Dynamics of Intramolecular Energy Hopping in Multi-Bodipy Self

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The Dynamics of Intramolecular Energy Hopping in MultiBodipy Self – Assembled Metallocyclic Species: A Tool For Probing Subtle Structural Distortions in Solution Elisabeth Martinou, Kostas Seintis, Nikolaos Karakostas, Anna A. Bletsou, Nikolaos S. Thomaidis, Mihalis Fakis, and George Pistolis J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12550 • Publication Date (Web): 15 Feb 2017 Downloaded from http://pubs.acs.org on February 16, 2017

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The Journal of Physical Chemistry

The Dynamics of Intramolecular Energy Hopping in Multi-Bodipy Self – Assembled Metallocyclic Species: A Tool For Probing Subtle Structural Distortions in Solution. Elisabeth Martinou,1 Kostas Seintis,2 Nikolaos Karakostas,1 Anna Bletsou,3 Nikolaos S. Thomaidis,3 Mihalis Fakis2 and George Pistolis*,1 1 2 3

NCSR "Demokritos" Institute of Nanosciences and Nanotechnology (INN) 153 10 Athens, Greece

Department of Physics, University of Patras, 26500 Patras, Greece

Department of Chemistry, National and Kapodistrian University of Athens, 15771 Athens, Greece

ABSTRACT: The intramolecular excitation energy transfer (EET) processes in a series of fluorescent – unquenched, self – assembled metallocycles consisting of spatially fixed – separated and parallel - aligned Bodipy chromophores, are investigated here by steady – state and fs – fluorescence upconversion measurements in solution phase. These multi-Bodipy macrocycles shown in Scheme 1, namely, the rhomboid (A1), the tetragon (A2) and the hexagon (A3), are formed via temperature – regulated Pt(II) – pyridyl coordination and consist, respectively, of 2, 4 and 6 Bodipy subunits, which are locked at the corners and aligned with their long molecular axis perpendicular to the rigid polygonal frame formed by the alternating B···Pt(II) connectivities. Extensive simulations and fits to the experimental fluorescence anisotropy decays r(t), show that EET within the cyclic scaffolds is quite uniform and much faster than the intrinsic decay rate of the Bodipys. The equalization of the excitation survival probabilities over time of all chromophores is found to be dependent upon the size of the macrocycle. From the observed dynamics supported by geometry optimization calculations, it is concluded that, in contrast to the model compound A1, in the large macrocycles the perfect parallel orientation of the Bodipy dipoles is lifted through limited out–of–plane distortions of the metallocyclic framework from a planar conformation. Additionally, we show that, as opposed to analogous covalent macrocycles, the survival probability of excitons as well as the degree of symmetry distortion and homogeneity in dipoles spacing remain nearly intact as the size of the macrocycle increases from tetragon to hexagon.

INTRODUCTION

To investigate the role of interchromophore distance and orientation on EET, numerous cyclic multichromophore arrays, either closed ring or dendritic architectures, have been explored. Various chromophores have been employed in these studies, including chlorophylls,17-20 porphyrins,1-6,21-37 phthalocyanines,38-40 rylene dyes,41,42 oligothiophenes,43-46 tetrathiafulvlenes,47,48 carbazoles,49 salphens,50 naphthoyloxy,51,52 Bodipys53,54 and perylene bisimides55-65. The bulk of these efforts have used covalent bonding to build the aforementioned architectures, though supramolecular bonding motifs, such as hydrogen bonding,66-68 π−π stacking69-71 and metal ion coordination25,26,53,54,72-78 have also been utilized.

Excitation energy transfer (EET) in artificial multichromophore arrays with cyclic architectures,1-6 has become a topic of increasing importance as can pave the way for understanding how nature captures and temporarily stores sunlight energy for initiating archetype photoreactions.7-10 The design and construction of cyclic light – harvesting macrocycles is quite demanding, since it requires precise control over the connectivity, spacing, orientation and energy levels of the participating chromophores.11-15 The minimization of structural deformations as the size of the macrocycle increases, is a particularly important task because it can preserve homogeneity in spacing, orientation and electronic coupling between chromophore dipoles.16 With these requirements being fulfilled, excitation energy migration (hopping) through the cyclic scaffold becomes sufficiently uniform and rapid, enabling the whole ensemble to function as light harvesting antenna in much the similar way as natural photosynthetic systems do.

While over the past decade much progress has occurred mainly by covalent cyclic architectures, well – defined structure - function correlations still lack sufficient awareness. Studies have shown that the structural heterogeneity becomes severe as the size of the macrocycles increases.16 Conformational disordering decreases the symmetry effect substantially and can seriously affect the

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uniformity in dipole–dipole coupling. Moreover, structural defects and strains often decrease the survival probability of the excitons by virtue of fast radiationless decay mechanisms that are competing against the EET pathway.16,55 Regarding the above, it motivated us to challenge the design and supramolecular synthesis of well – defined and fluorescent - unquenched antenna units that comprise as many as possible chromophoric units locked to a rigid, cyclic molecular framework. Provided that EET is much faster than the intrinsic decay rate of the chromophores, and no intervening traps halt the process, the captured excitation energy can uniformly spread and thereby be fleetingly stored (ns) across the circular multichromophoric scaffold. Coordination – driven self –assembly79-84 appears to be a powerful method for the above purpose, as the high directionality of the metal – to – ligand connectivity, can enable robustness and stiffness of a circular framework in which the rigid chromophoric building blocks are tightly fastened in a predefined arrangement, spacing and orientation. A limited variety of light – harvesting and fluorescent unquenched metallocyclic multichromophore assemblies,85 including our own efforts,53,54,86,87 have been prepared and studied by photophysical techniques. Their fine excited – state properties stem from the well – suited combination of properly designed emissive building blocks, consisting of well – known dyes such as anthracene88, carbazole89, pyrene90,91, chlorophylls20, porphyrins78, perylene – bisimides92,93 and Bodipys53,54,86,87. However, well - defined structure - EET correlation in these systems is rare, and little has been reported regarding this crucial issue. For example, Wasielewski et al. synthesized discrete self – assembled cyclic chlorophyll20 and porphyrin78 tetramers to study the effect of the strength, spacing and orientation of the transition dipole moments on the intramolecular EET. Rapid Förster type energy transfer through singlet - singlet annihilation, with rates of (0.83 ps)-1 and (0.26 ps)-1 between adjacent chlorophylls20 and porphyrin78 units, respectively, was observed. Würthner et al. synthesized a highly red - fluorescent planar square structure,92 after coordination of the ditopic ligand N,N'Di(4-pyridyl)- perylene bisimide (PBI) with the 90° platinum triflate Pt(dppp)(OTf)2 connector. Rapid energy hopping (~ 1 ps-1) between PBI subunits was observed.54 When subsequently suitable donor molecules were tethered covalently to the core of each PBI unit, tuning of the excitation energy funneling and emission sensitization of the antenna was achieved.93 In relation to the above planar square structure, recently our group synthesized and studied in detail, an advanced square structure wherein the photo-inert 900 platinum triflates at the corners of the aforementioned PBIs square, were replaced by a highly emissive Bodipy – based organoplatimun (II) tecton that promoted the establishment of an excited - state energy gradient.54 Efficient Förster - type energy transfer ~ (50 ps)-1 from the corners (Bodipys) to the PBIs lying at the sides, accompanied with energy hopping ~ (200 ps)-1 across the cyclically arranged PBIs units, was manifested. A multiple excited - state energy gradient was also estab-

