Delocalization of Coherent Triplet Excitons in ... - ACS Publications

Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)

Letter

Delocalization of Coherent Triplet Excitons in Linear Rigid-rod Conjugated Oligomers Christian Hintze, Patrick Korf, Frank Degen, Friederike Schütze, Stefan Mecking, Ulrich E. Steiner, and Malte Drescher J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02869 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 22, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry Letters

Delocalization of Coherent Triplet Excitons in Linear Rigid-Rod Conjugated Oligomers Christian Hintze, Patrick Korf, Frank Degen, Friederike Schütze, Stefan Mecking, Ulrich E. Steiner,∗ and Malte Drescher∗ Fachbereich Chemie, Universität Konstanz, 78457 Konstanz, Germany E-mail: [email protected]; [email protected]

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract In this work, the triplet state delocalization in a series of monodisperse oligo(pphenyleneethynylene)s (OPEs) is studied by pulsed electron paramagnetic resonance (EPR) and pulsed electron nuclear double resonance (ENDOR) determining zero-field splitting, optical spin polarization, and proton hyperfine couplings. Neither the zerofield splitting parameters nor the optical spin polarization change significantly with OPE chain length, in contrast to the hyperfine coupling constants, which showed a systematic decrease with chain length n according to a 2/(1+n) decay law. The results provide striking evidence for the Frenkel type nature of the triplet excitons exhibiting a full coherent delocalization in the OPEs under investigation with up to five OPE repeat units, and with a spin density distribution described by a nodeless particle in the box wavefunction. The same model is successfully applied to recently published data on π-conjugated porphyrin oligomers.

Graphical TOC Entry

EPR spectroscopy, ENDOR spectroscopy, Polymers, Triplet Excitons, Coherence, Delocalization

2


Page 2 of 18

Page 3 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Excited triplet states play a major role in organic electronics like organic light-emitting devices or solar cells. 1 In order to tailor the molecules involved for applications like displays and solar energy harvesting, understanding the nature of excited triplet states in polymers is essential because they can exhibit both, adverse or desirable effects. 1 Conjugated polymers are of high scientific interest as well as of great practical importance, e.g. as constituents of organic LEDs, 2 for photovoltaics 3 and nanoparticle formation. 4 A prominent class of conjugated polymers are poly(aryleneethynylene)s (PAEs), rigid rod-like polymers with use in molecular devices and organic electronics. 5–7 Oligo(p-phenyleneethynylene)s (OPEs) can serve as model compounds for PAEs. Monodisperse oligomers, as investigated in this work, allow precise extrapolation of structure-propertyrelationships to polymers. This is of particular interest if such polymers are difficult or even impossible to synthesize or to characterize analytically. 8 At the same time, monodisperse oligomers are highly relevant molecules in their own right, since they already entail the basis of the electronic and optical properties that are of interest in the polymer. With lengths in the nanometre range, conjugated oligomers are promising components of electronic devices of molecular scale 9 (cf. references in ref. 10). OPEs have been studied by various methods and for a variety of aspects, 9 such as photoluminescence and lasing properties in thin films, 11 the laser flash spectroscopy of singlet and triplet states in solution, 10 their molecular shape in solution 12,13 and their nanoparticles, 14 their electrooptical properties, 15 or photoinduced charge transfer. 16 Triplet state delocalization can be characterized by length in terms of the number of repeat units and by uniformity in terms of electronic spin density along the chain. The parameters from which such electronic structure related information may be derived are zero field splitting (ZFS), optical spin polarization (OSP), and hyperfine coupling (HFC), experimentally based on pulsed EPR and ENDOR, as has been convincingly demonstrated recently by Tait et al. 17–19 for linear π-conjugated porphyrin oligomers (Pn). In the current work we apply such methods to study the triplet state of a series of rigid rod-like, monodisperse OPEs

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

with up to 21 repeat units (cf. Scheme 1) for their relation between oligomer length and electronic delocalization. From the method of synthesis, odd numbers of repeat units were preferential targets. It will be shown that the experimental results provide strong evidence for a Frenkel type nature of the triplet excitons and their full coherent delocalization over at least 5 OPE units, behaving like a quantum mechanical particle in the box in its ground state.

