still scarce. Here, we investigate a ladder-type (para-phenylene) dimer and the corresponding polymer, methyl-substituted ladder-type poly-(para-phenylene), MeLPPP, with ca. 80 repeating units (Fig. 1, right). These materials are ideal model systems that allowed to perform several proof-of-principle experiments: very efficient two-photon absorption was reported,35,36 amplified spontaneous emission37 as well as stimulated emission and laser action was observed,38,39 and the formation of a Bose-Einstein condensate of exciton-polaritons at room temperature was demonstrated.40 Owing to the ladder-type backbone of this material, a high degree of order and symmetry prevails at least on length scales of the conjugation length even for the long-chain polymer,36 which is in contrast to other conjugated polymers such as poly(para phenylene vinylenes) and polyfluorenes, that can exhibit substantial bending.27,28,41 Moreover, the rigid backbone suppresses torsional degrees of freedom, which gives rise 4 ACS Paragon Plus Environment

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to a small Stokes shift and small inhomogeneous broadening of the ensemble spectra (Fig. S1 in the Supporting Information, SI). The ladder-type dimer and the polymer MeLPPP represent two extreme cases in terms of the conjugation length of two repeating units for the dimer and ~ 8 repeating units as effective conjugation length for MeLPPP.12,23 Since these systems feature the same conjugated backbone, they allow us to directly investigate the impact of π-electron delocalisation on their photophysical properties (line widths of electronic transitions, vibrational energies, electron-phonon coupling strengths) with high resolution using low-temperature single-molecule photoluminescence spectroscopy. Ultimately, this may help to develop a clear picture of how delocalisation of electronic excitations influences charge and energy transport.

Experimental: The synthesis of the ladder-type dimer and polymer MeLPPP is detailed elsewhere.42–46 The molecular weights are M = 1060 Da for the dimer and Mn = 55300 Da for MeLPPP. These materials were dissolved and further diluted in spectroscopic grade toluene (Aldrich). In the final step, a zeonex/toluene (dimer) or a polystyrene/toluene solution (MeLPPP) was added to obtain a concentration of about 1 µM for ensemble measurements and ca. 1 nM for single-molecule spectroscopy. These solutions were spin-coated onto quartz substrates, which were immediately mounted in a home-built liquid-helium bath cryostat and cooled to 1.5 K. To excite the dimers we used a pulsed diode laser (LDH-P-C-375, Picoquant) operating at 372 nm with a repetition rate of 40 MHz and a pulse duration of 40 ps. For MeLPPP the excitation source was a frequency-doubled titanium:sapphire laser (Tsunami, Spectra Physics) operating at 430 nm with a repetition rate of 81 MHz and a pulse width of 1.5 ps. The laser light was spatially filtered by a lens-pinhole-lens arrangement and directed to the cryostat via a suitable dichroic beam splitter (dimer: z375rdc; MeLPPP: z440DCSP; both AHF), a motorised scan mirror, and a telecentric lens system. A microscope objective (numerical aperture NA = 0.5 (dimer), NA = 0.85 (MeLPPP); both Microthek) that was immersed in liquid helium, was used to focus the laser light onto the sample. The PL was collected by the same objective and passed the dichroic mirror and dielectric long pass filters (dimer: 5 ACS Paragon Plus Environment


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LP390; MeLPPP: HQ500/100; both AHF) to suppress residual laser light. Finally, it was focussed onto the entrance slit of a spectrograph (SpectraPro-150, Acton Research Corporation), spectrally dispersed by a grating (600 lines/mm), and imaged onto a CCD-camera (SensiCam QE, PCO). We successively acquired several 100 PL spectra from each single object with integration times of 5 s for the dimer and 0.5 – 2 s for MeLPPP. In order to improve the signal-to-noise ratio of the spectra for data analysis, we subsequently performed a partial time average, i.e. we averaged for each dimer and MeLPPP-chain only similar spectra that were identified either by visual inspection or by a pattern recognition algorithm, see Refs. 32,33,47 and SI (Figs. S2 and S3) for details.