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lished by us, with the aid of a tailored host – guest rhomboid assembly consisting of pairs of unlike peripheral Bodipy dyes and a tetrasulfonated derivative of pyrene at the center of the cavity.53 Fast and unidirectional intrahost (7.1 ps)-1 and guest-to-host (0.2 ps)-1 energy transfer was demonstrated. The 4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes (Bodipy) dyes94,95 offer a number of attractive characteristics including high extinction coefficients in the “green gap” spectral range, sharp absorption and fluorescence bands with small Stokes shift, nearly collinear absorption and emission dipole moment vectors lying across the long molecular axis, high fluorescence quantum yields, photostability and excellent versatility towards chemical functionalization. These features have allowed us to build brightly fluorescent ~ 109.50 ditopic connectors (M1, M2; see Scheme 1) having the capacity to self-assemble into highly – ordered and fluorescent unquenched nanostructures via Pt(II) – pyridyl coordination.53,86,87,96,97 To conduct a detailed study for a deep understanding of the effects of the macrocycle’s size, spacing and mutual orientation of dyes on the electronic energy hopping dynamics, we have chosen to investigate three multichromophoric macrocycles formed via Pt(II) – pyridyl coordination (Scheme 1). Namely, the rhomboid (A1), the tetragon (A2) and the hexagon (A3) consisting respectively of 2, 4 and 6 Bodipy dyes. EET within self – assembled Bodipy arrays generally occurs via Forster resonance energy transfer45,46 (through-space dipole – dipole interactions), with the efficiency being determined by the separation distance and orientation between the donor and acceptor transition dipole moments and the overlap of donor absorption and acceptor emission spectra. The inherent rigidity and symmetry of the present materials allow the Bodipy dipoles to be well fixed in space; this enables uniform spacing between the dipoles, i.e., ~14 Å in A1 and ~20 Å in A2 and A3, that falls into the applicability domain of Förster model. Considering yet that the effective interaction radius, R0, the so – called critical (Förster) radius,85,86 for Bodipys is large (~ 50 Å), EET within the macrocycles is expected to be much faster than the intrinsic decay rate of the constituents Bodipy dyes. However, from the fact that EET occurs between “iso-energetic” like chromophores (homo – transfer), and hence spectrally indistinguishable, the usual method of analyzing the donor / acceptor time – resolved emission intensity decays is not applicable here. As such, ultrafast time – resolved fluorescence anisotropy, r(t), measurements were carried out. The time – dependent fluorescence anisotropy r(t), is a particularly powerful observable quantity for probing energy transfer phenomena, especially when the electronic excitation energy migrates among identical chromophores (homo-tranfer) in a finite multichromophore system.51,52 In particular, r(t) can provide information on both the eventual angular displacement between donor – acceptor transition dipole moment vectors, and the time that it takes for the donor’s oscillating dipole to transfer its excitation energy to the emitting acceptor. Notably,

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this technique not only yields information on the dynamics of energy transfer but it also provides a unique tool to study even subtle structural distortions of the metallocyclic frameworks in solution, for which little is known.100 The aim of this work is three - fold: firstly, to demonstrate a low temperature (- 800 C) spontaneous self – assembly route that yields a stable hexagonal structure, as opposed to the entropically favored tetragonal structure

formed normally at ambient conditions. Secondly, to elucidate comprehensively the effects of macrocyclic length, spacing and mutual orientation of dyes on the dynamics of electronic energy hopping dynamics and thirdly, to probe by r(t) subtle structural deformations in solution of the metallocyclic frameworks on which the Bodipys backbones are tightly bound.

Scheme 1. Schematic illustration of the supramolecular synthesis of the Rhomboid (A1), the Tetragon (A2) and the Hexagon (A3)



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RESULTS AND DISCUSSION Synthesis and Structural Characterization. The synthesis and characterization of the Bodipy dye – based building blocks M1, M2 and the rhomboid [2M1+2Ptdppp(OTf)2] (A1) itself have been published elsewhere.53,86 The angle between the linear arms in both ditopic connectors, M1 and M2, is c.a. ~ 109.50; this is translated into an intermediate angularity to that required for the formation of molecular tetragons (~ 900) and hexagons (~1200). Moreover, remarkable bending of the linear arms has been manifested by crystallography, leading to either closure86 or opening53 of the directional bonding angle. In this work we show that the above versatility of the directional bonding of the reactants M1 and M2, can lead to the formation and isolation of either of the two macrocycles i.e, [2M1+2M2] or [3M1+3M2], when the reactants are allowed to react at ambient or low temperature (- 80 0C), respectively. This is a key point of this work because, as far as materials for light harvesting are concerned, the construction of large macrocycles remains a core challenge in engineering systems with high molar absorptivities. The supramolecular synthesis of the tetragonal [2M1+2M2] (A2) and hexagonal [3M1+3M2] (A3) struc-

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ture was made by mixing of equimolar amounts of the above building blocks at ambient and low temperature (- 80 0C) respectively. More specifically, the reaction at room temperature of the ~ 109.50 donor M1 with an equimolar amount of the ~ 109.50 diplatinum acceptor M2 in a 3:1 (v:v) C2H2Cl2 – DMF mixture and the subsequent slow diffusion of ethyl – acetate into the mixture, facilitated the isolation of the entropically favored tetragonal structure as a polycrystalline salt. Contrarily, the isolation of the hexagonal structure (A3) in high purity (~95%) was achieved, when the above reactants were allowed to react for 1 hour at – 80 0C in dichloromethane, followed by slow elevation of temperature to ambient conditions, precipitation and washing with 10:1 Et2O/acetone mixture. The quantitative formation of the hexagon at low temperature could be primarily attributed to the lack of the energy needed for bending sufficiently the firm arms so as to fulfil the angularity requirements for the formation of the entropically favored tetragonal structure. Solutions of the above macrocycles were found to be stable for more than three days at ambient conditions, with no observable interconversion between the two forms.

