Deactivating Unproductive Pathways in Multichromophoric Sensitizers

Sep 4, 2014 - TEnT unfavorable. By switching to a low dielectric constant solvent, we are able to extend the lifetime of the 3MMLL′CT state to over ...
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Deactivating Unproductive Pathways in Multichromophoric Sensitizers Randy Pat Sabatini, Bo Zheng, Wen-Fu Fu,† Daniel J. Mark, Michael F. Mark, Emily Anne Hillenbrand, Richard Eisenberg,* and David W. McCamant* Department of Chemistry, University of Rochester, Rochester, New York 14627, United States S Supporting Information *

ABSTRACT: The effects of solvent and substituents on a multichromophoric complex containing a boron-dipyrromethene (Bodipy) chromophore and Pt(bpy)(bdt) (bpy = 2,2′-bipyridine, bdt =1,2-benzenedithiolate) were studied using steady-state absorption, emission, and ultrafast transient absorption spectroscopy. When the Bodipy molecule is connected to either the bpy or bdt in acetonitrile, excitation ultimately leads to the dyad undergoing triplet energy transfer (TEnT) from the redox-active Pt triplet mixed−metalligand−to−ligand′ charge transfer (3MMLL′CT) state to the Bodipy 3ππ* state in 8 and 160 ps, respectively. This is disadvantageous for solar energy applications. Here, we investigate two methods to lower the energy of the 3MMLL′CT state, thereby making TEnT unfavorable. By switching to a low dielectric constant solvent, we are able to extend the lifetime of the 3MMLL′CT state to over 1 ns, the time frame of our experiment. Additionally, electron-withdrawing groups, such as carboxylate and phosphonate esters, on the bpy lower the energy of the 3MMLL′CT state such that the photoexcited dyad remains in that state and avoids TEnT to the Bodipy 3ππ* state. It is also shown that a single methylene spacer between the bpy and phosphonate ester is sufficient to eliminate this effect, raising the energy of the 3MMLL′CT state and inducing relaxation to the 3ππ*.

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INTRODUCTION Multichromophoric species have tremendous application in terms of light harvesting because each individual unit can absorb a separate part of the solar spectrum. For the efficient utilization of solar photons, it is necessary to funnel the energy toward some long-lived redox-active state, from which electron transfer can take place. Multichromophoric species have been successfully implemented in dye-sensitized solar cells (DSSCs),1,2 capitalizing on the increase in absorption available from varied chromophores. However, one major challenge in designing multichromophoric species is the proper alignment of the energy levels of the components, where special care must be taken to avoid unproductive pathways.3 The photophysical properties of square-planar platinum (Pt) complexes have been well documented in the literature. An initial study by Balzani and von Zelewsky examined a series of ortho-metalated Pt(II) complexes, revealing the first emission with metal character from a d8 complex in solution.4 Work soon followed in the Eisenberg laboratory, synthesizing new Pt(II) complexes with strong solution luminescence and solvatochromic behavior.5 With increasing polarity of the solvent, the absorption maximum increased to higher energies due to the larger polarity of the ground state relative to the excited state. A number of Pt(diimine)(dithiolate) complexes, exemplified by 1, were subsequently studied, exhibiting similar solvatochromic characteristics.6−9 The excited-state energies of the Pt complexes, through ligand substitution, could also be tuned by ∼1 eV, and with the exception of a maleonitriledithiolate © 2014 American Chemical Society

(mnt) complex, all Pt(diimine)(dithiolate) complexes examined were found to undergo a mixed−metal-ligand−to−ligand′ charge transfer (MMLL′CT) transition, where electron density is moved from the Pt center and dithiolate ligand to the diimine ligand.7 Due to their long excited-state lifetimes, absorption in the visible range of the spectrum, and low oxidation potentials, these Pt(diimine)(dithiolate) complexes were deemed candidates for solar energy sensitization via attachment to TiO2 in DSSCs10,11 and for solar hydrogen production,12 achieving 3.0% cell efficiency and 80 turnover numbers (TONs) of hydrogen, respectively. In order to improve upon their modest molar absorptivities, Pt(bpy)(bdt) (bpy = 2,2′-bipyridine, bdt = 1,2-benzenedithiolate) (ε ≈ 104 M−1cm−1) was covalently attached to a more strongly absorbing boron dipyrromethene (Bodipy) dye (ε ≈ 105 M−1cm−1). Bodipy dyes have been used with Pt(II)−terpyridine complexes13,14 and various porphyrin chromophores in other multichromophoric species.15−20 Two Pt(diimine)(dithiolate)−Bodipy dyads were studied, one in which the Bodipy moiety was attached to the bdt (2) and the other with it attached to the bpy (3).21 Special Issue: Current Topics in Photochemistry Received: August 15, 2014 Revised: September 4, 2014 Published: September 4, 2014 10663

