Two is Better Than One? Investigating the Effect of Incorporating Re(CO)3Cl Side- Chains into Pt(II) Di-ynes and Poly-ynes

Pt(II) di-ynes and poly-ynes incorporating 5,5’and 6,6’-disubstituted 2,2’-bipyridines were prepared following conventional Sonogashira and Hagihara dehydrohalogenation reaction protocols. Using Pt(II) dimers and polymers as a rigid-rod backbone, four new hetero-bimetallic compounds incorporating Re(CO)3Cl as a pendant functionality in the 2,2’-bipyridine core were obtained. The new hetero-bimetallic Pt-Re compounds were characterized by analytical and spectroscopic techniques. The solid state structures of a Re(I)-coordinated diterminal alkynyl ligand and a representative model compound were determined by single-crystal X-ray diffraction. Detailed photo-physical characterization of the heterobimetallic Pt(II) di-ynes and poly-ynes was carried out. We find that the incorporation of the Re(CO)3Cl pendant functionality in the 2,2’-bipyridine-containing main-chain Pt(II) di-ynes and poly-ynes has a synergistic effect on the optical properties, red shifting the absorption profile and introducing strong longwavelength absorptions. The Re(I) moiety also introduces strong emission into the monomeric Pt(II) di-yne compounds, whereas this is suppressed in the poly-ynes. The extent of the synergy depends on the topology of the ligands. Computational modelling was performed to compare the energetic stabilities of the positional isomers and to understand the microscopic nature of the major optical transitions. We find that 5,5’-disubstituted 2,2’-bipyridine systems are better candidates in terms of yield, photophysical properties and stability than their 6,6’-substituted counterparts. Overall, this work provides an additional synthetic route to control the photo-physical properties of metalla-ynes for a variety of optoelectronic applications.


Introduction
5 nonetheless points to a high degree of polymerization in these poly-ynes. ESI mass spectrometry confirmed the molecular structures of the alkynyl ligands and the dinuclear Pt(II)-Re(I) acetylide complexes.

X-ray Diffraction
To complement the spectroscopic characterization of the newly synthesized materials, we attempted to determine the crystal structure of the reported complexes by single-crystal X-ray diffraction.
Single crystals of the mononuclear Re complex 1c were grown by slow diffusion of hexane into a solution of the complex in CH2Cl2. Crystallographic parameters for this structure are summarized in Table   S1. 1c crystallizes in the monoclinic space group P21/n. Figure 1a shows the molecular structure, and selected bond lengths and bond angles are given in Table 1. The crystal structure shows that the Re(I) centre adopts a distorted octahedral coordination environment with three carbonyl groups, a chelating bipyridine ligand and one chlorido ligand. The structure also confirms the successful attachment of the acetylene R-CCH groups to the bipyridine ligand. The bipyridine ligands adopt a cis configuration, as   Table 1.
The thermal ellipsoids are shown at 50 % probability. Table 1 Selected bond lengths (Å) and bond angles (°) from the crystal structures of 1c and M4.

Distance
[Å] Angle [°] 1c Re(1)-C(16) 1.877 (19) C(15)-Re(1)-N (1) 90.5(4) Re(1)-C(15) 1.908 (17)   Attempts were also made to grow crystals of the Re(CO)3Cl-incorporated model Pt(II) di-yne compounds M3 and M4. Single crystals of M4 suitable for X-ray diffraction were grown by slow diffusion of hexane to a concentrated solution of the complex in CH2Cl2. Crystallographic parameters for this structure are summarized in Table S1. Despite several attempts at doing so, we were not able to obtain crystals of M3 that were suitable for single-crystal diffraction studies. Moreover, the crystals of M4 were in general of poor quality and weakly diffracting, and thus while our structure is sufficient to confirm the overall molecular geometry, the bond parameters should be treated with caution.
M4 crystallizes in the triclinic space group P-1. The molecular structure comprises discrete trimetallic moieties defined by a central Re(2,2'-bipyridine-6,6'-diyl)(CO)3Cl unit attached to a pair of trans-[(Ph)(PEt3)2Pt-C≡C-] units (Figure 1b). The six-coordinate Re(I) metal center adopts a distorted octahedral geometry. In general, 2,2′-bipyridine derivatives of the fac-Re(CO)3Cl complex adopt an almost planar geometry with respect to the basal OC-Re-CO plane. 33 However, deviation from planarity can occur due to steric hindrance between functional groups on the 2,2′-bipyridine core and the carbonyl (CO) ligands on the metal, leading to changes in the properties of the spacer group. The two rings in the bipyridine spacer are slightly twisted with respect to one another, with a torsion angle of 13 ° defined by the N(1), C(6), C (7) and N (12) atoms. This hints at a degree of strain in the bipyridine ligand, and we anticipate that the deviation from planarity would lead to some disruption of the conjugation in the ligand system compared to a more planar geometry.
Selected bond lengths and angles from the M4 structure are summarized in Table 1

