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Cite This: J. Phys. Chem. C 2018, 122, 13848−13862

Positional Effects from σ‑Bonded Platinum(II) on Intersystem Crossing Rates in Perylenediimide Complexes: Synthesis, Structures, and Photophysical Properties James E. Yarnell,¶,† Iryna Davydenko,¶,‡ Pavel V. Dorovatovskii,§ Victor N. Khrustalev,∥,⊥ Tatiana V. Timofeeva,∥ Felix N. Castellano,*,† Seth R. Marder,*,‡ Chad Risko,*,# and Stephen Barlow‡ Downloaded via KAOHSIUNG MEDICAL UNIV on June 28, 2018 at 07:29:41 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States § National Research Center “Kurchatov Institute”, Moscow 123182, Russia ∥ Department of Chemistry, New Mexico Highlands University, Las Vegas, New Mexico 87701, United States ⊥ Department of Inorganic Chemistry, Peoples’ Friendship University of Russia (RUDN University), Moscow 117198, Russia # Department of Chemistry & Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky 40506-0055, United States ‡

S Supporting Information *

ABSTRACT: In this investigation, the synthesis and photophysical properties of a series of new chromophores featuring Pt(II) σ-bonded to perylenediimide (PDI) cores are reported. A Pt(PPh3)2X (X = Cl, Br) moiety was attached to PDI in either the ortho or the bay position (2- or 1-positions respectively) or a Pt(PPh3)2 subunit was used to bridge two bay positions (1- and 12-positions) forming a Pt(II) cyclometalate. Through a combination of steady-state and transient absorption and photoluminescence spectroscopy, the excited-state dynamics of these molecules were revealed, indicating that the Pt atom location on the PDI has a substantial impact on observed intersystem crossing (ISC) rates. The ISC time constants for the bay-substituted and cyclometalated PDIs are between 2.67 and 1.29 ns, respectively, determined by the singlet fluorescence decays from the initially populated singlet excited states. In the case of the ortho-substituted PDI, ISC to the triplet state occurs on the ultrafast time scale with a time constant of 345 fs, determined through ultrafast transient absorption spectroscopy. In all instances, the measured PDI-based fluorescence quantum yields quantitatively correlate with the measured ISC rates leaving little doubt that variation in the Pt(II) substitution position(s) markedly influence the resultant photophysics. Electronic structure calculations suggest differing amounts of metal-based contributions in the lowest excited states as a function of substitution position, supporting the trend of the observed ISC rates.

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INTRODUCTION Perylenediimide (PDI) chromophores have received immense research attention due to their exceptional set of chemical and photophysical properties.1−7 In addition to their fluorescence and absorption properties, these chromophores also exhibit high electron affinities and electron-transport properties, making them useful in arenas outside of the traditional dye and pigment market.8−10 These attributes have enabled PDIs to be used in niche applications such as electron-transporting semiconductors in organic field-effect transistors,11−13 lightemitting diodes,14 light-harvesting,15−20 and artificial photosynthetic systems. 21 Recently, a wide range of PDI chromophores have been incorporated, with some success, © 2018 American Chemical Society

into bulk-heterojunction organic photovoltaics as nonfullerene electron acceptors.12,22−26 The incorporation of nonfullerene chromophores in organic photovoltaics remains an active research area with PDIs being one of the main classes of acceptors being actively investigated.27 Additionally, molecular PDI species have been utilized as optical limiting materials,28,29 as acceptors/annihilators in photochemical upconversion Special Issue: Prashant V. Kamat Festschrift Received: January 29, 2018 Revised: February 22, 2018 Published: February 23, 2018 13848

