Energy Transfer in Dendritic Systems Having Pyrene Peripheral Groups as Donors and Different Acceptor Groups

In this feature article, a specific overview of resonance energy transfer (FRET) in dendritic molecules was performed. We focused mainly on constructs bearing peripheral pyrene groups as donor moieties using different acceptor groups, such as porphyrin, fullerene C60, ruthenium-bipyridine complexes, and cyclen-core. We have studied the effect of all the different donor-acceptor pairs in the energy transfer efficiency (FRET). In all cases, high FRET efficiency values were observed.


Introduction
Since their discovery at the end of the 80's, the scientific community has been very interested in dendrimers due to their well-defined nanostructures that confer them outstanding chemical and physical properties [1][2][3]. The special attention accorded to dendrimers is essentially due to their potential applications in diverse fields such as in catalysis, drug delivery and photoactive materials [4][5][6][7][8][9][10]. The investigation of the photophysical properties of these molecules is very attractive and depends in large measure on the architecture of these perfectly branched macromolecules. The incorporation of photoactive units into dendrimers can be achieved by attaching them covalently or noncovalently in three possible locations: periphery, branches, or core. Many photoactive chromophores have been linked to dendrimers through covalent bonds to produce highly luminescent materials [11,12].
Depending on the selected photoactive molecules and their position in the dendritic construct, different phenomena, such as fluorescence resonance energy transfer (FRET), excimers, and charge transfer (CT) can take place [13]. The study of those processes is very important, since it helps us to make innovations to improve the efficiency of existing photovoltaic devices [13][14][15][16][17]. With that aim, many scientists have focused their research on the design and study of various dendritic molecules introducing different chromophores with particular optical and photophysical properties [18][19][20][21].
Among the available chromophores, pyrene is considered the most used fluorescent dye in the study of labelled polymers [22], and there are some reviews covering the most important aspects of its fluorescent properties [23][24][25][26][27][28][29][30][31]. The use of pyrene as a fluorescent label to study polymers of different lengths and architectures is prevalent because of its unique photophysical behavior, mainly its tendency to form excimers. Pyrene moieties have been attached to numerous macromolecules [32,33] in order to investigate micellization, polymer, and dendrimer dynamics [34], and for the development [32,33] in order to investigate micellization, polymer, and dendrimer dynamics [34], and for the development of new π-conjugated polymers and oligomers with pyrene units [35,36]. This phenomenon has been also observed in dendritic structures bearing pyrene previously reported [37][38][39]. Numerous dendritic architectures with photoactive groups have been reported so far, but there are only a few examples where pyrene has been covalently attached jointly with the presence of an acceptor group to give an efficient energy transfer (FRET) for light-harvesting purposes.

Optical and Photophysical Features of Pyrene
Pyrene as a chromophore has outstanding optical properties, like elevated quantum yield, long fluorescence lifetime, and excimer emission, depending on its local concentration [22]. That is why it is often used as a probe to test dynamic processes of polymers in solution that take place in the pyrene lifetime regime [22][23][24][25][26][27]. Other interesting features of pyrene are its potential use in dyads for solar energy conversion and as its use in the exfoliation of carbon nanotubes (CNT) and graphene [40].
We have investigated the photophysical properties of various polymers and compounds bearing pyrene [36]. Pyrene has been also incorporated into various macromolecules with the aim to develop novel photoactive materials [35,36,[41][42][43][44][45][46]. Furthermore, we have designed a Fréchet-type dendron containing an increasing number of pyrene moieties with the generation number. It these constructs, pyrene acts as donor in a dual way, from the monomer or excimer emission, depending on the structure of the dendron. The absorption and emission spectra of the dendron of the first generation (Py2-G1OH) as well as of the dendron of the second generation (Py4-G2OH) is shown in Figure 1.  28(30), 11195-11205.). Copyright (2012) American Chemical Society [47].
Fluorescence spectra of Py2-TMEG1 and Py4-TMEG2 were recorded exciting at 344 nm in THF solution at room temperature ( Figure 3). The obtained spectra were showing two emission bands at 376 nm and 476 nm corresponding to the monomer and excimer bands of pyrene, respectively, that are comparable with the emission bands of the precursor pyrene-labelled dendrons. Nevertheless, we can also observe a new band at 651 nm due to the porphyrin emission. The ratios between the excimer intensity/monomer intensity (IE/IM) were calculated for Py2-TMEG1 and Py4-TMEG2. The obtained values were 0.58 and 1.46, respectively. Those values were much lower than the values obtained for the pyrene dendron precursors Py2-G1OH and Py4-G1OH (0.69 and 3.05, respectively).  Figure 2. Structure of the dendronized porphyrins containing first-generation and second-generation Fréchet-type dendrons [47].
Polymers 2018, 10, x FOR PEER REVIEW 4 of 25 bands, which appear in the range between 450 and 700 nm. Since the absorption spectra of the obtained dendronized porphyrins are the sum of the absorption spectra of their individual precursors having pyrene and porphyrin elements, we can affirm that there is no interaction between pyrene and porphyrin in the ground state [47].