Absorption Spectroscopy
Room temperature optical absorption spectra of the bis(ethynyl)bipyridine ligands 1b and 2b, Re(I)(CO)3Cl-chelated ligands 1c and 2c, and the Re(CO)3Cl incorporated Pt(II) di-and poly-ynes M3/M4 and P3/P4 were collected in 10 -5 M CH2Cl2 solutions (Figure 3). Table 2 compares the absorption maxima of the Pt(II) di-ynes and poly-ynes with and without the pendant Re(I) moieties, the latter taken from our previous work. 31 The absorption spectra of the ligands 1b/1c and 2b/2c are compared to those of the model compounds M1/M2 and M3/M4 in Figure S1.

8
The absorption spectra of the bis(ethynyl)-5,5'-bipyridine ligand 1b, the Re(CO)3Cl-chelated ligand 1c and the corresponding Pt(II) di-yne M3 and poly-yne P3 are compared in Figure 3a. The spectrum of 1b displays an intense absorption bands at max ≈ 315 and 328 nm and a shoulder feature around 348 nm.
The spectrum of 1c displays a noticeable shift in the absorption edge relative to 1b, with absorption bands at max ≈ 325, 332 and 350 nm. Absorption bands at ~270 and 300-350 nm can be assigned to → * transitions associated with the bipy/Ph and C≡C moieties, respectively, whereas broader bands at ~400-450 nm can be attributed to MLCT transitions. 35 This is in line with previous studies that have identified lowlying MLCT excited states involving a Re d donor orbital and a ligand * acceptor, 36,37 with chelation to the metal serving to make the ligand a better electron acceptor and possibly also forcing it into a more planar conformation and increasing the effective conjugation in the ligand orbitals. 38 The MLCT state has been shown to be long-lived and to luminesce at longer wavelengths in the visible spectrum. 36,37,39 The extended tail of this 1 MLCT band is a signature of the extended electronic delocalization in the alkynyl bipyridine derivatives. 35 Clear effects of incorporating the Re(I) core into the Pt(II) di-yne can be seen, for example, in the extended absorption of 1c and M3 relative to 1b and M1 (Figure 3, Figure S1). The steric hindrance due to the Pt(II) fragments is expected to be higher in systems based on 6,6'-bipyridine than in the 5,5'-conunterparts, leading to different levels of conjugation. This is evidenced by the optical properties and computational modelling (vide infra), although ideally this should be confirmed by further structural characterization.
The Pt(II) di-yne and poly-yne systems M3 and P3 both display strong long-wavelength absorptions, with a broad band centered around max ≈ 424 in M3 and an asymmetric feature in P3 comprising a primary peak at max ≈ 448 nm and a secondary feature around 419 nm. Both also show weaker secondary maxima, which occur at ~339 and 343 nm in M3 and P3 respectively. There is thus a notable red shift in both bands and an enhancement of the extinction coefficient of the longer-wavelength bands on going from the di-yne to the poly-yne. Interestingly, calculations show that the long-wavelength transition in M3 is not an MLCT band but is a bipyridine → * transition that is highly red shifted compared to 1b/1c due to a destabilised highest occupied molecular orbital (HOMO, see below). While somewhat surprising, this does account for the large enhancement of the extinction coefficient compared to the longwavelength MLCT band in 1c.
The spectra of the 6,6'-bis(ethynyl)bipyridine complexes in Figure 3b illustrate that, as for the 5,5'functionalized systems, chelation of the ligand to Re(CO)3Cl leads to a general red shift in the absorption profile and introduces an MLCT band, again at ~410 nm. Incorporation of the chelated spacer unit into the Pt(II) di-yne and poly-yne results in a further red shift and an increase in extinction coefficient ( max ≈ 395 and 388/402 nm respectively). Comparison of the spectra of the heterometallic di-ynes and poly-ynes to the corresponding homometallic Pt(II) species (M1/P1 and M2/P2) show that adding a second metal ion leads to significant changes ( Table 2). For example, a large bathochromic shift of the lower energy bands from 398 nm in P1 to 448 nm in P3, which we ascribe to stabilization of the LUMO. The 6,6'-bipyridine species generally show blue-shifted absorption maxima compared to the corresponding 5,5'-bipyridine species, which we account for by the different positions of the alkynyl groups and the steric strain in the Pt(II) systems hindering the conjugation. a ε was not measured in the work in Ref. 31 .
In principle, we might also expect the hindered conjugation in the 6,6'-bipyridine species to lead to lower extinction coefficients compared to the 5,5'-bipyridine analogues. While this is borne out for the 1b/2b and 1c/2c pairs, the M3/M4 pair, and to some extent also P3 and P4, show the opposite trend. As shown in Figure 2b, the model complex M4 exhibits molecular stacking in the solid-state, and it is possible that aggregation in solution through a similar mechanism may alter the extinction coefficient. 40 In particular, it has been reported that "V-shaped" conjugated molecules, such as M4, form aggregates in halogenated solvents, leading to enhancement of the absorption cross section. 41 Measurement of the absorption spectra of 1c/2c, M1/M2, and M3/M4 at solution concentrations from 1 × 10 -5 to 3 × 10 -5 M ( Figure S2) show that although the band shape and max remain the same, the molar extinction coefficient shows some sensitivity to the concentration. This suggests that the stacking and/or C-H••• /C-H•••Cl interactions visible in the Xray structure might influence the optical properties in solution. We also measured absorption and fluorescence emission spectra at varying concentration ( Figure S2) and found that increasing the concentration by up to 3× did not produce any major changes in spectral features, which is also in line with the results of the picosecond-nanosecond dynamics measurements presented below.