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different rates may also be due to different electronic coupling occurring when the Bodipy molecule is linked to either side of the Pt species. In most solar energy applications, the dyad would require anchoring groups to the surface of a semiconductor, which is most easily realized by functionalizing the bpy. In this motif, the directed CT character of the Pt MMLL′CT state sets up the electron for efficient transfer to the adjacent semiconductor. However, while the excited-state Pt lifetime of 2 is longer than that of 3, both dyads undergo the unproductive pathway of TEnT in under 200 ps, a process that would compete with potential electron injection from the redox-active MMLL′CT state of the Pt complex to an attached semiconductor such as TiO2 for charge separation. Charge injection from the ππ* states on Bodipy is not expected due to its distance from the surface and the increased oxidation potential of Bodipy relative to Pt(bpy)(bdt).21 This paper will demonstrate two methods of lowering the energy of the Pt 3MMLL′CT state, thereby deactivating the unproductive TEnT process and making the Pt 3MMLL′CT state last over 1 ns. The approach described in this work thus utilizes an intensely absorbing organic chromophore for light harvesting yet avoids being trapped in its characteristically lowenergy triplet state.

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EXPERIMENTAL SECTION The Pt−Bodipy dyad (3) and Pt(dbbpy)(bdt) complex (1) were synthesized according to literature procedures.21 The slightly modified synthesis of 2, as well as the synthesis of 1C, 1P, 1spP 2C, 2P, and 2spP will be presented elsewhere.22 Compounds were dissolved in each solvent and filtered before use. The optical densities (ODs) of the samples were kept between 0.3 and 0.7 at the pump wavelengths. Ground-state absorption spectra were measured with a Shimadzu UV-1800 or Cary 60 (Agilent) spectrophotometer. Emission and excitation spectra were measured with a Spex Fluoromax-P fluorimeter with a photomultiplier tube detector. The laser system for TA has been described previously.23 Femtosecond laser pulses were produced by a regeneratively amplified titanium:sapphire laser (Spectra-Physics Spitfire) with a 1 kHz repetition rate. The actinic pump was generated from a home-built noncollinear optical parametric amplifier. The probe was created by focusing an 800 nm beam through a sapphire or calcium fluoride crystal, generating a white-light continuum between 400 and 650 nm or 325 and 650 nm, respectively. A blue color filter (Schott BG40 or BG38) was used to eliminate the residual 800 nm light before the detector. The actinic pump, either 530, 600, or 660 nm, was kept between 70 and 100 nJ/pulse. The pump beam diameter was kept between ∼60 and 100 μm, and the time resolution was ∼150 fs. The time delay was adjusted by optically delaying the pump pulse, with time steps increasing logarithmically.24 Every other pump pulse was blocked by a chopper, and each probe pulse was measured by a CCD (Princeton Instruments, Pixis, 100BR) after dispersion by a grating spectrograph. (Acton, 150 gr/mm). Samples were held in a 2 mm cuvette and translated at 2 mm/s to ensure replenishment of the illuminated volume. Using the software program Igor Pro (WaveMetrics), kinetic traces were fit to the convolution of the Gaussian instrument response function (∼150 fs), with a sum of exponential decays. Global fitting analysis was used to determine time constants for 3. The chirp of the probe was corrected by allowing time zero to vary depending on the wavelength. Stated errors in the fit parameters are 1σ.

