Fate of Photoexcited Molecular Antennae - Intermolecular Energy

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Fate of Photo-Excited Molecular Antennae - Inter-Molecular Energy Transfer vs. Photodegradation Assessed by Quantum Dynamics Stephan Kupfer, Daniel Kinzel, Michael Siegmann, Jule Philipp, Benjamin Dietzek, and Stefanie Gräfe J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12190 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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The Journal of Physical Chemistry

Fate of Photo-excited Molecular Antennae - Inter-molecular Energy Transfer vs. Photodegradation Assessed by Quantum Dynamics Stephan Kupfer,1* Daniel Kinzel,2 Michael Siegmann,1 Jule Philipp,1 Benjamin Dietzek,1,2 Stefanie Gräfe1

1

Institute for Physical Chemistry and Abbe Center of Photonics, Friedrich Schiller University Jena,

Helmholtzweg 4, 07743 Jena, Germany 2

Leibniz Institute of Photonic Technology Jena, Albert-Einstein-Straße 9, 07745 Jena, Germany

Corresponding author: [email protected]

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Abstract The present computational study aims to unravel the competitive photo-induced intermolecular energy transfer and electron transfer phenomena in a light-harvesting antenna with potential applications in dye-sensitized solar cells and photocatalysis. A series of three thiazole dyes with hierarchically overlapping emission and absorption spectra, embedded in a methacrylate-based polymer backbone, is employed to absorb light over the entire visible region. Inter-molecular energy transfer in such antenna proceeds via energy transfer from dyeto-dye and eventually to a photosensitizer. Initially, the ground and excited state properties of the three push-pull-chromophores, e.g., with respect to their absorption and emission spectra as well as their equilibrium structures, are thoroughly evaluated using state-of-the-art multiconfigurational methods and computationally less demanding DFT and TDDFT simulations. Subsequently, the potential energy landscape for the three dyads, formed by the π-stacked dyes as occurring in the polymer environment, is investigated along linearinterpolated internal coordinates to elucidate the photo-induced dynamics associated to intermolecular energy and electron transfer processes. While energy transfer among the dyes is highly desired in such antenna, electron transfer, or rather a light-induced redox chemistry, leading to the degradation of the chromophores, is disadvantageous. We performed quantum dynamical wavepacket calculations to investigate the excited-state dynamics following initial light-excitation. Our calculations reveal for the two dyads with adjusted optical properties exclusively efficient inter-molecular energy transfer within 200 fs, while in the case of the third dyad inter-molecular electron transfer dynamics can be observed. Thus, this computational study reveals that statistical copolymerization of the individual dyes is disadvantageous with respect to the energy transfer efficiency as well as regarding the photostability of such antenna.


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1. Introduction Energy and electron transfer processes are of outstanding importance in nature, such as in photosynthesis1–5 and cellular respiration,6,7 as well as in artificial applications, i.e., in solar cells8,9 and photocatalysis.9–11 In photosynthesis, i.e. photosystem II, efficient light-harvesting is achieved by incorporating a manifold of chromophores, e.g. chlorophylls, carotenes and xanthophylls units into a supra-molecular antenna that enables absorption of sunlight covering a wide range of the visible spectrum.12,13,6,14 Inter-molecular energy transfer among the individual dyes incorporated in such an antenna and the photosensitizer may proceeds by Förster resonance energy transfer (FRET),15,16 while undesired inter-molecular electron transfer processes between the light-harvesting units lead to the deactivation or even the degradation of the antenna.17,18 In manmade applications - aiming at mimicking nature’s principles of photosynthesis to facilitate artificial solar energy conversion, i.e., by means dyesensitized solar cells (DSSCs) and photocatalytic devices - Ruthenium(II) and Osmium(II) polypyridine complexes are widely used due to their strong stability on light, heat and electricity as well as their visible absorption combined with redox and catalytic activity. However, such transition metal complexes commonly feature rather low-extinction coefficients. In order to maximize their overlap with the solar radiation spectrum, mainly two strategies have been perused: i) increasing the absorption toward the NIR range19 by increasing the π-system of the polypyridine ligands with additional dyes20 and ii) partially replacing the polypyridyl sphere with dyes absorbing in the visible region.21,22 A different strategy to enhance the light-harvesting efficiency mimics nature by arranging antennae in the vicinity of the transition metal based photosensitizer.8 To realize efficient energy transfer inside the antenna as well as from the antenna towards the photosensitizer, suitable dyes need to combine: i) high molar extinction coefficients (over a defined spectral range), ii) easily tunable excited state properties and iii) large Stokes shifts with iv) fluorescence quantum yields close to unity as well as v) a high redox and vi) photostability. A promising class of dyes meeting these requirements are 4-hydroxy-1,3-thiazoles structurally related to the naturally occurring luciferine, the light-emitting dye of fireflies. Thiazoles were already successfully applied as dyes in DSSCs due to their tailor-made optical properties and large Stokes shifts of more than 2000 cm-1.23,24 To allow efficient energy transfer processes between the chromophores the adjustment of the distance between the energy donor and the energy acceptor is of uttermost importance. One way to tune donor-acceptor distances and thus the efficiency of inter-molecular energy and electron transfer is to utilize (weak) inter3 ACS Paragon Plus Environment


molecular interactions such as electrostatics, dispersion or hydrogen bonds.25 Thiazoles are well known for their tendency to aggregate by virtue of π-stacking,24,26 and thus promote short-range energy transfer. A promising approach to construct such supra-molecular antenna has been shown by Breul et al., where a series of three different thiazoles - absorbing in the blue (D1: 2-(4-methoxy-5-phenylthiophen-2-yl)pyridine),27,28 yellow (D2: 4-(3-methoxy-5(pyridin-2-yl)thiophen-2-yl)-N,N-dimethylaniline)29,30 and red (D3: 2-((4'-methoxy-5'-(4methoxyphenyl)-2,2'-bithiophen-5-yl)methyl)malononitrile)31 spectral region - is statistically copolymerized with an Os(II) photosensitizer, see Scheme 1.31

Scheme 1: Antenna incorporating three 4-hydroxy-1,3-thiazole-based dyes (D1, D2 and D3) harvesting the entire visible spectrum. Unidirectional energy transfer between D1-D2 and D2-D3 is followed by energy transfer toward the Osmium photosensitizer; optical properties of D1 and D3 are not adjusted for energy transfer between these dyes.31 The linear polymer bears the dyes and the photosensitizer in the side-chain, while the incorporation of triethylene glycol mono-methyl ether methacrylate (TEGMA) allows for sufficiently high solubility in a wide range of solvents, e.g. water, hydrophilic and hydrophobic organic solvents.32,33 Energy transfer from dye-to-dye as well as to the Os(II) complex was confirmed by excitation-emission correlation spectroscopy for the respective ter-, quarter- and pentapolymers, but no experiments addressing the energy transfer efficiency in this complex environment were performed. However, energy transfer efficiencies of approximately 70% were determined between the energy donor D3 and an energy accepting tris-bipyridine-based Ruthenium(II) complex in acetonitrile (AcN)34, embedded in a statistical terpolymer.35 Even higher efficiencies of 70%-98% were obtained by Fréchet et al. between copolymerised pendant coumarin donors and Ruthenium(II) subunits.36,37