Scheme 1: Structure of the OPEn. Synthesis, structural characterization, and proof of monodispersity of the OPEs investigated (Scheme 1) have been reported earlier. 20 Optical absorption (cf. SI) and emission spectra (cf. ref. 20) both exhibit bathochromic shifts with increasing chain length and show that the effective conjugation length is not reached up to 43 repeat units, 20 which can be interpreted in terms of a full delocalization of the associated singlet states. Additionally, by extrapolating energies corresponding to the first absorption maximum to an infinite number of repeat units, a band gap of Eg = 2.70(1) eV for the polymer is found (cf. SI). The OPEs were dissolved in toluene-d 8 at a concentration of 5 mM (0.05 mM for OPE21). The OPEn were excited into their triplet state by short ( 6 ns) laser flashes of an excimer laser operating at a wavelength of 351 nm (XeF). Pulsed EPR experiments were performed at 50 K, Mims ENDOR at 10 K, both in X band. The spin system’s parameters were determined by best fit simulations performed with EasySpin. 21 Echo-detected field swept EPR spectra of the excited triplet states of OPEn, where n is the number of repeat units, are shown in Figure 1. Time-dependent measurements revealed triplet lifetimes on the order of 101 to 102 milliseconds and corresponding spinlattice relaxation (SLR) rates on the order of 101 to 102 microseconds (cf. SI). Thus, the OSP can be assumed to be constant throughout the whole pulsed EPR experiments with a

4


Page 4 of 18

Page 5 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


duration of about 300 ns. The experimental EPR spectra can be well described by spectral simulations. The most relevant simulation parameters, the ZFS parameters D and E with the widths of Gaussian distributions of those parameters (Dstrain and Estrain ) and relative sublevel populations pj (j = x, y, z), are summarized in Table 1. (For the convention on the sign of the ZFS parameters cf. SI.)

Figure 1: Echo detected field swept spectra of OPEn (black) with n = 1, 3, 5, 21 repeat units, recorded at 50 K in X band directly after the laser flash excitation (TDAF ≤ 2 µs) with a short pulse sequence (τ = 120 ns). The experimental data is shown together with best fit simulated spectra (red). A narrow single line around g ≈ 2, originating from radicals produced by UV irradiation, present in all spectra, was deleted from the raw data. In the inset, a detail of the spectrum of OPE5 is shown, including the radical signal (denoted with an asterisk).

Table 1: Parameters of the spectral simulations shown in Figure 1.

OPE1 OPE3 OPE5 OPE21

D Dstrain MHz MHz 2912 194 2927 295 2818 212 2750 100

E MHz -840 -619 -574 -550

Estrain MHz 0 0 23 100

px : py : pz 0.53:0.00:0.47 0.57:0.00:0.43 0.59:0.00:0.40 0.62:0.00:0.38

Mims ENDOR experiments (cf. Figure 2A) were performed in order to investigate the 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