DOI: 10.1021/acs.jpcc.8b01003 J. Phys. Chem. C 2018, 122, 13848−13862

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The Journal of Physical Chemistry C Chart 1. Pd(II) and Pt(II) PDI Derivatives and Numbering of the PDI Core

schemes,30,31 and to facilitate understanding of exciton coupling in polychromophoric systems.32−35 Numerous examples of PDI chromophores integrated into transition metal complexes have enabled triplet state photophysics to be readily accessed from the PDI platform as well.3,36−38 In some solar energy conversion applications, access to the long-lived triplet state has been shown to enhance device performance.39,40 Given the high fluorescence quantum yields of PDIs, the triplet excited states are rarely seen in this genre of chromophores unless sensitized by triplet donors or attached to heavy atoms (such as Pt) to increase spin−orbit coupling. Ford and Kamat first demonstrated bimolecular triplet sensitization of PDI in 1987,41 so the quest for generating triplet excited states in PDI chromophores now spans 3 decades and continues to garner interest from the scientific community. One of the first PDIs where a significant heavy atom effect was evident in a platinum-acetylide motif fused to the 1-position (“bay”) of the PDI.36,37 Effective quenching of the fluorescence in these Pt(II) acetylide PDI molecules was shown to be due to rapid singlet-to-triplet intersystem crossing (kISC > 1011 s−1).36,37 There are numerous examples of transition metal containing PDI structures already established featuring drastically different connectivity and electronic coupling between the subunits.3 Other methods to create triplet excitons in PDI’s include singlet exciton fission (SF) where the singlet exciton is energetically down converted to two triplet excitons.42 The SF process, in addition to the heavy atom effect, was demonstrated to efficiently access the triplet state in a 1,6,7,12-tetrabromo-PDI derivative.43 Charge-transfer states involving the metal are thought to play an important role in facilitating intersystem crossing in many molecules where metals are attached to organic chromophores. Many of these systems have focused on d6 metal complexes containing Ru(II), Os(II), Re(I), and Ir(III).44−54 More recently, there has been a shift of interest to d8 Pt(II) complexes of the type (triimine)PtL or (diimine)PtL, where L = acetylide, halide, isonitrile, nitrile, or thiolate.55−67 In the simplest Pt(II) polypyridyl complexes, the photophysical properties are often dominated by low-energy metal-to-ligand charge-transfer (MLCT) excited states where some portion of

the highest occupied molecular orbital (HOMO) originates from Pt d-orbitals and the lowest unoccupied molecular orbitals (LUMO) reside on the polypyridyl ligand.68−70 In the complexes containing bipyridine or terpyridine, the open coordination sites on the Pt can be occupied by arylacetylide moieties, introducing the possibility of ligand-centered (LC) 3 π,π* excited states to be isoenergetic with, or lower in energy than the 3MLCT excited states. There are some cases in the literature where late transition metals such as Pd(II) or Pt(II) were σ-bonded directly to perylene-based chromophores. In 2007, Rybtchinski and coworkers attached Pd(II) to the bay positions of PDI forming two 1,7-dipalladium PDI complexes.71 Despite having Pd(II) attached, both molecules displayed high fluorescence quantum yields (65% and 22%) demonstrating that intersystem crossing was not very efficient. The insensitivity of absorption or fluorescence emission to solvents of different polarity was taken as indicative of very little charge-transfer character in the electronic transition. More recently, Espinet and co-workers studied two series of chromophores with Pt(II) σ-bonded to both perylene and perylenemonoimide (PMI).72 Like the previous PDI examples, these molecules remained highly fluorescent (30−80%) with Pt(II) directly fused to the organic chromophore. Those authors demonstrated that the fluorescence energy of the perylene or PMI could be shifted by changing the ancillary ligands on the Pt(II) center, presumably through the polarization of the Ar−Pt bond since the platinum d-orbitals were shown to have weak interactions with the intraligand π,π* excited state. In the current study, the synthesis and photophysical properties of a series of new chromophores are reported in which a Pt(PPh3)2X (X = Cl, Br) moiety is attached in either the ortho- or the bay position (2- or 1-positions respectively) of the PDI, or in which a Pt(PPh3)2 moiety bridges the bay 1,12positions forming a cyclometalated PDI (Chart 1); these are compared to a model PDI chromophore that omits the metalcontaining fragment(s).33,73 One example of an ortho-Pd-PDI complex is also reported, although not studied from a photophysical point of view. Through the use of static and transient spectroscopy, the excited-state dynamics of the Pt(II)13849