Photoluminescence Spectroscopy
Re(I) complexes are well known for their intense, unstructured emission in the orange region of the visible spectrum, which originates from 3 MLCT excited states. 42 The energy, the intensity and the lifetime of the emission are highly sensitive to the nature of the diimine and ancillary ligands, 43 and a large range of photoluminescence quantum yields (PLQY) have been reported for Re(I) complexes. 42 The linkage of two metal atoms to the same chromophore has been shown to increase the metal d character in the frontier molecular orbitals, thereby enhancing the spin-orbit coupling between the emissive triplet state and the singlet manifold. 44 Linking two heavy metals to a single heterocyclic ligand is thus an interesting potential strategy for improving the PL properties of neutral Re(I) complexes.
Room temperature emission spectra of the Re(I)-coordinated ligands 1c/2c, the homometallic Pt(II) di-ynes M1/M2 and the heterobimetallic Pt(II)-Re(I) di-ynes M3/M4 were collected in 3 x 10 -5 M CH2Cl2 solutions (Figure 4, Table 3). We also collected room-temperature solid-state emission spectra of the compounds ( Figure S3), together with fluorescence excitation and emission spectra in CH2Cl2 solutions ( Figure S4). The spectra are somewhat complex and composed of multiple maxima and/or shoulder features. The fluorescence excitation spectra of 2c, M1, M2 and M4 are similar to the absorption spectra, providing evidence that the measured emission is from the metal complexes, whereas those of 1c and M3 show a significant red shift, which we tentatively ascribe to solution effects such as aggregation or ligand dissociation ( Figure S4). were found to be red shifted in the solid state ( Table 3).

