Long-lived photoinduced charge separation for solar cell applications in supramolecular complexes of multi-metalloporphyrins and fullerenes

Monomers, dimers, trimers, dendrimers and oligomers of metalloporphyrins form supramolecular complexes with fullerene derivatives via electrostatic interactions, π – π interactions and coordination bonds. Photoexcitation of the supramolecular complexes resulted in photoinduced electron transfer from the porphyrin moiety to the fullerene moiety to produce the charge-separated states as revealed by laser ﬂ ash photolysis measurements. The rate constants of photoinduced charge separation and charge recombination in supramolecular complexes of multi-metalloporphyrins and fullerenes were also determined by laser ﬂ ash photolysis measurements and the results depending on the number of porphyrins in the supramolecular complexes are discussed in terms of e ﬃ ciency of photoinduced energy transfer and charge separation as well as the lifetimes of charge-separated states. The photoelectrochemical performances of solar cells composed of supramolecular complexes of monomers, dimers, dendrimers and oligomers of metalloporphyrins with fullerenes are compared in relation to the rate constants of photoinduced charge separation and charge recombination.


Introduction
Photosynthesis is one of the most fundamental and indispensable processes in nature, because it converts light energy into chemical energy required to maintain life. 1,2Photosynthesis is initiated by the multistep electron-transfer reactions in the photosynthetic reaction centres following light energy harvesting by antenna chlorophylls, funnelled to a bacteriochlorophyll dimer, the so-called special pair, to attain the long-lived charge-separated (CS) state. 1,2The redox-active components such as chlorophyll, pheophytin and quinones are appropriately located in the protein matrix by non-covalent interactions. 1,2Extensive efforts have so far been devoted to the design of electron donor-acceptor composites using covalently and non-covalently linked systems to form the long-lived CS state upon photoexcitation for artificial photosynthesis.  Porrins, which are involved in a number of important biological electron-transfer systems including the primary photochemical reactions of chlorophylls ( porphyrin derivatives) in the photosynthetic reaction centres, are particularly attractive building blocks as electron acceptors as well as light-harvesting compounds for the construction of supramolecular electron donor-acceptor composites due to their excellent photophysical and electron-transfer properties.  Wit][32][33][34][35][36][37][38] Thus, combination of porphyrins and fullerenes is regarded as ideal donor-acceptor ensembles, because the combination results in a small reorganization energy, which allows to accelerate photoinduced electron transfer and to slow down charge recombination, leading to the generation of long-lived CS states with high quantum yields.  Howe][71][72][73] Muti-point binding sites can be introduced by using multimetalloporphyrins (dimers, trimers, dendrimers and oligomers), allowing strong binding between multi-metalloporphyrins and fullerenes in polar solvents. 29,30n this perspective, we review our recent development on photoinduced charge separation in supramolecular complexes of porphyrin anions and fullerene cations with electrostatic interactions and those composed of multi-metalloporphyrins and fullerenes, which are strongly bound in polar solvents, towards construction of supramolecular solar cells based on the long-lived photoinduced charge separation.
The transient absorption spectra taken by nanosecond laser flash photolysis shown in Fig. 2a demonstrate the formation of [ZnTPPS 4− ] •+ (λ max = 670 nm) and that of Li + @C 60 radical anion (λ max = 1035 nm). 78Thus, the electron transfer from ZnTPPS 4− to 3 [Li + @C 60 ]* or from 3 [ZnTPPS 4− ]* to Li + @C 60 occurs in the supramolecular complex to produce the triplet charge-separated (CS) state.The lifetime of the triplet CS state of the supramolecular complex was determined to be 300 μs for ZnTPPS 4− from the first-order decay of the CS state (Fig. 2b). 78It was confirmed that back electron transfer occurred in the supramolecular complex, because the firstorder decay rate constant remains the same irrespective of the difference in the laser intensity (inset of Fig. 2b). 78Similarly the CS lifetime of 310 μs was determined for [(H 2 TPPS 4− ) •+ -Li + @C 60 •− ]. 78 This is the longest lifetime of the CS state ever reported for monomer porphyrin/fullerene systems linked non-covalently in solution.The quantum yield of the CS state is determined to be 0.39 using the absorption of the CS state (Li + @C 60 •− : ε 1035 = 7300 M −1 cm −1 ). 78e activation enthalpies of the charge-recombination (CR) processes were determined to be 3.0 kcal mol −1 for ZnTPPS 4− -Li + @C 60 and 5.4 kcal mol −1 for H 2 TPPS 4− -Li + @C 60 . 78This indicates that there is a significant energy difference between the singlet and triplet CS states and that the CR processes may occur through the thermally activated singlet CS state.The lifetime of the CS state at 77 K is estimated as long as 60 h for H 2 TPPS 4− -Li + @C 60 . 78Such a long-lived triplet CS state was detected by the EPR measurements by photoirradiation of the H 2 TPPS 4− -Li + @C 60 complex in frozen PhCN as shown in Fig. 3.The spin-spin interaction in the triplet radical ion pair of the supramolecular complex is clearly shown at 77 K, where the fine structure due to the triplet CS state is clearly observed at g = 2. From the zero-field splitting values (D = 52 G for ZnTPPS 4− and 56 G for H 2 TPPS 4− ) the distances (r) between two electron spins were estimated using the relation, D = 27 800/r 3 , 79,80 to be 8.1 and 7.9 Å, respectively. 78These r values agree with the centre-to-centre distance of a reported crystal structure of porphyrin/C 60 .