Results: A partially averaged low-temperature PL spectrum of a single dimer embedded in zeonex is depicted in Fig. 1a (solid line, see also Fig. S2, SI). The strongest emission line at 25215 cm-1 is attributed to the electronic transition. Weaker features, labelled (i) – (iv) in Fig. 1a, are offset from the electronic transition to lower energies by 130 cm-1 (i), 760 cm-1 (ii), 1330 cm-1 (iii), and 1610 cm-1 (iv). These lines are assigned to a skeletal (or backbone) stretch (i), ring stretch (ii), carbon single bond (iii), and carbon double bond stretch mode (iv).20 A partially averaged low-temperature PL spectrum of a single chromophore on an individual MeLPPP chain embedded in polystyrene is presented in Fig. 1b (solid line, see also Fig. S3, SI). Here, the electronic transition appears as most intense line at 21370 cm-1. This line is accompanied by two very weak transitions at smaller energies of 1300 cm-1 and 1560 cm-1, that are assigned to (inter-ring and aromatic ring) carbon-bond stretch modes.48,49 These vibrational lines possess a smaller intensity with respect to the corresponding modes (iii) and (iv) in the single dimer spectrum. For MeLPPP intra-molecular vibrational modes with energies below 1000 cm-1 are not visible in our data. For both the dimer and MeLPPP, the corresponding low-temperature ensemble PL spectra exhibit the same vibrational structure as compared to the single-molecule spectra, although strongly washed out due to inhomogeneous broadening (dashed lines in Figs. 1a,b). In the dimer's bulk spectrum the inhomogeneous line width of the electronic transition at 25150 cm-1 is 360 cm-1 (full width at half 6 ACS Paragon Plus Environment

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maximum, FWHM), which does not allow to resolve the skeletal mode. The bulk spectrum of MeLPPP features the electronic transition at 21400 cm-1 with an inhomogeneous line width of 470 cm1

(FWHM). Importantly, the concentrations for these ensemble measurements have been chosen such

that the molecules are still isolated in their respective matrix (dimer: zeonex; MeLPPP: polystyrene). This suppresses energy transfer between adjacent dimers/polymer chains that would substantially narrow the measured ensemble PL line width (particularly for MeLPPP,50 although some narrowing due to intra-chain transfer may still occur). In the following, we consider a 300 cm-1 wide spectral region around the electronic transitions in the single-molecule spectra (grey filled areas in Figs. 1a,b). Expanded views of these regions are presented in Figs. 1c and d for the dimer and MeLPPP, respectively. In both spectra the shape of the main peak is clearly asymmetric with a shoulder in the low-energy wing. In our recent work on MeLPPP32,33 we identified the narrow line at high energies with the zero phonon line (ZPL), i.e. the purely electronic transition, and the shoulder with the phonon side band (PSB), i.e. an electronic transition and a simultaneous excitation of a low-energy (~ 20 cm-1) vibration ('phonon'). Moreover, we demonstrated that the PSB is caused exclusively by electron-phonon coupling to vibrational modes of the surrounding host material. The similar shapes and intensities of the PSBs in the dimer and MeLPPP spectra (Figs. 1c,d) indicate that the dimer's PSB stems from electron-phonon coupling to low-energy phonons of the surrounding amorphous zeonex matrix as well; the Huang-Rhys factor of this phonon mode appears to be largely independent of conjugation length.51 A detailed discussion of this line shape can be found in Refs. 32,33. Here we rather focus on a quantitative analysis of the spectral peak positions and line widths of the ZPLs. To this end, their high-energy tails were fitted by a Gaussian or Lorentzian function (dotted lines in Figs. 1c,d), whichever resulted in the best fit. For the dimer we find for the ZPL at 25215 cm-1 a line width of 29 cm-1 (FWHM, Fig. 1c), and for MeLPPP the ZPL at 21370 cm-1 has a width of 15 cm-1 (FWHM, Fig. 1d). In total we investigated the ZPLs in 33 spectra of single dimers and in 295 spectra of single chromophores on individual MeLPPP chains. The resulting histograms of ZPL positions are distributed around 25230 cm-1 with a width of 140 cm-1 (FWHM, Fig. 2a) for the dimer, 7 ACS Paragon Plus Environment