Figure 1. (a) Upper panel: Partial H NMR spectra of the donor M1 and diplatinum acceptor M2 (1.0mM 500MHz, 298 K, CDCl3) 31 1 and upon coordination in the tetragonal (midle pannel) and hexagonal structure (low panel).(b) P{ H} (121.4 MHz, 298 K, 31 1 CDCl3) of A2 (red curve) and A3 (blue curve). Inset: P{ H} spectrum of a mixture containing equimolar quantities of A2 and A3. 2+ 3+ (c) Electrospray ionization mass spectra of the tetragon (A2) [M – 2OTf] , [M-3OTf] (upper panel) and the hexagon (A3) [M – 4+ 5+ 4OTf] , [M-5OTf] (low panel). (d) DOSY spectra of A2 and A3. 1


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Examination of the 1H NMR spectra of A2 and A3, is fairly indicative of the formation of highly symmetrical structures with significant spectroscopic differences compared to their precursors M1 and M2. As shown in Fig. 1a, the α1 – and β1 - hydrogens atoms of the pyridine rings are significantly downfield shifted by 0.5 and 0.6 ppm, respectively, as the rings become poorer in electron density upon coordination of the pyridine – N atom with the Pt(II) metal center. This also leads to an alteration to the magnetic environment of the hydrogens atoms α2, adjacent to the Pt(II) metal center, which are found to be markedly shifted upfield by ~ 0.2 ppm. As it is obvious from Fig. 1, one cannot identify a tetragonal and a hexagonal structure on the basis of the 1H NMR alone, nor determine the number of their repeat units. However, the 31P{1H} NMR spectra of A2 and A3 in CDCl3 shown in Fig. 2b, are separated by each other, albeit weakly, and they are consistent with the formation of single and highly symmetrical species. Sharp singlets at 20.82 and 20.76 ppm were observed for A2 and A3, respectively, that are shifted approximately 0.3 ppm downfield relative to that of M2. This finding, in conjunction with the change in coupling of the flanking 195 Pt satellites (∆J = 40 Hz), suggests coordination of platinum with the pyridine donor. At this point we questioned if the validity of the 31P{1H} NMR methodology was capable of resolving the subtle difference observed between the independently detected spectra of A2 and A3. To this end, we deliberately prepared a mixture composed of equimolar quantities of A2 and A3. The 31P{1H} NMR analysis of the above mixture clearly showed the confidence limits in the discrimination between the observed singlets of A2 and A3 (see Figure 1b inset). In contrast to the small rhomboid A1, the large macrocycles [2M1+2M2] (A2) and [3M1+3M2] (A3), do not appear to be very stable under the ESI – MS conditions. However, we were able to clearly observe distinct charge states for each macrocycle. Two charge states were observed for the cyclic tetramer (A2) with mass-to-charge ratios (m/z) of 2552.9 and 1652.1, corresponding respectively to [M – 2OTf]2+, [M - 3OTf]3+ species. The experimental isotopic partners centered at these m/z values were found to be in very good agreement with their theoretical isotopic distribution, as is seeing in Fig. 1c (upper panel). Similarly, two peaks at m/z = 1877.4 and 1472.2 attributed to the loss of respectively four and five triflate counterions from a [3M1+3M2] hexagonal macrocycle (A3) were observed (Fig. 1c; low panel). These peaks were isotopically resolved and they agreed very well with the theoretical distribution. The 2D-DOSY (diffusion-ordered spectroscopy) 1H NMR spectra further support the presence of discrete structures in CDCl3 solution (Figure 1d). In the above

spectra the macrocycles appear as discrete bands with diffusion coefficients (D) of (5.91 ± 0.12) x 10-10 and (4.30 ± 0.12) x 10-10 m2 s-1 for the cyclic tetramer A2 and hexamer A3 at 25 0C, respectively. No signals attributed to polymers or parent molecules were detected. The “hollow – ring” shape of the assemblies diverts considerably from that of a sphere; therefore a reliable estimation of their “actual” radius cannot be derived. Nevertheless, the ratio of the diffusion coefficients, DA2/DA3 ≈ 1.36, nearly matches the inverse ratio of the hydrodynamic radii, rA3/rA2 ≈ 1.37, calculated from the optimized structures of the aforementioned macrocycles (see Supporting Information).

Steady-State and Fluorescence Lifetime Characterization. The optical properties of dilute solutions of the Bodipy - based building blocks (M1, M2) and their cyclic assemblies A1, A2, and A3, have been studied with UV-vis absorption and emission spectroscopy in 1,2dichloroethane. The stability of the assemblies upon dilution was evaluated by 1H NMR dilution experiments in freshly distilled CDCl3. The results showed the absence of either aggregation or disintegration of the assembled macrocyclic units even upon dilution down to ~ 2.0 x10-6 M. Figure 2 shows the UV-Vis absorption and fluorescence spectra of all compounds in 1,2-dichloroethane at ambient conditions. Fluorescence decay curves (ns) of the supramolecular assemblies are shown in the inset and the most important photophysical properties are summarized in Table 1. The ground-state optical properties of all three macrocycles are found to be quite similar to those of the free subunits, M1 and M2. As Fig.2 shows, neither the shape nor the maximum of the absorption spectra of the metallocyclic assemblies is affected upon coordination with respect to the free components. Yet, the molar extinction coefficient (ε) of each multichromophoric assembly, can be roughly expressed as the sum of the molar extinction coefficients of its constituent chromophoric building blocks (see Table 1). The fluorescence spectra, on the other hand, of all three assemblies remain virtually identical to those of the free tectons, suggesting that the emissive state in the complexed state is not influenced by coordination. As a consequence, both the fluorescence quantum yield Φf, and the fluorescence lifetime τf, of the macrocycles hold respectively - within experimental error - the same values as their constituent subcomponents, M1 and M2 (see Table 1). Hence, the absence of hypo- or hyperchromic effect and noticeable broadening of the steady - state spectra, along with the invariability of Φf and τf before and after coordination, definitely preclude the existence of significant ground state electronic interactions between the Bodipy dipoles in the self - assembled entities.



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Figure 2. Absorption (left) and normalized fluorescence spectra (right) of the free building blocks M1, M2 and their supramolecular macrocyclic assemblies A1, A2 and A3 in 1,2-dichloroethane at room temperature. Insets: (left) normalized absorption spectra; (right) fluorescence decay curves (ns) of A1, A2 and A3 probed at 535 nm (exc. at 500 nm).