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The Journal of Physical Chemistry C Scheme 1. Chromophore Synthesis

containing complexes are revealed, indicating that the Pt(II) atom substitution location on the PDI subunit has a substantial impact on ISC rates and single fluorescence quantum yields. When Pt(II) is bonded to the ortho-position of the PDI, ISC to the triplet state is nearly instantaneous, resulting in no observed fluorescence from the PDI moiety. Attaching the Pt(II) center to yield bay-substituted or cyclometalated structures does induce some ISC, however, there is still measurable fluorescence observed in both molecules. Electronic structure calculations suggest differing amounts of metal contribution in

the lowest excited states, consistent with the observed trends in the experimentally determined ISC rates.

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EXPERMENTAL METHODS Synthesis and Characterization. Complete details concerning the synthesis and characterization of all compounds are provided in the Supporting Information. The synthetic procedures used to make all the chromophores are outlined in Scheme 1. The identity and purity of all compounds was confirmed using 1H, 13C, and 31P NMR spectroscopy, highresolution mass spectrometry, and elemental analysis; X-ray 13850


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The Journal of Physical Chemistry C crystal structures were obtained for bay-Br/cyclo-Pt-PDI as well as bay-2Pt-PDI. Time-Correlated Single Photon Counting. Timeresolved photoluminescence experiments made use of a timecorrelated single photon counting (TCSPC) spectrometer (Edinburgh Instruments, LifeSpec II) as described previously.74 For the measurements, the samples were excited at 500 nm and pulse picked to a 4 MHz repetition rate. The TCSPC emission intensity decays were analyzed using Origin 2016 software. Ultrafast Transient Absorption Spectroscopy. The transient absorption measurements were performed at the NCSU Imaging and Kinetic Spectroscopy (IMAKS) Laboratory using a mode-locked Ti:sapphire laser (Coherent Libra) as described previously.75 The pump beam was directed into a parametric amplifier (Coherent OPerA Solo) to generate the 535 and 650 nm excitation. The probe beam was focused onto a sapphire crystal to generate a white light continuum between 425 to 775 nm or a NIR crystal to generate a light continuum between 825 to 1200 nm. The pump beam was focused and overlapped with the probe beam through a 2 mm path length cuvette. The ground-state absorption spectra were taken before and after each experiment to ensure there was no sample photodegradation during the experiment. The transient kinetic data at specific wavelengths was evaluated using the fitting routines available in OriginPro 2016. The global fit analysis was performed using Surface Xplorer available from Ultrafast Systems. Nanosecond Transient Absorption Spectroscopy. Transient absorption measurements were collected using a LP920 laser flash photolysis system (Edinburgh Instruments) described previously.76 Samples were prepared to have a 0.5 optical density at the excitation wavelength (3.0 mJ/pulse) in a 1 cm path length quartz optical cell and degassed using the freeze−pump−thaw technique. All transient absorption experiments were performed at room temperature. The reported difference spectra and kinetic data are the average of 50 laser shots. The ground-state absorption spectra were taken before and after each experiment to ensure there was no sample degradation. The transient kinetic data were evaluated using the fitting routines available in OriginPro 2016. X-ray Crystallography. Single-crystal X-ray structure determinations were carried out for bay-Br/cyclo-Pt-PDI and bay-2Pt-PDI. In the former case, data were acquired using synchrotron radiation due to the small, poor-quality crystal obtained, whereas for the latter, data were obtained using a conventional diffractometer. In bay-Br/cyclo-Pt-PDI there is disorder between the bay C−Br and C−H positions, with the major and minor molecular orientations present in an approximate 85:15 ratio. Disorder of the N,N′-dialkyl substituents, which, at least in part, is likely due to the chirality of the specific substituents used and, therefore, the formation of a diastereomeric mixture, was modeled as 55:45 and 50:50 mixtures of two orientations. Disorder in the alkyl groups of bay-2Pt-PDI was not modeled, but likely results in the large thermal parameters seen for these atoms. Further details of the data collection and refinement are provided as part of the main Supporting Information document and in CIF format. The crystallographic data (CCDC1814201 and CCDC1814202 for Br/cyclo-Pt-PDI and bay-2Pt-PDI respectively) may also be obtained from the Cambridge Crystallographic Data Center (http://www.ccdc.cam.ac.uk/conts/retrieving.html). Density Functional Theory (DFT) Calculations. All density functional theory (DFT) and time-dependent DFT