hyperfine interactions of the triplet state with protons of the oligomer chain backbone in the triplet state. Due to the considerably weaker spin echo intensity of OPE21 it was not investigated by Mims ENDOR. The protons of the sidechains do not show up in the ENDOR spectra, so we can conclude that triplet spin density is negligible in the sidechains. This seems also to be the case for the terminal ethyne protons. Neither did they show up in the Mims ENDOR spectra of OPE1 nor of the longer oligomers, where their HFCC is expected to be even more reduced than in the monomer. The relevant ENDOR signals of the ring protons were identified by the criterion of peaks showing systematic shift on variation of chain length. Hyperfine coupling constants (HFCCs) were determined by Gaussian fits to the signals indicated by gray arrows of the Mims ENDOR spectra (Figure 2A). In Figure 2B the shifts of ENDOR signals are plotted versus chain length. The main purpose of this work is to clarify the question of triplet state delocalization in the series of OPEs. The experimental findings on which we will base our discussion may be summarized as follows: 1. The ZFS parameters show very little variation with OPEn length between n = 1 and 21. (Table 1) 2. OSP displays rather weak changes, indicating slightly larger polarization as n increases. (Table 1) 3. The dominant HFCCs exhibit a systematic decrease with n. (Figure 2) Using the point-dipole approximation one can estimate an average separation of the unpaired spins of about 3.0 Å from the values of the D-parameters ranging between 2750 and 2927 MHz. Such a separation is somewhat larger than for the isolated benzene ring (2.5 Å) but definitely within the scope of a triplet localized on one repeat unit. On the other hand, the systematic decrease of the HFCCs indicate a wide delocalization of the triplet exciton. As will be shown in the following, our findings regarding the ZFS parameters and 6


Page 6 of 18

Page 7 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2: A: Mims ENDOR spectra of the triplet state of OPE1 (red), OPE3 (green), and OPE5 (blue) for both transitions (1 and 2) of the three canonical orientations (x, y, and z). Recorded at 10 K in X band. B: HFCCs determined via Gaussian fits to the signals indicated by gray arrows of the Mims ENDOR spectra shown in A for both transitions (negatively shifted ENDOR signals in the case of transition 1) of the three canonical orientations, x (red squares), y (violet circles), and z (orange diamonds). The error bars indicate 95 % confidence . The intervals of the Gaussian fits. The dashed black lines indicate the function A(n) = A(1) n 2A(1) colored lines indicate the function A(n) = n+1 expected from the triplet exciton model (cf. eq. 6). 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 18

HFCCs are entirely compatible with the delocalization of Frenkel type excitons, where close spatial correlation of hole and electron goes along with the delocalization of the center of gravity of the electron-hole pair. 22 23 Let us now consider a delocalization of the local excitations by a hopping process, e.g. for the trimer:

M∗ MM ⇀ ↽ MMM∗ . ↽ MM∗ M ⇀

(1)

Hopping of triplet excitons between two sites in molecular contact may be expected to occur at hopping rates much faster than 109 s-1 . 24,25 Thus, under the conditions of the EPR experiments many jumps have occurred and equilibrium should have been established within the OPEn. During the hopping process, the spin orientation in the laboratory frame will be conserved, but the initial spin polarization will be modified in the transfer process if the molecular axes of the repeat units functioning as donor and acceptor are tilted towards each other. Denoting the angles between the corresponding axes of the ZFS tensors of donor and acceptor by θij , with the indices i and j running over x, y, and z, the polarization of the acceptor is given: 26–28

pA i =

X

cos2 θij pD j .

(2)

j

We may assume that in the rod-like OPEs the rod axis represents one of the ZFS tensor axes. Hence for this axis, the sublevel populations of donor and acceptor do not mix. We argue that the direction of the the triple bonds remains the x-axis throughout the series of OPEn (cf. SI). As documented in Table 1, the population of the respective ZFS state Tx changes indeed very little, with only a slight tendency to increase. On the other hand, there is no transfer of polarization between the directions y and z perpendicular to the rod axis, as follows from the fact, that the population of Ty remains zero throughout the series. Thus a stochastic hopping mechanism of delocalization is rather unlikely. The same reasoning

8


Page 9 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


applies to the series of π-conjugated porphyrin oligomers Pn investigated by the Timmel group 17,18 (cf. SI). Another argument against the hopping mechanism follows from the conclusion that fast hopping would lead to equal probability of triplet excitation at each repeat unit, which in turn would lead to a uniform dilution of spin density at each repeat unit. Hence a 1/n dependence of the HFCC should be expected, which is clearly not very well fulfilled (cf. Figure 2B). To describe coherent delocalization of a localized Frenkel type triplet exciton we use superpositions of wavefunctions φi for the (tightly bound) localized states:

∗

φi = φ(M1 M2 . . .3 Mi . . . Mn ).