Table 1. Photophysical Properties of the building blocks M1, M2 and their assemblies A1, A2 and A3 in 1,2 dichloroethane at room temperature λabs (nm)a c d -1 e f g h -1 -1 Compounds Φf τ (ns) kf (ns ) R0 (Ǻ) μ (D) r (ε Μ cm ) λfl (nm)b M1 516 (78000) 534 0.83 6.0 0.139 47.7 6.6 0.375 i M2 516 (77300) 534 0.86 6.4 0.134 47.1 6.6 0.370 A1 516 (185400) 534 0.86 6.9 0.125 A2 516 (299750) 534 0.80 6.2 0.130 A3 516 (472383) 534 0.83 6.5 0.128 (a) λmax (± 0.5 nm) of the absorption spectrum and extinction coefficient (ε) at λmax (error ~ 4%). (b) Wavelength (± 0.5 nm) of the fluorescence peak. (c) Fluorescence quantum yield Φf (± 0.03). (d) Fluorescence lifetime τ (± 0.1 ns) . (e) Radiative rate constant kf (ns). (f) R0 (Å) is the critical (Förster) radius at which the rate for energy transfer equals the decay rate of the chromophore (1/τ) and was calculated using spectral properties of the molecule (see text for details). μ (expressed in terms of Debye units) is the transition dipole moment of the low - energy absorption band (S0 → S1 transition) of the Bodipy backbone. (h) r is the steady-state fluorescence anisotropy (± 0.005) across the S0→ S1 transition of the Bodipy backbone, obtained in glassy solution of 2-methyl-tetrahydrofuran (2-MTHF) at 93 K . (i) Taken from ref. 46.

Time - Resolved Polarization Spectroscopy: Evidence for Intramolecular Energy Hopping. With the aim of getting insight into the influence of molecular topology on the dynamics of energy hopping between the weakly coupled, “iso-energetic” Bodipy – dyes, ultrafast time – resolved fluorescence anisotropy measurements were carried out. Given that the output of the “directional bonding” approach is dictated by the geometric information encoded in the building units, it is

predicted in the presented materials the Bodipy dipoles to be fixed at the corners and aligned perpendicular to the polygonal frame formed by the alternating B··· Pt(II) connectivities. Unambiguous evidence provided by the available crystal structure of A1 validates the above features.86 As such, for a perfect polygon wherein the Bodipy dipoles are strictly parallel to one another, the timedependent fluorescence anisotropy r(t) will not decay at all. On the contrary, any distortion of the rigid framework will cause orientation fluctuations of the Bodipy


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dipoles, the amplitude of which is reflected in the distinct decay features of r(t). Figure 3 depicts the 0 – 100 ps time-dependent fluorescence anisotropy r(t), probed at 530 nm, upon excitation of the Bodipy subunit at 380 nm. For comparison the time – resolved fluorescence intensity decays (0 - 100 ps) are also depicted (see inset). As can be seen, while r(t) versus time remains nearly static for the small bichromophore assembly (A1), it decays distinctly within ~ 30 ps for the larger ones (A2 and A3) and then tend to level off retaining a high residual value, rres ≈ 0.07, compared to the time-zero anisotropy, r(0) ≈ 0.11. In obvious contrast to the above, the respective decays of the time – dependent fluorescence intensity, are totally indistinguishable, matching perfectly those of the free building blocks (M1, M2). The latter finding clearly demonstrates the absence of any intramolecular quenching mechanism in the ultrafast time domain, within the multichromophoric scaffolds. The above experimental facts, furthermore, rule out the presence of excitation energy migration via exciton – exciton annihilation78 which, in fact, is a quenching mechanism since two excitons are fused to produce one exciton to a higher excited-state. Such a type of excitation energy migration has been commonly observed in various cyclic multichromophore architectures when high excitation fluencies are used.20,55,78 We note that to avoid the above type of quenching we kept the excitation fluence low (~30 pJ pulse-1). On the basis of the above results, it can be concluded that the fast depolarization process in the large macrocycles (A2, A3), can only be accounted for by excitation

energy hopping among adjacent Bodipy - dipoles that are not ideally parallel to one another (vide infra). Correlation of the EET mechanism with Förster Theory. The high directionality of the Pt(II) - pyridyl connectivity along with the fact that the present metallocyclic assemblies are built up by rigid building blocks, allows the chromophore dipoles to be well-fixed in space. This leads to a certain separation and orientation between the Bodipy dipoles wherein - in the absence of a structural deformation - their long molecular axes are strictly parallel to one another (see Scheme 1). The above structural features allow to adopt the Förster Coulombic mechanism - the so-called ideal dipole approximation (IDA) - that accounts for throughspace electronic excitation energy transfer between relatively closely – spaced and weakly - coupled dipole transition moments of electronically allowed emission (donor) and absorption (acceptor) transitions.42,43,101,102 This can be rationalized by both the appropriate distance Rj (j =A1, A2, A3) between adjacent donor – acceptor pairs of Bodipys in the macrocyclic scaffolds (RA1 ≈ 14 Å and RA2 ≈ RA3 ≈ 20 Å) and the fact that the Bodipy dipoles remain electronically independent to one another after coordination. More precisely, it has been well demonstrated that Förster theory can usually completely account for experimental kEET values at separations that are typically larger than the sum of the donor/acceptor transition dipole moment vectors divided by the electronic charge,101-103 while for more closely spaced reactants significant deviations are observed.104

Figure 3. Experimental anisotropy decays r(t) vs time probed at 530 nm 0f the rhomboid (A1), tetragon (A2) and hexagon (A3) after excitation at 380 nm. Inset: Time – resolved fluorescence decays (0 – 100 ps) at 530 nm (exc: 380 nm) of the free building blocks M1, M2 and their supramolecular metallocyclic compounds A1, A2 and A3. All experiments were made in 1,2-dichloroethane at ambient conditions.



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In addition, as mentioned above and supported by the photophysical findings (see Table 1), the inability of the dipoles to exchange electronic energy through the metal – to - ligand (Pt(II) ← Py–C≡C-B) connectivity, conclusively implies that through - bond electron-exchange interactions105 (Dexter–type EET) can be ruled out in the present supramolecular scaffolds. For a comprehensive description of the calculations used to estimate the effective interaction radius (R0), the spectral overlap integral (JDA), the transition dipole moment (μ) for the S0→S1 transition of the Bodipy dipoles, the electronic coupling (VDA) for coulombic interactions between the donor´s emission (D* → D) and acceptor´s absorption (A → A*) transition dipole moments, and the orientation factor κ2 (that is a function of the angle θDA formed by the above dipoles and of the angles θD and θA between these dipoles and the intermolecular separation vector RDA joining the centroids of the donor and the acceptor), the reader may refer to the Supporting Information. Previous studies have established both experimentally107 and theoretically53 that the absorption and emission dipole moment vectors of the low - lying optical transition (emissive state) of Bodipys, deviate by only θAA ≈ 100 from being ideally collinear to each other across the long molecular axis. Similar results were obtained for M2 and have been published elsewhere.54 We confirmed that the same holds true for M1 by monitoring its steady – state excitation and fluorescence anisotropy spectra in an optically dilute glassy solution of 2MTHF at 93 K (when molecular tumbling has ceased) and details can be found in the Supporting Information.