(TDDFT) calculations were carried out with the M06 functional77 and the 6-31G(d,p) − for the first-row atoms− and LANL2DZ − for Pt, Pd, Cl, Br, and P − basis sets.78−82 For each system reported, the geometries of the neutral, ground singlet (S0) and lowest-lying triplet (T1) states were optimized; the N-alkyl groups were truncated to methyl groups to reduce computational cost. Unconstrained geometries were confirmed as minima on the potential energy surface through normalmode analyses. We note that for the molecule denoted bay2Pt-PDI that the optimized T1 state possessed one negative frequency at −3.9 cm−1, attributed to a torsion across the entire molecule; given the size of bay-2Pt-PDI and that the triplet states were not characterized experimentally, we report only general aspects of the T1 state in this system. The Gaussian09 (Revision A.02) software suite was used for the DFT and TDDFT calculations.83

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RESULTS AND DISCUSSION Synthesis and Structural Characterization. Recently, it was reported that trans-Pd(PPh3)2Cl substituents can be readily incorporated into the meso-positions of polymethines and merocyanines, and Pd(PPh3)2Br into the 1,7-positions of PDIs, through reaction of Pd(PPh3)4 with the corresponding halogen derivatives, and that these substituents are effective disaggregating groups that disrupt intermolecular dye−dye π−π interactions.84,85 This reaction was extended to obtain molecules in which Pt(II) is σ-bonded to a PDI core and molecules bay-Pt-PDI − where, in contrast to its Pd(II) analogues, the two phosphines are cis to one another−and cyclo-Pt-PDI were obtained.84 Given the rapid intersystem crossing to the triplet manifold seen for Pt-acetylide-functionalized PDIs,36−38,86 we were interested in the effect of Pt directly bound to the PDI and so here we study the previously reported examples, bay-Pt-PDI and cyclo-Pt-PDI, in more detail, along with related new compounds ortho-Pt-PDI, orthoPd-PDI, and bay-Br/cyclo-Pt-PDI (Scheme 1). The starting material for the ortho-metalated derivatives, ortho-Bpin-PDI, was obtained using a literature procedure.87−89 Treatment of ortho-Bpin-PDI with CuCl2 yields ortho-Cl-PDI in good yield (78%). Pt(PPh3)4 reacts with ortho-Cl-PDI to give ortho-Pt-PDI (yield 49%). 1H and 31P NMR spectra confirm a trans configuration for the PPh3 ligands; interestingly, this differs from what has been seen for bay-Pt-PDI, where a cis arrangement of phosphine ligands is obtained.84 This reaction was also conducted using Pd(PPh3)4 to give ortho-Pd-PDI with 67% yield, which also, according to 1 H and 31P NMR spectra, has trans PPh3 ligands. DFT calculations indicate that, regardless of the PDI position, the metal, and the halogen, the trans-isomers are more thermodynamically stable than the cis. Accordingly, the formation of the cis isomer for bay-Pt-PDI is likely due to kinetic effects; specifically for this reaction the oxidative addition to the metal center may proceed in a single concerted step, whereas for the other position/metal/halogen combinations examined, the reaction may be stepwise, consisting of a nucleophilic attack by the metal on the halogenated PDI derivative, followed by loss of halide and reaction of that halide ion with the resulting metal complex. The reaction of bay-2Br-PDI with Pt(PPh3)4 (1:2 eq ratio) was less straightforward than the ortho-metalations: thin-layer chromatography indicated the formation of several red products with similar Rf values, along with a green product. The green product was isolated by column chromatography 13851


