Photo-Annealing of Merocyanine Aggregates - The Journal of Physical

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Photo-Annealing of Merocyanine Aggregates Felix Herrmann-Westendorf, Torsten Sachse, Martin Schulz, Martin Kaufmann, Vladimir Sivakov, Rainer Beckert, Todd J. Martínez, Benjamin Dietzek, and Martin Presselt J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b09048 • Publication Date (Web): 30 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018

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The Journal of Physical Chemistry

Photo-Annealing of Merocyanine Aggregates Felix Herrmann-Westendorf1,2, Torsten Sachse1,2, Martin Schulz1,2, Martin Kaufmann1,3, Vladimir Sivakov2, Rainer Beckert3, Todd Martínez5,6, Benjamin Dietzek1,2, Martin Presselt1,2,6,7,* 1

Institute of Physical Chemistry, Friedrich Schiller University Jena, Helmholtzweg 4, 07743 Jena, Germany

2

Leibniz Institute of Photonic Technology Jena (IPHT), Department Functional Interfaces, Albert Einstein

Straße 9, 07745 Jena, Germany 3

Institute of Organic and Macromolecular Chemistry, Friedrich Schiller University Jena, 07743 Jena,

Germany 4

Department of Chemistry and PULSE Institute, Stanford University, Stanford, CA 94305, USA

5

SLAC National Accelerator Laboratory, Menlo Park, CA 94309, USA

6Center

for Energy and Environmental Chemistry Jena (CEEC Jena), Friedrich Schiller University Jena, Jena,

Germany 7

sciclus GmbH & Co. KG, Moritz-von-Rohr Str. 1a, 07745 Jena, Germany

* [email protected]

ABSTRACT In this work we elucidate the fundamental difference between aggregate formation of donor-π-acceptor merocyanines in their electronic ground and excited states. While increasing the π-bridge size favors formation of π-stacked aggregates in the dark, irradiation with visible light causes reorientation of the dyes to form prototype H-aggregates with compensating dipole moments. This photo-annealing changes the supramolecular structure and its UV-vis spectroscopic properties dramatically, thus being of importance for the function of active layers composed of these dyes. Aggregates of the ground state dyes are bound cooperatively through ππ-London dispersion interactions and hydrogen bonds between the polar α-cyano-carboxylic acid groups. However, charge transfer upon photoexcitation leads to repulsion of the polar acid groups. Electronic excitation of the dyes approximately doubles the ground state dipole moment, thus driving molecular reorientation into prototype H-aggregate structures. We show that this photo-induced supramolecular rearrangement can disrupt the large polymeric aggregates formed in the dark. The photo-induced supramolecular structural changes reported in this work will 1 ACS Paragon Plus Environment

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influence the performance of optoelectronic devices composed of these structures and must be controlled to avoid morphological decomposition of active layers upon operation.

INTRODUCTION Supramolecular structures of dyes used for optical layers1-3, active layers in organic4-10, dye sensitized1113,

or hybrid solar cells14-16 (OSCs, DSSCs, HSCs, respectively), sensors17-18, as well as organic light emitting

diodes3, 19-21 (OLEDs), essentially determine the resulting thin film22-25 and device properties26. While the molecular interactions between many dyes with extended π-systems are governed by dispersion interactions27, the interactions between push-pull dyes, such as merocyanines28-29, are typically dominated by dipole-dipole interactions30. The molecular dipoles of the merocyanines arise from the resonance between the non-polar polyene and the polar zwitterionic forms and allow for controlling supramolecular structures, e.g. by orientation of the dipoles in electric fields.31 Prototype aggregates of dipolar dyes were discovered by Scheibe32 and Jelley33-34 and named as H- and Jaggregates, respectively. A theoretical understanding of their electronic spectra has been derived35-36 and essential work has been conducted towards understanding and controlling molecular aggregate formation in solution via self-assembly28, temperature- or concentration-induced formation in thin solid films37-38, and controlled interface assembly3, 24. The aggregate formation mechanisms reported in the literature are highly diverse and involve basic dimerization30, formation of seeds39-40, growth of and transitions between distinct supramolecular structures41-42, as well as autocatalysis39,