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In the literature, several theoretical and joint theoretical-spectroscopic investigations addressing energy transfer processes in the scope of biology and biochemistry38–43 as well as in artificial light-harvesting44–46 - mostly based on molecular dynamical (MD) simulations coupled to quantum chemical or QM/MM (quantum mechanical/molecular mechanical) calculations - are available. The present computational study aims at elucidating the hierarchical inter-molecular energy and electron transfer processes in an antenna based on the dyes D1, D2 and D3 (see Scheme 1) by virtue of quantum chemical and quantum dynamical simulations. Of particular interest are the competitive excited state dynamics in these softmatter-embedded dyes associated to energy transfer and electron transfer. While hierarchical and unidirectional inter-molecular energy transfer from D1 to D2 and from D2 to

D3 (and eventually to the photosensitizer) is highly desired, inter-molecular electron transfer between the dyes is disadvantageous.17,18 Such photo-induced redox processes (i.e. photooxidation/photoreduction of the dyes) may alter the carefully adjusted photophysical properties of the dyes, which leads in consequence to malfunction and degradation of the entire antenna. Preliminary to the rationalization of inter-molecular interactions, an elaborate evaluation of the photophysics and the photochemistry of the isolated dyes is essential for the later assessment of the properties in such antenna. Therefore, the dyes D1, D2 and D3 are investigated RASPT2

47,48

using

state-of-the-art 49,50

/RASSCF

multiconfigurational

methods

namely

MS-

(multi-state restricted active space perturbation theory of second-

order on a restricted active space self-consistent field reference wavefunction) as well as economical density functional theory (DFT) and time-dependent DFT (TDDFT) simulations to unravel the electronic transitions underlying their absorption and emission spectra. Subsequently, energy and electron transfer phenomena in the scope of the dyads D1D2, D2D3 and D1D3 are studied by means of quantum dynamics (QD) on the potential energy curves obtained at the DFT and TDDFT levels of theory along effective reaction coordinates. The optical properties of these dyes were tailor-made to realize heteroenergy transfer from D1 to

D2 and to D3 but not directly from D1 to D3. The quantum mechanical calculations presented in the following aim to evaluate the efficiency of inter-molecular energy transfer as well as the photostability of a statistical copolymerized antenna and thus to function as a guideline for synthetic strategies.



2. Computational details All DFT as well as TDDFT calculations presented in this study were performed using the Gaussian 09 program,51 while all multiconfigurational simulations were carried out in Molcas 8.0.52 The ground state equilibrium structures of the present series of dyes based on the 4-methoxy-1,3-thiazole chromophore, namely D1, D2 and D3 as well as the respective dimers D1D2, D2D3 and D1D3 incorporated in a methacrylate-based polymer backbone, were investigated by means of DFT. The global hybrid functionals PBE0,53 B3LYP3521,54 (a functional based on B3LYP55,56 combining 35 % of exact exchange, 58.5 % of non-local B8857 exchange and the LYP56 correlation) and M06-2X58 with increasing amount of exact exchange from 25 % to 35 % and 54 % were employed. As shown recently for structurally, closely related push-pull chromophores, the amount of exact exchange influences the localization of carbon-carbon single and double bonds, which leads in consequence to pronounced variations of the ground and excited state properties for this class of compounds.59 The fully relaxed ground state equilibrium structures of the three dyes (D1, D2 and D3) were obtained by means of all three functionals in cooperation with Ahlrich's SVP double-ζ basis set.60 A subsequent harmonic vibrational analysis revealed that all optimized structures correspond to minima of the potential energy (hyper-)surface (PES). Excited state properties, such as vertical excitation energies and transition dipole moments, were computed within the adiabatic approximation at the TDDFT level of theory using the same basis set and XC functionals as for the preliminary ground state calculations. The UVabsorption spectra of the three dyes were simulated based on the ten lowest singlet excited states, while the same functional was used for the ground and excited state calculations, respectively. The S1 equilibrium structures were obtained at the aforementioned levels of theory to elucidate the fluorescence spectra of D1, D2 and D3. Solvent effects (acetonitrile, AcN; ε = 35.688, n = 1.3442) were taken into account for the (ground and excited state) equilibrium geometries as well as for excited state properties such as excitation energies and transition dipole moments by means of the integral equation formalism of the polarizable continuum model (IEFPCM).61 The nonequilibrium procedure of solvation was applied for processes, where only the fast reorganization of the electronic distribution of the solvent is important, i.e. UV-vis absorption, while the equilibrium procedure of solvation was used for excited state optimizations. In addition, excitation energies and transition dipole moments in the optimized S0 and S1 structures (in AcN) of D1, D2 and D3 were computed in gas phase to address solvent stabilization for the respective states. 6 ACS Paragon Plus Environment

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Furthermore, the computational results obtained by the economical TDDFT simulations were validated against multiconfigurational methods, namely RASPT247,48/RASSCF.49,50 The ANO-S basis set62 with the contractions S[7s7p4d]/C,N,O[7s6p3d]/H[4s3p] was applied, while the Cholesky decomposition63 was used to generate the two-electron integrals. RASPT2/RASSCF calculations were exclusively carried out in gasphase within the solvated (TD-)B3LYP35 ground and excited state geometries (S0 and S1). To label the restricted active spaces (RASs) utilized in the RASSCF calculations for D1, D2 and D3, the notation RAS (n,l,m;i,j,k) of Gagliardi and co-workers64 was used, where n labels the number of active electrons, l is the maximum number of holes in the RAS1, m is the maximum number of electrons in RAS3, and i, j and k are the number of active orbitals in the RAS1, RAS2, and RAS3 subspaces, respectively. The RAS1 typically comprises orbitals with large occupation numbers, while only a maximum number of electron holes is allowed. In contrast, the RAS3 includes virtual orbitals with occupation numbers close to zero, here, only a defined maximum number of electrons is allowed. The RAS2 is equivalent to the active space in the CASSCF method and allows all possible electronic configurations. The RASs were constructed to describe the singlet ground state as well as the first excited singlet state of the respective push-pull chromophore. Previous joint experimentalcomputational studies revealed that the visible absorption and fluorescence spectra of related 4-methoxy-1,3-thiazoles is rationalized by virtue of these states.24,21,65,59,66 State-average RASSCF calculations comprising the first two roots were carried out. Dynamical correlation was added my means of multi-state (MS-)RASPT267 on the SA(2)-RASSCF reference wave functions, while the core electrons were kept frozen and a real level shift68 of 0.3 a.u. was applied. Transition dipole moments for the S0 → S1 excitation were obtained at the SARASSCF and the MS-RASPT2 levels of theory using the CAS state interaction method.69 The S0 → S1 excitation is mainly described by the HOMO/LUMO transition (highest occupied molecular orbital/lowest unoccupied molecular orbital). Thus, the RASs for D1, D2 and D3 were designed as follows: The RAS for D1, depicted in Figure 1, comprises the HOMO and LUMO within the RAS2 subspace. The RAS1 includes the sulfur p-orbital in the aromatic plane, the remaining bonding π-orbital of the thiazole as well as two pairs of π-orbitals of the phenyl and the pyridine moiety, respectively, while the two total bonding π-orbitals were neglected. Accordingly, the RAS3 comprises the total antibonding thiazole π*-orbital and two pairs of π*-orbitals (localized on the phenyl and the pyridine fragments); the respective two total antibonding π*-orbitals were not incorporated. Interactions among the subspaces were 7 ACS Paragon Plus Environment