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The Journal of Physical Chemistry C and found to be pure bay-Br/cyclo-Pt-PDI. This is analogous to the observation of the formation of cyclo-Pt-PDI alongside that of bay-Br-PDI (and of cyclo-Pt-PDI′ and bay-Pt-PDI′, which differ in their N,N′-alkyl substituents). However, in this case, it was possible to use X-ray diffraction to confirm the cyclic structure of these species inferred from NMR spectra, mass spectra, and elemental analysis. The red material from the reaction was not fully purified; however, a single crystal obtained from the mixture was found, using single-crystal X-ray diffraction, to be bay-2Pt-PDI, which showed a cis configuration of PPh3 ligands, as deduced by NMR for the reaction products of Pt(PPh3)4 with other bay-brominated PDIs. The M(PPh3)2X (M = Pd/Pt, X = Cl/Br) group significantly increases the solubility of the corresponding unsubstituted PDIs in common organic solvents; however, the cycloplatinum compounds (bay-Br/cyclo-Pt-PDI and cyclo-Pt-PDI) are much less soluble than the M(PPh3)2X species (soluble in THF and toluene; insoluble in methanol, acetone, and acetonitrile). Thermogravimetric analysis (5 °C min−1, Supporting Information) indicates that the organo-Pd and Pt derivatives exhibit reasonably good thermal stability, albeit much lower stability than simple nonmetalated PDIs (decomposition temperatures are 210, 280, 240, and 400 °C for ortho-PdPDI, bay-Pt-PDI, cyclo-Pt-PDI, and ortho-Cl-PDI, respectively; Figure S1). Crystal Structures. Crystal structures were determined for bay-Br/cyclo-Pt-PDI and bay-2Pt-PDI. Although neither of these compounds was a focus of the photophysical work described below, these two structures support the structures inferred from other techniques for cyclo-Pt-PDI and bay-PtPDI, respectively, as well as giving insight into the molecular conformations in these molecules. Figures 1 and 2 show views