43

and

polymerization40, 44. Recently, Petrenko and Dimitriev distinguished between different mechanisms for aggregate formation in the dark and under irradiation with visible light.45 Such a difference might originate from the driving force, i.e. the dipole moment to be compensated through aggregation, that can be further enhanced upon photo-excitation11, 46-49. Thus, in excited states different supramolecular structures might form as compared to the electronic ground states. Consequently, it appears to be essential for the development and processing of photoactive materials to determine the influence of light on supramolecular structure formation, as recently reported for cyanine dyes45, and to discriminate the effects of photo-aggregation from photodegradation50. In contrast to the small dipolar donor-acceptor (D-A) dyes studied by Petrenko and Dimitriev45, more recently developed D-π-A dyes48 contain π-bridges to extend the charge transfer range and shift the 2 ACS Paragon Plus Environment

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absorption towards red and NIR spectral ranges. Absorption spectra of their solutions or thin films do not necessarily resemble those of prototype H- or J-aggregates48, thus indicating that the additional πbridges lead to aggregation mechanisms deviating from those leading to prototypical H- or J-aggregates, cf. dispersion interaction- and multipole interaction-driven aggregation and polymerization of porphyrins22, 51-52, perylenes53, and azaacenes54. To investigate whether photo-control of aggregation, as observed by Petrenko and Dimitriev45, applies for D-π-A dyes48, we compare the spectral signatures of D-π-A-aggregate formation as induced by concentrating and irradiating solutions containing D-π-A dyes in the present work. The D-π-A dyes used are dipolar merocyanines, which vary in the length and chemical nature of the π-bridge between donor and acceptor moieties. These variations tune the balance between charge transfer (CT) and π-π* character for the S0→S1 electronic transition. Thus, supramolecular structure formation will be varied by both the importance of dipole-dipole interactions compared to other intermolecular interactions and also the molecular shape, which changes with increasing π-bridge length. We investigate the supramolecular structures formed by means of UV-vis absorption and fluorescence spectroscopy, dynamic light scattering (DLS), atomic force microscopy (AFM), and quantum chemical calculations. These combined experimental-theoretical investigations unravel the different aggregation mechanisms of dipolar merocyanines in bright and dark conditions. Awareness of this difference is of utmost importance for the fabrication of supramolecular structures, particularly in active materials for optics and optoelectronics.

RESULTS AND DISCUSSION The merocyanines used in this work contain α-cyano carboxylic acid groups as electron acceptors (A) and triarylamine groups as donors (D)55, as shown in Figure 1. If we consider one of the aryl groups as the first part of the π-bridge between D and A, substance D2A might be viewed as containing a bridge consisting of two thiophene rings. The π-bridges of D3A and D4A, as described by Menzel et al.56, are successively extended by one and then two additional aromatic rings. However, D3A is distinctly different from D2A since the thiophene of the triaryl moiety is replaced by a 4-methoxy-5-phenylenethiazole moiety.24, 56 Finally, D4A combines both motifs, the 4-methoxy-5-phenylene-thiazole and the bithiophene moieties, in its bridge.56 In the following, we will first compare the electronic properties of the three derivatives as derived from quantum chemical calculations and analyze their experimental UV-

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vis absorption and emission spectra in solution. Subsequently, differently induced aggregation and subsequent dissolution is investigated.

Figure 1: Lewis structures of the merocyanines D2A, D3A, D4A.

Electronic Structure of Individual Merocyanines All three derivatives feature an intense absorption band at ~520 nm; as the π-system is increased, the absorption maximum shifts slightly to shorter wavelengths (λmax(D2A, D3A, D4A): 530, 520, 493 nm), cf. the results of Menzel et al. for D3A and D4A56. According to our time dependent functional theory (TDDFT) calculations, these electronic transitions generally are energetically well-separated from excitation to higher states and are dominantly described as π-π*, with rather weak CT character. This character of the electronic transitions is visualized by means of charge difference density plots49, 57 in Figure 2, which show the change in total electron density upon photoexcitation into the S1-states. The centers of charge depletion are mainly localized at the triphenylen-amines, while charge accumulation predominantly 4 ACS Paragon Plus Environment