taken into account up to double excitations, which leads in consequence to a RAS (14,2,2;6,2,5) spanning over 2821 configuration state functions (CSFs).

Figure 1: RAS (14,2,2;6,2,5) for D1 within the fully optimized ground state (B3LYP35/SVP) structure, the distribution of active orbitals over the subspaces RAS1, RAS2 and RAS3 is displayed. The occupation within the Hartree-Fock (HF) reference wavefunction is indicated by the gray dashed line. The RAS partition of D2 was designed accordingly yielding an equivalent RAS (14,2,2;6,2,5). The RAS of D3 includes the two sulfur p-orbitals (thiazole and thiophene) as well as the bonding thiazole π-orbital in RAS1, two pairs of phenyl π/π*-orbitals, two pairs of thiophene π/π*-orbitals and one pair of π/π*-orbitals from the ethylene group in RAS1/RAS3, while the HOMO and LUMO are assigned to RAS2. This leads in consequence to a RAS (18,2,2;8,2,6) spanning over 6700 CSFs. The corresponding RASs for D2 and D3 are shown in Figure S1 and S2 in the supporting information. In addition to the isolated dyes, dyads of D1, D2 and D3, denoted D1D2, D2D3 and D1D3, incorporated in a methacrylate-based polymer backbone were studied, see Figure S3a)-i). An analogous computational protocol was applied as introduced before for the isolated dyes, while all (TD)DFT simulations were exclusively performed with the B3LYP35 functional (in AcN). However, due to the interaction of the planar π-systems in the dimer, which may lead to π-stacking, Grimme's D2 dispersion correction was incorporated in the DFT simulations.70 The absorption spectra of dyads D1D2 and D2D3 were simulated by means of the five lowest excited singlet states, while for D1D3 the seven lowest excited singlet states were obtained. Subsequently, the lowest local ππ*-state of the energy donor dye within the respective dyad was optimized using TDDFT that is: S3 (πD1πD1*) for D1D2, S3 (πD2πD2*) for D2D3 and S4 (πD1πD1*) for D1D3. In order to enable the investigation of energy and electron transfer processes upon photoexcition, the computational demand was reduced by replacing the polymer backbone by two methyl groups - a partial geometry optimization followed. The 8 ACS Paragon Plus Environment

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energy and electron transfer kinetics among the donor and acceptor states of the respective dyads (D1D2, D2D3 and D1D3) were described along linear-interpolated internal coordinates (LIICs) connecting the at the DFT and TDDFT levels of theory fully optimized equilibrium structures. The correlating adiabatic and diabatic PESs were constructed along the LIIC, denoted R, by means of TDDFT single point calculations. One-dimensional quantum dynamical (QD) simulations in the diabatic representation were carried out for each dyad along the LIIC. Very recently such QD simulations were performed by our group to unravel the photo-induced electron transfer process in a supra-molecular photocatalyst model71 and compared to semi-empirical Marcus theory.72–75 Here, we extend this computational approach to four states in case of D1D2 and D2D3 as well as to five states for D1D3. A straight forward diabatization of the PESs was achieved by manually following the electronic character or rather the electronic transitions for each states of interest by the along the LIIC, which is feasible due to the minor multiconfigurational character of these states. The time-dependent Schrödinger equation (TDSE) was numerically integrated on a spatial grid for the nuclear wavefunctions, (), in each state i. In the diabatic representation,

the TDSE for a n-state model reads as:

1 ( , ) 11 ( ) ⋮ ℏ

= ⋮ ψn ( , ) n1 ( )

⋯ 1n ( ) 1 ( , ) ⋱ ⋮

⋮ , ⋯ nn ( ) ψn ( , )

Eq. 1

= ( ) − ( ) ∙ !() ,

Eq. 2

with matrix elements of the Hamiltonian, given as:

= −

ℏ

+ ( )

and

where M is the reduced mass and ( ) are the electronic diabatic PESs computed along the

reaction coordinate R, for all states, respectively. ( ) are the potential couplings retrieved

by an unitary transformation of the adiabatic potential matrix for each R:

1

⋮ n1

⋯ 1n %&' # ⋱ ⋮ =" $ ⋮ ⋯ n 0

⋯ 0 ⋱ ⋮ *", ⋯ )&'

Eq. 3

where U is a general rotation matrix. In order to identify the inter-molecular energy transfer coordinate R, we analyzed the most prominent geometry changes occurring for the three dyads. As stated above, these are the LIICs connecting the fully optimized equilibrium structures of the electronic ground state and the energy donor state; D1D2: S3 (πD1πD1*), 9 ACS Paragon Plus Environment


D2D3: S3 (πD2πD2*) and D1D3: S4 (πD1πD1*). Thus, the dimensionality of the PESs is decreased from 3N-6 to merely one. Consequently, the energy acceptor state - D1D2: S2 (πD2πD2*), D2D3: S2 (πD3πD3*) and D1D3: S1 (πD3πD3*) - as well as the inter-molecular charge-separated (CS) state(s) - D1D2: S1 (πD2πD1*), D2D3: S1 (πD2πD3*) and D1D3: S3 (πD3πD1*) and S2 (πD1πD3*) - are not fully relaxed along the LIICs. For D1D2 and D2D3, the most dominant geometry changes are rationalized by means of a translation of one dye with respect to the other, which is given for D1D2 by the intermolecular hydrogen-hydrogen distance, rD1D2, of D1's methoxy group and the amino group of

D2 and for D2D3 - the distance of the donor group of D2 (amino group) and D3 (4-methoxy group), rD2D3. In D1D3 the LIIC is approximated by the dihedral angle δD1D3. All three coordinates rD1D2, rD2D3 and δD1D3 as well as the respective LIICs are illustrated in Figure S4a)-c) in the supporting information. These LIICs define the reduced masses to

be + = 1.2 ∙ 10. a. u. (relative vibration of the dye pair) for D1D2 and D2D3, and to

+ = 2.5 ∙ 102 a. u. (moment of inertia of the D1 dye rotating around C2) for the D1D3 pair.