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among the cyclically arranged Bodipy dipoles has been constructed using the same rationale as described in an earlier publication54. Owing to the fact that the present materials are inherently highly symmetrical and rigid, we have made the assumption of uniform (a) rate constants for energy hopping (khopp) (b) distances for all nearest – neighboring Bodipy pairs, Rj (j = A1, A2, A3), of a given macrocycle and (c) orientation θDA for all donor – emitting acceptor Bodipy pairs. The fluorescence anisotropy decays were simulated and fitted with a general kinetic model which describes the dynamics of electronic excitation energy hopping between cyclically arranged Bi (i=1,2…N) identical chromophores.25,51,52,94,108,109 An explanatory illustration for N = 6 is given in Scheme 2. In the above kinetic scheme, with each chromophore having an equal probability of being initially excited, B1 (donor) is the chromophore directly excited by the pulse, and B2, B3, B4, ... Bi-1 (acceptors) stand for the chromophores whose the emissive state is populated via energy transfer from a neighbor excited Bodipy moiety. Γ is the intrinsic decay rate (1/τf), and khopp is the rate constant for energy hopping between adjacent chromophores. As mentioned above, the fact that khopp dominates over Γ means that the initial excitation energy after several energy hops migrates uniformly among N chromophores. In other words, at long times after excitation an equalization of the excitation survival probabilities occurs, leading to the establishment of an excited-state equilibrium among all chromophores. This could be understood by the time it takes for all chromophores being equally likely to be in the excited-state.

As such, given the high rigidity and symmetry of the presented metallosupramolecular scaffolds, and ignoring for the moment limited structural deformations that could cause orientation fluctuations from the perfect parallelism (κ2 ≈ 1) between the donor–acceptor transition dipole moments, we obtain an estimation of the strength of Coulombic interactions using eq. S8 (see Supporting Information). With μ1 = μ2 = 6.6 D, RA1 ≈ 14.4 Å and RA2 ≈ RA3 ≈ 20.0 Å we calculate respectively VDA ≈ 74 cm-1 and 27 cm-1. It is remarkable that both values fall into the very weak coupling (Förster) region. Hence, by virtue of eq. S7 or the well-known Förster expression98,99 (equation 1)

If depolarization due to rotational motion is negligible (Θrot-1 ≈ 0) at the time scale of energy hopping, the overall fluorescence anisotropy r(t) is given by eq. 9

3 κ 2 R0 6 ( ) 2 τ f Rj

There are three important aspects of the time - evolution of the fluorescence anisotropy that must be clearly considered when formulating excitation energy migration within a finite multichromophoric system. First, the value of the time-zero anisotropy r(0), which is the recovered value of t = 0. In the absence of reorientation of the initially excited exciton dipole direction, that is not resolved by the instrument, r(0) should be closely equal to the fundamental (limiting) anisotropy (r0) for a given λexc, as indeed is the case for

kj

calc

=

(1)

the rate constant kjcalc (j=A1, A2, A3) for energy transfer between adjacent donor-acceptor Bodipy pairs in the macrocycles, is calculated to be about 0.29 ps-1 (3.4 ps)-1 for A1 and 0.04 ps-1 (25 ps)-1 for A2 and A3. This suggests that in the cyclic multichromophoric arrays, EET predominates significantly over the deactivation path of the excited-state of Bodipys (1/τf ≈ 1.5 x 10-4 ps-1).

r (t ) = rd (t ) + ∑ ri (t )

(2)

where rd(t) is the anisotropy of the initially excited chromophore (donor) and ri(t) of the ith acceptor given by eq. S12 and eq.S13, respectively (see Supporting Information). For a detailed description concerning the mathematical background of the time – evolution of the excitation survival probabilities after pulsed excitation the reader could also refer to the Supporting Information.

Dynamics of Intramolecular Excitation Energy Hopping. The theoretical model for energy hopping


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Scheme 2. Electronic energy hopping Scheme between six identical Bodipy chromophores arranged cyclically.

the initially excited Bodipys (r0 ~ 0.10 – 0.12; λexc = 380 nm) of this study (see Figure S7 and ref. 54). Second, the slope of the decay curve which reflects the rate at which energy transfer occurs and third, the residual anisotropy rres which is the late - time leveling off of the emission anisotropy (on a time – scale where rotational diffusion is minimal). It is important to note that this conceptual parameter (rres) is strongly correlated to the eventual reorientation of the emissive exciton from its initially photogenerated position (donor). In other words, rres provides a unique means by which the angular displacement (θDA) between donor – acceptor transition dipole moment vectors can be measured. Hence, it encodes information about chromophoric arrangement in a finite multichromophoric ensemble. It has been shown,51,52 for example, both theoretically and experimentally, that for a multichromophoric finite system consisting from N randomly oriented chromophores in equilibrium in the excited – state, the residual anisotropy is definitely r0d/N. It is worth noticing that in the opposite condition, when donor – acceptor dipoles are parallel to one another, i.e., θDA ≈ 00, equation (2) becomes r(t) = rd(t). This is translated into a seemingly non – dependence of r(t) on energy hopping, with r(t) versus time being static and equal to rres = r0d. On the basis of the above formulated principles, we performed several simulations and fits to the experimental fluorescence anisotropy decay curves shown in Figure 3, with the purpose of exploring and investigating (a) the dynamics of energy hopping, evolution of the survival probabilities, the excited-state equilibrium among the cyclically arranged Bodipy dipoles and (b) critical issues relating mutual orientation of the fluorescent dipoles with structural distortions of the closed B ··· Pt(II) polygonal framework on which the chromophoric backbones are tightly bound. Calculated energy hopping rate constants (khoppcal) between adjacent Bodipy pairs were obtained using the well-known Forster expression98,99 (eq. 1). The mean nearest-neighbor dipoles distance Rj (j=A1, A2, A3) and also the two angles θD and θA