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happens at the α-cyano carboxylic acid groups, and pronounced π-π* excitations are observed at all hetero-cycles. However, as the length of the π-bridge increases, the CT character of the S0S1 transitions increases as reflected in the charge difference density plots. The experimentally observed adjacent energetically higher absorption peaks are located at 348, 370, and 410 nm for D2A, D3A, and D4A, respectively, as shown in Figure 2. Because the TD-DFT-derived spectra shown in Figure 2 resemble the absorption spectrum of D3A quite well in the investigated wavelength range, the 370 nm-peak is assigned to the S0S2 transitions. However, in case of D4A the S0S2 transition is predicted at higher energies than the experimentally observed 410 nm-peak. Alternatively, the latter might originate from absorption by H-aggregates (see below), which might form even at low dye concentrations58. In the case of D2A the uniformly applied energetic shift (280 meV) of the TD-DFT-derived energies is too small to match the experimentally determined energies, presumably due to the different CT/π-π* ratio of the energetically lowest transitions of D2A as compared to D3A and D4A. Taking further shifts of the TD-DFT-derived transition energies into account, the experimentally observed D2A-peak at 348 nm can be assigned to the S0S2 transition. All dyes show spectrally broad fluorescence ranging to the near-IR, i.e. displaying a significant Stokes shift. The Stokes shift gets larger upon increasing the length of the π-bridges and is determined to 420, 640 and 790 meV, for D2A, D3A and D4A, respectively. This Stokes shift increase indicates a stronger geometric change in the S1-states with increasing π-bridge length.



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Figure 2: Experimental molar extinction (ε) and fluorescence spectra of all molecules in chloroform solution in comparison to the stick spectra derived from theory. All theoretically derived absorption energies where shifted red-shifted by 280 meV to facilitate comparison to the experimental results. The volumetric data plotted on the molecular structures represent photo-induced changes in the electron density distribution at the Franck-Condon point of the S0S1 transition (orange: electron depletion, blue: electron accumulation; chloroform solvation is modeled using linear response COSMO).


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Aggregate Formation As shown in Figure 3, aggregate formation as induced by traditional increasing the concentration of D2A, D3A, and D4A give rise to markedly different changes in the absorption spectra as compared to those observed upon successive irradiation of the solutions with intense blue light (455 nm, approx. 100 mW/cm²): While the absorption spectra change just little, even if varying the dye concentration by almost two orders of magnitude (Figure 3a-c), the main absorption band at ≈500 nm successively vanishes and a band peaking between 350 and 400 nm grows upon irradiation with intense white light (Figure 3d-f). Therefore, we assume that concentration-increase and irradiation lead to different supramolecular structures with distinct absorption spectroscopic properties as discussed in more detail in the following.



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Figure 3: Experimental extinction coefficient and absorption spectra of D2A, D4A in chloroform and D3A in THF at systematically varied concentrations (a-c), irradiation times (absorbance (d-f) and significant components C1, C2, C3 (g-i) together with their kinetics (see inset) as revealed by a singular-value decomposition - SVD), and derived from time-dependent density functional theory calculations for different dimers (j-l). The geometries of the latter model dimers AA, AN, ANπ, Aπ for D2A, D3A, D4A, respectively, are depicted in Figure 5 and Figure 6. Their TD-DFT-derived spectra were calculated using CAM-B3LYP. The reference monomer is calculated using two molecules separated by 10 Å. The extinction coefficient of D2A was divided by a factor of two to facilitate comparison. 8 ACS Paragon Plus Environment

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Ground State Aggregation Increasing the concentrations of D2A, D3A and D4A causes small absorption decreases accompanied by minute blue-shifts of the S0S1 absorption peaks while the ones assigned to the S0S2 transitions are rather unaffected by concentration changes, as shown in Figure 3a-c. These small absorption changes do not resemble those reported for merocyanines that form prototype H-aggregates to compensate their molecular dipole moments28, 58. In the latter case typical spectral series of aggregate formation show the rise of a blue shifted dimer peak at the expense of a low-energy monomer peak, involving an isosbestic point28. In contrast, the concentration-induced aggregation observed in this work appears to follow a different aggregation-mechanism than the typical dipole-dipole compensation mechanism. In addition to the latter, London dispersion interactions are expected to significantly contribute to the intermolecular interactions27. Their weight is expected to grow with increasing length of the π-bridge from D2A, D3A, to D4A. Additionally, the heteroatoms in the π-bridges of D3A and D4A induce polarity to the π-bridges as shown by the electrostatic potentials at the van der Waals surfaces in Figure 4. Furthermore, the α-cyano-carboxylic acid moiety might be involved in hydrogen bonding by means of the acid59-60 and the cyano group61-62. Moreover, both, hydrogen bonds63 as well as π-stacks64-67, can give rise to step-growth to larger aggregates.



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Figure 4: Electrostatic potential at the van der Waals surface of the investigated merocyanines in the S0 state together with the S0 and S1 dipole moments µS0 and µS1, respectively (in vacuum relaxed geometries). The blue and red arrow represents the dipole vector of the S0 and S1 state, respectively.