For the quantum dynamics, PESs of the ground state and the low-lying excited states along the LIICs were plotted in the ranges of [2.21 Å : 3.28 Å] for rD1D2, [8.30 Å : 13.13 Å] for rD2D3 and [38.28° : 47.15°] for δD1D3. The total spatial grids are then cubically splined to consist of 1024 points. The potential couplings, determined by Eq. 3, were assumed to be Gaussian functions centered at the diabatic crossings and were fitted accordingly (for more details see Figure S5). The diabatic TDSE, Eq. 1, is solved with the help of the split-operator method76,77 for the first 2500 fs with a time discretization of ∆t = 0.01 fs. The initial nuclear wavefunction is prepared on the S0 potential curve by imaginary time propagation.78 The resulting nuclear eigenfunction in S0 is then dipole coupled to an excitation laser pulse of Gaussian shape with a xyz-polarization (in order to simulate a statistical orientation of the dyes in the polymer), centered at 30 fs with a full width at half maximum of 20 fs and a maximum amplitude of 1 GV/m (Eq. 2). We couple the laser field to the corresponding Franck-Condon transition dipole moments µ0i in x, y, and z-polarization, respectively, for each S0i transition and approximate them to be constant over the grid. While the laser parameters are chosen rather arbitrarily, these pulses ensure a quick and efficient population transfer to the desired bright diabatic state, and hence interfering to a minimum with the wavepacket propagation on the excited states. 10 ACS Paragon Plus Environment

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3. Results and Discussion 3.1 Isolated Dyes Initially, the electronic transitions underlying the absorption and emission spectra of the isolated dyes D1, D2 and D3 are analyzed by RASPT2 and TDDFT simulations. In acetonitrile the lowest energy absorption features are experimentally observed at 3.34 (371 nm), 3.02 (411 nm) and 2.49 eV (497 nm), while emission energies of 2.76 (448 nm), 2.59 (478 nm), 1.96 eV (632 nm) were measured for D1, D2 and D3, respectively. The corresponding UV-vis and fluorescence spectra are depicted in Figure S6a) in the supplementary information. To allow a thorough assessment of ground and excited state structures as well as of excited state properties, i.e. absorption and emission spectra, preliminary benchmark calculations were carried out. All dyes subject to this investigation are push-pull chromophores, where light-induced charge transfer (CT) proceeds from an electron rich to an electron poor moiety. Since the CT character increases from D1 via D2 to D3, the DFT and TDDFT benchmark was restricted to D1, featuring the lowest CT character, and to D3 with the most pronounced CT character. Previous studies revealed that hybrid functionals with a moderate and high amount of exact exchange as well as long-range corrected functionals allow good estimations of ground state structures, excitation energies, excited state gradients and fluorescence energies.21,24,59,66 Therefore, the functionals PBE0, B3LYP35 and M06-2X were selected. For

D1 (and D2), two rotamers are conceivable, here, the trans configuration is favored by approximately 0.2 eV due to the repulsion of the nitrogen atoms of the thiazole and the phyridyl moietes as obtained by B3LYP35. Thus, all data shown in the following for D1 (and

D2) is exclusively obtained for the more stable trans configuration. As expected, slightly twisted D1, D2 and D3 equilibrium structures are predicted with torsional angles ranging from 15.6° to 17.7° (B3LYP35 and M06-2X), see δ1 in Table S1. Such values are typical for this class of thiazole-based push-pull chromophores.21,24,59,65,66,79–81 However, PBE0 yielded a completely planar structure. In order to elucidate this unexpected result further optimizations at the MP2 (Møller-Plesset perturbation theory of second-order)82,83 level of theory were performed for D1 and D3, yielding - in agreement with the functionals B3LYP35 and M062X - a twisted ground state (D1: δ1 = 31.4°, D3: δ1 = 29.8°). All DFT methods and MP2 predict the thiazole-acceptor moiety to be (almost) planar, see dihedral angle δ2 in Table S1. 11 ACS Paragon Plus Environment


TDDFT associates the lowest bright excited state (S1) for all three dyes to the experimental absorption maxima between 370 and 500 nm, which is characterized by the HOMO-LUMO transition independent of the XC functional, see Figure 2.

Figure 2: Frontier orbitals of D1, D2 and D3 involved in the leading transitions associated to the bright S1 state of the respective dye. For D1 PBE0, B3LYP35 and M06-2X yield excitation energies between 3.22 (PBE0) and 3.63 eV (M06-2X). D3 - with increased CT character - features S1 excitation energies between 2.16 (PBE0) and 2.65 eV (M06-2X). Excited state relaxation within S1 leads to a planarization of all three dyes, for more details see δ1 and δ2 in Table S1. Emission energies obtained within the equilibrated S1 state range between 3.15 (PBE0) and 3.56 eV (M06-2X) for D1 and 1.80 (B3LYP35) and 2.57 eV (M06-2X) for D3. As studied recently by Kupfer et al., the pronounced dependency of S0-S1 excitation energies in thiazole-based push-pull chromophores is only partially associated with the distinct description of the excited state with respect to the XC functional but also correlated to the equilibrium structure - namely the extent of single/double bond localization correlating with the amount of exact exchange.59 A similar trend is evident by comparison of the bond lengths r1 and r2, shown in Table S1, with the excitation energies listed in Table S2. The B3LYP35 functional was found to yield the smallest deviations with respect to the experimental values for absorption and emission of D1 and D3, as shown in Table S2. B3LYP35 predicts the bright S0→S1 excitations within the FC region at 3.43 (362 nm), 3.01 (412 nm) and 2.38 eV (520 nm) for D1, D2 and D3, respectively, experimental values of 3.34 (371 nm), 3.02 (411 nm) and 2.49 eV (497 nm) were measured. Detailed information with respect to the excited state properties are listed in Table 1. 12 ACS Paragon Plus Environment

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Dye D1

Method MS-RASPT2

B3LYP35 D2

MS-RASPT2 B3LYP35

D3

MS-RASPT2 B3LYP35

Dye D1

Method MS-RASPT2

B3LYP35 D2

MS-RASPT2 B3LYP35

D3

MS-RASPT2 B3LYP35

Transition π7 → π8* HF π(70) → π*(71)

π7 → π8* HF π(82) → π*(83) π9 → π10* HF π(98) → π*(99)

Character π7 → π8* DE π(70) → π*(71)