By mixing PhCN solutions of the supramolecular complexes of MTPPS 4− and Li + @C 60 with acetonitrile (MeCN), nanoclusters were produced and they were deposited on an optically transparent electrode (OTE) of nanostructured SnO 2 (OTE/SnO 2 ) by application of a dc electric field (∼100 V cm −1 ) to construct photovoltaic cells. 81The (MTPPS 4− /Li + @C 60 ) n films are composed of closely packed Li + @C 60 clusters of about 80 nm size, which render a nanoporous morphology to the film as shown in the TEM images in Fig. 4. 81 The photoelectrochemical measurements of a robust thin film of OTE/SnO 2 /(MTPPS 4− /Li + @C 60 ) n were performed using a standard two-electrode system consisting of a working electrode and a Pt wire gauze electrode in air-saturated MeCN containing 0.5 M LiI and 0.01 M I 2 (Fig. 5). 77[84][85] IPCE ð%Þ ¼ 100 Â 1240 Â i sc =ðI inc Â λÞ ð 1Þ   where i sc is the short circuit photocurrent (A cm −2 ), I inc is the incident light intensity (W cm −2 ) and λ is the wavelength (nm).
The IPCE value of OTE/SnO 2 /(ZnTPPS 4− /Li + @C 60 ) n is much higher than the sum of the two individual IPCE values of the individual systems OTE/SnO 2 /(ZnTPPS 4− ) n and OTE/SnO 2 / (Li + @C 60 ) n in the visible region (Fig. 6).The maximum IPCE value of OTE/SnO 2 /(ZnTPPS 4− /Li + @C 60 ) n was 77% at 450 nm.Such a high IPCE value indicates that photocurrent generation is initiated via photoinduced electron transfer from ZnTPPS 4− to Li + @C 60 , followed by the charge transport to the collective surface of an OTE/SnO 2 electrode (Fig. 5).When ZnTPPS 4− was replaced by H 2 TPPS 4− , a significantly low IPCE value was observed as 7% at 440 nm probably because of the self-aggregation of H 2 TPPS 4− without binding with Li + @C 60 . 81he power conversion efficiency (η) of the OTE/SnO 2 / (ZnTPPS 4− /Li + @C 60 ) n electrode was calculated by using eqn (2): [82][83][84][85] in which the fill factor (FF) is defined as FF = [IV] max /I sc V oc and V oc is the open-circuit photovoltage and I sc is the short-circuit photocurrent.The OTE/SnO 2 /(ZnTPPS 4− /Li + @C 60 ) n electrode has an overall power conversion efficiency (η) of 2.1% at an input power (W in ) of 28 mW cm −2 , whereas FF = 0.37, V oc = 460 mV and I sc = 3.4 mA cm −2 in the OTE/SnO 2 /(ZnTPPS 4− / Li + @C 60 ) n .The η value is two orders of magnitude greater than that of the previously reported simple porphyrin and C 60 composite system (∼0.03%). 83Such a significant enhancement of the η value indicates that the strong ordering in the clusters and the efficient charge separation in (ZnTPPS 4− /Li + @C 60 ) n improved the light energy conversion properties.