Because of interplay between the aforementioned different intermolecular interactions, the aggregate formation is highly complex. Therefore, energetically favorable dimer structures were identified by systematic grid-based scanning of dimer geometries, using the recently introduced molecular modeling tool EnergyScan51, rather than by chemical intuition-guided manual dimer creation. With EnergyScan all dimer geometries with a binding or dimerization energy up to 300 meV above the global minimum energy were identified and the 20 most distinct geometries were automatically detected. Out of the twenty dimers identified this way the ones with the most distinct absorption spectra are shown in Figure 3j-l and can be assigned to either of the following archetype binding motifs (see Figure 5 for the dimer geometries, Figure 1 of the supporting information (SI) for electrostatic potentials at the van der Waals surface of the dimers; selected geometric parameters of the dimers can be found in Table 1 of the SI):


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AA: Double hydrogen bond between the two carboxylic acids (abbrev. AA); limited to dimerization, allows no further aggregate growth AN: Double hydrogen bond between a carboxylic acid and the nitrile together with its neighboring β-hydrogen (abbrev. AN); not limited to dimerization, allows for further aggregate growth ANπ: AN-binding motif, but rotation around the C(OOH)C(CN)-bond enables ππ-interactions (abbrev. ANπ); not limited to dimerization, allows for further aggregate growth Aπ: Interaction between the α-cyano carboxylic acid and the π-bridge (abbrev. Aπ); presumably not limited to dimerization, allows for further aggregate growth

Table 1: Dimerization energies Edim, dipole moments of the ground and S1 excited states(µdim-GS, µdim-S1, respectively) of merocyanine dimers and angles between the individual molecular groundstate dipole moments within the identified archetype dimers

Dimer

AA

AN

ANπ

Aπ

D2A

108.1

56.9

63.7

39.6

D3A

94.6

54.6

74.3

39.6

D4A

109.0

73.3

101.4

42.5

D2A

0.42

2.99

13.05

1.62

D3A

0.53

6.12

15.38

5.49

D4A

0.35

11.69

1.44

6.36

D2A

0.44

9.35

26.99

0.85

D3A

1.39

17.63

36.93

14.84

D4A

1.80

30.22

14.70

15.19

D2A

175

8

25

164

D3A

177

155

21

134

Type Edimmin

µdim-GS

µdim-S1

α(µ1,µ2)

[kJ/mol]

[D]

[D]

[°]



D4A StepGrowth

170

105

11

161

no

yes

yes

pos-

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sibly

Figure 5: Structures of the dimers discussed in this paper together with their binding energies. Binding energies were obtained as the difference in total energies between the dimers depicted here and two molecules separated by 10 Å. More details can be found in the methods section.

According to the DFT-derived dimerization energies (Table 1 and Figure 5) AA-type interactions are energetically favorable in vacuum and presumably in very non-polar media as compared to the other investigated interactions mentioned above. In the rather polar solvents used in this study we expect a considerable screening of the surface charges at the acid groups68. Thus, the binding energies, 12 ACS Paragon Plus Environment

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particularly for the AA and AN binding motifs, are expected to decrease upon increasing solvent polarity. Furthermore, for all binding motifs except AA polymerization is possible, i.e. the product of aggregation is still a reactive species, thus increasing the probability of finding these other binding motifs rather than AA. To determine the prevalent species in the experiment, we compare TD-DFT and experimentally derived absorption peak shifts upon aggregate formation, see Figure 3a-c,j-l, as detailed in the following. In case of D2A virtually no aggregation-induced shift of the S0S1 absorption energy can be observed experimentally, but slight-blue shifts are present in case of D3A (λmax: 505  480 nm) and D4A (λmax: 510  490 nm), respectively (see Figure 3a-c). In contrast, the TD-DFT calculations yield red-shifted S0S1 absorptions for AA-dimerization for all of the merocyanines (see Figure 3j-l and a schematic in Figure 6). Thus, the AA-binding motif does not appear to dominate in ground-state aggregate formation. In case of D3A, AN-binding also involves a slight red-shift, which is again not in accordance with the experimental blue shift. For the other two merocyanines AN-binding causes no spectral change, which fits the experimental observation, just as in the case of D2A. Finally, the experimentally observed slight blue-shift of the S0S1 absorption peak in case of D3A and D4A can be reproduced when the π-bridge is involved in dimerization, i.e. in ANπ- and Aπ-type dimers. This S0S1 absorption peak of the dimer does not shift further to the blue upon successive aggregate growth in case of ANπ-type binding, as shown by the comparison of the TD-DFT-derived absorption spectra of a D3A-hexamer with the dimer in Figures SI 2 and SI 3. After the AA-binding motif, the ANπbinding motif is energetically most favorable for all investigated merocyanines and the binding energy scales with the π-bridge lengths (D2A: 0.66 eV, D3A: 0.77 eV, D4A: 1.05 eV). However, the number of geometrical realizations of this binding motif is rather limited due to distinct interactions between the polar heads, while a larger number of realizations can be expected for the somewhat energetically less favorable Aπ-dimers. As the position of the absorption peak is not affected by concentration changes of D2A, the blue-shifted ANπ- and Aπ-type dimers are not expected to dominate in case of D2A because the spectral changes due to their formation would need to be balanced by formation of AA-dimers. However, in case of D2A the rather concentration-independent S0S1 absorption energy can be reproduced by various linear combinations of all the above discussed binding motifs.