Fluorescence spectra of Py2-TMEG1 and Py4-TMEG2 were recorded exciting at 344 nm in THF solution at room temperature ( Figure 3). The obtained spectra were showing two emission bands at 376 nm and 476 nm corresponding to the monomer and excimer bands of pyrene, respectively, that are comparable with the emission bands of the precursor pyrene-labelled dendrons. Nevertheless, we can also observe a new band at 651 nm due to the porphyrin emission. The ratios between the excimer intensity/monomer intensity (IE/IM) were calculated for Py2-TMEG1 and Py4-TMEG2. The obtained values were 0.58 and 1.46, respectively. Those values were much lower than the values obtained for the pyrene dendron precursors Py2-G1OH and Py4-G1OH (0. 69  Fluorescence spectra of Py 2 -TMEG1 and Py 4 -TMEG2 were recorded exciting at 344 nm in THF solution at room temperature ( Figure 3). The obtained spectra were showing two emission bands at 376 nm and 476 nm corresponding to the monomer and excimer bands of pyrene, respectively, that are comparable with the emission bands of the precursor pyrene-labelled dendrons. Nevertheless, we can also observe a new band at 651 nm due to the porphyrin emission. The ratios between the excimer intensity/monomer intensity (I E /I M ) were calculated for Py 2 -TMEG1 and Py 4 -TMEG2. The obtained values were 0.58 and 1.46, respectively. Those values were much lower than the values obtained for the pyrene dendron precursors Py 2 -G1OH and Py 4 -G1OH (0.69 and 3.05, respectively).
Time-resolved fluorescence experiments revealed that the pyrene fluorescence quenching happens with an energy transfer rate constant value of 2.3 and 1.8 × 10 9 s -1 for Py 2 -TMEG1 and Py 4 -TMEG2, respectively. Such rate constants are the proof of a highly efficient quenching mechanism. Regardless the I E /I M ratio, a more than 30-fold decrease in fluorescence intensity can be seen when the dendrons and the porphyrin are covalently attached, exhibiting a very efficient FRET from an excited pyrene monomer or excimer to the porphyrin. Indeed, pyrene fluorescence in the porphyrinic construct Py 2 -TMEG1 and Py 4 -TMEG2 suffer a quantitative quenching (99% and 97%, respectively) as shown in Table 1.
The fluorescence band observed at 651 nm, exciting at 344 nm, comes from the porphyrin unit of compounds Py 2 -TMEG1 and Py 4 -TMEG2. This emission is more intense than that observed after direct excitation of TME (precursor porphyrin without pyrene) in solution ( Figure 3). The donor (pyrene monomer and excimer) showed a remarkable decrease in fluorescence intensity while the acceptor (porphyrin) exhibited an enhancement in fluorescence intensity, which is a clear indication that FRET is occurring in the pyrene-porphyrin dendritic constructs Py 2 -TMEG1 and Py 4 -TMEG2. E FRET values and quantum yields are summarized in Table 1. a All reported fluorescence quantum yields were determined using the fluorescence quantum yield of pyrene in cyclohexane as a reference which has been reported to equal 0.32 [65]. b This value is the fluorescence quantum yield of the porphyrin core having undergone FRET from an excited pyrene to the porphyrin. It is calculated by integrating the porphyrin fluorescence intensity in Figure 3 between 580 and 680 nm, after the fluorescence spectrum was correct to account for the direct excitation of porpyrin. c E FRET is the FRET efficiency, calculated using the following equation: where I (py+por) is the absolute fluorescence intensity of one mole of pyrenyl pendant in a dendron and I (py) is the absolute fluorescence intensity of one mole of pyrene attached to the corresponding dendron. d All experiments were conducted in triplicate.
In function of the distance pyrene-porphyrin (d Py−Por ), we can expect a decrease in FRET efficiency in Py 4 -TMEG2, with respect to Py 2 -TMEG1. Nevertheless, if the increase of the distance d Py−Por is not enough to overcome the Förster radius (R 0 ), this change in the FRET efficiency will not be observed. As a result, very high FRET efficiencies were observed for both generations of our dendrimer leading to the conclusion that the pyrene and porphyrin units in our constructs are located well within the R 0 value (5.2 nm) determined for this pair of chromophores. This assumption was confirmed by molecular mechanics calculations to estimate the longest possible pyrene-porphyrin distance in a stretched conformation of the molecules [47]. It is not realistic to expect that the dendritic molecules remain stretched in solution since they tend to adopt a coiled conformation. The donor-acceptor distance in these constructs was found to be 3.0 and 3.5 nm for Py 2 -TMEG1 and Py 4 -TMEG2, respectively, well under the 5.2 nm R 0 value, showing that efficient FRET occur in these dendritic molecules (Table 2).