accounting for the orientation of the dipoles with respect to the intermolecular separation vector, were estimated from the crystal structure for the model compound A1, and the 3D structures shown in Scheme 3 for the tetragon (A2) and hexagon (A3) after performing geometry optimization using the PM6 semi-empirical method.110 In order to minimize the contribution of depolarization due to the slow rotational motion of the macrocycles, fittings and simulations were performed using short time – windows namely, 0 - 50 ps for the small bichromophore assembly (A1) and 0 – 100 ps for the larger ones (A2 and A3). In an effort to obtain a reliable assessment of the depolarization contribution of the slow rotational molecular motion to the above short time – windows of r(t), we probed rotational dynamics by measuring the time-dependent anisotropy up to 1 ns. The long – time dynamics of r(t) shown in SI (Figure S8), was used to determine the slow rotational diffusion time (Θrot) of A1, A2 and A3, which is found approximately 610, 1340 and 1800 ps respectively. Admittedly, these values, especially the large ones suffer from increased uncertainty as the upper temporal limit of our measurements is 1 ns. However, they exhibit the intuitively expected increase upon going from the bichromophore compound A1 to the large macrocycles A2 and A3. In addition, the above dynamics exhibit a ~ 30 ps anisotropy decay component for the large macrocycles, similar to the value found from the 0 - 100 ps measurements, the magnitude of which is larger in A3 than in A2. In the following analysis focused on EET within the macrocycles, the time – dependent fluorescence anisotropies are evaluated over a short time – window of increased temporal resolution (0-50 ps for A1 and 0-100 ps for A2 and A3) after having taken into account the minor amount of depolarization due to the slow molecular rotational motion. For this case the model was properly modified by multiplying the right terms of the master eq. 2 by exp(-t/Θrot), where the rotational relaxation time Θrot was introduced as a fixed value obtained from the above mentioned long – time dynamics of r(t) for each macrocycle. In general, the fluorescence anisotropy fits are found not to be significantly affected by rotational depolarization (for a comparison of fits with and without contribution of rotational depolarization see the Supporting Information; Figure S9) and the results are summarized in Table 2. The Rhomboid [2M1+2Ptbppp(OTf)2] (A1) Structure. The rhomboid structure (A1) bearing only two metal centers, has been fully characterized including the determination of the X-ray crystal structure.86 Both the crystal and the geometry optimized structure, agree to a highly symmetrical structure wherein the two Bodipy dipoles are perpendicularly aligned to the strictly planar rhomboidal – shaped frame (see Scheme 3). Figure 4 compares the experimental with the calculated r(t)cal curve generated by means of the master eq. 9



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(donor 1 and acceptor 2) properly modified to account for rotational depolarization. For the generation of the simulation curve we set, Θrot = 610 ps, θD = 800, θA = 900, θDA = 100, θAA = 100, RA1 = 14.44 Å, r10 = 0.10 and τf = 6900 ps. These values afford an orientation factor of κ2 = 0.97, which is very close to that predicted for ideally parallel donor - acceptor dipoles (κ2 = 1). As can be seen, the simulation curve r(t)cal fits properly the experimental curve which appears to be nearly static (at least in a time - window of 0 – 50 ps where depolarization due to rotational motion is minimal) as is expected for the given system in the absence of a structural distortion. In other words, the Bodipy dipoles manifest themselves as being nearly perfectly parallel, suggesting that the structure of the rhomboid is very similar both in solution and in the solid state.

Figure 4. Comparison between the experimental (filled circles; probed at 530 nm) and theoretical (red curve) anisotropy decay curves after 380 nm excitation of the Bodipy subunit in A1 (For details on the adjustment of the parameters see text). Colored curves are the individual anisotropy components r1(t) and r2(t) of the donor 1 and the acceptor 2 respectively. Inset: time – evolution of the excitation probabilities. The present bichromophore assembly (A1) wherein parallel orientation is strictly maintained between the dipoles, represents a fascinating prototype for providing the mathematical basis underlying to the non – dependence of r(t) with energy hopping. More precisely, the seemingly static appearance of the overall anisotropy, r(t) = r1(t) + r2(t), is the result of the nearly complete cancelation between the individual anisotropy components i.e., r1(t) and r2(t), which, in fact, decay distinctly with reverse slopes and a rate of khoppcal = 0.28 ps-1 (3.57 ps)-1 as is shown in Fig. 4. This leads to the rapid equalization of the excitation survival probabilities and to the

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establishment of the excited-state equilibrium among the populations of the two dyes within ~ 8 ps after pulsed excitation (see inset of Fig. 4). It is also worth of noticing that simulations have shown that r(t) is not sensitive to the angles θDA and θAA as long as they are ≤ 100Hence, the implicit influence of the above angles (adopted in the simulations i.e., θDA ≈ θAA ≈ 100) on the overall anisotropy curve of A1 has a minimal effect that cannot be resolved at all by the experimental resolution. The Tetragonal [2M1+2M2] structure (A2). The experimental anisotropy decay curve r(t) versus time of the tetragonal structure, in contrast to that of the rhomboid, shows distinct decay features within the first 0-50 ps shown in Fig. 3. Having demonstrated the almost static behaviour of r(t) versus time - when the Bodipy dyes position their long molecular axis perpendicular to the strictly planar rhomboidal frame of A1 – the above finding can only be accounted for by limited distortion of the quadrangle frame. In spite of several efforts, we were unable to get suitable crystals for a single crystal structure determination of the tetragonal structure. Hence, we examined closely the geometry optimized structure. The energy minimized structure of A2 is shown in Scheme 3 along with key distances and angles. The results are in line with the notion that the cyclic [2M1+2M2] tetramer (A2) closely retains the geometric characteristics of a slightly distorted tetragon. This view can be strengthened by the near equality of the four platinum – platinum (14.02 – 14.22 Å) and the four boron – boron (17.85 – 17.97 Å) sides length as well as the four angles formed at the corners of the platinum (88.50 – 91.40) and boron (88.20 – 91.10) quadrangle frames, respectively. Nevertheless, as can be seen from the side view of the structure (Scheme 3), not all boron and platinum atoms are coplanar as they are in the rhomboid (A1). This leads to a slight distortion from the planarity of the frame defined by the alternating B ··· Pt(II) connectivities, with the boron atoms being more pronounced to be found out – of - plane (e.g., a value of ~ 130 was found for the intersection angle of the planes defined by the boron atoms 4, 1, 2 and 2, 3, 4 respectively). As a consequence, the tightly bound Bodipy dipoles at the corners of the slightly distorted tetragon are no longer strictly parallel to each other as they are in the small assembly A1. Therefore, although the geometry optimized structure of A2 does not reflect the overall magnitude of the conformational changes (considering yet that temperature, solvent and counterions effects were not taken into account), it nevertheless provides a valuable insight into the observed distinct decay of the fluorescence anisotropy r(t) versus time.


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Scheme 3. Top and side view of the crystal structure of the rhomboid (A1) and of the geometry optimized structure of the tetragon (A2) and hexagon (A3) along with key distances and angles. Counterions are not shown. The direction of the absorption transition dipole moment μa of the Bodipy unit is shown on the right side view of A1.