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Figure 6: Schematic representation of theoretically derived dimer binding types that do or do not cause red-shifted absorption peaks, AA, AN, respectively, or blue-shifted absorption spectra, ANπ, Aπ, with respect to the monomer-absorption peak. Experimentally, exclusively blue-shifts are observed.

Because all but the AA-type dimers allow for successive growth of aggregates, analysis of the aggregate size might further support the above deduced importance of ANπ- and Aπ-binding in case of D3A and D4A. Such analysis of particle sizes, as investigated by means of dynamic light scattering (DLS) studies, reveal that D4A forms large aggregates in chloroform (1.5 mM) with a mean diameter of 70 nm as shown in Figure 6 of the SI. Also in case of D3A the particle size systematically increases from 1.5 nm (presumably monomers: maximum molecular lengths: 2.3 nm, distance between the oxygen of the OCH3-groups: 1 nm) to 1.9 nm (dimers) to 2.0 nm (dimers and higher aggregates) with rising concentration (0.2, 1, 2 mM, respectively) as revealed by the DLS results shown in Figure 7. Additionally, the DLS results indicate the presence of aggregate sizes between 6 and 8 nm, which we assign to different principal axes of the before-mentioned aggregates as we are expecting anisotropic, i.e. not totally rotationally symmetric spherical, aggregates. These aggregates grow further with time and finally formed a gel within a few days from THF and chlorobenzene solutions of D3A, as shown by the photograph in Figure 7. To ensure the van der Waals nature of the formed aggregates, their dissolution was investigate by means of DLS as well. The DLS data in Figure 7 of the SI show dissolution of aggregates (solid state powder) by the polar and protic solvent methanol, thus supporting the above assumption of prevailing van der Waals type aggregates formed in the dark. Finally, the analysis of our quantum chemical results, the experimental absorption spectra and DLS data reveal that the molecular shape determines the ground-state aggregation in the dark. For D3A and D4A, the ANπ and Aπ binding motifs (which allow for successive aggregation) prevail. However, a dominant binding motif could not be unambiguously determined for D2A, where either AN dominates or many 14 ACS Paragon Plus Environment

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differently bound dimers are formed without causing a net spectral absorption shift. Thus, for the former dyes aggregation in the dark is determined by cooperative binding that involves both dispersion binding between π-bridges and the distinct interactions between the polar heads. These aggregates grow to large van der Waals polymers, finally forming gels, without showing large UV-vis spectral changes upon aggregate formation, what is in stark contrast to what is observed upon external illumination, compare Figure 3a-c with Figure 3d-f, respectively.

Figure 7: Particle size distributions, represented by the hydrodynamic radius (RH), at different concentrations of D3A in tetrahydrofuran as determined by means of dynamic light scattering measurements. Since spherical particles are usually assumed the particle size is the diameter of the single particles. In the inset a jelly worm made from D3A in THF-d6 is shown.

Excited State Aggregation In stark contrast to the changes in absorption spectra upon increasing the dye concentration, irradiating the solutions of all investigated merocyanines causes strong changes in the absorption spectra: The S0S1 absorption peaks vanish, while peaks close to those assigned above to S0S2 transitions evolve and give rise to isosbestic points in each case, as shown in Figure 3. This type of absorption change is known for the formation of H-aggregates, where aggregation is driven by the energetically favorable compensation of dipole moments28, 58. However, even if such ground-state aggregation is apparently 15 ACS Paragon Plus Environment