We can observe that FRET efficiency is not affected by the quantity of excimer emission arising from the pyrene-containing dendrons since the FRET efficiency is not affected by the amount of excimer formed in the pyrene dendron precursors. This fact reveals that excimer formation would takes place after the FRET process occurs from the monomer emission to the porphyrin acceptor and that it does not have a contribution to the whole FRET process. Time-resolved fluorescence experiments showed that FRET from excited pyrene monomer to the porphyrin core take place more than 10 times faster than the formation of pyrene excimers in the pyrene dendrons. From these results, we can affirm that FRET from the excited pyrene monomer to the porphyrin is very efficient and occurs before the formation of the excimer.
A second series of dendrimers bearing first-generation Fréchet-type pyrene-labelled dendrons and a porphyrin as the core was synthesized and characterized by our group [66]. The obtained free base porphyrins were further metallated with Zn. The structure of the obtained porphyrins was characterized by 1H NMR spectra and confirmed by MALDI-TOF mass spectrometry [66]. The electrochemical properties and charge transfer character of such systems has been also studied [67]. The structure of this series of dendrimers is illustrated in Figure 4.
Polymers 2018, 10, x FOR PEER REVIEW 6 of 25 located well within the R0 value (5.2 nm) determined for this pair of chromophores. This assumption was confirmed by molecular mechanics calculations to estimate the longest possible pyrene-porphyrin distance in a stretched conformation of the molecules [47]. It is not realistic to expect that the dendritic molecules remain stretched in solution since they tend to adopt a coiled conformation. The donor-acceptor distance in these constructs was found to be 3.0 and 3.5 nm for Py2-TMEG1 and Py4-TMEG2, respectively, well under the 5.2 nm R0 value, showing that efficient FRET occur in these dendritic molecules (Table 2). We can observe that FRET efficiency is not affected by the quantity of excimer emission arising from the pyrene-containing dendrons since the FRET efficiency is not affected by the amount of excimer formed in the pyrene dendron precursors. This fact reveals that excimer formation would takes place after the FRET process occurs from the monomer emission to the porphyrin acceptor and that it does not have a contribution to the whole FRET process. Time-resolved fluorescence experiments showed that FRET from excited pyrene monomer to the porphyrin core take place more than 10 times faster than the formation of pyrene excimers in the pyrene dendrons. From these results, we can affirm that FRET from the excited pyrene monomer to the porphyrin is very efficient and occurs before the formation of the excimer.
A second series of dendrimers bearing first-generation Fréchet-type pyrene-labelled dendrons and a porphyrin as the core was synthesized and characterized by our group [66]. The obtained free base porphyrins were further metallated with Zn. The structure of the obtained porphyrins was characterized by 1H NMR spectra and confirmed by MALDI-TOF mass spectrometry [66]. The electrochemical properties and charge transfer character of such systems has been also studied [67]. The structure of this series of dendrimers is illustrated in Figure 4.
The absorption spectra of the porphyrinic dendrimers in THF solution are shown in Figure 5. In the absorption spectra of these compounds, we can see the typical S0 → S2 absorption band of the pyrene group centered 344 nm, as well as the Soret band of the porphyrin (free base or metalated), which appear at about 420 and 426 nm, respectively. Usually, the free porphyrin derivatives exhibit four Q bands situated between 513 and 647 nm, whereas the zinc metallated porphyrins show only two Q bands in the same UV-VIS region. The absorption spectra of the porphyrinic dendrimers in THF solution are shown in Figure 5. In the absorption spectra of these compounds, we can see the typical S 0 → S 2 absorption band of the pyrene group centered 344 nm, as well as the Soret band of the porphyrin (free base or metalated), which appear at about 420 and 426 nm, respectively. Usually, the free porphyrin derivatives exhibit four Q bands situated between 513 and 647 nm, whereas the zinc metallated porphyrins show only two Q bands in the same UV-VIS region.            The absorption spectra of the free-base dendronized porphyrins Py 2 -TMEG1 and Py 4 -TMEG2 resulted to be the sum of the absorption spectra of 1-pyrenebutanol and porphyrin, so that we can realize that negligible to no electronic interactions occur between both chromophores.
The fluorescence emissions of all dendrimers based on a porphyrin core were recorded and the absolute emission spectra are shown in Figure 6. If we compare the Y-axes on the left and on the right corresponding to the porphyrinic compounds and 1-pyrenebutanol (quantum yield of 0.52), respectively, we can realize how efficient is FRET from an excited 1-pyrenebutoxy to the ground-state of the porphyrin unit. A reduction in the fluorescence emission intensity of more than two orders of magnitude was observed.
A remarkable FRET efficiency was expected in these dendritic constructs given that the distance between the center of the porphyrin and the center of the pyrenyl unit in a fully extended conformation (d Por−Py EXT ) varies between 18 and 35 Å depending on the dendritic construct. The Förster radius (R 0 ) value was to equal 51.8 ± 0.2 and 48.7 ± 0.3 Å for a free-base and a Zn-metalated porphyrin, respectively ( Table 2). The distance d Por−Py EXT included in Table 3 was calculated by means of molecular mechanics optimizations, using the program Hyperchem (Hypercube, ink., Gainesville, FL, USA). Since d Por-Py EXT is much smaller than R 0 for all the dendronized porphyrins, FRET is expected to take place on a faster time scale than the formation of excimers within a Py 2 -G1OH or Py 4 -G2OH dendron.