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We next fitted the master eq. 2 (donor 1 and acceptors 2-4), modified to account for rotational depolarization (Θrot =1340 ps), to the experimental anisotropy decay curve in a time – window of 0 – 100 ps, by keeping θD, θA, θAA and RDA constant, (θD ≈ θA ≈ 900, θAA ≈ 100, RA2 = 20.05 Å) and allowing θDA and r(0) to run as free parameters (Figure 5). A value of θDA = 29.4 ± 10 was extracted from the best fit curve, affording an orientation factor of κ2 = 0.73 and an energy hopping rate of khopp = 0.031 ps-1 (32.3 ps)-1. The time evolution of the excitation survival probabilities shown in the inset of Fig. 5, clearly shows that the excited-state equilibrium among the four Bodipy dyes in A2, is reached on a time scale much longer (~ 60 ps) than that in the bichromophoric rhomboid A1 (~ 8 ps). This can be rationalized by both the reduced khopp in A2 (0.031 ps-1 versus 0.28 ps-1 in A1 as a result of a larger interchromophore spacing in the former) and the longer time needed for the excitons to spread among four dyes (i = 4) in the larger macrocycle (Figure 5; inset). It is also remarkable to note that the excitation probability versus time of the acceptor 2 equals that of 4, as both they are symmetrically positioned about the donor 1.

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Table 2. Nearest – neighbor distances RAj, orientation angles θDA between donor – acceptor Bodipy dipoles, energy hopping rate constants khopp and time-zero anisotropies r(0) used for the simulations or extracted from fits to the time – resolved anisotropy decays. The two angles θD and θA obtained from the 3D structures, and the angle θAA ≈ 100 that accounts for the slight deviation from the collinearity between absorption and emission transition moment vectors of the Bodipys, were introduced as fixed parameters. -1 e Compd Θrota /ps RAj/Å r(0) θDAd (0) khopp /ps A1 A2

A3 a

610 1340 1340 1800 1800

14.44b 20.05b 20.05b 21.1c 19.57b 19.57b 20.8c

10 31.2 29.4 30.2 35.2 34.0 35.8

0.28 0.032 0.031 0.028 0.031 0.030 0.026

0.10 0.112 0.112 0.110 0.115 0.115 0.112

Rotational diffusion times Θrot were introduced as fixed parameters to the properly modified eq. 2 (see text for deb tails). Nearest – neighbor distances calculated from the c d optimized structure. extracted from the fits (±0.5 Å). 0 e Uncertainty of ± 1 . Uncertainty of ± 0.001

constant all the remaining parameters. For simplicity rotational depolarization was not considered. The collection of curves is shown in Fig. 6, and provides unambiguous evidence for the strong dependence of r(t), and in particular of the levelling off part (rres), on θDA. At this point, it must be emphasized that although in the cyclic tetramer (A2) we do not have the perfect parallel orientation of dipoles observed in A1, we do not have a totally random orientation of dipoles either, because in such case the fluorescence anisotropy ought to be leveling off51,52 at 0.028 (rres = r0d/4), whereas Figure 6

Figure 5. Experimental anisotropy decay r(t) vs time, probed at 530 nm (filled circles; exc. at 380 nm) and best fitting curve (red line) using eq. 2 modified to include Θrot = 1340 ps. Fitting results: r(0) = 0.112 ± 0.001 and θDA = 29.4 ± 0 1 (see text for details). Colored curves are the individual anisotropy components r1(t), r2(t), r3(t) and r4(t) of the donor 1 and the acceptors 2, 3, and 4 respectively. Inset: time – evolution of the donor - acceptors excitation probabilities. To obtain a realistic assessment of the expected variability and confidence limits in the above estimation of the angular displacement between donor – acceptors transition dipole moment vectors, we evaluated the sensitivity of r(t) against θDA. This was made by comparing the experimental decay curve with theoretical ones generated by varying θDA in a narrow range (± 100) centered at the value ~ 300 extracted from the fit, and keeping

Figure 6. Comparison between the experimental (filled circles) and theoretical fluorescence anisotropy decay 0 0 curves generated by varying θDA from 20 to 40 with an 0 increment of 10 and keeping constant all the remaining parameters (For details on the adjustment of the parameters see text).


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shows that rres is quite larger c.a. ~ 0.07. The latter, along with the fact that the simulations were deteriorating as θDA was slightly varied (± 100) above and below the value found from the best fit (~ 300), strongly support (a) the significance and confidence limits of our measurements and (b) that the degree of deviation of the Bodipy dipoles from a mutual parallel orientation is not misquoted to a substantial extent. Up to this point, fittings and simulations have been performed by treating the mean distance of the nearest neighboring Bodipy dipoles (RA2) as a fixed parameter calculated from the optimized structure (RA2cal = 20.05 Å). Given that the slope of the decay curve reflects the rate at which energy transfer occurs, and considering that the rate is strongly dependent on RA2 (kEET ∝ 1/RA26; see equation 1), it would be exceptionally revealing to estimate the true RA2 from the experimental data. To this end, both RA2 and θDA were allowed to run freely during the fitting curve procedure. The new fitting curve was found to be nearly superimposable with that shown in Fig. 5, giving RA2 = 21.1 ± 0.5 Å and θDA = 30.20 ± 10. It is obvious that the new value of θDA remains within experimental error equal to that found before (θDA = 29.4 ± 10), whereas the value of RA2 extracted from the fit, turns out to just barely exceed the value calculated from the optimized structure (RA2cal = 20.05 Å). Therefore, apart from a limited distortion of the quadrangle frame, the very good agreement between the calculated and experimental value of RA2, can lead to the conclusion that homogeneity in spacing is maintained very close to that predicted for an equivalent, nearly regular four – sided polygon. The hexagonal [3M1+3M2] Structure (A3). An inspection in Fig. 3 clearly shows that the fluorescence anisotropy r(t) versus time of the hexagon (A3), is not significantly differentiated from that of the tetragon (A2). The fitting of the experimental points to the appropriately modified master eq. 2 (donor 1 and acceptors 2-6; Θrot =1800 ps) is shown in Fig. 7 and was performed by keeping fixed the angles θD, θA and θAA (θD ≈ θA ≈ 900; θAA ≈ 100) and the nearest-neighbor distance RA3 (RA3cal = 19.57 Å) estimated from the optimized structure (Scheme 2), while θDA and r(0) were left free. The values of the above parameters obtained from the fitting were θDA = 34.0± 10 and r(0) = 0.115 ± 0.001, with the former being hardly increased from its counterpart (θDA = 29.4± 10) in the tetragon (A2). From the above values we calculate κ2 = 0.67 and khopp = 0.030 ps-1 (33.3 ps)-1 that almost equals the hopping rate found in A2, i.e., 0.031 ps-1 (32.3 ps)-1. The above findings clearly indicate that in the hexagon (a) the degree of structural distortion is virtually similar to that in A2 and (b) the migration of the excitation energy among the cyclically arranged Bodipys, takes place at an equal rate as in A2. In spite, however, of the equality of the excitation energy hopping rates, the equalization of the excitation survival probabilities and the establishment of the excited-state equilibrium

Figure 7. Experimental anisotropy decay r(t) vs time detected at 530 nm (filled circles; exc. at 380 nm) and best fitting curve (red line) using eq. 9 modified to include the rotational diffusion time Θrot =1800 ps ; fitting results: r(0) = 0.115 ± 0.001 and θDA = 34.0 ± 10 (see text for details). Colored curves are the individual anisotropy components ri(t) of the donor 1 and the acceptors 2 - 6. Inset: time – evolution of the donor - acceptors excitation probabilities.