(3)

The energy eigenvalues in this function space are determined by the coupling matrix elements between pairs of φi . In the case of triplet excitons, the coupling is of Dexter type and therefore rather short range. Thus only the coupling between adjacent repeat units needs to be taken into account. If, furthermore, we assume that the φi are orthogonal and if we neglect any effect of the left terminal ethynyl group completing the oligomer, the eigenvalues and eigenvectors are analogous to the LCAO-MO or Hückel problem of a conjugated linear (k)

hydrocarbon with n carbon atoms. The coefficients ci

Ψ

(k)

=

n X

of the n eigenstates Ψ(k)

(k)

ci φi

(4)

i=1

are given by 22

(k) ci

=

2 n+1

21

sin

kπ i . n+1

(5)

Since it will be of importance below, we note here, that there is also a perfect equivalence between the LCAO-MO model and a free electron model, if the terminal bounding potential walls are placed one bondlength beyond the terminal C-atoms in a conjugated hydrocarbon 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 18

chain. 29 The squares of the coefficients represent the probabilities of the triplet excitation to reside at the pertinent repeat unit site or else, the spin density at that repeat unit. The HFCC measured by ENDOR are proportional to the spin density. Taking the lowest state of the exciton band (k = 1), in case of n being odd the square of the central coefficient, ic = (n + 1)/2, is given by

(1)

cic

2

=

π 2 2 sin2 = n+1 2 n+1

(6)

Thus we expect the HFCC in the OPEn series to drop as 2/(n + 1). In fact, the corresponding lines shown in Figure 2B are in excellent agreement with the experimental data points, thus providing strong evidence of the coherent, Frenkel-type nature of delocalization of the triplet exciton in the OPEs. Having specified the exciton wave function by equations (3) and (4), we can cast our argument about the insensitivity of the ZFS parameters to the length of the OPEn in more rigor3 ous terms. The ZFS parameters are expectation values of operators of the form HZFS (1/r12 ),

where r12 denotes the distance of the electrons in the SOMO orbitals. The expectation values can be expanded as follows:

3 EZFS =hΨ(1) |HZFS (1/r12 )|Ψ(1) i * n + n X X (1) (1) 3 ci φi HZFS (1/r12 ci φ i ) = i=1

=

n X i=1

+

i=1

(1)

3 ) φi |ci |2 φi HZFS (1/r12

n D X

i6=j=1

=EZFS

3

M

E (1) 3 (1) ) cj φ j ci φi HZFS (1/r12

∗

+

n D X

i6=j=1

(7)

(1) 3 (1) ci φi HZFS (1/r12 ) cj φj

10


E

.

Page 11 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Here the first term corresponds to the EZFS value of the monomer triplet. The second term represents an interference term, receiving its main contributions from nearest neighbors 3 (j = i±1). Since 1/r12 rapidly decreases with the distance, the interference term contributes

only little if the wavefunctions of the monomer triplets do not significantly overlap. In any case, a good approximation should be obtained by concentrating on the contributions EZFS,ifnn of nearest neighbors. In the SI, it is shown that this contribution converges to a value 3 of 2 hφ1 |HZFS (1/r12 )|φ2 i which may be positive or negative, depending on the character of the

monomer wavefunctions. In case of the OPEn, the linkage of monomers tends to decrease the absolute values of D and E. In a qualitative sense, the observed values follow the predicted convergence. In case of the Pn, such a convergence is observed too. However, the linkage of monomers tends to increase the absolute values of D and E which may be just explained as a weak continuation of the effect which leads to the sign change of the ZFS parameters when going from the monomer to the dimer. In case of the Pn, too, the largest HFCC shows a general decrease with n, but as in the case of OPEn it does not follow well a 1/n relation and is not even monotonic (cf. case n = 3 in Figure 3). It has been found that the standard procedure, with the one-unit-length extension rule for applying the FE MO model to calculate the coefficients of the LCAO model, leads to a good fit of the HFCCs in case of the OPEn. For the porphyrin oligomers Pn, the central spin density derived from the HFCCs is found somewhat higher than from the standard model. In order to adapt the model, we took a heuristic approach and varied the only parameter which is variable in the free-electron molecular orbital (FEMO) model, namely the extension of the free-electron (FE) wavelength. Thus we modify the standard FEMO function by indenting its bounds by a fraction w of one lattice unit (cf. Figure 4). For a reduction in wavelength by 2w, equation (5) is modified to