Polymers 2018, 10, x FOR PEER REVIEW 8 of 25 The absorption spectra of the free-base dendronized porphyrins Py2-TMEG1 and Py4-TMEG2 resulted to be the sum of the absorption spectra of 1-pyrenebutanol and porphyrin, so that we can realize that negligible to no electronic interactions occur between both chromophores.
The fluorescence emissions of all dendrimers based on a porphyrin core were recorded and the absolute emission spectra are shown in Figure 6. If we compare the Y-axes on the left and on the right corresponding to the porphyrinic compounds and 1-pyrenebutanol (quantum yield of 0.52), respectively, we can realize how efficient is FRET from an excited 1-pyrenebutoxy to the ground-state of the porphyrin unit. A reduction in the fluorescence emission intensity of more than two orders of magnitude was observed.
A remarkable FRET efficiency was expected in these dendritic constructs given that the distance between the center of the porphyrin and the center of the pyrenyl unit in a fully extended conformation (dPor−Py EXT ) varies between 18 and 35 Å depending on the dendritic construct. The Förster radius (R0) value was to equal 51.8 ± 0.2 and 48.7 ± 0.3 Å for a free-base and a Zn-metalated porphyrin, respectively ( Table 2). The distance dPor−Py EXT included in Table 3 was calculated by means of molecular mechanics optimizations, using the program Hyperchem (Hypercube, ink., Gainesville, FL, USA). Since dPor-Py EXT is much smaller than R0 for all the dendronized porphyrins, FRET is expected to take place on a faster time scale than the formation of excimers within a Py2-G1OH or Py4-G2OH dendron.   The absorption spectra of the free-base dendronized porphyrins Py2-TMEG1 and Py4-TMEG2 resulted to be the sum of the absorption spectra of 1-pyrenebutanol and porphyrin, so that we can realize that negligible to no electronic interactions occur between both chromophores.
The fluorescence emissions of all dendrimers based on a porphyrin core were recorded and the absolute emission spectra are shown in Figure 6. If we compare the Y-axes on the left and on the right corresponding to the porphyrinic compounds and 1-pyrenebutanol (quantum yield of 0.52), respectively, we can realize how efficient is FRET from an excited 1-pyrenebutoxy to the ground-state of the porphyrin unit. A reduction in the fluorescence emission intensity of more than two orders of magnitude was observed.
A remarkable FRET efficiency was expected in these dendritic constructs given that the distance between the center of the porphyrin and the center of the pyrenyl unit in a fully extended conformation (dPor−Py EXT ) varies between 18 and 35 Å depending on the dendritic construct. The Förster radius (R0) value was to equal 51.8 ± 0.2 and 48.7 ± 0.3 Å for a free-base and a Zn-metalated porphyrin, respectively ( Table 2). The distance dPor−Py EXT included in Table 3 was calculated by means of molecular mechanics optimizations, using the program Hyperchem (Hypercube, ink., Gainesville, FL, USA). Since dPor-Py EXT is much smaller than R0 for all the dendronized porphyrins, FRET is expected to take place on a faster time scale than the formation of excimers within a Py2-G1OH or Py4-G2OH dendron.

Compound E FRET (SS) E FRET (SPC) d Por-Py EXT (Å) E FRET (SS) E FRET (SPC) d Por-Py EXT (Å)
Por- ( This conclusion was verified experimentally by time-resolved fluorescence experiments. The fluorescence decays of the pyrene monomer and porphyrin in compounds Py 2 -TMEG1 and Py 4 -TMEG2 were analyzed applying the model free analysis (MFA) [66]. In these constructs an excited pyrene units transfers their excess energy so efficiently to the porphyrin core that it is deactivated before having the possibility to interact with a ground-state pyrene to give an excimer. Therefore, it can be deduced that the presence of an excimer emission band in the recorded spectra ( Figure 6) for the porphyrin-cored dendritic structures is due to the small amount of impurities of pyrene derivatives.

Dendritic Molecules Bearing Peripheral Pyrene Groups as Donors and a Fullerene as Acceptor
Recently, interest for materials based on fullerene C 60 derivatives has increased since this type of materials has promising properties for the development of new materials for technological applications. Particularly, performances of fullerene-based compounds have been reported for solar energy conversion [68,69]. After this discovery [70], many strategies to functionalize the fullerene C 60 were proposed [71][72][73][74][75] in order to generate new kinds of materials. The reason for this interest is due to its chemical and physical proprieties, such as for example its absorption in the UV region of the electromagnetic spectra [76]. The main drawback of the fullerene C 60 is that it is poorly soluble in organic solvents [77,78]. Fullerene C 60 can be synthetically modified in order to increase its solubility without affecting its photophysical and electrochemical properties [76,79]. Fullerene C 60 derivatives possess a large variety of applications that are ranging from chemosensors [80][81][82][83] to biological probes [84][85][86] and photocatalyst [87][88][89][90]. A large number fullerene based dyads have been published, however, there are only a few reports about pyrene-fullerene C 60 derivatives [91][92][93][94].