As shown in Figure 1, when the Bodipy moiety of 2 or 3 was selectively excited with 530 nm light, singlet energy transfer (SEnT) to the Pt moiety was observed, followed by rapid intersystem crossing.21 However, back triplet energy transfer (TEnT) to the Bodipy moiety then followed (Figure 1). Both energy transfers were attributed to a Dexter-type mechanism. The associated time constants were greatly affected by the choice of linking the Bodipy molecule to either the dithiolate or diimine side of the complex, with TEnT occurring in 160 ps for 2 and 8 ps for 3, respectively. At that time, the difference in TEnT rates was attributed to the variation in the energy of charge-separated intermediate states in 2 versus 3; however, the

Figure 1. Energy level diagram of the Bodipy−Pt dyad. Initial excitation of the Bodipy moiety by 530 nm light results in rapid SEnT to the Pt singlet mixed−metal-ligand−to−ligand′ charge transfer (1MMLL′CT) After intersystem crossing (ISC), TEnT results in reexcitation of the Bodipy, this time to the 3ππ* state. The ISC and TEnT processes occur identically if initiated by the absorption of 600 nm light by the Pt species. 10664

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Figure 2. (a) Absorption spectra of the bpy-bound Bodipy−Pt dyad (3) in different solvents. (b) Absorption spectra of Pt(dbbpy)(bdt) (1) in different solvents. (c) Absorption spectra of the bdt-bound Bodipy−Pt dyads 2, 2C, 2P, and 2spP in acetonitrile. (d) Absorption spectra of Pt(diimine)(dithiolate) complexes 1, 1C, 1P, and 1spP in acetonitrile.

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RESULTS The ground-state absorption spectrum of the Pt(diimine)(dithiolate)−Bodipy dyad, 3 (where the Bodipy moiety is attached to bpy) was measured in eight different solvents, with dielectric constants ranging from toluene (εr = 2.38) to dimethyl sulfoxide (εr = 46.7). The large absorption peak at ∼530 nm is due to the Bodipy, and the weaker low-energy peak at 600−700 nm is due to the Pt(bpy)(bdt). As seen in Figure 2a, as the dielectric constant of the solvent is decreased, the Bodipy absorption (∼530 nm) changes only slightly. However, the Pt(diimine)(dithiolate) MMLL′CT absorption (which will be referred to as the Pt absorption throughout the rest of the article) red shifts dramatically, from ∼560 nm in dimethyl sulfoxide, where it is convoluted with the Bodipy absorption band, to 692 nm in toluene. Due to the negative solvatochromism of the Pt complex, the energy gap between the Bodipy and Pt absorption becomes much larger in solvents with low dielectric constants. Figure 2b displays the absorption spectra of Pt(dbbpy)(bdt) (1) in the different solvents, showing its solvatochromism more clearly without obfuscation by the Bodipy peak. Note that there is an energy offset between the absorption of the Pt moiety in 3 and the Pt complex (1), but the shape of the overall peaks remains the same (Figure S1, Supporting Information). New complexes studied in this work are shown in Chart 1. Complexes 1C, 1P, and 1spP are derivatives of 1 containing possible linking moieties for attachment to an oxide semiconductor. In 1C, the linker is a carboxylate ester, while for 1P, a phosphonate ester is present. In 1spP, the phosphonate ester is separated from the bipyridine by a methylene spacer. Ultimately, for semiconductor attachment, the esters will be hydrolyzed. Compounds 2C, 2P, and 2spP are dyads analogous to 2 in which the Bodipy dye is linked directly to the benzenedithiolate ligand in order that attachment to the semiconductor can be through the diimine substituent. Figure 2c displays the ground-state absorption of the series 2 dyads (where the Bodipy moiety is attached to bdt) in acetonitrile, illustrating the effects of substituents. For these

Chart 1. New Bodipy−Pt Dyads and Pt Complexes Used in This Study

compounds, the Bodipy absorption is unaffected, remaining at 522 nm. However, the Pt absorption is strongly dependent on the effects of the bpy substituent. With no substituent on bpy (2), the Pt absorption is observed at ∼525 nm. When the bipyridine is functionalized with a carboxylate ester (2C) or a phosphonate ester (2P), the Pt absorption red shifts to 612 and 600 nm, respectively. Less of a red shift is observed when a CH2 spacer is added between bpy and the phosphonate ester, with 2spP showing an absorption maximum of ∼541 nm. The absorption spectra of the corresponding Pt complexes (1, 1C, 10665

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was examined, exhibiting rapid ISC in ∼1 ps (Figure S7, Supporting Information). See Tables S1−S8 (Supporting Information) for a list of time constants of 3 in different solvents. When dyad 3 is dissolved in toluene or diethyl ether, however, different dynamics are observed. Figure 4 displays the