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and around 21390 cm-1 with a width of 390 cm-1 (FWHM, Fig. 2b) for MeLPPP. These distributions are compared to the electronic transitions in the corresponding low-temperature ensemble PL spectra (dashed lines in Figs. 2a,b). Interestingly, for single dimers the distribution of ZPL positions is shifted by about 80 cm-1 into the high-energy wing of the ensemble PL. Moreover, the width of this distribution is significantly narrower than the width of the electronic transition in the bulk spectrum (140 vs. 360 cm-1). These data suggest that the width of the dimer's ensemble spectrum is not determined by the inhomogeneous distribution of ZPLs,52 i.e. further line broadening mechanisms must play a role (see discussion below). In contrast, for MeLPPP we find a reasonable agreement between the single-molecule histogram and the bulk spectrum in terms of their widths (390 vs. 470 cm-1), indicating that the bulk spectrum is essentially determined by static disorder. The maximum of the ZPL distribution is slightly shifted by 50 cm-1 to lower energies, which can be attributed to the fact that MeLPPP is a multichromophoric system and features on average 2 – 3 emission lines of single chromophores per chain.12 In the centre of the inhomogeneous band these lines are often spectrally closely spaced and in the high-energy tail the ZPLs can be less intense due to (partial) intra-chain energy transfer to energetically lower-lying chromophores.34 Hence, in both situations the ZPLs are difficult to analyse, which gives rise to an apparent red-shift of the ZPL distribution with respect to the bulk spectrum of MeLPPP. The distribution of ZPL widths of single dimers is depicted in Fig. 2c, which is spread between 10 and 80 cm-1 with a maximum at 40 cm-1. The ZPL widths of the polymer MeLPPP are on average narrower, and are distributed between 7 and 45 cm-1 with a peak at 12 cm-1 (Fig. 2d). Finally, for single dimers we retrieved the energies as well as the Huang-Rhys factors of the lowenergy skeletal mode (which is absent in the spectra of single MeLPPP chains), by fitting a Gaussian function to the corresponding line in the dimer spectra (Fig. 1c, thick solid line). The energies of this mode, measured relative to the corresponding ZPL positions, range from 110 to 180 cm-1 with a FWHM of 36 cm-1 and a peak at 150 cm-1 (Fig. 3a). The Huang-Rhys factor S was calculated according to


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−S

e =

I ZPL =α, I ZPL + I Skel

where IZPL (ISkel) are the integrated intensities of the ZPL (skeletal mode). We find that S ranges from 0.1 to 0.95 with a mean value of 0.43 (Fig. 3b), which corresponds to a Debye-Waller factor α = 0.65 and indicates weak electron-phonon coupling strength for the skeletal mode.

Discussion: Electron-phonon coupling to intra-molecular vibrations. The single-molecule PL spectra of both dimer and MeLPPP feature two Franck-Condon active intra-molecular vibrations of the conjugated backbone with energies in the range between 1300 and 1650 cm-1 (Fig. 1a,b). These vibrations are ascribed to carbon-bond stretch modes based on Raman spectra and quantum chemical calculations.20,48,49 In the MeLPPP single-molecule spectra the intensities of these vibrational lines, relative to the corresponding ZPL intensities, are typically weaker as compared to the situation in the single dimer spectra. This observation is in line with the trend in the ensemble spectra (Fig. 1a,b and Fig. S1). In the single dimer spectra the ring stretch mode at 760 cm-1 is visible as well, whereas in the spectra of individual MeLPPP chains this mode is absent. Low-temperature ensemble PL spectra of MeLPPP films exhibit a very weak feature at ~ 800 cm-1,23,36 indicating that the ring stretch is too weak to be visible in the spectra of single polymers. Hence, all higher-energy (> 500 cm-1) intramolecular vibrations show a decrease in intensity, or equivalently a decrease in electron-phonon coupling strength (Huang-Rhys factor), from dimer to MeLPPP. This observation is related to the larger chromophore size, i.e. a greater delocalisation of electronic excitations in the polymer as compared to the dimer.18–22 It is also in agreement with Spano's approach to model PL spectra of conjugated polymers using J-aggregate theory, in which the intensity of the vibrational lines decrease relative to the purely electronic transition as a function of the number of monomers.53 Skeletal stretch mode of the dimer. The single dimer spectra feature a skeletal stretch vibration at ~ 150 cm-1, that does not appear in the spectra of single MeLPPP chains (Figs. 1c,d). There are two possibilities to account for this observation: First, the electron-phonon coupling strength of this mode 9 ACS Paragon Plus Environment