We have reported a new series of pyrene-fullerene C 60 dyads, and we have studied their optical and photophysical properties (Figure 7) [48].
We have reported a new series of pyrene-fullerene C60 dyads, and we have studied their optical and photophysical properties (Figure 7) [48]. UV-VIS analysis for the pyrene-fullerene C60 dyads was performed in toluene ( Figure 8). It showed that the series exhibited the characteristic bands of the pyrene chromophore at 346 nm due to S0 → S2 transition. No remarkable change was observed for this transition in comparison with 1-pyrenebutanol, the compound used as model in this experiment. Moreover, for all the pyrene-fullerene C60 dyads the absorption band at 330 nm was more intense that the band observed in the spectra of 1-pyrenebutanol at the same wavelength and the absorption was extended in the visible region. Those two features were attributed to the fullerene C60 cage. In the case of PyFPy, a strong band at 330 nm followed by another intense band at 346 nm were observed. The first one was attributed to the C60 cage and the second one to the pyrene absorption. Contrarily, in the case of PyFC12 and Py2FC12 the pyrene bands are more resolved due to the higher concentration of pyrene in the molecules. UV-VIS analysis for the pyrene-fullerene C 60 dyads was performed in toluene ( Figure 8). It showed that the series exhibited the characteristic bands of the pyrene chromophore at 346 nm due to S 0 → S 2 transition. No remarkable change was observed for this transition in comparison with 1-pyrenebutanol, the compound used as model in this experiment. Moreover, for all the pyrene-fullerene C 60 dyads the absorption band at 330 nm was more intense that the band observed in the spectra of 1-pyrenebutanol at the same wavelength and the absorption was extended in the visible region. Those two features were attributed to the fullerene C 60 cage. In the case of PyFPy, a strong band at 330 nm followed by another intense band at 346 nm were observed. The first one was attributed to the C 60 cage and the second one to the pyrene absorption. Contrarily, in the case of PyFC 12 and Py 2 FC 12 the pyrene bands are more resolved due to the higher concentration of pyrene in the molecules. The fluorescence spectra of the compounds show the typical emission of pyrene monomer at 376 nm as illustrated in Figure 9. Only compounds PyFPy, Py2FC12 and Py2NF are able to form excimers due to the presence of two pyrene units in the structures. In the case of PyFPy, the excimer formation is blocked by the presence of the C60 cage that obstruct rapprochement of both pyrene units and in the spectra, there are no signal for the excimer emission (Figure 9). On the other hand, compounds Py2FC12 and Py2NF present the two pyrene units that are free of the fullerene steric influence and, therefore, the excimer formation is possible. This behavior is supported by the presence of the excimer band at 470 nm (Figure 9). The relative quantum yield was calculated using 1-pyrenebutanol as standard. The obtained values are shown in Table 4. The emission of the malonate precursors that are not containing a The fluorescence spectra of the compounds show the typical emission of pyrene monomer at 376 nm as illustrated in Figure 9. Only compounds PyFPy, Py 2 FC 12 and Py2NF are able to form excimers due to the presence of two pyrene units in the structures. In the case of PyFPy, the excimer formation is blocked by the presence of the C 60 cage that obstruct rapprochement of both pyrene units and in the spectra, there are no signal for the excimer emission (Figure 9). On the other hand, compounds Py 2 FC 12 and Py 2 NF present the two pyrene units that are free of the fullerene steric influence and, therefore, the excimer formation is possible. This behavior is supported by the presence of the excimer band at 470 nm ( Figure 9). The fluorescence spectra of the compounds show the typical emission of pyrene monomer at 376 nm as illustrated in Figure 9. Only compounds PyFPy, Py2FC12 and Py2NF are able to form excimers due to the presence of two pyrene units in the structures. In the case of PyFPy, the excimer formation is blocked by the presence of the C60 cage that obstruct rapprochement of both pyrene units and in the spectra, there are no signal for the excimer emission (Figure 9). On the other hand, compounds Py2FC12 and Py2NF present the two pyrene units that are free of the fullerene steric influence and, therefore, the excimer formation is possible. This behavior is supported by the presence of the excimer band at 470 nm ( Figure 9). The relative quantum yield was calculated using 1-pyrenebutanol as standard. The obtained values are shown in Table 4. The emission of the malonate precursors that are not containing a The relative quantum yield was calculated using 1-pyrenebutanol as standard. The obtained values are shown in Table 4. The emission of the malonate precursors that are not containing a fullerene unit was much higher than the emission of the pyrene-fullerene constructs. Low quantum yield values obtained for the pyrene-fullerene C 60 constructs and the corresponding values of quenching corroborated that efficient FRET between pyrene and fullerene C 60 chromophores is taking place [48]. Table 4. Relative quantum yield and FRET efficiencies of the pyrene-fullerene C 60 compounds [48].