π7 → π8* DE π(82) → π*(83) π9 → π10* DE π(98) → π*(99)

Absorption Weight ∆E / eV 79 3.59 6 99 3.43 (99) (3.48) 80 3.30 5 98 3.01 (98) (3.13) 76 2.72 7 98 2.38 (99) (2.51) Emission Weight ∆E / eV 69 3.10 14 100 2.79 (100) (2.89) 73 2.82 11 99 2.30 (100) (2.67) 69 2.40 12 99 1.80 (98) (2.22)

λ / nm 345

f 0.6436

362 (356) 375

0.8013 (0.6923) 0.7061

412 (396) 457

0.9222 (0.8179) 0.9830

520 (495)

1.1904 (1.0556)

λ / nm 401

f 0.6421

445 (429) 439

0.8610 (0.7599) 0.7121

540 (465) 517

1.2631 (0.9565) 0.9758

690 (558)

1.6377 (1.2904)

∆Eexp / eV

∆∆E / eV

3.34

+0.20#

+0.09 3.02

+0.17# -0.01

2.49

+0.10# -0.11

∆Eexp / eV

∆∆E / eV

2.76

+0.23#

+0.03 2.59

-0.14# -0.29

1.96

+0.01# -0.16

Table 1: Absorption and emission properties for the bright S1 state of D1, D2 and D3: leading electronic transitions (with weights; double excitations denoted DE), excitation energies (∆E), wavelengths (λ), oscillator strengths (f) obtained at the MS-RASPT2 (gas phase) and B3LYP35 (in AcN, gas phase values in parentheses) level of theory. Deviations with respect to experimental data (∆Eexp) are given by ∆∆E. #Values obtained by incorporating TDDFT solvent stabilization. The impact of solvent stabilization on the excited state energy (S1) was assessed by gas phase TDDFT single point calculations within the solvated ground state equilibria. For S1 of D1, moderate solvent stabilization of 0.05 eV is observed, while more pronounced stabilizations of 0.12 and 0.13 eV are obtained for D2 and D3. This finding is in line with the predicted increase of the CT character from D1 to D3. Thus, a polar solvent like AcN stabilizes the S1 CT state accordingly. Upon relaxation in S1, planarization is induced based on the partially populated LUMO, which does not feature a nodal plane between the thiazole moiety and the respective donor (D1: phenyl; D2: dimethylanilyl, D3: 4-methoxy phenyl) and acceptor fragments (D1 and D2:



phyridyl; D3: thiophenyl-methylmalonnitril). This planarization is accompanied by a decrease of the bond lengths r1 and r2 with respect to the ground state (see Table S1). The simulated fluorescence energies of D1-D3 - obtained by means of B3LYP35 and within the adiabatic approximation - of 2.59 (478 nm), 2.30 (540 nm) and 1.80 eV (690 nm) follow the experimental trends (2.76 (448 nm), 2.59 (478 nm) and 1.96 eV (632 nm)), see Table 1. Pronounced solvent stabilizations (including S1 solvent relaxation) of 0.30, 0.37 and 0.42 eV are predicted. The computational results determined at the PBE0 and M06-2X levels of theory are summarized in Table S2. In order to verify the accuracy of the TDDFT simulations and to evaluate the multiconfigurational character of the dyes, MS-RASPT2 calculations were performed using the ASs introduced in the previous section (see Figure 1 as well as Figures S1 and S2). The multiconfiguational calculations were carried out within the S0 and S1 equilibrium structures of D1, D2 and D3 optimized at the B3LYP35 level of theory, Table 1 summarizes the respective MS-RASPT2 results for the three dyes. For D1 MS-RASPT2 predicts the bright S1 state at 3.59 eV (345 nm), which is approximately 0.25 eV above the experimental value. Considering a slight solvent stabilization of 0.05 eV, as predicted by TDDFT, an approximate S0→S1 excitation energy in AcN of 3.54 eV is yielded. For D2 an excitation energy of 3.30 eV (376 nm) is calculated for the S1 state, AcN environment decreases the excitation energy to 3.18 eV; this energy is 0.16 eV above the experimental value of 3.02 eV. A similar, slight overestimation of 0.10 eV (∆Eexp = 2.49 eV) is predicted for the S0→S1 excitation of

D3; here MS-RASPT2 computes an excitation energy of 2.72 eV (456 nm), which is further lowered by 0.13 eV in AcN. For all three dyes, no pronounced multiconfigurational character is observed (for S0 and S1), while the HOMO-LUMO transition contributes with 79%, 80% and 76% (D1, D2 and D3) to the electronic wavefunction of S1. Likewise, MS-RASPT2 calculations were performed within the S1 equilibrium structures optimized at the TDDFT level of theory (B3LYP35). Here, the vertical S0-S1 gap - associated to the fluorescence maximum - is lowered to 3.10, 2.82 and 2.40 eV. Taking into account the considerable solvent effect of 0.3-0.4 eV, approximate fluorescence energies of 2.80 (443 nm), 2.45 (506 nm) and 1.98 eV (626 nm) are obtained. Here a more pronounced multiconfigurational character is observed for S1, which is apparent from double excitations (DE) contributing to the wavefunction of S1, see Table 1. Comparison of the multiconfigurational and the TDDFT results clearly reveals the ability of both methods to describe the ground and excited state properties of all three dyes accurately. 14 ACS Paragon Plus Environment

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For the absorption, B3LYP35 overestimates the S1 excitation energy by 0.20 eV for D1 - the dye with the lowest charge transfer character - while it slightly underestimates the excitation energies for the dyes featuring higher charge transfer character (D2: -0.01eV and D3: 0.11 eV). A more consistent picture is given by MS-RASPT2, here all S0→S1 excitation energies are slightly overestimated by 0.1-0.2 eV. Similarly, the B3LYP35 results for the emission are in good agreement with the experiment, however, variations between +0.03 and -0.29 eV are obtained; comparable deviations between -0.14 and +0.23 eV are calculated with MS-RASPT2. The simulated absorption and emission spectra for D1-D3 obtained at the MSRASPT2 and B3LYP35 levels of theory are collected in Figure S6b)-d).