(1) ci

= N sin

π (i − w) n + 1 − 2w 11


(8)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 18

Figure 3: HFCC measured along the y axis of the ZFS tensor for the H1 protons of porphyrin oligomers Pn, from ref. 18 (red squares) and theoretical values (blue points) from the coherent exciton model assuming w = 0.38 (cf. text). The black dashed line represents the function 1/n, the other two lines represent the theoretical values for w = 0 (upper line) and w = 1 (lower line). with the normalization constant N given by

N=

n X i=1

sin

π (i − w) n + 1 − 2w

2 !− 21

.

(9)

The squares of the coefficients for n = 1 . . . 6 are given in Table 2. Taking out the central coefficients, representing the spin densities responsible for the measured HFCC, we obtain for OPEn (n = 1, 3, 5, case w = 0): 1 : 0.5 : 0.333, for Pn (n = 1 . . . 6, case w = 0.38): 1 : 0.5 : 0.610 : 0.408 : 0.381 : 0.300. The former values correspond to the 2/(n + 1) dependence represented for the OPEn in Figure 2B, the latter values correspond to the relative HFCC values represented in Figure 3. The best fit value of 0.38 for w indicates that the zero point of the FE MO is still 62 % outside the terminal repeat units. Thus both, the butadiyne linked porphyrin oligomers and the phenyleneethynylene oligomers are shown to represent systems with coherent delocalization of Frenkel type triplet excitons over the full π-chain. In this work we have shown that the ZFS of the excited triplet states of a series of monodisperse, rigid rod-like OPEs does not change significantly with oligomer chain length, a 12


Page 13 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4: Function for

(1) cx

2

(where x denotes the position along the chain backbone, line 2 (1) plots, top abscissa and right ordinate) and coefficients ci for w = 0 (red) and w = 0.38 (blue) over 5 local centers (bar charts, bottom abscissa and left ordinate). result similarly observed for monodisperse porphyrin oligomers. 17–19 The spectral simulation of echo-detected, field swept spectra of the OPE triplet states shows that the ISC mechanism is not substantially affected by chain length. Mims ENDOR data of a series of monodisperse OPEs show a systematic decrease of the detectable HFCC with chain length n according to a 2/(n + 1) dependence. Our findings are quantitatively accounted for by the model of a coherently delocalized Frenkel exciton, with an envelope described by a nodeless particle in the box wavefunction. Delocalization extends over at least five repeat units in OPE5, corresponding to a chain length of 35 Å. 14 The model also accounts for the ZFS and spin density variation reported for up to six repeat units of porphyrin oligomers connected by butadiyne linkers. 17,18 Thus, triplet ENDOR spectra have been shown to hold a unique potential for specifying the coherent nature of conjugated oligomers.

Acknowledgement The authors thank Adelheid Godt for fruitful discussions. This work was financially supported by the DFG (SFB 1214). 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 18

(1)

Table 2: Squares of the coefficients ci , that account for a reduction in wavelength of the particle in the box envelope by 2w, as shown in equation (8). case w = 0 1 0.5 0.25 0.138 0.0833 0.0538

0.0302

0.362 0.25

0.175

0.5 0.5

0.25 0.362

0.333 0.272

0.138 0.25

0.272

0.0833 0.175

case w = 0.38 (best fit to the data in ref. 18, cf. Figure 3) 1 0.5 0.5 0.195 0.610 0.195 0.0923 0.408 0.408 0.0923 0.0502 0.259 0.381 0.259 0.0502 0.170 0.300 0.300 0.170

0.0538

0.0302

Supporting Information Available The following files are available free of charge. • Experimental details, UV/Vis spectroscopic data, EPR triplet kinetic data, details on ZFS tensor orientation, EPR experimental and data analysis details, estimation of interference term for nearest neighbors, additional spin density plots.