Dalton Transactions Perspective
4][105] The observed value along the b axis of the single crystal of C 60 ⊂ Ni 2 -CPD Py is comparable to that of the single crystal of C 60 (∑μ = 0.50 cm 2 V −1 s −1 measured by TOF). 106The observed high electron mobility along the b axis is due to the well-ordered linear arrangement of C 60 in the porphyrin nanotube.However, the expected charge-separated state could not be observed in the time-resolved transient absorption spectra of C 60 ⊂ Ni 2 -CPD Py because the singlet excited state of the nickel porphyrin immediately changes to the triplet excited state by intersystem crossing and the low energy triplet excited state of C 60 ( 3 C 60 *) is formed by energy transfer. 97The estimated energy level of the charge-separated state (1.98 eV) is higher than that of 3 C 60 * (1.60 eV). 97When Ni 2 -CPD Py was replaced by a free base porphyrin dimer (H4-CPDPy), a complete charge-separated state {H 4 -CPDPy •+ + C 60 •− )} was observed by femtosecond laser flash photolysis of C 60 ⊂ H 4 -CPD Py in the solid state with a lifetime of 470 ps. 107he photovoltaic activity of C 60 ⊂ Ni 2 -CPD Py and C 60 ⊂ H 4 -CPD Py was evaluated by using solar cells composed of modified electrodes and I − /I 3 − solution. 107The C 60 ⊂ H 4 -CPD Pymodified electrode exhibited IPCE of 17% and a power conversion efficiency (η) of 0.33%, which was more than 16 times larger than that of OTE/SnO 2 /(C 60 ⊂ Ni 2 -CPD Py ) n (0.02%). 107uch a significant enhancement of the η value demonstrates that the formation of highly ordered clusters and the efficient charge separation of (C 60 ⊂ H 4 -CPD Py ) n contributes to the improvement of the light energy conversion properties. 107hen C 60 is replaced by Li + @C 60 , porphyrin dimers with four long alkoxy substituents on the meso-phenyl groups (MCPD Py (OC 6 ) in Fig. 9) form strong supramolecular complexes in even a polar solvent such as PhCN. 108The association constants (K assoc ) of Li + @C 60 ⊂ MCPD Py (OC 6 ) in PhCN at 298 K were determined to be 2.6 × 10 5 M −1 for Li + @C 60 ⊂ H 4 -CPD Py (OC 6 ) and 3.5 × 10 5 M −1 for Li + @C 60 ⊂ Ni 2 -CPD Py (OC 6 ). 108pon laser excitation of Li + @C 60 ⊂ Ni 2 -CPD Py (OC 6 ), transient absorption bands due to Ni 2 -CPD Py (OC 6 ) •+ and Li + @C 60 were observed as shown in Fig. 10a. 108In this case, electron transfer occurs from Ni 2 -CPD Py (OC 6 ) to the triplet excited state of Li + @C 60 ( 3 Li + @C 60 *) rather than from 3 [Ni 2 -CPD Py (OC 6 )]* to Li + @C 60 as indicated by the disappearance of the absorption band at 750 nm due to 3 Li + @C 60 *, accompanied by the appearance of the absorption band at 1035 nm due to Li + @C 60 •− (Fig. 10b). 108The rate constant of electron transfer from Ni 2 -CPD Py (OC 6 ) to 3 Li + @C 60 * to produce the CS state was determined from the rise in the absorbance at 1035 nm due to Li + @C 60 •− to be 5.7 × 10 7 s −1 . 108The absorbance at 1035 nm due to Li + @C 60 •− in the CS state decayed obeying first-order kinetics with the same slope irrespective of the difference in the laser intensity (Fig. 10c). 108This clearly indicates that the decay of the CS state occurs via intrasupramolecular back electron transfer rather than a bimolecular back electron-transfer reaction between the CS states.The CS lifetime was determined from the slope of the first-order plots in Fig. 10c to be 0.67 ms, which is the longest value ever reported for noncovalent monomer dimer porphyrin-fullerene supramolecules in solution. 108The CS state was also observed for Li + @C 60 ⊂ H 4 -CPD Py (OC 6 ).The quantum yields of the CS states were estimated to be 0.13 for Li + @C 60 ⊂ Ni 2 -CPD Py (OC 6 ) and 0.32 for Li + @C 60 ⊂ H 4 -CPD Py (OC 6 ) and by means of the comparative method with the absorption intensities of the CS states (Li + @C 60 •− : ε(1035 nm) = 7300 M −1 cm −1 ). 108When Li + @C 60 was replaced by pristine C 60 , no CS states were produced as predicted by their higher energy levels than those of the triplet excited states of CPD Py (OC 6 ) and C 60 . 