of the molecular structures of the two molecules (two independent molecules in the case of bay-2Pt-PDI), in each case omitting the alkyl and phenyl groups and, in the case of bay-Br/cyclo-Pt-PDI, the minor Br positions for clarity. The structure of bay-Br/cyclo-Pt-PDI confirms the presence of a platinum-containing five-membered ring. The structure shows evidence for some ring and steric strain. The C−Pt−C and P−Pt−P angles of 80.1(4) and 99.15(14)°, respectively, are unremarkable for a cis-Pt(PPh3)2Ar2 derivative (C−Pt−C angles are 81.6, 82.8, and 85.3° and P−Pt−P angles are 100.9, 100.4, and 99.8° for Ar = o-MeOC6H4,90 o-O2NC6H4,91 and 7-methyl-1-naphthyl derivatives,92 respectively). However, the distortion from planarity of Pt and the atoms coordinated to it is much more significant: the sum of bond angles at Pt is 363.9° and the angle between the PtC2 and PtP2 planes is 22.0°, while for cis-Pt(PPh3)2Ar2 these values are typically closer to 360 and 0°, respectively (360.0−360.2° and 1.1−5.3° for the three examples cited above).90−92 The distortions are, however, similar to those seen for other structures in which the two carbon atoms are constrained in a five-membered ring fused to two conjugated rings, in particular to those in two structures of Pt(PPh3)2(2,2′-biphenyl)93,94 (sum of angles, 361.9−363.0°; PtC2/PtP2 angle, 6.3−15.8°) and in the structure of Pt(PPh3)2(2,2′-biheptatrienyl)95 (365.1, 25.5°). In addition, as in these other three structures, the five-membered ring is puckered; in bay-Br/cyclo-Pt-PDI there is an angle of 8.7° between the PtC2 plane and the plane formed by the four C atoms (cf. 6.3−26.4° in literature structures).93−95 The perylene core remains fairly planar (torsion angles defined by the four adjacent atoms of the bay region are 0.1° in the metalated bay region and 3.3° in the monobrominated bay. The apparent Pt−C bond lengths (2.012(13) and 2.012(15) Å) are rather short compared to those in typical trans-Pt(PPh3)2Ar2 compounds (e.g., 2.08−2.09 Å for Ar = Ph),96,97 cisPt(PPh3)2Ar2 derivatives (2.03−2.08 Å),90−92 or in Pt(PPh3)2(2,2′-biphenyl)93,94 (2.07−2.09 Å), but are determined somewhat imprecisely and are perhaps affected by the disorder present in the structure. The crystal packing consists of centrosymmetric slipped π-stacked dimers, in which the unmetalated side of one molecule substantially overlaps with that of another, with a distance of 3.41 Å between the two perylene planes (Figure S4). As noted above, the structure of bay-2Pt-PDI contains two crystallographically independent molecules, each of which has crystallographic inversion symmetry. The Pt(II) coordination is almost perfectly square planar (sum of bond angles around Pt ca. 360.2° for both independent molecules, bond angles varying from 84.3(3) to 100.1(1)°, with the P−Pt−P angles being largest due to the bulk of the PPh3 ligands), the cisconfiguration of the two phosphines on each metal center is clearly confirmed, and the planes formed by the Pt atoms are its coordinated atoms are roughly perpendicular to the average plane of the perylene core (85.6 and 89.7° for the two independent molecules). As in other cis-Pt(PR3)2PhX derivatives,98−100 the Pt−P bond lengths trans to the PDI C atoms (2.347(3) and 2.338(3) Å) are significantly longer than those trans to Br (2.239(3) and 2.235(3) Å), consistent with the greater structural trans influence of aryl vs halide ligands. The crystallographic centrosymmetry, coupled with the cis-configuration of phosphines, leads to (for each molecule) one PPh3 projecting above the perylene and the other below. The perylene cores in the structure of bay-2Pt-PDI are not perfectly planar. One way of quantifying this distortion is the torsion

Figure 1. Two views of the molecular structure of bay-Br/cyclo-PtPDI (C gray, N blue, O red, P orange, Br brown, Pt green; 50% thermal ellipsoids) in its crystal structure (H atoms, the minor Br position, the alkyl chains apart from the C atoms directly bound to N, and the phenyl rings apart from the C atoms directly bound to P are excluded for clarity to emphasize the geometry of the PDI and the metallocycle; the full structure is shown in Figure S2). 13852


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The Journal of Physical Chemistry C

Figure 2. Two views of the molecular structures (C gray, N blue, O red, P orange, Br brown, Pt green; 50% thermal ellipsoids) of the two crystallographically independent centrosymmetric molecules in the structure of bay-2Pt-PDI (H atoms, alkyl chains apart from the C atoms directly bound to N, and phenyl rings apart from the C atoms directly bound to P are excluded for clarity; full structure shown in Figure S3).