As is commonly observed in
At room temperature, the photo-luminescence quantum yield Φ (PLQY; chromophores. 29 While we were not able to assign the longer-wavelength transitions definitively, quantumchemical calculations (see below) suggest that they may be associated with emission from formally spinforbidden triplet states. The PLQY measured for the heterobimetallic complexes are also higher than for Re(bpy)Cl(CO)3 (3.1 × 10 -3 in CH2Cl2), 35 which indicates that the incorporation of a second metal limits nonradiative decay pathways. In general, our data indicates that incorporation of the Re(I) fragment into the heterobimetallic di-ynes improves the PLQY and leads to a red shift of emission maxima, although the emission intensity from the heterobimetallic complexes are lower than that from the homometallic Re(I) complexes at shorter wavelengths. As expected, the emission properties of the 5,5'-and 6,6'-systems differ significantly, which can be attributed to geometric constraints limiting the conjugation between the bipyridine and Pt(II) cores separated by the ethynyl units.   35 In contrast to the solution spectra, the position of the band centers in the solid-state spectra of 1c and 2c are very similar ( Figure S3). The emission profiles of the model compounds M3 and M4 are both red shifted compared to the chelated spacers, and the shift in emission wavelength of M3 is more prominent than that of M4. Finally, in contrast to M3 and M4, the polymers P3 and P4 were found to exhibit only weak emission both in solution and in the solid state at room temperature ( Figure S5). The same was found for P1 and P2. 31 The solution emission profiles of P3 and P4 are very similar to those of 5,5'-dibromobipyridine and 6,6'-dibromoipyridine, respectively, suggesting a dominant bipyridine-centered emission and suppression of the reverse MLCT transition seen in the chelated spacers 1c/2c and the model compounds

Picosecond-Nanosecond Dynamics
Time-resolved fluorescence spectra of 1c, 2c, M3 and M4 in CH2Cl2 were measured at room temperature close to the emission maxima in Table 3. The transients are shown in Figure 5 and parameters from a series of multi-exponential fits are summarized in Table 4. Decay transients measured over a longer observation window, in order to accurately characterise longer-lived components, are shown in Figure S6.
Two components with lifetimes = 684 ps and 2.43 ns were obtained for 1c. The shorter-lived component can be assigned to the relaxation of the MLCT state, whereas the nanosecond lifetime of the other component is typical of π  π* decay and can thus be ascribed to the bipyridine ligand. In 2c, the 6,6' substitution pattern reduces the lifetime of the MLCT state to 597 ps compared to the 5,5'-substituted 1c but raises the lifetime of the π* decay to 2.74 ns and introduces a new, long-lived state with a lifetime of 20-23 ns. The latter are indicative of improved conjugation increasing the local heterogeneity of the system.
The presence of the Pt(II) fragments in M3 further reduces the lifetime of the MLCT state to 200 ps but slightly increases the π* lifetime to 3.14 ns. This can be accounted for through increased stability of the π* state due to the more extended conjugation in the planar structure. M3 also exhibits a very small contribution from the longer-lived component (20-23 ns) seen in 2c. In M4, the steric effects due to the Pt(II) fragments in the 6,6' positions lead to an increased contribution from the long-lived state (20-23 ns) at the expense of the MLCT state, as indicated by the fitting weights in Table 4. These effects also increase the π* lifetime to 5.66 ns.
Increasing the concentration of the four species from 1 × 10 -5 to 3 × 10 -5 M did not produce any notable changes to the measured fluorescence decay transients ( Figure S7). This indicates that increasing the concentration does not lead to excimer formation, as this would typically cause a buildup of signal (rise time) in the transients similar to that found in other systems such as pyrenes. 45 The increase in concentration may however still lead to aggregation in the ground state, which explains the increase in the extinction coefficient observed in the absorption spectroscopy measurements.   Table 4. Note that the artefact at ~4 ns in the 1c decay transient is due to "afterpulsing" in the photomultiplier tube detector and is excluded from the fitting, so does not affect the values reported in Table 4.