Compound
Relative Quantum Yield a % Quenching

Dendritic Molecules Bearing Peripheral Pyrene Groups as Donors and an Organometallic Complex as Acceptor
It is of interest to include metal ions in the structure of dendrimers in order to obtain complex molecules combining the properties of both entities; the structural ordered properties from the dendritic shell and the redox properties from metal ions [95,96]. In the literature, many examples of metal ions incorporated in dendrimers have been reported with different applications, such as drug carrier, catalysts, and enzyme mimics [97,98]. In some cases, the resulting organometallic complexes are able to accept energy transfer from a donor moiety [99][100][101].
The specific case of the incorporation of ruthenium bipyridine complexes into photoactive dendrimer has been studied [102][103][104][105]. Ruthenium bypiridine complexes have attracted the attention of many researcher due to chemical stability, redox properties, as well as fluorescence emission and excited state lifetime [106]. The complexes of ruthenium with bipyridine ligands are presenting typical UV-VIS spectra. The first band observed at about 280 nm corresponds to the ligand centred (LC) transition and the second band observed in a range from 450 to 480 nm corresponds to the metal to ligand charge transfer (MLCT) band. The long-lived emission occurs from the 3 MLCT state with low quantum yield [107,108]. Ruthenium bipyridine complexes are able to accept energy transfer from fluorescent donors through FRET [109]. Moreover, one application of this type of compound include the field of dye-sensitized solar cells [110,111].
Coordination complexes of ruthenium incorporating pyrene units have been reported [112]. Moreover, ruthenium bipyridine complex covalently linked to pyrene units have shown prolonged emission lifetime due to the fact that the energy levels of the 3 MLCT state of the ruthenium complex and of the pyrene triplet state can be tuned to almost match each other. Therefore, we used our pyrene-based dendrimer to design new organometallic complexes of ruthenium. The benzylic core of our dendrimer was modified to a bipyridine core in order to obtain a suitable ligand to form ruthenium complexes [49].
We designed and synthesized new organometallic complexes bearing 2, 6, and 12 pyrene units in the periphery and a bipyridine core ([Ru(Bpy) 2 (Bpy-Py2)] 2+ , [Ru(Bpy-Py2) 3 ] 2+ and [Ru(BpyG1-Py4) 3 ] 2+ ) (Figures 10 and 11). The obtained compounds showed absorption spectra that are reflecting the absorption properties of a pyrene moiety and of the corresponding bipyridine complex measured separately ( Figure 12). This is an indication that, when the compounds are present in the ground state, both chromophores are not interacting. The absorption band of pyrene is observed at 344 nm for the S 0 → S 2 transition for the three complexes. The absorption bands of the ruthenium bipyridine complexes depend on the substitution of the bipyridine ligand.    complexes. The absorption band of pyrene presents a high extinction coefficient and increases proportionally with the increasing number of pyrene units. The extinction coefficient of the MLCT transition band presents values of about 12,000 M −1 cm −1 which is comparable to the reported values for MLCT transitions of ruthenium bipyridine complexes. Figure 10. Structures of the ruthenium bipyridine complexes containing two and six pyrene units [49]. Figure 11. Structure of the dendritic ruthenium bipyridine complex possessing 12 pyrene units [49]. Figure 11. Structure of the dendritic ruthenium bipyridine complex possessing 12 pyrene units [49]. The obtained compounds present very efficient energy transfer from the pyrene moiety to the metal complex core. When they are excited at the maximum of absorption of the pyrene unit at 344 nm, weak residual emission from the pyrene units is observed but clearly quenched compared to the free bipyridine ligands. Only 2% of residual emission of pyrene was observed for [Ru(Bpy)2(Bpy-Py2)] 2+ , 1% for [Ru(Bpy-Py2)3] 2+ , and 4% for [Ru(BpyG1-Py4)3] 2+ indicating an efficient energy transfer in all cases. For the three compounds, emission is observed in the red part of the spectrum between 600 and 700 nm corresponding to the typical emission of ruthenium bipyridine complexes ( Figure 13). As for the porphyrin dendrimers described in the previous section, the quenching of the fluorescence from the pyrene donor is a good indication of efficient FRET process. It is important to note that when excited at 344 nm the percentage of light absorbed by the pyrene units increases with the increasing amount of pyrene units from 91% for [Ru(Bpy)2(Bpy-Py2)] 2+ to 97 and 99%, respectively, for compounds [Ru(Bpy-Py2)3] 2+ and [Ru(BpyG1-Py4)3] 2+ (Table 5).  