References (1) Köhler, A.; Bässler, H. Triplet States in Organic Semiconductors. Mater. Sci. Eng., R 2009, 66, 71–109. (2) Grimsdale, A. C.; Leok Chan, K.; Martin, R. E.; Jokisz, P. G.; Holmes, A. B. Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices. Chem. Rev. 2009, 109, 897–1091. (3) Dou, L.; You, J.; Hong, Z.; Xu, Z.; Li, G.; Street, R. A.; Yang, Y. 25th Anniversary 14


Page 15 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Article: A Decade of Organic/Polymeric Photovoltaic Research. Adv. Mater. 2013, 25, 6642–6671. (4) Pecher, J.; Mecking, S. Nanoparticles of Conjugated Polymers. Chem. Rev. 2010, 110, 6260–6279. (5) Bunz, U. H. F. Poly(aryleneethynylene)s: Syntheses, Properties, Structures, and Applications. Chem. Rev. 2000, 100, 1605–1644. (6) Bunz, U. H. F. In Poly(arylene etynylene)s; Weder, C., Ed.; Advances in Polymer Science 177; Springer: Berlin, Heidelberg, 2005; pp 1–52, DOI: 10.1007/b101374. (7) Bunz, U. H. F. Poly(aryleneethynylene)s. Macromol. Rapid Commun. 2009, 30, 772– 805. (8) Müllen, K.; Wegner, G. Electronic Materials: The Oligomer Approach; Wiley-VCH: Weinheim, 2008. (9) Martin, R. E.; Diederich, F. Linear Monodisperse pi-Conjugated Oligomers: Model Compounds for Polymers and More. Angew. Chem., Int. Ed. 1999, 38, 1350–1377. (10) Sudeep, P. K.; James, P. V.; Thomas, K. G.; Kamat, P. V. Singlet and Triplet ExcitedState Interactions and Photochemical Reactivity of Phenyleneethynylene Oligomers. J. Phys. Chem. A 2006, 110, 5642–5649. (11) Maillou, T.; Le Moigne, J.; Dumarcher, V.; Rocha, L.; Geffroy, B.; Nunzi, J.-M. Oligo(phenylethynylene) as a High Photoluminescence Quantum Yield Material and its Distributed Feedback Laser Emission in Thin Films. Adv. Mater. 2002, 14, 1297– 1301. (12) Godt, A.; Schulte, M.; Zimmermann, H.; Jeschke, G. How Flexible Are Poly(paraphenyleneethynylene)s? Angew. Chem., Int. Ed. 2006, 45, 7560–7564.