Computational Modelling
To better understand the changes in optical properties on chelating the bipyridine units to the Re(I) centres and subsequently incorporating the chelated spacers into the Pt(II) model complexes, we carried out molecular quantum-chemical calculations using density-functional theory (DFT) on the bipyridine-based diterminal alkynyl ligands 1b and 2b, chelated diterminal alkynyl ligands 1c and 2c, and the Pt(II) di-ynes M3 and M4. To match the conditions of the solution measurements as closely as possible, the calculations were performed with an implicit solvent of CH2Cl2.
Images of the optimised structures are given in Figures S8-S13 and the Cartesian coordinates are provided in Listings S1-S6. For all three pairs, we found that the 5,5'-bis(acetylide) bipyridine compounds were more energetically favourable (in CH2Cl2) than the 6,6'analogues, with 1b, 1c and M3 calculated to   Simulated optical absorption spectra obtained using time-dependent DFT (TD-DFT; Figure 6) reproduce the key trends in the measured spectra in The calculations also predict that all six compounds possess low-lying triplet (spin-forbidden) excited states 100-150 nm below the onset of the absorption from the lowest-energy singlet states (marked by stars and dashed lines in Figure 6). The density of these triplet states generally increases on going from the spacer ligands 1b/2b to the chelated spacers 1c/2c to the model compounds M3/M4. As noted in the previous section, these states may be associated with the weaker long-wavelength features in the solution emission spectra in Figure 4. Although it is in principle possible to model emission processes using TD-DFT, the procedure is considerably more involved than calculating the transition wavelengths and oscillator strengths to model absorption spectra, and we do not consider it feasible to do so for the six compounds being examined in this work.
The brightest singlet (spin-allowed) transitions and the 2-3 lowest-lying triplet (spin-forbidden) transitions in the simulated spectra in Figure 6 were analysed by inspecting the molecular orbitals involved and, for transitions comprising more than one significant excitation between occupied and virtual states, by using the method of natural transition orbitals (NTOs) to visualise the composite occupied "particle" and unoccupied "hole" states. 41 Table 5 lists the calculated wavelengths, oscillator strengths and our assignments of the brightest singlet (spin-allowed) transitions in each of the six complexes, while Table 6 lists the wavelengths and the assignments of the low-lying triplet (spin-forbidden) states. A full breakdown of the states listed in Tables 5 and 6 into transitions between pairs of occupied and virtual orbitals, and isosurfaces showing the NTOs, are given in Tables S2-S13 and Figures S14-S27/S31-S47 respectively. is most likely due to destabilisation of the LUMO. The lower degree of conjugation also leads to poorer spatial overlap with the HOMO, which explains the mixing of the LUMO + 2 orbital into the → * excited state. This would further raise the transition energy, and the poorer spatial overlap between the HOMO and LUMO may explain the lower oscillator strength of the transition compared to the 5,5'-bipyridine analogue 1b.
Chelation of 1b results in new frontier Re-based orbitals, of which the HOMO -1 orbital has the best spatial overlap with the bipyridine-based LUMO and gives rise to a weak MLCT band at 355 nm ( Figure   6c). The HOMO -3 orbital remains similar in form to the HOMO in 1b, and the much stronger → * transition occurs at 305 nm. The particle and hole states obtained from the NTO analysis of the two transitions are shown in Figure 8.
The spectrum of 2c also shows a weak MLCT band with a comparable, if slightly lower, oscillator strength, which again occurs between a Re-based HOMO -1 and the bipyridine ligand-based LUMO (Figure 6d, Figure S18). As for 1b/2b, this transition is blue shifted compared to that in 1c due to the higher-energy LUMO ( Figure S29). The strong shorter-wavelength feature in Figure 6d is a combination of three bands at 295, 284 and 280 nm. The longer-wavelength band at 295 nm was assigned as predominantly a → * transition based on the NTOs but with some MLCT character (Figure S19), the latter of which may explain its ~3× smaller oscillator strength than the corresponding → * transition in 1c, relative to the smaller ~2× difference in oscillator strengths of the electronic transitions in 1b/2b (c.f. Table 5). This reduction in oscillator strength can also be seen in the measured spectra in Figure 3. The two higher-energy transitions at 284 and 280 nm were assigned as predominantly MLCT, which is consistent with their weak oscillator strengths (Figures S20/S21). The low-lying triplet states in 1b and 2b were assigned based on the NTOs as → * excitations ( Table 6; Figures S31-S36). The lowest-lying triplet excitation in 1c is predicted to occur at 492 nm and was also characterised as a → * (Figure S37), whereas the next-highest triplet excitations at 391 and 377 nm are primarily MLCT bands (Figures S38/S39). In 2c, on the other hand, the lowest-lying triplet states at 443 and 402 nm are of mixed MLCT and → * character (Figures S40/S41), while the higherlying state at 374 nm is predominantly an MLCT excitation ( Figure S42). The simulated spectrum of M3 in Figure 6e shows two triplet states below the bright, long-wavelength singlet excitation, which were both characterised as a → * transitions (Figures S43/S44). In M4, there are three triplet states at notably longer wavelengths than the absorption onset, and these were again characterised as → * transitions ( Figures S45-S47). The change in the nature of the triplet states on going from the chelated spacers to the heterobimetallic di-ynes is consistent with the shortening of the MLCT state lifetime and lengthening of the * lifetime observed in the picosecond-nanosecond dynamics measurements (c.f. Table 4). The mixed character of the triplet states in the 6,6'-bipyridine species may also explain the larger contribution of the longer-lived → * states to the transients in Figure 5.  therefore not shown (c.f. Figure S24). The isosurfaces are drawn to a contour value of 2.5 × 10 -2 e bohr -3 .