The obtained compounds present very efficient energy transfer from the pyrene moiety to the metal complex core. When they are excited at the maximum of absorption of the pyrene unit at 344 nm, weak residual emission from the pyrene units is observed but clearly quenched compared to the free bipyridine ligands. Only 2% of residual emission of pyrene was observed for [Ru(Bpy) 2 (Bpy-Py2)] 2+ , 1% for [Ru(Bpy-Py2) 3 ] 2+ , and 4% for [Ru(BpyG1-Py4) 3 ] 2+ indicating an efficient energy transfer in all cases. For the three compounds, emission is observed in the red part of the spectrum between 600 and 700 nm corresponding to the typical emission of ruthenium bipyridine complexes ( Figure 13). As for the porphyrin dendrimers described in the previous section, the quenching of the fluorescence from the pyrene donor is a good indication of efficient FRET process. It is important to note that when excited at 344 nm the percentage of light absorbed by the pyrene units increases with the increasing amount of pyrene units from 91% for [Ru(Bpy) 2 (Bpy-Py2)] 2+ to 97 and 99%, respectively, for compounds [Ru(Bpy-Py2) 3 ] 2+ and [Ru(BpyG1-Py4) 3 ] 2+ (Table 5). The obtained compounds present very efficient energy transfer from the pyrene moiety to the metal complex core. When they are excited at the maximum of absorption of the pyrene unit at 344 nm, weak residual emission from the pyrene units is observed but clearly quenched compared to the free bipyridine ligands. Only 2% of residual emission of pyrene was observed for [Ru(Bpy)2(Bpy-Py2)] 2+ , 1% for [Ru(Bpy-Py2)3] 2+ , and 4% for [Ru(BpyG1-Py4)3] 2+ indicating an efficient energy transfer in all cases. For the three compounds, emission is observed in the red part of the spectrum between 600 and 700 nm corresponding to the typical emission of ruthenium bipyridine complexes ( Figure 13). As for the porphyrin dendrimers described in the previous section, the quenching of the fluorescence from the pyrene donor is a good indication of efficient FRET process. It is important to note that when excited at 344 nm the percentage of light absorbed by the pyrene units increases with the increasing amount of pyrene units from 91% for [Ru(Bpy)2(Bpy-Py2)] 2+ to 97 and 99%, respectively, for compounds [Ru(Bpy-Py2)3] 2+ and [Ru(BpyG1-Py4)3] 2+ (Table 5).    3 ] 2+ , respectively. The same measurements were performed without degasing the solutions and the following quantum yields were obtained: 0.0032, 0.0016, and 0.0025 (Table 5).
The difference between the quantum yield in degased solution and in air-equilibrated solutions is much lower for the complex bearing three times the first generation dendron (12 pyrene units) indicating that the dendrimer branches are functioning as a protection toward oxygen quenching. For the pyrene/ruthenium bipyridine complex pair of chromophores, we have calculated that the Förster radius is in the order of 3.30 nm. As previously calculated for the pyrene-porphyrin couple the expected distance between the chromophore is within the Förster distance, confirming an efficient FRET process.

Dendritic Molecules Bearing Peripheral Pyrene Groups as Donors and Cyclen Core as Potential Ligands for Metal Ions
As mentioned above, the incorporation of different metals in photoactive dendrimers have been performed through the inclusion of porphyrin or bipyridine ligands in the structures and this has shown potential applications in sensors, catalysis, or nanomaterials for drug delivery [95,113]. Another efficient family of ligands for metal ions is the family of aza-macrocycles. Efficient chelating effects have been reported from this kind of ligand towards metal ions [114][115][116][117][118][119]. 1,4,7,10-teraazacyclododecane (cyclen) is part of this family and has exhibited a good coordination with a wide variety of metals. Moreover it has been included in a wide range of constructs for diverse applications, such as ion sensing, ion carrier, and metal diagnosis [120][121][122][123][124]. The coordination of cyclen construct containing various chromophores with lanthanides has also been reported, leading to sensitized emission from the photoactive metal ions [125,126].
Only a few reports of FRET from pyrene to lanthanide photoactive ions have been reported. Using the cyclen as core of our pyrene-based dendrimer, we could obtain a construct that is able to complex lanthanide ions and, consequently, a FRET process could be observed from pyrene to the photoactive metal ion [50].
