Article pubs.acs.org/JPCA
Excimer Formation and Symmetry-Breaking Charge Transfer in Cofacial Perylene Dimers Rita E. Cook,† Brian T. Phelan,† Rebecca J. Kamire, Marek B. Majewski, Ryan M. Young,* and Michael R. Wasielewski* Department of Chemistry and Argonne−Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, United States S Supporting Information *
ABSTRACT: The use of multiple chromophores as photosensitizers for catalysts involved in energy-demanding redox reactions is often complicated by electronic interactions between the chromophores. These interchromophore interactions can lead to processes, such as excimer formation and symmetrybreaking charge separation (SB-CS), that compete with efficient electron transfer to or from the catalyst. Here, two dimers of perylene bound either directly or through a xylyl spacer to a xanthene backbone were synthesized to probe the effects of interchromophore electronic coupling on excimer formation and SB-CS using ultrafast transient absorption spectroscopy. Two time constants for excimer formation in the 1−25 ps range were observed in each dimer due to the presence of rotational isomers having different degrees of interchromophore coupling. In highly polar acetonitrile, SB-CS competes with excimer formation in the more weakly coupled isomers followed by charge recombination with τCR = 72−85 ps to yield the excimer. The results of this study of perylene molecular dimers can inform the design of chromophore−catalyst systems for solar fuel production that utilize multiple perylene chromophores.
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INTRODUCTION Covalent chromophore−catalyst systems with carefully selected linkages have been shown to demonstrate improved efficiency for CO2 reduction over solution mixtures due to improved charge separation rates.1−5 Further improvements have been observed for covalent multichromophore−catalyst systems for proton and CO2 reduction over covalent systems with only one chromophore or solution mixtures containing extra photosensitizer units.6−8 However, the use of multiple chromophores can also introduce new competitive processes that may be detrimental to the activity of the system. Specifically, both symmetry-breaking charge separation (SB-CS) and excimer formation may compete with the desired charge transfer (CT) pathway and reduce the catalytic activity of the system. For example, Sakai and co-workers observed that the covalent attachment of two [Ru(bpy)2(phen)]2+-type photosensitizers to the bipyridyl ligand of the proton reduction catalyst Pt(bpy)Cl2 resulted in no photocatalytic activity despite being derived from an active system that utilized only one chromophore.9,10 This lack of activity was attributed to rapid excited-state quenching via intramolecular CT between the two chromophores and highlights one potential mechanism that can reduce the activity of a multichromophoric system. SB-CS between chromophores has also been observed in weakly coupled molecular systems of perylene-3,4;9,10-bis(dicarboximide) (PDI), 11−14 perylene-3,4-dicarboximide (PMI),15 boron-dipyrromethene (BODIPY),16 and perylene.17 Multichromophore systems can also undergo excimer for© XXXX American Chemical Society
mation where the excitation energy is shared between chromophore units. Formation of the excimer state has been observed in systems with strongly coupled chromophores in cofacial or slip-stacked arrangements in dimers or trimers of tetracene,18 PDI,19−24 PMI,25 and BODIPY.26,27 Excimer formation in PDI and PMI dimers occurred within a few ps, which places a rate limit on how fast electron transfer to a catalyst must be in order to out-compete excimer formation. Perylene is a useful chromophore for photodriven reduction reactions due to its strong visible absorption (ε440 nm = 38 500 M−1 cm−1)28 and strongly reducing excited state (−1.7 V vs SCE). Importantly, it contains no scarce elements, such as Ru, that could hinder its deployment in large-scale applications. However, due to its comparatively short singlet excited-state lifetime of approximately 4 ns, the coupling between perylene and the catalyst must be sufficiently strong to facilitate efficient electron transfer, meaning that the covalent connection between the units should be short and fairly rigid. If used in a multichromophore system, this arrangement can place the chromophores in close proximity to one another, creating the potential for interchromophore electronic coupling and resulting in the associated competing processes described above. For this reason, detailed understanding of interchromophore interactions in perylene dimers is important for the Received: December 15, 2016 Revised: February 7, 2017 Published: February 13, 2017 A
DOI: 10.1021/acs.jpca.6b12644 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A
impart more conformational flexibility to the system and reduce the coupling between chromophores.