1P, and 1spP) are displayed in Figure 2d, showing a slight shift in the absorption maximum of the Pt complex, relative to its absorption in the dyad. (See detailed comparisons in Figure S1, Supporting Information.) However, the results of the Pt complexes agree with those of the linked Bodipy−Pt(diimine)(dithiolate) dyads, that the carboxylate or phosphonate esters dramatically red shift the absorbance and that a single methylene spacer is sufficient to mollify this effect. The basis for this effect arises from the effect of the substituent on the energy of the π* (diimine) orbital that acts as the electron acceptor upon excitation. It has previously been witnessed with 3 (Bodipy moiety attached to bpy) that in dichloromethane, excitation of the Bodipy moiety leads to fast SEnT to Pt, rapid ISC crossing, and then subsequent back TEnT to the Bodipy moiety.21 Transient absorption spectroscopy was performed on dyad 3 in different solvent environments. In chloroform, tetrahydrofuran, acetone, acetonitrile, and dimethyl sulfoxide, spectral changes mirror the previous results in dichloromethane. Figure 3a displays a

Figure 4. Transient spectra (a) and kinetic traces (b) of bpy-bound Bodipy−Pt dyad 3 in toluene following excitation at 530 nm.

transient spectra and kinetics traces of the dyad in toluene (see Figure S8, Supporting Information, for 3 in ether). As expected, after 530 nm excitation, the 0.05 ps spectrum resembles the singlet excited state of the Bodipy moiety, with ESA at 400− 475 nm, and a negative signal from 475 to 625 nm, composed of both GSB and SE. With a time constant of 0.35 ps, the SE signal at 585 nm decays. The negative signal associated with the Bodipy bleach is still present but strongly attenuated and shifted slightly to the red. Very slight adjustments in the spectra then occur with a 3.8 ps time constant, which are attributed to ISC and intramolecular vibrational redistribution (IVR) in the Pt 3MMLL′CT state. This signal persists for the remainder of the experiment (1 ns, Figure S9, Supporting Information). The spectral signature is similar to what is observed when 660 nm light is used to selectively excite the Pt moiety of 3 in toluene (Figure S10, Supporting Information), as well as the Pt(dbbpy)(bdt) complex (1) in toluene (Figure S11, Supporting Information). In order to probe the effect of the bpy substituent, TA spectra were obtained for series 2 (dyads with the Bodipy moiety on bdt) in acetonitrile. Excitation of dyad 2 with 530 nm light (Figure 5) results in initial population of the singlet excited state of Bodipy, with spectral features consisting of a positive ESA at 350 nm and a negative feature composed of the GSB and SE from 465 to 600 nm. (Note that the Bodipy ESA at 350 nm is visible in these experiments because the CaF2 white light continuum extends to the near-UV, unlike the sapphire continuum used for the experiments presented in Figures 3 and 4.) The Bodipy ESA is consistent with previous measurements.25 In 0.48 ps, SEnT to the Pt 1MMLL′CT state occurs, resulting in a loss of the 350 nm ESA and the SE to the red of the bleach. After slight changes occurring with an 18 ps time constant, presumably due to ISC/IVR, a positive ESA

Figure 3. (a) Transient spectra (a) and kinetic traces (b) of bpybound Bodipy−Pt dyad 3 in chloroform following excitation at 530 nm. In (a) and in later figures, the ground-state absorption spectrum is shown as the dotted line for comparison.

representative TA spectrum of dyad 3 in chloroform, with Figure 3b showing kinetics at a selection of wavelengths (see Figures S2−S6 (Supporting Information) for 3 in other polar solvents). At 0.25 ps, the spectrum is composed of a broad positive region from 400 to 475 nm and a negative signal from 475 to 650 nm. This is consistent with population of the Bodipy singlet excited state.25 With a time constant of 0.56 ps, decreases in the excited-state absorption (ESA) at 400−475 nm, the Bodipy ground-state bleach (GSB) at 530 nm, and stimulated emission (SE) at 550−600 nm are observed, corresponding to SEnT from the 1ππ* state of the Bodipy moiety to the 1MMLL′CT state of the Pt moiety. Rapid ISC follows due to the spin−orbit coupling induced by the heavy Pt atom. A positive peak at 425 nm then grows in with a time constant of 13.6 ps, concurrent with a rebleaching of the Bodipy ground-state absorption, which is consistent with back TEnT to the Bodipy moiety, forming its triplet state. For comparison, the Pt(dbbpy)(bdt) complex (1) in chloroform 10666