3.2 Dyads In order to allow efficient inter-molecular energy transfer between the different dyes, the individual dyes need to be arranged in spatial proximity to each other, which was rationalized by Breul et al. by embedding the dyes in a polymer strand.31 In the following we investigate possible alterations of the photophysical properties of the dyes upon incorporation in polymer environment and assess the excited state relaxation pathways associated to inter-molecular energy and electron transfer processes. This is realized by virtue of three dyads (D1D2, D2D3 and D1D3) illustrated in Figure S3a)-c). To facilitate efficient locally confined radiationless energy transfer from dye-to-dye, the emission spectrum of the photo-excited dye and the absorption spectrum of the acceptor dyes need to overlap. A key feature to allow for efficient inter-molecular energy and/or electron transfer is the relative orientation as well as the distance of the softmatter-embedded dyes. The tendency of thiazole donor-acceptor dyes among other aromatic systems to form aggregates based on inter-molecular π-π-interactions is well known in the literature.24,26 As expected, π-stacking is observed for all dyads in polymer environment [see Figure S3d)-f)]. Inter-molecular distances between the aromatic planes of the individual dyes of approximately 3.2-3.5 Å are calculated upon including dispersion correction with the optimized S0 structures, while binding energy of 2.5-3.0 eV were obtained for the three dyads. The equilibrium structures of the dyads are comparable to the structures of the isolated dyes. For all dyes within the dyads, slightly twisted structures are predicted (8.3 ≤ 6% ≤ 19.3°), these values are

in accordance with the corresponding dihedral angles of 15.6-16.2° for the isolated dyes, see Table S1 and S3 for more details.



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Analogous to the investigation of the excited state properties in the previous section, TDDFT simulation by means of the B3LYP35 XC functional were applied for D1D2, D2D3 and

D1D3. However, no multiconfigurational calculations were performed due to the enormous computational costs describing the dyads. The UV-vis absorption spectra were calculated with the polymer backbone as well as without the backbone - in the latter case the polymer was replaced by methyl groups. The excited state properties are congruent for both models, thus only the results obtained for the methyl-substituted systems are shown here. In general, the low-lying bright excited states in the dyads are identical to the results obtained for the individual dyes, see Table 2, which is in agreement with experimental studies on the absorption and emission of the dye monomers and dye containing polymers.31 Absorption Dye

D1D2

D2D3

D1D3

State

Transition

Weight / %

∆E / eV

λ / nm

f

S1

πD2(152) → πD1*(153)

98

2.71

457

0.0463

S2

πD2(152) → πD2*(154)

96

2.96

418

0.6096

S3

πD1(151) → πD1*(153)

96

3.33

373

0.7755

S1

πD2(180) → πD3*(181)

98

1.69

734

0.0167

S2

πD3(179) → πD3*(181)

97

2.32

535

0.8515

S3

πD2(180) → πD2*(182)

91

2.99

414

0.8106

S1

πD3(168) → πD3*(169)

98

2.30

538

1.0231

S2

πD1(167) → πD3*(169)

98

2.47

501

0.0180

S3

πD3(168) → πD1*(170)

97

3.27

380

0.0580

S4

πD1(167) → πD1*(170)

94

3.35

370

0.6836

Table 2: Electronic properties, i.e., excitation energies (∆E), excitation wavelengths (λ), oscillator strengths (f) and electronic characters (with weights), of low-lying excited singlet states of D1D2, D2D3 and D1D3 associated to intra- and inter-molecular CS within FC region. However, besides the intra-molecular CT states localized on one dye also inter-molecular CS states are present that may lead to an undesired redox chemistry upon population. For D1D2, the bright intra-molecular CT states, centered on D2 (S2) and D1 (S3), are calculated at 2.96 (418 nm) and 3.33 eV (373 nm). Thus, the local bright CT states are slightly stabilized upon spatial proximity of the dyes. In addition, the weakly absorbing inter-molecular CS state, S1, leading to the photo-induced oxidation of D2 and the reduction of D1 is predicted at 2.71 eV (457 nm). The respective charge density differences (CDDs) of the excited states S1-S3 in

D1D2 are depicted in Figure 3. 16 ACS Paragon Plus Environment

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Figure 3: Charge density differences for the inter-molecular charge-separated (CS) state S1 and the intra-molecular charge transfer (CT) states S2 and S3 of dyad D1D2 within the FranckCondon region; CT occurs from red to blue. A similar behavior is observed by the quantum chemical simulations for the dyad D2D3: The bright intra-molecular CT states of D3 and D2, S2 and S3, are calculated with excitation energies of 2.32 (535 nm) and 2.99 eV (414 nm), while a low-lying weakly absorbing intermolecular CS state (from D2 to D3) is found at 1.69 eV (734 nm), see Table 2 for more details. Again, the excitation energies within the dyad are slightly lower than the respective states of the isolated dyes. The CDDs for the low-lying excited states in D2D3 are depicted in Table S4. Although no distinct overlap of D1's emission and D3's absorption bands is present, D1D3 was investigated to assess the possibility of energy and electron transfer among these dyes, i.e., to verify a potential disruption of the antenna at the D1D3 interface in statistical polymers 17 ACS Paragon Plus Environment


as well as the photostability. The bright intra-molecular CT states of D3 (S1) and D1 (S4) are calculated at 2.30 (538 nm) and 3.35 eV (370 nm), while opposed to the dyads D1D2 and

D2D3 two weakly absorbing inter-molecular CS states, S2 and S3, are found in D1D3 between these bright intra-molecular states (S1 and S4). S2 is associated to an electron transfer from D1 to D3, while population of S3 leads to charge migration from D3 to D1. The respective CDDs are illustrated in Table S4 of the supporting information.

3.3 Photophysics and excited-state processes in the dyads In order to obtain further insight into the photophysics and the photochemistry of the three dyads, the potential energy landscape of the low-lying intra-molecular CT states - associated to energy transfer between the dyes and inter-molecular CS states between the dyes, leading to photoreduction and -oxidation of the dyes, are investigated. Antennae are designed to convey energy in a dye-to-dye like fashion and eventually to a photosensitzer, while the present system aims to transfer energy from D1 to D2 and finally to D3. Thus, the bright intra-molecular CT state of the energy donating dye in each of the three dyads was optimized, that is: S3 in case of D1D2 (πD1π*D1) and D2D3 (πD2π*D2) and S4 in case of D1D3 (πD1π*D1). In agreement with the equilibrium structures of the respective excited states of the isolated dyes (Table S1), photo-induced planarization is observed. However, planarization of the energy donor dye (in the dyad) is less pronounced due to steric hindrance introduced upon πstacking (see Table S3 for more details). Initially, energy dissipation within the excited state of the energy donor dye occurs towards its equilibrium geometry, thus the landscape of the low-lying excited states is studied along a LIIC connecting the FC region with the excited state equilibrium structure of the bright energy donor state. The respective PESs for the dyads

D1D2, D2D3 and D1D3 are shown in Figure 4a)-c).