design of chromophore−catalyst systems employing two or more perylene units so that the nonproductive competitive processes can be avoided. Perylene dimers have been studied extensively in crystals,29−32 Langmuir−Blodgett films,33−35 concentrated solutions,36 and one weakly coupled molecular dyad,17 but no studies of strongly coupled, rigid molecular dimers have been reported to the best of our knowledge. Such systems permit the study of an isolated dimer for which the energy landscape is simpler than that in crystals and other extended structures37 and also provide the ability to tune the coupling by controlling the distance between chromophores or by introducing substituents on the chromophore that influence the dimer structure.19,21 For example, our group has used a 2,5dimethylphenyl (xylyl) group as a spacer between perylene and a 1,8-naphthalimide acceptor to facilitate efficient electron transfer and slow charge recombination in a system for photodriven proton reduction,38 and the presence of this group could be expected to alter the degree of coupling between perylene units. To understand the effects of interchromophore interactions in perylene dimers relevant to the design of multichromophore−catalyst systems, we report the synthesis and photophysical study of two molecular dimers of perylene on a xanthene scaffold. To create a strongly coupled dimer that closely resembles the structure observed in α-perylene crystals, two perylene molecules were directly attached to the xanthene scaffold to produce dimer 1, as shown in Figure 1. Dimer 2 includes a xylyl spacer between xanthene and perylene to mimic structures relevant to perylene-based photosensitizers for catalytic proton reduction.38 The xylyl spacer is expected to
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RESULTS AND DISCUSSION Synthesis and Structure. Two molecular perylene dimers (Figure 1) were prepared by cross-coupling reactions of 4,5dibromo-2,7-di-tert-butyl-9,9-dimethylxanthene with the boronic ester of perylene or 3-(2,5-xylyl)perylene, as outlined in Scheme 1. Full synthetic details are provided in the Supporting Information. Both dimers are present as a mixture of isomers (Figure 1) due to the asymmetric attachment to the 3-position of perylene and the asymmetry of the 2,5-xylyl spacer. The isomers of dimer 1 are observable by 1H NMR spectroscopy only at low temperatures and could not be separated by HPLC, while the presence of isomers in 2 is evident by 1H NMR spectroscopy at room temperature and could be separated by HPLC. However, isomerization of an isolated band of 2 occurs on the order of minutes to restore the mixture of isomers (see the Supporting Information). These differences in the isomerization rates of the dimers are likely due to reduced steric interactions in 1 compared to those in 2, in which the xylyl spacer hinders the rotations about the xanthene−xylyl and xylyl−perylene bonds. The energy-minimized molecular structures of dimers 1 and 2 computed using DFT are shown in Figures 2 and 3, and the relevant angles and distances are summarized in Table 1. The angle ψ is the angle between the long axes of the perylene units (the direction of the transition dipole moments).39 These axes in 1 are nearly parallel, while in 2 they are splayed by 12°. The dihedral angles φ1 and φ2 between the plane of each perylene and that of the xanthene scaffold are measured between the xanthene oxygen, the 4- or 5-carbon of xanthene that is attached to perylene or to the xylyl spacer, the 3-carbon of perylene, and the 2- or 4-carbon of perylene. The similar φ1 and φ2 values obtained for the two perylene units in 1 indicate that the perylene planes are nearly parallel, while in 2 the planes intersect one another with an angle of 47°. The π−π overlap is therefore greatest in 1, where the distance between the centers of mass of the perylene units d is 4.6 Å, compared to 7.2 Å in 2. Due to the herringbone-type structure and the greater distance between chromophores in 2, this system is expected to demonstrate weaker electronic interaction between perylene units. Steady-State Spectroscopy. The normalized steady-state absorption and emission spectra of monomeric Per-xy and dimers 1 and 2 in hexanes and MeCN are shown in Figure 4, and the spectral properties are summarized in Table 2. The absorption spectra of the dimers differ significantly from those of Per-xy, with apparent enhancement of the higher-energy transitions of the dimers compared to Per-xy. These differences can be understood using Kasha’s zero-order exciton coupling model,40 in which strong coupling between the transition dipole moments of the perylene units in the dimers causes the singlet excited state of perylene to split into two exciton states. For chromophores in a cofacial arrangement, such as in dimers 1 and 2, the electronic transition from the ground state to the upper exciton state is allowed, which results in a blue shift in the absorption spectra and an apparent enhancement of the higher-energy bands. Extension of the zero-order model to include vibronic coupling relaxes the selection rules and causes the additional observed transitions.19,41−44 The ratio of the 0−1 vibronic band to the 0−0 vibronic band is an indication of the strength of electronic coupling in these systems,21,25,45 and a
Figure 1. Structures of the model chromophore Per-xy and dimers 1 and 2, showing the possible rotational isomers. B
DOI: 10.1021/acs.jpca.6b12644 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A Scheme 1. Synthesis of Dimers 1 and 2a
a
Reaction conditions: Triphenylphosphine-3,3′,3″-trisulfonic acid trisodium salt, Na2CO3, Pd(OAc)2, 2:1 dioxane/H2O, 15−17 h, 85% (1) and 39% (2).
group in 2 imparts some flexibility to the system, as indicated by the optimized geometries in Figures 2 and 3. The stronger coupling in MeCN compared to that in hexanes is likely due to the weaker ability of MeCN to solvate perylene as the result of the large difference in polarity. The effects of interchromophore coupling are also observed in the steady-state emission spectra and fluorescence quantum yields of dimers 1 and 2. Per-xy is strongly emissive, with ΦF = 0.73 ± 0.02 in hexanes and ΦF = 0.83 ± 0.02 in MeCN, and the emission spectra in both solvents exhibit a vibronic progression mirroring that of the absorption spectra. In comparison, the emission of the dimers is significantly quenched, with ΦF < 0.01 for 1 in both solvents and ΦF = 0.08 ± 0.01 in hexanes and ΦF = 0.02 ± 0.01 in MeCN for 2. The normalized emission spectra of both dimers exhibit a broad, red-shifted band that is characteristic of the excimer state of perylene, which is formed as a result of strong coupling between the chromophores in the excited state.36,46−48 The excimer band is more red-shifted in 1 than that in 2 and in MeCN compared to that in hexanes for both dimers (Table 4). In addition to an excimer band, the spectra of dimer 2 also contain higher-energy features that dominate in hexanes, while the excimer band is larger in MeCN. The high-energy bands are approximately mirror images of the ground-state absorption of the dimer, which suggests that these bands are due to emission from the exciton state of the dimer and not to excimer emission or a monomeric impurity. Furthermore, the excitation spectra observed for 2 in hexanes (Figure S4) at both the high-energy emission and excimer emission closely resemble the absorption spectrum but suggest that they originate from different populations with differing degrees of coupling between the individual perylene moieties. This transition from the lower exciton state is partially allowed in systems where the transition dipoles of the chromophores are not parallel, such as in dimer 2, for which the angle between the long axes of the two perylenes is 12° (Table 1).42 The origin of this population will be discussed further below. The small shoulder on the blue edge of the emission spectra of dimer 1 is likely from a minor monomeric impurity (