108he mechanisms of intrasupramolecular photoinduced charge separation in Li + @C 60 ⊂ Ni 2 -CPD Py (OC 6 ) are shown in Scheme 2. 108 The singlet excited state of Ni 2 -CPD Py (OC 6 ) ( 1 [Ni 2 -CPD Py (OC 6 )]*) is generated upon photoexcitation of Li + @C 60 ⊂ Ni 2 -CPD Py (OC 6 ) at 420 nm, where the porphyrin moiety is exclusively excited.Even if the Li + @C 60 moiety is excited, energy transfer from 1 [Li + @C 60 ]* (E s = 1.94 eV) 77 to Ni 2 -CPD Py (OC 6 ) (E s = 1.97 eV) occurs to produce  108 Then, electron transfer occurs from 3 [Ni 2 -CPD Py (OC 6 )]* to Li + @C 60 with the driving force of 0.30 eV to produce the CS state.The CS state decays slowly via intrasupramolecular BET with the lifetime of 0.67 ms (Scheme 2).108

Supramolecular complex of a porphyrin tripod with C 60
1][112][113] The association constant of TPZn 3 with PyC 60 (1.1 × 10 5 M −1 in toluene) determined from the UV-vis absorption spectral titration (Fig. 12a) is much larger as compared with those of the corresponding monomer (MPZn) and dimer porphyrin (DPZn 2 ). 109The 1 H NMR signals of TPZn 3 exhibit downfield shifts upon complexation with PyC 60 , whereas the pyridyl protons of PyC 60 exhibit large upfield shifts by the complexation, which is ascribed to the influence of the large porphyrin aromatic ring current. 113This result clearly shows that the Scheme 3 Formation of a supramolecular complex between TPZn 3 and PyC 60 .

Dalton Transactions Perspective
pyridyl group of PyC 60 coordinates to the central zinc ions of TPZn 3 .The encapsulation of PyC 60 into the cavity of TPZn 3 was supported by the DFT-optimized structure (B3LYP/3-21G(*) basis set) in Fig. 12b. 113he occurrence of photoinduced electron transfer from 1 TPZn 3 * to PyC 60 was confirmed by femtosecond laser flash photolysis measurements in Fig. 13a, where the transient absorption spectrum due to 1 TPZn 3 * changes as time elapses to afford the absorption bands at λ max = 1000 nm due to the monofunctionalized fullerene radical anion 114,115 and at 670 nm due to the one-electron oxidized species of TPZn 3 (TPZn 3 •+ ). 113,116,117 sharp contrast to the TPZn 3 -PyC 60 complex, the transient absorption spectrum of the monomer porphyrin (MPZn) in the presence of PyC 60 (Fig. 13b) exhibits the absorbance change due to the energy transfer from 1 MPZn* to PyC 60 to give the singlet excited state 1 PyC 60 * (1.76 eV), followed by the conversion to the triplet excited states 3 MPZn* and 3 PyC 60 * at 2800 ps (green line in Fig. 13b), accompanied by the recovery of the ground state. 113he energy diagrams of photodynamics for TPZn 1.9 × 10 9 s −1 ), the lifetime of the CS state is determined to be τ CS = 0.53 ns.In contrast, only energy transfer from 1 MPZn* to PyC 60 occurs to produce 1 PyC 60 *, in competition with intersystem crossing to 3 MPZn*. 113PZn 3 also forms a stable 1 : 1 complex with gold(III) tetra(4-pyridyl)porphyrin (AuTPyP + ) in nonpolar solvents. 118he strong binding of TPZn 3 with AuTPyP + results from the encapsulation of AuTPyP + inside the cavity of TPZn 3 through multiple coordination bonds.The efficient quenching of the singlet excited state of TPZn 3 occurs via a photoinduced electron-transfer pathway in the TPZn 3 -AuTPyP + complex as the case of TPZn 3 -PyC 60 complex. 118

Supramolecular complexes of porphyrin oligopeptides and C 60
Multiple photosynthetic reaction centres composed of lightharvesting multiporphyrin units and charge-separation units were obtained by using both the coordination bond and π-π interaction.Zinc porphyrinic oligopeptides with various numbers of porphyrin units [P(ZnP) n ; n = 2, 4, 8] 119,120 were used as light-harvesting multiporphyrin units (Fig. 14), which are bound to electron acceptors of fulleropyrrolidine bearing a pyridine (PyC 60 ) 113 or imidazole coordinating ligand (ImC 60 ) 82 as shown in Fig. 15. 