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Figure 4: PESs for ground and low-lying excited states contributing to photo-induced energy and electron transfer processes along the LIICs in a) D1D2, b) D2D3 and c) D1D3. Adiabatic PESs are given by dashed (black) cubic splines connecting the (TD)DFT single points (circles), diabatic PESs are given by solid curves (black for intra-molecular CT and gray for inter-molecular CS). The initial excitation in the FC region is indicated by a red arrow. Ground state structures and LIICs are illustrated. As visualized in Figure 4a), population of the bright S3 state of πD1πD1* nature and relaxation along its diabatic PES leads to a crossing with S2 (πD2πD2*) localized on D2 before the equilibrium of πD1πD1* is reached. In the vicinity of this crossing population transfer - and thus energy transfer - from the photo-excited D1 toward D2 may proceed. This energy transfer from D1 to D2 is also evident by means of the equilibrium of S2 (πD2πD2*) along the LIIC, which is found in the vicinity of the FC region: The twisted D1 structure is recovered from its planar excited state equilibrium, which is only possible upon depopulating D1's LUMO, i.e., upon inter-molecular energy transfer to D2. Accordingly, planarization and relaxation in the excited state of D2 is achieved. Please note that two-dimensional PESs, e.g., by means of a second LIIC connecting the equilibrium structures of the energy donor state 19 ACS Paragon Plus Environment


(πD1πD1*) and the energy acceptor state (πD2πD2*) are not part of the present study. Along the LIIC illustrated in Figure 4a), a further crossing of the excited D2 (πD2πD2*) with the intermolecular electron transfer state S1 (πD2πD1*) (gray line) is observed. Population of this (intermolecular) electron transfer state leads, in consequence, to the undesired photooxidation of

D2 and reduction of D1. A very similar excited state landscape is obtained along the LIIC connecting the equilibrium structures of S0 and S3 (πD2πD2*) in D2D3. In this dyad, the crossing between the energy donor state S3 (πD2πD2*) and the acceptor state S2 (πD3πD3*) is found close to the equilibrium configuration of the energy donor state. Analogous to D1D2, energy transfer in D2D3 proceeds from D2 to D3 accompanied by recovering the twisted D2 structure from the planar donor equilibrium (πD2πD2*), while subsequent relaxation of D3 upon energy transfer is not investigated. In contrast to D1D2, no crossing between the energy acceptor and the intermolecular electron transfer state S1 (πD2πD3*) is found. For D1D3 TDDFT predicts a different picture with respect to the excited state relaxation channels, see Figure 4c). Here, the bright intra-molecular CT state of D1, S4 (πD1πD1*), at 3.35 eV is almost degenerate with the inter-molecular CS state S3, associated to electron transfer from D3 to D1. Both states cross along the LIIC toward the equilibrium of πD1πD1*, while no crossing with the locally excited state of D3, S1 (πD3πD3*), is found between the FC region and the optimized S4 (πD1πD1*) structure. Substantial displacement along the LIIC is necessary to reach the crossing between the respective diabatic curves. The required energy of 0.30 eV to reach this crossing is gained upon relaxation within S4 from the FC region (0.35 eV). Thus, energy transfer from D1 to D3 and relaxation of D1 towards the FC region proceeds, while πD3πD3* crosses with a second inter-molecular electron transfer state, S2 (πD1πD3*).

3.4 Energy vs. Electron Transfer Dynamics In order to yield a qualitative picture of the photo-induced excited state relaxation processes associated to inter-molecular energy and electron transfer phenomena, QD simulations were performed for the three dyads. Therefore, the potential coupling matrix elements between the respective ground and excited states were evaluated by Eq. 3 (see section 2). For D1D2 a maximum potential coupling of 0.05 eV is obtained in the vicinity of the crossing of the diabatic states S3 (πD1πD1*) and S2 (πD2πD2*), which is associated with energy transfer from 20 ACS Paragon Plus Environment

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D1 to D2. The potential coupling correlated to inter-molecular electron transfer from D2 toward D1 is found to be merely 0.03 eV. In case of the dyad D2D3, a similar value of

9 9: = 0.06 eV is predicted for the energy transfer process from D2 to D3, while the electron

transfer state S1 (πD2πD3*) does not couple to any state along the LIIC. A more complex situation is obtained for D1D3. In the vicinity of the FC region, the energy donor state, S4

(πD1πD1*), is coupled with up to 0.02 eV to the almost degenerate inter-molecular CS state, S3 (πD3πD1*). Upon pronounced displacement along the LIIC, energy transfer may occur at the crossing region of the diabatic intra-molecular CT states of D1 and D3, where a maximum coupling of 0.04 eV is calculated. A further crossing between S1 (πD3πD3*) and S2 (πD1πD3*) is present close to the FC region, for this inter-molecular electron transfer process a coupling of 0.03 eV is predicted. Thus, for the energy transfer processes in D1D2 and D2D3 - the dyads with matched optical properties - couplings of 0.05 and 0.06 eV are simulated, while a slightly smaller coupling of 0.04 eV is obtained for D1D3. The couplings associated to electron transfer processes where found to be merely 0.02-0.03 eV for all three dyads. The calculated potential couplings along the LIICs (or rather the projected coordinates rD1D2, rD2D3 and δD1D2) are depicted in Figure S5 for D1D2, D2D3 and D1D3, along with the respective adiabatic and diabatic PESs.

Figure 5: Population of low-lying excited states in a) D1D2, b) D2D3 and c) D1D3 within the first 500 fs normalized with respect to the population of the energy donor state at t = 50 fs,

D1D2: S3 (πD1πD1*), D2D3: S3 (πD2πD2*) and D1D3: S4 (πD1πD1*). Black curves depict intramolecular CT states and gray curves inter-molecular CS states, laser pulses are indicated in red.



Upon laser excitation at λexc = 373 nm into the bright energy donor state of D1D2, population transfer from S0 to S3 takes place. The populations of the excited states S1-S3 within the first 500 fs are illustrated in Figure 5a), with all populations being renormalized with respect to the population of S3 (πD1πD1*) at the end of the laser pulse (t = 50 fs). Figure S7 depicts the QD results for the wavepacket propagation within the first 2.5 ps. Upon excitation the wavepacket starts to spreads along the PES of S3 (πD1πD1*), population transfer into S2 (πD2πD2*) associated to inter-molecular energy transfer from D1 to D2 - occurs already at approximately 25 fs and onward. At t = 190 fs approximately 22% of the initially excited population is transferred from D1 to D2. Subsequently, the populations of S2 (πD2πD2*) and S3 (πD1πD1*) oscillate, while almost no population transfer to S1 (πD2πD1*) - associated to inter-molecular electron transfer - proceeds. However, the inter-molecular CS state (S1 (πD2πD1s*)) is initially populated of up to 4% by the laser within the first 50 fs, this is reasoned by the pronounced xcomponent of its transition dipole moment. Subsequently, depletion of this population takes place, while later on merely relative populations of up to 1% are calculated. For the dyad D2D3, similar excited state relaxation dynamics are predicted by the QD simulations as illustrated in Figure 5b) and Figure S7b). The energy donor state, S3 (πD2πD2*), is populated upon laser excitation at 414 nm, while the excited wavepacket spreads along the PES of the donor state until the crossing with S2 (πD3πD3*) is reached. The strong potential coupling of 0.06 eV between the adiabatic states enables an efficient energy transfer from D2 to D3. At t = 285 fs 80% of the excited population is dissipated into the energy transfer channel. Subsequently, the wavepacket oscillates between S2 (πD3πD3*) and S3 (πD2πD2*), and no inter-molecular electron transfer into S1 (πD2πD3*) proceeds. Such long-lasting oscillations of the population can be attributed to the absence of environmental deactivation channels in the QD simulations.84 In case of the third dyad, D1D3, excitation at λexc = 370 nm leads to the population of the energy donor state; however due to the crossing of S4 (πD1πD1*) and S3 (πD3πD1*) in the vicinity of the FC region, ultrafast population transfer into the electron transfer channel, occurs, leading in consequence to the photo-induced oxidation of D3 and the reduction of D1, see Figure 5c). At approximately 355 fs, more than 22% of the population is transferred into the undesired inter-molecular CS state. Inter-molecular energy transfer is initiated once the wavepacket in S4 (πD1πD1*) reaches the crossing region with S1 (πD3πD3*) at ~70 fs. The population of S1 (πD3πD3*) builds up to roughly 80% at 130 fs of propagation, while a further 22 ACS Paragon Plus Environment