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may decrease strongly from dimer to MeLPPP, as observed for the high-energy modes detailed above. Second, because the skeletal mode is a collective vibration and involves the motion of essentially all nuclei of the dimer's backbone, its energy follows a 'particle-in-a-box' behaviour and scales with the inverse oligomer (chromophore) length.20,21 Hence, for MeLPPP with its larger chromophore size this mode may possess a substantially smaller energy, such that it is hidden under the ZPLs in the singlemolecule spectra. Since the ZPL widths for MeLPPP are smaller than 50 cm-1 (Fig. 2d), the energy of its skeletal mode has to be ~ 10 cm-1 only. We note that this mode cannot contribute to the PSB in the spectra of single MeLPPP chains in Figs. 1b,d, because the PSBs are caused exclusively by lowenergy matrix vibrations; in other words, without surrounding host matrix we observed only symmetric ZPLs without PSB.33 Unfortunately, from our data we cannot discriminate between these options. Quantum chemical calculations predicted a slight increase of the Huang-Rhys factor of the skeletal mode with increasing oligomer length,20,21 which favours the second option, a large energy shift of the skeletal mode. An interesting aspect of the dimers' skeletal stretch mode is the broad distribution of energies and Huang-Rhys factors (Fig. 3). Although variations of vibrational energies were observed in singlemolecule spectra of small organic molecules as well as of conjugated polymers,54–58 the energies of the dimers' skeletal mode are distributed over a surprisingly large range from 110 to 180 cm-1. Generally, the energy of a vibration (for a given size of a molecule) is strongly influenced by the specific local interactions between molecule and environment, that can be attractive or repulsive depending on the polarisability and local density of the matrix.54 Within the Born-Oppenheimer approximation these interactions influence the curvature of the potential energy surface for the nuclei in the ground and excited electronic state, and as a consequence, influence the energy for a specific vibrational mode for each molecule. For instance, a high local density of the matrix with a small cavity around a dimer provides a small free volume for the skeletal stretch mode, which is a breathing motion of the dimer along its long molecular axis. In this case repulsive interactions between the matrix and the dimer probably dominate. Hence, the potential energy surfaces are steeper (the radii of curvature are smaller) and this mode is shifted to higher energies as compared to a situation with a larger free volume around 10 ACS Paragon Plus Environment

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a dimer. In addition to the curvature, varying interactions between dimer and environment modify the mutual displacement of the potential energy surfaces in the ground and excited electronic state along the configuration coordinate of the skeletal mode (linear electron-phonon coupling strength). As this mode involves a motion of all nuclei of the backbone, its electron-phonon coupling strength can be expected to be very sensitive to different local interactions.54 The wide spread of both the energies and Huang-Rhys factors of the skeletal mode is therefore a result of the heterogeneous local surroundings from dimer to dimer. The same mechanism also gives rise to an inhomogeneous distribution of ZPL positions for the dimer (vide infra), yet, the distribution of the skeletal mode energy is narrower with respect to the histogram of ZPL positions (36 cm-1 vs. 140 cm-1, Figs. 2a and 3a). Hence, the heterogeneous environment influences the energies of the skeletal mode to a smaller extend compared to the energies of the ZPLs. Finally, we note that quantum chemical calculations predicted an energy of 160 cm-1 and a Huang-Rhys factor of 0.4 for the dimer's skeletal mode.20 Those numbers are in excellent agreement with the mean values determined from our histograms (150 cm-1 and 0.43, Fig. 3), given that calculations typically overestimate vibrational energies by about 10 %.20 Zero phonon lines: line widths. The ZPL widths measured for single dimers are widely distributed around 40 cm-1, and thus appear on average about 2 – 3 times broader than the ZPL widths of single chromophores on individual MeLPPP chains (Fig. 2c,d). For MeLPPP and other long-chain conjugated polymers it is known that the ZPLs are subject to strong spectral diffusion processes,32,33,59– 61

i.e. random spectral jumps of the emission line that are caused by structural fluctuations in the local

environment of the emitting site. We previously showed that the ZPL widths are apparently broadened due to unresolved spectral diffusion processes,33 i.e. spectral jumps that are faster than the acquisition time for a single spectrum irrespective of the choice of the matrix material. Since spectral diffusion occurs on all time scales, the measured ZPL widths strongly depend on the acquisition time (see Figs. S3, S4 and Refs.