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grows in at 450 nm, and the Bodipy ground-state absorption rebleaches, consistent with TEnT back to the triplet state of Bodipy in 138 ps. These kinetics are similar to, although faster than, those observed when 2 had previously been studied in dichloromethane.21 When the bpy is functionalized with a carboxylate ester (2C) (Figure 6), 530 nm excitation of the dyad results again in population of the Bodipy singlet state, with a positive ESA at 350 nm and a negative region, composed of the GSB and SE from 475 to 650 nm. SEnT occurs with a 0.73 ps time constant, evident by the decay of the ESA at 350 nm as well as the loss of SE at 575 nm. A broader positive region from 325 to 475 nm is seen, along with the bleach of the Pt moiety from 575 to 650 nm. The GSB of the Bodipy moiety becomes dispersive in shape during this time. These features are evident throughout the remainder of the experiment. The same is observed when dyad 2P (phosphonate ester substituent on bpy) is run in

acetonitrile (Figure 7), with a 0.69 ps time constant (SEnT) giving rise to a long-lived state. For comparison, 600 nm was also used to excite 2C and 2P, as well as both of the Pt complex analogues, 1C and 1P (Figures S12 and S13, Supporting Information). In all four cases, only rapid ISC with no TEnT to the Bodipy 3ππ* state was observed. Dyad 2spP was excited by 530 nm light in acetonitrile (Figure 8) in order to probe the effect of a methylene spacer between the bpy and the phosphonate ester. At 0.2 ps, a positive ESA peak at 350 nm is present, along with a negative signal (GSB and SE) from 475 to 600 nm. With a 0.51 ps time constant, the ESA, GSB, and SE decrease, consistent with SEnT. Following an 11.7 ps time constant (ISC/IVR), the GSB increases, with a positive peak growing in at ∼450 nm with a 139 ps time constant, consistent with TEnT back to the Bodipy 3ππ* state. In essence, 2spP proceeds through a relaxation pathway akin to the parent compound 2 (Figure 1).

Figure 5. Transient spectra (a) and kinetic traces (b) of bdt-bound Bodipy−Pt dyad 2 in acetonitrile following excitation at 530 nm.

Figure 7. Transient spectra (a) and kinetic traces (b) of the phosphonate substituent dyad (2P) in acetonitrile following excitation at 530 nm.

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DISCUSSION Relative Energies of the Bodipy and MMLL′CT Triplet States. The transient absorption results in toluene and ether give evidence of a clear difference in the dynamics of dyad 3 in which Bodipy is attached to bpy relative to the results in more polar solvents. In all solvents, initial 530 nm excitation results in fast SEnT from the Bodipy to the Pt moiety (0.24−5.8 ps), followed by ISC (0.8−3.8 ps) to form the 3MMLL′CT state. In the more polar solvents, back TEnT then occurs to the Bodipy moiety (10.7−32 ps), evident by the growth of the characteristic 450 nm ESA. In toluene and ether, however, the 450 nm absorption of the Bodipy 3ππ* state never grows in, implying that the 3MMLL′CT remains for the duration of the experiment. This is supported by comparison of the transient spectra of Bodipy−Pt(bpy)(bdt) dyad (3) and the Pt(dbbpy)(bdt) complex (1) in toluene (Figures 4 and S11, Supporting Information, respectively). Besides the initial SEnT, similar spectra are also observed with a direct excitation of the Pt moiety in 3 using 660 nm light (Figure S10, Supporting

Figure 6. Transient spectra (a) and kinetic traces (b) of the carboxylate substituent Bodipy−Pt dyad (2C) in acetonitrile following excitation at 530 nm. 10667

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Figure 8. Transient spectra (a) and kinetic traces (b) of phosphonate with spacer substituent Bodipy−Pt dyad (2spP) in acetonitrile following excitation at 530 nm.

Figure 9. (a) Δt > 1000 ps spectra of the carboxylate Pt complex (1C) and carboxylate Bodipy−Pt dyad (2C) using 530 and 600 nm excitation, showing the same ESA. (b) Δt > 1000 ps spectra of the phosphonate Pt complex (1P) and phosphonate Bodipy−Pt dyad (2P), using 530 and 600 nm excitation, showing the same ESA. Note that the absence of data near 600 nm is due to pump scattering.