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increase of up to ~85% is observed once the wavepacket in S3 (πD3πD1*) again reaches the FC region and back-populations transfer into S4 (πD1πD1*) follows, which decays into the energy transfer channel. Evolution of the wavepacket in S1 (πD3πD3*) also leads to relaxation into a further inter-molecular electron transfer channel by means of S2 (πD1πD3*) starting at 115 fs. On the longer time scales, see Figure S7c) for QD results up to 2.5 ps, efficient energy transfer from D1 to D3 takes place accompanied by the population of both inter-molecular electron transfer channels. The QD results reveal efficient inter-molecular energy transfer for the three dyads within roughly 200 fs, while almost no inter-molecular electron transfer processes are evident for

D1D2 and D2D3. However, significant population transfer into two inter-molecular CS states is predicted for D1D3, which proceeds immediately upon laser excitation. The approximation of the 3N-6-dimensional problem by merely one dimension (LIIC) is sufficient to obtain a qualitative picture of the excited state relaxation processes in the dyads, while further coordinates need to be incorporated to yield detailed quantitative insight, i.e., by additional LIICs connecting the energy donor equilibrium and the equilibria of the energy acceptor and the inter-molecular CS states.

4. Conclusions The present contribution aims at elucidating the competitive excited state relaxation channels in a light-harvesting antenna in the scope of solar energy conversion by quantum chemical and quantum dynamical methods. A set of three recently investigated thiazole-based push-pull chromophores (D1, D2 and D3) with successively overlapping emission and absorption spectra covering the entire visible region is embedded in a polymer backbone to allow efficient energy transfer from D1 to D2 and D3 (and finally to a photosensitizer). However, excited state inter-molecular energy transfer competes with inter-molecular electron transfer processes leading to the deactivation of the antenna. A comprehensive understanding of the photo-induced energy and electron transfer processes in such antennae is of uttermost importance for future theory-spectroscopy guided design strategies in light-harvesting applications, i.e., in the fields of DSSCs and photocatalysis. The photophysics of the three isolated dyes are studied by DFT and TDDFT as well as by MS-RASPT2. For the present series of dyes with successively increasing CT character from D1 to D3, the B3LYP35 XC functional and MS-RASPT2 provide an accurate description of the absorption and 23 ACS Paragon Plus Environment


fluorescence spectra when compared to experimental data. The nature of the low energy absorption and emission band is assigned for all dyes to the bright S0→S1 intra-molecular CT excitation. Photo-induced planarization is observed from the twisted FC region. A subsequent evaluation of the ground and excited state properties of the dyads (D1D2, D2D3 and D1D3) revealed their tendency to aggregate based on π-stacking, with dye-dye distances of 3.2-3.5 Å being obtained. Excited state optimization of the respective energy donor state within the dyads lead to an analogous planarization, while the structural reorganization is partially hindered by the second dye (energy acceptor). LIICs connecting the FC region and the excited state equilibrium of the respective energy donor state were constructed for D1D2, D2D3 and

D1D3. Potential energy curves along these coordinates were calculated at the DFT and TDDFT levels of theory comprising the low-lying energy and electron transfer states of the dyads, whereas the photoinactive methacrylate-based polymer backbone was omitted, as it did not influence the excited-state properties substantially. Quantum dynamical wavepacket propagation for the dyads D1D2 and D2D3 along the LIICs yielded the desired intermolecular energy transfer from D1 (πD1πD1*) to D2 (πD2πD2*) and from D2 (πD2πD2*) to D3 (πD3πD3*). After 100-200 fs (upon laser excitation) approximately 22%, 80% and 82% of the population is transferred onto the energy acceptor in D1D2, D2D3 and D1D3, respectively, while almost no population is transferred into the undesired inter-molecular electron transfer channel for the dyads with adjusted optical properties (D1D2 and D2D3). In case of D1D3, inter-molecular electron transfer from D3 to D1 is directly stirred upon excitation of the wavepacket from the electronic ground state, a second electron transfer associated to the photooxidation of D1 and the reduction of D3 occurs from 115 fs and onward. Thus, the photostabiliy of antennae obtained by statistical copolymerization of the individual dyes (monomers) is considerably lower than in synthetically more challenging hierarchical polymers. Future studies on similar antennae will be rationalized by means of multidimensional QD simulations incorporating several excited state relaxation coordinates, e.g. based on additional LIICs from the energy donor equilibrium to the inter-molecular energy and electron transfer equilibria, the multi configuration time-dependent Hartree (MCTDH) algorithm or MD simulations. Furthermore, elucidating the successive energy transfer from

D1 via D2 to D3 based on the triad D1D2D3 is envisioned.


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Acknowledgement Support by the COST Action CM1202 Perspect-H2O is gratefully acknowledged. J.P. thanks the Förderverein Chemie-Olympiade e.V for supporting a two-week internship. S.G. and S.K. thank the Abbe Center of Photonics for financial support within the ACP Explore project. All calculations were performed at the Universitätsrechenzentrum of the Friedrich-Schiller University of Jena.

Supporting Information RASs for D2 and D3, ground and excited state equilibrium structures of polymer-embedded dyads, LIICs and projected coordinates rD1D2, rD2D3 and δD1D3, selected structural and electronic properties as well as experimental and simulated absorption and emission spectra of

D1-D3, CDDs for low-lying excited states in the dyads as well as QD results for 2.5 ps. This information is available free of charge via the Internet at http://pubs.acs.org.



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Fate of Photoexcited Molecular Antennae - Intermolecular Energy

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