33,62,63

). Here, we used longer integration times for the single dimer spectra (5 s) as

compared to MeLPPP (0.5 – 2 s). Hence, the ZPLs observed for the dimer are very likely broadened due to these different experimental conditions, see Figs. S3 and S4 in the SI, and not due to a different delocalisation of electronic excitations in the dimer and MeLPPP (motional narrowing), as previously 11 ACS Paragon Plus Environment


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suggested for similar ladder-type systems.12 Zero phonon lines: distribution of spectral positions. The distribution of the spectral positions of the ZPLs for the dimer is by a factor > 2 narrower with respect to that for MeLPPP (Fig. 2a,b). Generally, the spectral positions of ZPLs are determined by electrostatic dispersion interactions with the local dielectric environment, which vary from molecule to molecule in a disordered solid matrix (static disorder) and give rise to an inhomogeneously broadened distribution of ZPL positions.52 Although these interactions are present for both dimer and polymer, their variation is expected to be smaller for MeLPPP due to the stronger π-electron delocalisation.10 For conjugated polymers, however, the presence of conformational disorder further broadens the ZPL distribution owing to a distribution of conjugation lengths and thus of transition energies of individual chromophores.11,13 Such conformational disorder is present even in the rigid, rod-like polymer MeLPPP64 (but it is absent for the dimer with its defined length). Since the distribution of spectral positions of ZPLs is broader for MeLPPP, the influence of intra-molecular conformational disorder is likely to dominate over variations in the dispersion interactions with the environment. We note that this finding of an increasing width of the ZPL distribution with increasing degree of conformational disorder of conjugated polymer chains is in agreement with the continuous energy disorder model that has been put forward by Tilgner et al.65 and more recently by Barford and co-workers.66,67 In this model structural defects do not necessarily interrupt π-electron delocalisation and thus break the conjugation, as assumed in the classical “chromophore picture” of conjugated polymers. The spectral distribution of ZPLs – the so called inhomogeneous distribution function – is expected to resemble the inhomogeneously broadened electronic ensemble line shape for weak electron-phonon coupling to low-energy phonon modes of the surrounding host,52,68–72 as indeed observed for MeLPPP (Fig. 2b). In contrast, the dimer's ZPL distribution is substantially narrower and additionally shifted by ~ 80 cm-1 into the high-energy tail of the electronic transition in the bulk spectrum (Fig. 2a). The surprisingly narrow distribution of ZPL positions indicates a low degree of static energetic disorder, probably related to the the rigid geometry and synthetic purity of the dimer. The second peculiar observation – the blue shift of the ZPL distribution – can be understood, if we 12 ACS Paragon Plus Environment

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consider that in single dimer spectra the ZPLs are accompanied by the rather prominent skeletal mode with an energy of ~ 150 cm-1 and a Huang-Rhys factor of ~ 0.4. In Fig. 4 we therefore show the (normalised) histograms of the absolute spectral positions of the dimers' ZPLs (open bars) and skeletal stretch modes (hatched bars). The low temperature bulk PL spectrum provides a nearly perfect envelope for these distributions (dashed line). This overlay clearly demonstrates that the ensemble line width of the dimer spectrum is broadened by significant electron-phonon coupling to the low-energy skeletal mode.

Concluding Remarks: We presented single-molecule PL spectra of π-conjugated ladder-type dimers and polymers at low temperatures. The spectra show a non-trivial behaviour when going from the dimer to the polymer MeLPPP, i.e. as a function of conjugation length. Our results indicate that mainly intra-molecular parameters are affected by increasing delocalisation of electronic excitations: intra-molecular vibrations with energies > 500 cm-1 show a decrease in the electron-phonon coupling strength; the intra-molecular skeletal mode features a decrease of its energy and/or Huang-Rhys factor; the distribution of ZPL positions is substantially broader for MeLPPP, which we attributed to intramolecular conformational disorder of its backbone. In contrast, factors that are determined by interactions with the local environment do not lead to substantial changes in the single-molecule spectra as a function of π-electron delocalisation. In particular, the line widths of the ZPLs are determined by fast unresolved spectral diffusion processes due to temporally fluctuating interactions with the local surrounding, and do not exhibit line narrowing due to different π-electron delocalisation when going from dimer to MeLPPP. In fact, these observations seem reasonable, because intermolecular interactions (i.e. between conjugated backbone and matrix) are likely to be substantially weaker as compared to intra-molecular interactions (i.e. within the conjugated backbone) owing to the larger distances involved in the former. The ZPL widths of the ladder-type dimers and polymers are not subject to motional narrowing, which is in line with the theoretical treatment by Knapp14 and Knoester15. Substantial motional 13 ACS Paragon Plus Environment