Information), indicating that for nonpolar solvents, the resting state is in fact the 3MMLL′CT state of Pt. This effect is also seen when comparing the functional group series 2, where the Bodipy moiety is attached to bdt, in the same solvent. When 2, the carboxylate derivative (2C), the phosphonate derivative (2P), and the phosphonate with methylene spacer derivative (2spP) are excited with 530 nm light, SEnT and ISC occur. In both 2 and 2spP, back TEnT follows, resulting in the population of the Bodipy 3ππ* state. However, in 2C and 2P, the Bodipy 3ππ* state is never populated, with the 3MMLL′CT state of the Pt complex remaining for the duration of the experiment. As shown in Figure 9, the similarity between the spectra of 2C and 2P, excited at either 530 or 600 nm, and the spectra of 1C and 1P indicates that all of these carboxylate- and phosphonatefunctionalized compounds relax to a common excited state, which must necessarily be the 3MMLL′CT state. The solvent and substituent effects can be linked together through an analysis of the difference in energy between the ground and excited states of the Pt complex. As observed in Figure 2, both the solvent and functional group dramatically affect the absorption of the Pt moiety while having little effect on the absorption energy of the Bodipy moiety. As the solvent is varied from toluene (ε = 2.38) to dimethyl sulfoxide (ε = 46.7), the Pt absorption maximum changes dramatically from 687 to ∼560 nm, a change of over 300 meV (Figure 2b). As seen in similar Pt(diimine)(dithiolate) complexes, this “negative solvatochromism” stems from either an excited state that is less polar than the ground state or an inversion of the dipole moment between the ground and excited states. The Pt excited state has been defined as MMLL′CT in character, where an electron is moved from an orbital that has appreciable Pt and bdt character to an orbital that is primarily on the bpy ligand (π*).7,16 The consequence of this electron shift is to significantly reduce and invert the dipole moment of the molecule in its excited state relative to that in its ground state. Alternatively, when the bipyridine is functionalized with direct binding to the carboxylate (1C, 2C) or phosphonate ester groups (1P, 2P), the electron-withdrawing nature of the

substituents pulls electron density away from the bipyridine. The LUMO, which is centered on the bipyridine, is therefore lowered in energy,9 red shifting the Pt absorption of 2C and 2P by ∼ 2600 and 2200 cm−1, respectively, relative to that of 2 (Figure 1c,d). In contrast, the CH2 spacer between the phosphonate ester and the bipyridine in 1spP and 2spP mitigates the electron-withdrawing effect, resulting in a spectral shift of much lower magnitude in 2spP, only ∼400 cm−1 relative to 2. In different solvents, the energies of the Pt moiety’s excited state shift with respect to the Bodipy’s mostly static 1ππ* energy level. This is due to the different changes in dipole moments between the ground and excited states for the two moieties. The Pt complex exhibits negative solvatochromism as the MMLL′CT transition moves electron density from the Pt center and electron-rich bdt to the bpy. Hence, the dipole moment of the ground state is opposite of that in the excited state, causing a dramatic increase in the energy of the absorption band as the solvent dielectric is increased (Figure 1).9 The ground and excited states of the Bodipy moiety, however, have similar dipole moments, as evidenced by its small Stokes shift, and the absorption maximum does not change appreciably with changes in solvent. As a result, in polar solvents, the 3MMLL′CT state lies above the 3ππ* energy of Bodipy, but in toluene and ether, the 3MMLL′CT state of Pt lies below the 3ππ* state of Bodipy. Therefore, in toluene and ether, the undesirable TEnT becomes unfavorable and thus is not observed during the time frame of this experiment (>1 ns for 3 and >1.6 ns for 2C and 2P). A similar situation in terms of the relative energies of the 3MMLL′CT and Bodipy 3ππ* states occurs for dyads 2, 2C, 2P, and 2spP in which the bpy substituents vary. For 2C and 2P, with the strongly electronwithdrawing substituents −COOR and −P(O)(OR)2, the 3 MMLL′CT state lies lower in energy than the Bodipy 3ππ* state, whereas for 2 and 2spP, the Bodipy 3ππ* state is lower than the 3MMLL′CT state. 10668