among the dyes in the hexagonal structure, last longer (~ 120 ps) compared to the corresponding time – dependent phenomena in the tetragon (~ 60 ps) as is shown in the inset of Fig. 7. This is not unexpected since in reality the excitons in the six – sided polygon, migrate to a greater distance of about 118 Å (that is, the length defined by the perimeter of the closed polygonal chain connecting the centroids of the chromophores) compared to the corresponding distance of ~ 80 Å in the smaller macrocycle (A2). The delay of excitons in reaching the excited – state equilibrium in A3 compared to A2, is reflected in the late–time decay features of r(t) in Fig. 7 and is a manifestation of the excited-state populations (i*; i = 1 – 6) taking time to balance themselves following photoproduction of the donor 1* (see Scheme 2). Clearly, as is seeing by carefully inspecting the 40 < t < 100 ps time window, while r(t) becomes essentially static in A2 (see Figure 5), it shows a gentle slope that gradually is levelling off as the excitation survival probabilities of the dyes in A3 tend to be equalized over time. When RA3 is treated as a free parameter in the fitting curve procedure, the extracted value RA3 = 20.8 ± 0.5 Å, closely approximates the value calculated from the geometry optimized structure of A3 (RA3cal = 19. 6 Å). The above structure is shown in Scheme 3 along with key geometric parameters. As can be seen, the sides length of the hexagonal frame of A3, defined by either the six platinum – platimum (15.3 – 16.05 Å) or the six boron boron sides (18.04 – 18.07 Å), shows very little deviation from the normality. The same is true by analogy for the



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six center–to–center nearest-neighbor interchromophore distances RA3 (19.49 – 19.62 Å) adopted in the fittings. However, while the hexagon formed by the platinum atoms at the corners appears to be nearly equiangular (118.3 – 121.60), that made by the boron atoms (113.5 – 125.60) shows distinct deviation from the regularity (1200). The above molecular modeling based structural distortions, tentatively indicate that the boron atoms are more susceptible to deviate from planarity, most likely through out–of–plane bending of the linear arms connected to the boron atom. The above may in fact provide a reasonable explanation for the experimentally observed deviation from the perfect parallelism of the Bodipy dipoles.

CONCLUSIONS Multi – Bodipy self - assembled metallocycles wherein the Bodipy dipoles reside in a parallel alignment at the two opposite vertices of a rhomboid (A1) and the vertices of a tetragon (A2) and hexagon (A3), respectively, have been investigated in solution phase to explore the influence of molecular topology on exciton dynamics. Steady–state and time–resolved fluorescence spectroscopy, validate that the excited-state of Bodipys retains its highest survival probability as the size of the macrocycle increases, in contrast to that observed in analogous covalent macrocycles. Time-dependent ultrafast spectroscopy reveals that the small bichromophore assembly (A1) exhibits static fluorescence anisotropy transients r(t), at early times after excitation. Simulations to the experimental data derived with a theoretical kinetic model, show that the two Bodipy dipoles in A1 are nearly perfectly parallel as they are in the crystal state. Excitation energy is exchanged very fast between the dipoles with a rate of about (3.6 ps)-1 leading to the completion of the excited-state equilibrium within ~ 8 ps after pulsed excitation. In contrast to the small rhomboid, the fluorescence anisotropy r(t) vs time shows distinct decay features that are nearly indistinguishable between the tetragonal and the hexagonal structure. The results suggest that limited distortions from the planarity of the macrocyclic frameworks, cause orientation fluctuations through which the perfect parallel mutual arrangement of the Bodipy dipoles breaks down. From the late-time anisotropy decays of A2 and A3, the eventual reorientation of the emissive exciton from its initially photogenerated position was found to be very similar in both macrocycles. Extensive simulations and fits to the fluorescence anisotropy decays - supported by geometry optimization calculations - show that in the large macrocycles, i.e., tetragon and hexagon, (a) the degree of structural distortion is nearly similar and (b) intramolecular energy hopping takes place with almost the same rate, khopp ≈ (32 ps)-1. Distances Rj (j = A2, A3) between nearest – neighboring Bodipy dyes extracted from the fits, are found to correlate very well with those predicted by calculations for a slightly distorted four- and six – sided regular poly-

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gon, respectively. The high homogeneity in dipoles spacing along with the fact that the excitation energy hopping rate khopp ≈ (32 ps)-1 is much faster than the intrinsic deactivation rate of Bodipys, 1/τf ≈ (6500 ps)-1, allows for rapid and uniform distribution of the excitation probability over all Bodipy subunits of the macrocycles. The attainment of the excited-state equilibrium among the populations of the excited Bodipys is found to be dependent upon the size of the macrocycle. In summary, temperature – regulated supramolecular synthesis via Pt(II) – pyridyl coordination, not only appears to be a powerful tool in the synthesis of large and highly emissive macrocycles, but it also provides a means to enable homogeneity in critical structural issues. As such, advantageous uniform migration and temporal storage (ns) of the excitation energy across the circular assembly can be achieved. Considering yet that the presented macrocycles are potential host materials, the findings of this work may open exciting new possibilities for fast and quantitative funneling of excitation energy from the periphery to a suitable guest molecule at the center of the cavity, amplifying thus the final output (e.g., light tunability, charge separation e.t.c.,)

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Materials and methods: Detailed synthetic procedure for 1 the tetragonal (A2) and hexagonal structure (A3), full H 31 1 NMR (500 MHz) and P { H} NMR spectra of A2 and A3, Pulsed field gradient NMR spectroscopy, ESI – MS experimental conditions, calculations and modelling of the cyclic self - assemblies. Dipole – dipole Förster formulation, steady – state excitation and fluorescence anisotropy spectra, dynamics of intramolecular excitation energy hopping, measurements of rotational relaxation times, fluorescence anisotropy fits with and without contribution of rotational depolarization.

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was performed in the framework of “Advanced Materials and Devices for Collection and Energy Management” project within GSRT’s KRIPIS action, funded by Greece and the European Regional Development Fund of the European Union under NSRF 2007-2013 and the Regional Operational Program of Attica.

ABBREVIATIONS Pt(dppp)OTf2,


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[dppp[1,3-Bis(diphenylphosphino)propane]platinum(II) triflate

18. Gunderson, V. L.; Wilson, T. M.; Wasielewski, M. R. Excitation Energy Transfer Pathways in Asymmetric Covalent Chlorophyll a Tetramers. J. Phys. Chem. C 2009, 113, 11936−11942.

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Dynamics of Intramolecular Energy Hopping in Multi-Bodipy Self

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