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hampered in solution as discussed above, presumably due to the bulky substituents28, 30, drop casting a D4A solution reveals that the peak developed at 400 nm upon irradiating D4A solutions perfectly coincides with an aggregate specific absorption peak, as shown in Figure 4 of the SI. To understand why the prototype H-aggregate formation absorption changes are observed when irradiating the solutions but not for aggregates formed in the dark, the (TD)DFT-derived molecular dipole moments of ground and excited states are considered in the following. According to these (TD)DFT calculations, the S1 exited states have significantly larger dipole moments μ (μS1(D2A, D3A, D4A)=21.8, 29.8, 26.3 D) compared to the S0 electronic ground states (μS0(D2A, D3A, D4A)=9.1, 9.1, 10.5 D) as depicted in Figure 4. Thus, the driving force for compensating the dipole moments upon dimerization is significantly enhanced upon photo-excitation and might support overcoming steric hindrances hampering formation of prototype H-aggregates in the electronic ground state. This H-type dimerization might occur from associated monomers or via interconversion of aggregates1, as discussed in the literature for H- to J-type transition45, 69, within the typical fluorescence lifetime in the order of nanoseconds reported by Menzel et al.56. The hypothesis that photo-enhanced dipole moments are the essential driving force for formation of prototype H-aggregates is supported by a control experiment, where protonation of the amine function was used to hamper photo-induced charge transfer, i.e. photoenhancement of the dipole moment. According the hypothesis, the absorption spectra shown in Figure 5 of the SI reveal that photo-induced formation of H-aggregates is hampered upon protonation of the amine function. Because dye aggregates can be tightly bound, cf. binding energies of few hundred meV for dyes bound by cooperative hydrogen and London dispersion bonds70 that approach the binding energies calculated in this work, they might hardly dissolve in solution. However, in case large aggregates of the polymerizable types (sampled in Figure 5 and Table 1) have formed before illumination, photoenhancement of molecular dipoles apparently causes molecular reorientation leading to aggregate dissociation, as shown by the DLS data in Figure SI 6. This reduction of aggregate size upon irradiation might be indicative of van der Waals binding within the aggregates, rather than of covalent binding. However, even if the presence of isosbestic points in the absorption spectra series detected after illumination, Figure 3d-f, indicates formation of distinct dimers, which are likely van der Waals bound according the above considerations, spectral changes of the D4A-peak at ≈400 nm after 10 min of note that the rigidification of the dyes within aggregates extends fluorescence lifetimes, see Ref. 69. Rosch, U.; Yao, S.; Wortmann, R.; Wurthner, F., Fluorescent H-Aggregates of Merocyanine Dyes. Angewandte ChemieInternational Edition 2006, 45, 7026-30. 1


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irradiation hints to photochemical reactions after long and intense irradiation in addition to the initially prevailing photo-induced formation of prototype H-aggregates.

Kinetics of Excited State Aggregation To discriminate between photo-aggregation and -degradation we analyzed the kinetics of the absorption changes. While in case of photo-degradation basically 1st-order kinetics would be expected, 2nd order kinetics might be found for dimerization. Plotting our data according to these orders (1st-order function f1=ln(E0/Et)=kt, 2nd-order function f2=1/Et - 1/E0=kt; see Figure 8 in the SI) reveals that the photo-induced changes in absorption spectra clearly do not follow 1st order kinetics, i.e. photo-degradation can be excluded as dominant reason for the observed absorption changes as already indicated by the presence of isosbestic points in Figure 3d-f, but 2nd order kinetics also does not describe our data accurately in case of D2A, D3A, D4A. For the example of D4A we tested for the order of the reaction by comparing reaction rates r for different starting concentrations and evaluated the extinctions E according to r = (E(t) - E(t0))/t (see Figure 8 of the SI). When increasing the concentration fourfold from 7.7 mM to 30.8 mM, the reaction rate increases by a factor of 12. This initial ratio quickly decays and converges at approx. 6 for long times as shown in Figure SI 9, thus, again neither fitting to 1st or 2nd order kinetics. Instead of simple 1st or 2nd order models, a stretched exponential model with the free monomer concentration decreasing exponentially as a power of time, cf. the work of Pasternack et al.39 and Petrenko et al.45, fits our data sufficiently for all investigated merocyanines as shown in Figure 8. In the applied stretched exponential function

E  t   E  t0    E  t   E  t0    1  e  kt     n

E(t), E(t0), E(t∞) are the extinctions at a given wavelength at the variable time t, at the start t0 and the end t∞ of the experiment, respectively. k is the reaction rate constant, and n accounts for possible further growth of merocyanine assemblies as a power function of time. The fit was applied to the extinction spectra and to the significant component C2, which was determined by a singular-value decomposition (SVD, see supporting information for details and Figure 3g-i) and describes the temporal spectral change. The kinetic parameter resulting from fitting the stretched exponential model to the C2data derived from the SVD are in qualitative agreement to those obtain from fitting the model directly