Conclusions
We have synthesized and characterized two Re(I)-coordinated diterminal alkynyl ligands, viz.

General Procedures
All reactions were performed in a dry argon atmosphere using standard Schlenk techniques.
Solvents were distilled and pre-dried before being used according to standard procedures. 42  Viscotek Model 200 differential refractometer/viscometers, which were used to calculate the molecular weights.

X-ray Crystallography
Single-crystal X-ray diffraction experiments were performed at 150 K on a STOE IPDS (II) diffractometer using monochromatic Mo-Kα radiation ( = 0.71073 Å) with the sample temperature controlled using an Oxford Diffraction Cryojet. The X-Area software was used for data collection and indexing. The structure was solved and refined using full-matrix least squares on F 2 in SHELX2014 45 from the WinGX suite. 48 A multi-scan absorption correction was applied. There was extensive disorder in the alkyl groups of the phosphine ligands, which were modelled over two or three sites using partial occupancies, constrained to sum to unity, and with additional constraints placed on the bond parameters to maintain reasonable bond lengths and angles. With the exception of some of the disordered carbon atoms in the alkyl chains of the phosphine ligands, all non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were included using rigid methyl groups or a riding model, with partial occupancies used as appropriate. Refinement was continued until convergence, and in the final cycles of refinement a weighting scheme was used that gave a relatively flat analysis of variance.

Time-Resolved Fluorescence
Lifetime measurements were performed using time-correlated single photon counting (TCSPC).
The TCSPC setup, described elsewhere, 47  Fluorescence was attenuated and directed to the detector, and a monochromator was used to adjust the detection wavelength. Decays were recorded to ~ 10,000 counts in the peak channel. The decay transients were fitted to multi-exponential functions convolved with the IRF.

Computational Modelling
Molecular quantum-chemical calculations were carried out using the density-functional theory (DFT) formalism as implemented in the Gaussian09 software. 49 The CAM-B3LYP hybrid functional 50 was used in conjunction with Pople split-valence basis sets 51 of 6-31g and 6-31g** quality for the H and non-H atoms, respectively. The LANL2DZ pseudopotential 52 and corresponding double-zeta basis sets were used to describe the Pt and Re atoms. Initial models of 1b/2b, 1c/2c and M3/M4 were prepared from X-ray structures or using the Avogadro software. 53 The molecular structures were optimised in the gas phase and the minima confirmed to be stationary points from the absence of imaginary modes in the vibrational Hessian matrix. Time-dependent DFT (TD-DFT) calculations were carried out on the optimized models using adiabatic B3LYP to identify the 50 lowest-energy singlet and triplet states, a subset of which were characterized using natural transition orbitals. 41 Visualisation of the frontier orbitals was performed using VESTA. 54

Supporting Information
Crystal structure data for 1c and M4 is available under the CCDC reference numbers 2009579 and

Data-Access Statement
The crystal structures of 1c and M4 are available under the CCDC reference numbers 2009579 and 1964156. Raw data from the spectroscopic characterization and computational modelling is available from the authors on request.

Notes
The authors declare no competing financial interests.

Table of Contents Graphic
Two new heterobimetallic Pt(II) di-ynes and poly-ynes are prepared by incorporating Re(CO)3Cl moieties via chelation of the bipyridine spacer group. A chelated spacer and model di-yne are crystallised and studied by X-ray difraction, and the photophysical properties of the chelated spacers, di-ynes and poly-ynes are examined using absorption and emission spectroscopy and quantum-chemical modelling. The synergistic effect of the two metals on the optical properties highlights a novel route to fine-tuning the optoelectronic properties of these organometallic poly-ynes.

Figure S6
Room temperature fluorescence decay transients of 1c/2c and M3/M4 in CH2Cl2 (1 × 10 -5 M). This is the same data as Figure 5 in the text shown to a longer decay time.