Pt-PDI (αS0 = 18.1°; αT1 = 14.2°) and bay-2Pt-PDI (αS0 = 15.1°; αT1 = 11.4°), as one may expect, demonstrate a more twisted structure in the bay region. The value for the ground state of bay-2Pt-PDI can be compared with values of 8.3 and 14.2° for two independent molecules in the corresponding crystal structure (see above), where intermolecular interactions likely affect the torsion angle. The Pt−C bond lengths show almost no change for both the bay-Pt-PDI (lS0 = 2.04 Å; lT1 = 2.04 Å) and bay-2Pt-PDI (lS0 = 2.04 Å; lT1 = 2.04 Å; lS0 = 2.026(10) and 2.051(12) Å for the two independent molecules in the crystal structure) between the S0 and T1 states. Closure of the bay rings to form cyclo-Pt-PDI and bay-Br/cyclo-Pt-PDI result in very similar trends: Neither the bay torsions − αS0 = 2.2° and αT1 = 1.3° for cyclo-Pt-PDI and αS0 = 1.7° and αT1 = 0.5° for bay-Br/cyclo-Pt-PDI − nor the Pt−C bonds − lS0 = 2.06 Å and lT1 = 2.05 Å for cyclo-Pt-PDI and lS0 = 2.06 Å and lT1 = 2.04 Å for bay-Br/cyclo-Pt-PDI − reveal much difference between the two states. The closed cyclo-Pt rings in both structures have a (puckered) geometry and the metal coordination is significantly distorted from square planar toward tetrahedral, consistent with the crystal structure for bay-Br/cyclo-Pt-PDI. Finally, a similar pattern holds for the ortho-Pt-PDI and ortho-Pd-PDI systems. The geometries of the PDIs are slightly distorted from planarity in the two states, as defined by bay torsions αS0 = 7.0° and αT1 = 5.4° for orthoPt-PDI and αS0 = 3.4° and αT1 = 4.0° for ortho-Pd-PDI. Further the metal−carbon bonds show almost no difference when comparing the S0 and T1 stateslS0 ∼ lT1 ∼ 2.01 Å for both structures.

angles defined by the 4 adjacent atoms of the bay region (C1− C12b−C12a−C12, equivalent by symmetry in this structure to the C6−C6a−C6b−C7 torsion; α in the following discussion of DFT results). For the two independent molecules in the crystal these values are found to be 8.3 and 14.2°. Such distortions from planarity are often found in PDIs with bulky “bay” substituents,84 although in the analogue of bay-2Pt-PDI with trans-Pd(PPh3)2Br substituents, where there are large phosphines both above and below the perylene in both bay positions, this angle is a negligible ca. 1.5°. Although the molecular geometry and, therefore, molecular packing in bay2Pt-PDI is quite different to that in its previously reported Pd analogue,84 due to the presence of cis-Pt(PPh3)2Br rather than trans-Pd(PPh3)2Br groups, there is no PDI−PDI π-stacking in either structure. DFT Evaluations of the Geometric and Electronic Structures. Prior to our discussion of the optical characteristics of these systems, we present a brief DFT-based analysis of the molecular geometric and electronic structures as determined at the M06/6-31G(d,p)/LANL2DZ level of theory. Seven different molecular systems were investigated: PDI, bayPt-PDI, bay-2Pt-PDI, cyclo-Pt-PDI, bay-Br/cyclo-Pt-PDI, ortho-Pt-PDI, and ortho-Pd-PDI (with, in each case, the N,N′-alkyl groups simplified to methyl groups). We begin with how the metal-substitutions perturb the geometric characteristics of PDI. To focus the discussion, we emphasize two main geometric parameters in the S0 and T1 states: (i) the dihedral angle or torsion (α) among the bay carbon atoms in the PDI ligand (C1−C12b−C12a−C12 and C6−C6a−C6b−C7 torsions), and (ii) the length of the metal− carbon bond (l). For both the S0 and T1 states of PDI, there is no torsion in the bay region (αS0 = 0.0°; αT1 = 0.0°). The bay13853


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The Journal of Physical Chemistry C Table 1. Steady-State Photophysical Data of the Chromophores in This Studya molecule

λabs max (nm) (ε, 104 M−1cm−1)b

estimated oscillator strength, f

λem max (nm)b

ΦPL (%)c

PDI ortho-Pd-PDI ortho-Pt-PDI bay-Pt-PDI cyclo-Pt-PDI bay-Br/cyclo-Pt-PDI

523 (8.06) 544 (6.9) 546 (6.4) 565 (5.4) 655 (3.0) 656 (2.8)

0.58 0.65 0.68 0.50 0.29 0.29

535 562 n.d. 585 682 679

100 1 − 37 5 1

a Chromophores measured in chloroform at room temperature. bAbsorption and emission maxima, ± 2 nm. cQuantum yield measurements were performed using an integrating sphere, values ±5%.