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to the temporal evolution of the extinction coefficient spectra and are not discussed separately in the following. In the stretched exponential model shown above, the exponent n describes if the aggregation occurs upon formation of aggregates from single molecules or small clusters (n1)71; cf. the initial work of Leyvraz72 and the more recent papers of Pasternack et al.39 and Petrenko and Dimitriev45. The rather small n-values reported in Table 2, indicate aggregation proceeds primarily through combination of monomers rather than larger aggregates. This is in line with the above discussed model of photo-induced dimerization without further aggregate growth. While the rate constants k are similar for D2A and D3A, see Table 2, the rate constant for D4A is significantly greater. The larger rate constant of D4A as compared to D2A and D3A is attributed to the long π-bridge of D4A that increases torsional degrees of freedom and enables ππ-binding for many different mutual molecular orientations and positions, see Aπ binding motif discussed above and shown in Figure 5. Because additionally the sterically demanding aryl-amine moiety has a smaller intramolecular geometric weight, the probability for low-energy barrier aggregation is expected to be higher in the case of D4A as compared to D2A and D3A.

Table 2: Parameters obtained by fitting the stretched exponential model to the decay of monomer extinctions E (low-energy maximum as detailed in Figure 8) and to the C2-component, which describes the temporal spectral change and was derived from a singular-value decomposition (SVD). Additionally, the significance, as represented by the values in the S-vector of the SVD, of the 3 significant components (C1, C2, C3) determined for all merocyanine derivatives are given.

Molecule

c

c

S (C1,C2,C3)

Snorm (C1,C2,C3)

k(C2), k(E)

n(C2), n(E)

(mg/ml)

(mM)

D2A

10

23.4

31.6, 11.0, 1.0

1.00, 0.35, 0.03

0.082, 0.073

0.89, 0.87

D3A

10

16.8

27.6, 4.8, 0.2

1.00, 0.17, 0.01

0.059, 0.062

0.95, 0.78

D4A

20

30.8

63.8, 7.5, 2.8

1.00, 0.12, 0.04

0.278, 0.332

0.68, 0.84

D4A

5

7.7

8.7, 1.6, 0.3

1.00, 0.18, 0.03

0.114, 0.139

0.91, 0.92


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Figure 8: Changes of the extinction E with illumination time for the three merocyanines D2A, D3A, D4A (top to bottom) in chloroform (c=10, 10 and 5 µg/ml, top to bottom, respectively) and line fits according to the stretched exponential model.

CONCLUSION We show that dipolar D-π-A merocyanine, which finds applications in optoelectronic devices48, 56, forms different aggregates in the dark and under irradiation with vis-light. The absorption properties of the monomers are largely retained if aggregates grow in the dark, while those formed upon irradiation show vanished low-energy absorption and increased hypsochromic absorption, as is typical for H-aggregates35. The type of aggregates that form is of particular importance because it essentially determines the range of light that is absorbed28, the fluorescence quantum yields28, energy dissipation, exciton splitting and charge injection in dye sensitized solar cells73. The π-bridge of the investigated D-π-A dyes causes intermolecular binding beyond dipole-dipole interactions, cf. the work of Petrenko and Dimitriev45, that demonstrated light-induced interconversion between H- and J-aggregates of small dipolar dyes. In the dark, with increasing length of the π-bridge of the merocyanines, the weight of London dispersion interactions on intermolecular binding and conformational flexibility naturally increase. Therefore, also the total binding energy increases in case of cooperative binding, which involves hydrogen bonds, London dispersion, and dipole interactions. Within the series of compounds investigated the DFT-calculations show a 60% increase of the binding energy from 0.66 eV to 1.05 eV with increasing π-bridge in case of cooperative binding. Importantly, these types 19 ACS Paragon Plus Environment


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of aggregates formed in the dark are able to polymerize non-covalently and finally gels are obtained in this work. In stark contrast to ground-state aggregation, irradiation with visible light causes formation of prototype H-aggregates, as deduced from systematic changes in the absorption spectra. This H-aggregate formation, which is not dominant in the dark, is facilitated upon photo-enhancement of the molecular dipoles by 2x within the series of investigated merocyanines. This photo-enhancement of molecular dipoles apparently also causes dissociation of larger aggregates formed in the dark because of molecular reorganization. Depending on the impact on the performance of optoelectronic devices, like dye sensitized solar cells, such supramolecular reorganization might be utilized as photo-annealing or must be controlled upon adding aggregation inhibitors56 or embedding dyes in solid matrices26, respectively. While such photo-induced supramolecular reorganization of dipolar dyes has been reported for small and sterically less hindered dyes, the present work for the first time demonstrate the effect for the extended and sterically hindered class of merocyanines employed in modern optoelectronic devices. Therefore, the present work clearly stresses the impact of photoannealing for fabrication and operation of modern organic optoelectronic devices.