121he binding constant (K) of PyC 60 to P(ZnP) n increased with increasing number of zinc porphyrins in an oligopeptide unit. 121No supramolecular complex formation was observed in the case of zinc tetraphenylporphyrin in PhCN. 121The strong binding between P(ZnP) 8 and PyC 60 results from the strong π-π interactions between two zinc porphyrins and PyC 60 in addition to the axial coordination of PyC 60 to zinc ions of the porphyrins.In the case of ImC 60 , however, the highest K value was obtained in the P(ZnP) 4 -ImC 60 complex.This indicates that ImC 60 is much more strongly bound by the oligopeptide, P(ZnP) 4 , than PyC 60 . 121The apparent binding constants (K) determined from the fluorescence quenching of P(ZnP) n were significantly larger than those determined from the UV-vis spectral change, and the difference in the values increased with increasing the generation of porphyrinic oligopeptides (with increasing the number of the porphyrin units). 121This indicates that the excited energy migration between the porphyrin units occurs efficiently prior to the electron transfer to the bound C 60 moiety.An extremely efficient energy transfer also occurs in P(ZnP The occurrence of photoinduced electron transfer in the supramolecular complex in PhCN was confirmed by the transient absorption spectra of the supramolecular complex of P(ZnP) 8 with PyC 60 using nanosecond laser flash photolysis. 121he laser photoexcitation at 561 nm of the supramolecular complex of P(ZnP) 8 with PyC 60 results in formation of the CS state as indicated by the transient absorption spectra in Fig. 16a, where the absorption band due to PyC 60 •− is clearly observed at 1000 nm together with that due to ZnP •+ at 630 nm. 121The CS state detected decays obeying first-order kinetics (Fig. 16b) and the first-order plots at different initial CS concentrations afford linear correlations with the same slope (inset of Fig. 16b). 121If there is any contribution of intermolecular back electron transfer from unbound PyC 60 •− to ZnP •+ , the second-order kinetics would be involved for the decay time profile.In fact, the corresponding second-order plots (Fig. 16c) are clearly non-linear and the initial slope varies depending on the CS concentration.Thus, the decay process is ascribed to back electron transfer in the supramolecular complex rather than intermolecular back electron transfer between ZnP •+ and PyC 60 •− . 121The CS lifetimes of the supramolecular complexes of other porphyrins [P(ZnP) n ] and the fullerene derivative (ImC 60 ) become longer with increasing generation of porphyrinic oligopeptides (with increasing the number of the porphyrin units). 121Such elongation of the CS

Dalton Transactions Perspective
lifetimes results from efficient hole migration between the porphyrin units following the photoinduced electron transfer in the supramolecular complexes.
Multiple photosynthetic reaction centres have also been constructed using supramolecular complexes of zinc porphyrin dendrimers [D(ZnP) n : n = 4, 8, 16] with PyC 60 . 122Efficient energy migration occurs more efficiently between the ZnP units of dendrimers prior to the photoinduced electron transfer with increasing the generation of dendrimers to attain an extremely long CS lifetime e.g., 0.25 ms for the D(ZnP) 16 -PyC 60 complex in PhCN at 298 K. 122 Multiple photosynthetic reaction centres composed of supramolecular complexes of harvesting multiporphyrin units and charge-separation units have enabled us to construct supramolecular organic solar cells by the electrodeposition of mixed porphyrin-peptide oligomers [P(H 2 P) n or P(ZnP) n ] and  123 The IPCE value increased with increasing the number of porphyrins in a polypeptide unit in both (P(H 2 P) n + C 60 ) m and (P(ZnP) n + C 60 ) m (n = 1, 2, 4, 8, 16) systems as shown in Fig. 17.Such a good photoelectrochemical performance results from efficient photoinduced electron-transfer from the excited state of the porphyrin unit to C 60 in the supramolecular complex with longer CS lifetimes as the number of porphyrins in a polypeptide unit increases (vide supra).The maximum IPCE value of (P(ZnP) 16 + C 60 ) m (56%) is larger than that of (P(H 2 P) 16 + C 60 ) m (48%) probably because of the larger driving force of the photoinduced electron transfer.