The series of dendrimers of generation zero (1), one (2) and two (3) bearing 4, 8, and 16 pyrene units, respectively, was synthesized (Figures 14 and 15). The obtained compounds are showing the typical absorption properties of the pyrene chromophore, increasing its extinction coefficient linearly with the amount of pyrene units in the construct. The emission properties of those compounds were also studied. The amount of excimer emission in those dendrimers is increasing as expected with the increasing number of pyrene units due to the increased local concentration of pyrene [50]. linearly with the amount of pyrene units in the construct. The emission properties of those compounds were also studied. The amount of excimer emission in those dendrimers is increasing as expected with the increasing number of pyrene units due to the increased local concentration of pyrene [50].  At the time of incorporating metal ions into those dendrimers, we faced some difficulties. The ligands appeared to be less effective as foreseen towards the complexation of lanthanide ions. This may be due to the fact that the ligand has only four nitrogen atoms for the coordination of metal ions and is, therefore, a teradentate ligand. Lanthanides are known to form hexa-to octa-coordinated complexes and our ligand may not be suitable for such metal ions. Therefore, new ligands including linearly with the amount of pyrene units in the construct. The emission properties of those compounds were also studied. The amount of excimer emission in those dendrimers is increasing as expected with the increasing number of pyrene units due to the increased local concentration of pyrene [50].  At the time of incorporating metal ions into those dendrimers, we faced some difficulties. The ligands appeared to be less effective as foreseen towards the complexation of lanthanide ions. This may be due to the fact that the ligand has only four nitrogen atoms for the coordination of metal ions and is, therefore, a teradentate ligand. Lanthanides are known to form hexa-to octa-coordinated complexes and our ligand may not be suitable for such metal ions. Therefore, new ligands including At the time of incorporating metal ions into those dendrimers, we faced some difficulties. The ligands appeared to be less effective as foreseen towards the complexation of lanthanide ions. This may be due to the fact that the ligand has only four nitrogen atoms for the coordination of metal ions and is, therefore, a teradentate ligand. Lanthanides are known to form hexa-to octa-coordinated complexes and our ligand may not be suitable for such metal ions. Therefore, new ligands including carbonyl group linked to the cyclen core (DOTA) were designed and synthesized (4 and 5) (Figure 16) [127]. carbonyl group linked to the cyclen core (DOTA) were designed and synthesized (4 and 5) ( Figure  16) [127]. The synthesis was performed and the optical properties of the obtained dendrimers were studied. The obtained photophysical characterization is very similar to that of dendrimers 1 and 2. Titration experiments, followed by fluorescence, were carried out with compounds 4 and 5 in order to evaluate their potential for FRET studies. Sm 3+ , Eu 3+ , Gd 3+ , Tb 3+ , Er 3+ , and Zn 2+ were tested and fluorescence quenching was observed for all metals except for Zn 2+ . Figure 17 illustrates the titration of compound 4 with Gd 3+ [127]. This result is a primary indication that the complexes are formed and that energy transfer could occur. Chelation experiments with lanthanide ions are currently being performed in our laboratory. The synthesis was performed and the optical properties of the obtained dendrimers were studied. The obtained photophysical characterization is very similar to that of dendrimers 1 and 2. Titration experiments, followed by fluorescence, were carried out with compounds 4 and 5 in order to evaluate their potential for FRET studies. Sm 3+ , Eu 3+ , Gd 3+ , Tb 3+ , Er 3+ , and Zn 2+ were tested and fluorescence quenching was observed for all metals except for Zn 2+ . Figure 17 illustrates the titration of compound 4 with Gd 3+ [127]. This result is a primary indication that the complexes are formed and that energy transfer could occur. Chelation experiments with lanthanide ions are currently being performed in our laboratory.

Conclusions
In our interest to study pyrene dynamics in macromolecules, we have designed dendritic molecules bearing an increasing number of pyrene peripheral groups and various acceptor moieties such as porphyrin, fullerene, or ruthenium bipyridine at the core. Those molecules were expected to function as light-harvesting antennas to funnel the energy absorbed from the pyrene groups to the acceptor moiety.
Thus, we reported that the FRET process occurs in a very efficient manner in all the designed constructs. When the pyrene moiety was excited at 344 nm, the energy was transferred to the ground

Conclusions
In our interest to study pyrene dynamics in macromolecules, we have designed dendritic molecules bearing an increasing number of pyrene peripheral groups and various acceptor moieties such as porphyrin, fullerene, or ruthenium bipyridine at the core. Those molecules were expected to function as light-harvesting antennas to funnel the energy absorbed from the pyrene groups to the acceptor moiety.
Thus, we reported that the FRET process occurs in a very efficient manner in all the designed constructs. When the pyrene moiety was excited at 344 nm, the energy was transferred to the ground state porphyrin, fullerene, or ruthenium bipyridine complex. The quenching of fluorescence of pyrene is the first indication to determine the efficiency of the FRET process. It was quantified and information about pyrene dynamics was obtained. As a result, we could determine through MF analysis that the excimer formation was much slower than the FRET phenomenon. A value of 11.2 × 10 7 s -1 was found for the average rate constant of intramolecular pyrene excimer formation <k E >, which is much slower than the average rate constant for FRET <k ET > (1.8 × 10 9 s -1 ). For this reason, we observed that the FRET process is more competitive than the excimer formation, it happens first, and the formation of excimer is precluded even in the case of higher-generation dendrons.
Dendritic structures with the donor-acceptor pair pyrene-porphyrn exhibited FRET values between 97-99% with a drastic quenching of the pyrene emission and increased emission of the porphyrinic moiety. In the case of the pyrene-fullerene C 60 constructs, FRET efficiency values were determined in a similar range, however, unlike for the case of porphyrins, we could not observe a final emission of the fullerene C 60 acceptor group in the UV-VIS range. On the other hand, ruthenium bipyridine complexes exhibited FRET values in the order between 96-98%, showing an emission of the acceptor group after FRET at 661 nm. Finally dendritic constructs bearing pyrene units as peripheral groups and a modified cyclen unit as the core also showed the FRET process after metallation with lanthanides, which was monitored by fluorescence titrations. The quenching of the emission of pyrene could be observed after the addition of the different lanthanide ions.