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narrowing occurs only if dynamic disorder as well as correlations in the site energies between neighbouring building blocks (here: repeating units of the conjugated backbone) are absent. However, in our single-molecule data we clearly observe spectral diffusion of the emission lines, i.e. dynamic disorder, on all time scales even at 1.5 K. Additionally, in conjugated oligomers and polymers the building blocks are covalently bound, which may introduce significant site energy correlations.73 Hence, motional narrowing can be expected to play only a minor role for the observed ZPL widths in conjugated polymers. Our data also show that the determination of the true inhomogeneous line widths of ensemble spectra of conjugated polymers and oligomers is very difficult. Significant electron-phonon coupling to low-energy vibrational modes (here the dimers' skeletal stretch mode) can substantially broaden ensemble spectra even at low temperatures and for the comparably well-defined ladder-type systems studied here. As a final remark, we add that the product of the Huang-Rhys factors and the energies of the corresponding vibrational modes – the so called reorganisation energy – is one of the key parameters for energy and charge transfer in organic materials.4,74 For the dimer a significant contribution to the reorganisation energy comes from its Franck-Condon active skeletal mode (that is not visible in the spectra of MeLPPP). The dimer therefore features a higher reorganisation energy, i.e. smaller energy and charge transfer rates, as compared with MeLPPP. In other words, from our data we expect devices based on MeLPPP to possess a higher performance, as has indeed been observed recently.8,9


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Acknowledgements: Financial support from the German Research Foundation (DFG/GRK 1640, HI1508/2) and the Bavarian State Ministry of Science, Research, and the Arts for the collaborative research network “Solar Technologies Go Hybrid” is gratefully acknowledged. We also thank Marin van Heel (University of Leiden, NL) for providing the MSA-algorithm for data analysis.

Supporting Information: Room temperature ensemble spectroscopy; Analysis of low-temperature single-molecule spectra; Figures S1 - S4



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Figure Captions: Figure 1: Low-temperature photoluminescence (PL) spectra of a single dimer and of a single chromophore on a single MeLPPP chain. (a) Single-molecule (solid) and ensemble (dashed) PL spectra of dimers embedded in zeonex. The lines labelled (i) – (iv) arise from electron-phonon coupling to intra-molecular vibrational modes of the dimer's backbone. (b) Single-molecule (solid) and ensemble (dashed) PL spectra of the polymer MeLPPP embedded in polystyrene. The single-molecule spectra for both the dimer and MeLPPP are obtained after partial averaging over several individual spectra acquired with short integration times. (c), (d): Expanded views of the single-molecule spectra in the grey filled areas in (a) and (b). The dotted lines represent fits to the high-energy tail of the zerophonon lines. The thick solid line in (c) is a fit to the intra-molecular skeletal mode of the dimer, which is absent in the MeLPPP spectrum (d). The chemical structures of the dimer (top) and of MeLPPP (bottom) are shown to the right; R1: n-hexyl, R2: methyl, R3: 1,4-tertbutyl, R4: 1,4decylphenyl; number of repeating units of MeLPPP: n ≈ 80.

Figure 2: Distributions of the spectral positions and widths of the zero-phonon lines for the dimer and MeLPPP obtained from fits to single-molecule spectra as shown in Figure 1. (a), (b) Histograms of the zero-phonon line positions (bars) of the dimer (a) and MeLPPP (b), which are overlaid with the corresponding low-temperature ensemble PL spectra (dashed lines). (c), (d) Distributions of the line widths of the zero-phonon lines. The data in (d) have been taken from Ref. 33.

Figure 3: Distribution of the energies (a) and Huang-Rhys factors S (b) of the dimer's skeletal stretch mode.

Figure 4: Normalised distributions of the absolute spectral positions of the zero-phonon lines (open bars) and of the skeletal mode (hatched bars) of single dimers. The histograms are overlaid with the low-temperature ensemble PL spectrum of the dimer embedded in zeonex (dashed line), demonstrating an apparent line broadening mechanism due to electron-phonon coupling to the skeletal mode. 22 ACS Paragon Plus Environment

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Figure 1:



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Figure 2:


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Figure 3:



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Figure 4:


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Table of Contents Graphic:


Influence of the Conjugation Length on the Optical Spectra of Single

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