EXPERIMENTAL AND THEORETICAL METHODS Dyes Dyes D3A and D4A have been synthesized according previously reported protocols56, D2A was obtained from the Würthner group (Universität Würzburg).

UV-vis absorption and fluorescence spectroscopy UV-vis spectra where recorded with a Varian CARY 5000 spectrometer using quartz glass cuvettes with a length of 1, 4 or 10 mm. Time-dependent experiments where measured on a JASCO 530 UV-vis spectrometer with an additional LED port for illumination of the samples. The illumination was performed with a 455 nm high power LED obtained from Thorlabs. The LED was controlled using a Thorlabs DC4100 LED driver. The current limit was set to 700 mA giving a light intensity of roughly 100200 mW/cm² at the cuvette. The JASCO spectrometer and the LED driver were controlled using a self20 ACS Paragon Plus Environment

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written Labview program for an automatic measurement with constant illumination times and intensities. Emission spectra were recorded on home build setup which was already described elsewhere24.

Dynamic light scattering experiments DLS spectra were recorded on a CGS3 compact goniometer system from ALV using a 22 mW HeNe-laser with a wavelength of 633 nm.

Molecular mechanics and quantum chemical modeling All quantum chemical computations performed were of the (time-dependent74 (TD)) density functional theory75 (DFT) type. They were carried out using the GPU-accelerated program TeraChem76-77. We chose the CAM-B3LYP density functional and the polarizable variant of the double-ζ valence basis set (Ahlrichs pVDZ78) developed by Ahlrichs and coworkers79, which have been shown to yield reliable geometries and energies at reasonable computational cost.52,

80-83

The TD-DFT-derived transition energies were

uniformly shifted by 280 meV to facilitate comparison to the experimental absorption spectra. Dipole moments of the S1-states refer to the Franck-Condon point. The electrostatic potential on a van der Waals (vdW)84 surface in Figure 4 was computed using the inhouse program package EnergyScan (freely available online) based on the quantum mechanical orbital data. The same program was used to automatically generate dimer geometries. A detailed description of EnergyScan can be found elsewhere.51 Binding energies were obtained as the difference in total energies between the dimers and two molecules separated by 10 Å. Within EnergyScan, in a first step the supramolecular potential energy surface of each individual merocyanine was scanned with a scanning molecule (using the MMFF94 forcefield85-89) that was moved in steps 0.5 Å along the three Cartesian axes and rotated in steps of 7.5° around its main axes. No energy evaluation was performed for dimers whose scaled vdW surfaces overlap or are farther apart from each other than 2 Å. All vdW radii were scaled by a factor of 0.9. In a second step the local energy minimum geometries are determined from all calculated dimer geometries. In the third and last step, a set of diverse dimer geometries among all local energy minimum (with energies less than 300 meV above the global minimum) is automatically selected using a minimum RMSD requirement: Starting with



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a value of RMSD=3Å, that RMSD-cutoff was successively increased in steps of 0.25 Å until no more than 50 dimers were left.

Supporting Information Available Within the SI additional results from our quantum chemical calculations are available. This includes electrostatic potentials of the dimers, a geometry optimized hexamer and the respective calculated absorption spectra. Furthermore, details on different methods for aggregation inhibition are shown. More DLS results and graphs on the UV-vis kinetics of photo aggregation can also be found in the SI.

ACKNOWLEDGEMENTS The authors thank the Bundesministerium für Bildung und Forschung (BMBF FKZ 03EK3507) and the Deutsche Forschungsgemeinschaft (DFG Grant No. PR 1415/2-1) for financial support. T. Sachse acknowledges the German Federal Environmental Foundation for his fellowship. The authors would like to thank Karin Kobow for support in the lab and Prof. Felix Schacher for supporting the DLS measurements. The authors like to express their special gratitude to Prof. Frank Würthner and his group for providing dye D2A and for fruitful discussion of our results.

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Photo-Annealing of Merocyanine Aggregates - The Journal of Physical

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