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Figure 10. (top) Schematic showing the effect of the solvent dielectric and bpy substituent. (bottom) Energy level diagram of series 2 dyads, as well as 3 in select solvents (DMSO = dimethyl sulfoxide; CLF = chloroform; ETH = diethyl ether; TOL = toluene). The energy scale is relative to the ground electronic state. Dark green bars correspond to Bodipy’s 1ππ* state, light green to the Bodipy 3ππ*, dark red to the Pt 1MMLL′CT, and orange to the Pt 3MMLL′CT state. See Table S13 (Supporting Information) for a complete list of values.

To illustrate this analysis, the energy levels of the dyads are shown in Figure 10. The energies of the 1ππ* state of the Bodipy moiety were measured directly from the ground-state absorption of dyads 2 or 3 in the appropriate solvent. The energies of the 1MMLL′CT states of the Pt moiety were found by measuring the ground-state absorption of the Pt(diimine)(dithiolate) complexes, series 1, in each solvent and then applying an energy offset obtained from comparing the spectra of the dyad to the Pt complexes (Figure S1, Supporting Information). Due to the different vibrational structure in different solvents, the actual E0,0 transition for dyad 3 in each solvent was found by fitting the Franck−Condon progression of the absorption spectrum (see Figure S14 (Supporting Information) for example). The energies of the 3MMLL′CT state of the Pt moiety were estimated by measuring the phosphorescence of series 1 and applying the same offset (see Figures S15 and S16 (Supporting Information) for emission and excitation spectra), as was determined to align the absorption spectra of series 1 with those of series 2 and 3. The energy of the 3ππ* state of the Bodipy moiety was approximated by comparing several literature values.25−28 Error bars are placed to illustrate the range of literature values. Because TEnT requires resonance between the Pt 3MMLL′CT emission and Bodipy S0 to 3ππ* absorption, a constant equal to the Stokes shift of Bodipy was added to the energy of the 3ππ* state. On the basis of the transient absorption data, we propose that the triplet energy for series 2 (Bodipy attached to bdt) is in the higher range of values (∼1.70 eV), where it lies above the Pt 3MMLL′CT state for 2C and 2P. However, we propose that the Bodipy 3ππ* state for dyad 3 (Bodipy attached to bpy) is in the lower range of values (∼1.64 eV), where it lies above the Pt 3 MMLL′CT state only in toluene and ether. Figure 11 displays the rates of SEnT versus the energy gap between the Bodipy 1ππ* state and the Pt 1MMLL′CT state. According to the Energy Gap law, for a similar set of molecules,

Figure 11. (a) SEnT rates of the bpy-bound Bodipy−Pt dyad 3 in various solvents versus the energy gap of the Bodipy 1ππ* state and Pt 1 MMLL′CT state, showing no real dependence on energy gap. (b) SEnT rates of bdt-bound Bodipy−Pt dyads (series 2) versus the energy gap of the Bodipy 1ππ* state and Pt 1MMLL′CT state, showing a strong correlation.

there should be an increase in the rate of relaxation as the energy difference between those two states decreases. A correlation would also be expected if the SEnT process occurred by electron exchange or fluorescence resonance energy-transfer processes, although in this case, the rate would reach a maximum when the energy gap induced maximum donor/acceptor resonance. In Figure 11a, no correlation is observed for the solvent study of dyad 3, presumably due to a contribution of other effects associated 10669

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Figure 12. Transient spectra of the bpy-bound Bodipy−Pt dyad (3) in toluene (a) and chloroform (b), displaying the dispersive nature of the Bodipy feature while the Pt complex is in the MMLL′CT state. (c, d) Transient spectra of the bdt-bound Bodipy−Pt dyad 2C and 2, displaying the dispersive nature of the Bodipy feature while the Pt complex is in the MMLL′CT state.

when the molecule has populated the 3MMLL′CT state. Likewise, in 2C and 2P at 25 ps (Figure 12c), the region between ∼525 and 550 nm indicates a red shift in the Bodipy absorption band. These shifts cause a dispersive feature in the transient spectra, where the positive region is to the blue of the ground-state absorption of 3 in toluene and to the red of the ground-state absorption of 2C and 2P. This dispersive feature is also transiently present at around ∼2 ps when 3 is dissolved in solvents with a higher dielectric constant (Figure 12b), as well as 2 and 2spP (Figure 12d), appearing after SEnT to the Pt complex and disappearing upon TEnT to the Bodipy moiety. In future work, this dispersive feature can be used as a marker band for the presence of a CT state adjacent to Bodipy in multichromophoric light-harvesting systems.