The maximum IPCE values of (P(ZnP) 16 + PyC 60 ) m (20%) and (P(ZnP) 16 + ImC 60 ) m (15%) are much smaller than that of (P(ZnP) 16 + C 60 ) m (56%), whereas the binding constant of P(ZnP) 16 -C 60 is smaller than those of P(ZnP) 16 -ImC 60 and P(ZnP) 16 -PyC 60 . 123The lower IPCE values of P(ZnP) 16 -ImC 60 and P(ZnP) 16 -PyC 60 systems as compared with that of P(ZnP) 16 -C 60 system may result from the poor electron-transport properties of C 60 derivatives due to the steric hindrance of the ligand moiety. 123Thus, a key element for efficient photocurrent generation is mainly the hole and electron transport in   the thin film rather than the charge separation between porphyrins and C 60 . 123/V characteristics of (a) (P(H 2 P) 16 + C 60 ) m , (b) (P(H 2 P) 8 + C 60 ) m and (c) (P(H 2 P) 1 + C 60 ) m modified electrodes under visible light irradiation (λ > 400 nm) are shown in Fig. 18.The (P(H 2 P) 16 + C 60 ) m system has a larger fill factor (FF) of 0.47, an open circuit voltage (V oc ) of 320 mV, a short circuit current density (I sc ) of 0.36 mA cm −2 , and the overall power conversion efficiency (η) of 1.6% at input power (W in ) of 3.4 mW cm −2 . 123he η values of the (P(H 2 P) 16 + C 60 ) m system was remarkably enhanced (around 40 times) in comparison with the (P(H 2 P) 1 + C 60 ) m modified electrode (η = 0.043%) under the same experimental conditions.The η value of (P(ZnP) 16 + C 60 ) m is also determined as 1.4% and this value is much larger than that of (P(ZnP) 1 + C 60 ) m (0.047%) as shown in Fig. 18B. 123

Conclusions
As described above, porphyrin monomers, dimers, trimers and oligomers form supramolecular complexes with fullerene derivatives via electrostatic interactions, π-π interactions and coordination bonds.Photoexcitation of the supramolecular complexes resulted in efficient photoinduced electron transfer from the porphyrin moiety to the fullerene moiety to produce the long-lived CS states as revealed by laser flash photolysis measurements.In particular, a supramolecular complex of a cyclic Ni porphyrin dimer with Li + @C 60 [Li + @C 60 ⊂ Ni 2 -CPD Py (OC 6 )] affords a long-lived triplet CS state with 0.63 ms lifetime.A high IPCE value (77% at 450 nm) was achieved for a supramolecular solar cell using the OTE/SnO 2 /(ZnTPPS 4− / Li + @C 60 ) n electrode.The use of porphyrin oligomer peptides has also enabled to construct multiple photosynthetic reaction centres composed of light-harvesting multiporphyrin units and charge-separation units linked by both the coordination bond and π-π interactions, which afforded long-lived CS states.Supramolecular organic solar cells composed of porphyrinic oligopeptides and C 60 exhibited higher overall power conversion efficiency with increasing the number of porphyrin units.Supramolecular complexes formed between porphyrins and fullerenes in particular Li + @C 60 provide promising materials for more efficient solar energy conversion.
3 and MPZn in the presence of PyC 60 in toluene are shown in Scheme 4a and 4b, respectively. 113The energy level (1.49 eV) of the CS state (TPZn 3 •+ −PyC 60 •− ) is lower than the energy level of the triplet excited state of PyC 60 moieties (1.56 eV).The rate constant (k ET ) of photoinduce electron transfer from 1 TPZn 3 * to PyC 60 is larger than the rate constant of intersystem crossing.From the rate constant of back electron transfer (k BET =

) 8 -
ImC 60 judging from the large difference in the K values determined by the absorption change and by the fluorescence quenching (1.5 × 10 4 vs. 3.3 × 10 5 M −1 ).