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(13) Jeschke, G.; Sajid, M.; Schulte, M.; Ramezanian, N.; Volkov, A.; Zimmermann, H.; Godt, A. Flexibility of Shape-Persistent Molecular Building Blocks Composed of pPhenylene and Ethynylene Units. J. Am. Chem. Soc. 2010, 132, 10107–10117. (14) Hintze, C.; Schütze, F.; Drescher, M.; Mecking, S. Probing of Chain Conformations in Conjugated Polymer Nanoparticles by Electron Spin Resonance Spectroscopy. Phys. Chem. Chem. Phys. 2015, 17, 32289–32296. (15) Castruita, G.; Arias, E.; Moggio, I.; Pérez, F.; Medellín, D.; Torres, R.; Ziolo, R.; Olivas, A.; Giorgetti, E.; Muniz-Miranda, M. Synthesis, Optical Properties and Supramolecular Order of pi-Conjugated 2,5-di(Alcoxy)phenyleneethynylene Oligomers. J. Mol. Struct. 2009, 936, 177–186. (16) Linton, K. E.; Fox, M. A.; Pålsson, L.-O.; Bryce, M. R. Oligo(p-phenyleneethynylene) (OPE) Molecular Wires: Synthesis and Length Dependence of Photoinduced Charge Transfer in OPEs with Triarylamine and Diaryloxadiazole End Groups. Chem. - Eur. J. 2015, 21, 3997–4007. (17) Tait, C. E.; Neuhaus, P.; Anderson, H. L.; Timmel, C. R. Triplet State Delocalization in a Conjugated Porphyrin Dimer Probed by Transient Electron Paramagnetic Resonance Techniques. J. Am. Chem. Soc. 2015, 137, 6670–6679. (18) Tait, C. E.; Neuhaus, P.; Peeks, M. D.; Anderson, H. L.; Timmel, C. R. Transient EPR Reveals Triplet State Delocalization in a Series of Cyclic and Linear pi-Conjugated Porphyrin Oligomers. J. Am. Chem. Soc. 2015, 137, 8284–8293. (19) Tait, C. E.; Neuhaus, P.; Peeks, M. D.; Anderson, H. L.; Timmel, C. R. Excitation Wavelength-Dependent EPR Study on the Influence of the Conformation of Multiporphyrin Arrays on Triplet State Delocalization. Phys. Chem. Chem. Phys. 2016, 18, 5275–5280.

16


Page 16 of 18

Page 17 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(20) Schütze, F.; Krumova, M.; Mecking, S. Size Control of Spherical and Anisotropic Fluorescent Polymer Nanoparticles via Precise Rigid Molecules. Macromolecules 2015, 48, 3900–3906. (21) Stoll, S.; Schweiger, A. EasySpin, a Comprehensive Software Package for Spectral Simulation and Analysis in EPR. J. Magn. Reson. 2006, 178, 42–55. (22) Barford, W. Excitons in Conjugated Polymers: A Tale of Two Particles. J. Phys. Chem. A 2013, 117, 2665–2671. (23) Since such a distinction has not been made in ref. 18, the misleading conclusion of a failure of the point-dipole approximation for judging “delocalization” could ensue. (24) Namdas, E. B.; Ruseckas, A.; Samuel, I. D. W.; Lo, S.-C.; Burn, P. L. Triplet Exciton Diffusion in fac-tris(2-Phenylpyridine) iridium(III)-Cored Electroluminescent Dendrimers. Appl. Phys. Lett. 2005, 86, 091104. (25) Mahato, P.; Monguzzi, A.; Yanai, N.; Yamada, T.; Kimizuka, N. Fast and Long-Range Triplet Exciton Diffusion in Metal-Organic Frameworks for Photon Upconversion at Ultralow Excitation Power. Nat. Mater. 2015, 14, 924–930. (26) El-Sayed, M. A.; Tinti, D. S.; Yee, E. M. Conservation of Spin Direction and Production of Spin Alignment in Triplet–Triplet Energy Transfer. J. Chem. Phys. 1969, 51, 5721– 5723. (27) El-Sayed, M. A. Optical Pumping of the Lowest Triplet State and Multiple Resonance Optical Techniques in Zero Field. J. Chem. Phys. 1971, 54, 680–691. (28) Akiyama, K.; Tero-Kubota, S.; Ikoma, T.; Ikegami, Y. Spin Polarization Conservation during Intramolecular Triplet-Triplet Energy Transfer Studied by Time-Resolved EPR Spectroscopy. J. Am. Chem. Soc. 1994, 116, 5324–5327.

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(29) Ruedenberg, K.; Scherr, C. W. Free-Electron Network Model for Conjugated Systems. I. Theory. J. Chem. Phys. 1953, 21, 1565–1581.

18


Page 18 of 18

Delocalization of Coherent Triplet Excitons in ... - ACS Publications

Recommend Documents