with solvent change. We attribute the lack of correlation of the SEnT rate with the energy gap to complications arising from specific solute/solvent couplings that cause large changes in the electronic coupling of the two states as the solvent changes. In general, it is well-known that the square-planar geometry of these Pt compounds makes them susceptible to coordination with the solvent in the axial positions.29 For instance, acetonitrile and dimethyl sulfoxide have the possibility of coordinating with the Pt center, and chloroform and dichloromethane contain easily polarizable but noncoordinating Cl atoms. It is clear from the steady-state spectroscopy of these and previous Pt complexes9 that specific solvent interactions impact the absorbance and emission maxima rather than simply being an effect of dielectric constant. This is also consistent with the large range of inhomogeneous broadening seen in the Franck−Condon structure of the absorption spectrum in the different solvents (Figure 2). However, a strong correlation of the SEnT rate and singlet energy gap does exist for series 2 containing varying bpy substituents (Figure 11b). This suggests that while the bpy substituent does red shift the Pt absorption, no strong perturbation to the electronic structure of the system occurs. In transient spectra where only the Pt MMLL′CT excited state should be observed (2C, 2P, 3 in toluene and ether), a signal is still present near the Bodipy ground-state absorption. The question remains regarding why this signal appears despite the fact that the Bodipy is in its ground electronic configuration. This additional signal indicates a shift in the Bodipy absorption band due to the close proximity of the highly polar MMLL′CT state. The MMLL′CT nature of the Pt moiety in 3 (Bodipy attached to bpy) and series 2 (Bodipy attached to bdt) results in a negative and positive charge residing next to the Bodipy moiety, respectively, which shifts the Bodipy ground-state absorption. This results in an opposite spectral shift when Bodipy is attached to the different sides of the Pt complex and the adjacent Pt chromophore is residing in an MMLL′CT state. When 3 is dissolved in toluene (Figure 12a), the region from ∼500 to 525 nm is indicative of a blue shift in the Bodipy absorption band in the 17.8 ps spectrum,

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CONCLUSION

The effects of solvent environment and electron-withdrawing substituents were studied in two Bodipy−Pt(diimine)(dithiolate) multichromophoric species. The lowest-energy state of the dyad is usually the 3ππ* state of the Bodipy moiety. When the Bodipy chromophore is attached to either bpy or bdt, excitation ultimately leads to population of this state in under 200 ps. This is disadvantageous in terms of solar energy applications as the Pt moiety’s charge-transfer state can facilitate necessary electron transfer. By turning to more nonpolar solvents or by using electron-withdrawing substituents on the bpy, the energy level of the Pt 3MMLL′CT state becomes lower in energy than the Bodipy 3ππ* state, effectively shutting off the unproductive TEnT pathway to the Bodipy 3 ππ* state. Using these methods, the 3MMLL′CT state of Pt remains for over 1 ns, the time frame of our experiment. These results indicate that by sensitizing the Pt complex with an organic dye, we are able to enhance light harvesting using both chromophores and set up the possibility for efficient electron transfer from the Pt 3MMLL′CT state into a semiconductor, for purposes of both solar hydrogen generation and DSSCs. 10670

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ASSOCIATED CONTENT

S Supporting Information *

Ground-state absorption spectra of 1 and 3 in all solvents and series 1 and 2, showing the superposition of the Pt peaks upon an energy offset. Transient absorption spectra and kinetic traces of dyads and Pt controls in various solvents. Tabulated time constants for 2 and 3. Energy values for excited states. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.W.M.). *E-mail: [email protected] (R.E.). Present Address †

W.-F.F.: Chinese Academy of Sciences, Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Beijing 100190, Peoples Republic of China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Science Foundation Collaborative Research grant, CHE-1151789. Additionally, D.W.M. was supported as an Alfred P. Sloan Research Fellow, and R.P.S. was supported by a National Science Foundation Graduate Research Fellowship. W.-F.F. was supported by the China Scholarship Council under the State Scholarship Fund.

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