While the geometric comparisons show, at best, modest differences in geometry, there are notable differences in the electronic structures that will be of importance in understanding the nature of the excited states (see below). Pictorial representations of select frontier molecular orbitals are in the Supporting Information. Again, we start with PDI as the reference system, where both the highest-occupied molecular orbital (HOMO, −6.29 eV) and lowest-unoccupied molecular orbital (LUMO, −3.37 eV) are delocalized across the πconjugated structure; the HOMO−LUMO gap (ΔHL) is 2.91 eV. For the four bay- and cyclo-Pt structures, the HOMO and LUMO are PDI-centered, with more metal dcharacter in the HOMO than in the LUMO. For these systems, the HOMOs (by 0.71−1.15 eV) and LUMOs (by 0.62−1.03 eV) are energetically destabilized relative to the respective PDI orbitals, resulting in ΔHL being somewhat smaller (by 0.08− 0.34 eV), the smallest values being found for the two metallocycles. The trends in orbital energies for PDI, bay-PtPDI, and the metallocycles are broadly consistent with those in electrochemical oxidation and reduction potentials (see Table S2, Supporting Information). Notably, the energetic gap between the HOMO and HOMO−1 for these four systems ranges from 0.50−1.17 eV; unlike the HOMO and LUMO, the HOMO−1’s are strongly metal-centered orbitals. The situation differs for the two ortho-substituted systems. While the HOMO and LUMO remain mainly PDI centered with some metal d-character, there is notable wave function delocalization in the HOMO across the Pt and onto the transCl atom. The HOMO and LUMO are energetically destabilized compared to those of PDI, but now only by ∼0.30−0.40 eV; ΔHL ∼ 2.88 eV for both systems. The smaller shifts seen for ortho trans-M(PPh3)2Cl-substitution relative to what is seen for bay cis-Pt(PPh3)2Br substitution are again consistent with electrochemical potentials (Table S2) and indicate less metalto-PDI back-bonding in the ortho position than the bay. A cisPt(PPh3)2X substituent might be expected to be a less effective π-donor than a trans-Pt(PPh3)2X group due to competition for back-bonding in the former from the PPh3 group trans to the PDI; indeed calculations for model compounds (Table S4) confirm that, for a given substitution position, both HOMO and LUMO are more destabilized by a trans-Pt(PPh3)2X group than by a cis (and that there is negligible difference between X = Cl and Br derivatives in a given position with a given stereochemistry). This is presumably outweighed by the smaller coefficients for both the HOMO and LUMO of PDIs in the ortho vs bay position. Further, the energetic gap between the HOMO and HOMO−1s are also smaller than in the case of the bay-substituted structures: 0.13 eV for ortho-Pd-PDI and 0.12 eV for ortho-Pt-PDI. Again, the HOMO−1 (and HOMO−2 and HOMO−3) for the ortho-substituted structures are strongly metal-centered, and all three molecular orbitals are

in close energetic proximity, i.e. the orbitals lie within an energy range of 0.41 eV for ortho-Pd-PDI and 0.36 eV for ortho-PtPDI) with respect to the HOMO. These results are relevant to the nature of the low-lying excited-state transitions in the singlet manifold, discussed in the following section alongside the experimental absorption and emission spectra. Absorption and Photoluminescence Spectroscopy. Pertinent results from the absorption and emission spectra of PDI, ortho-Pt-PDI, bay-Pt-PDI, and cyclo-Pt-PDI are summarized in Table 1. The low-energy transitions for these four chromophores are presented in Figure 3. The lowest

Figure 3. Electronic absorption spectra of PDI, ortho-Pt-PDI, bay-PtPDI, and cyclo-Pt-PDI in CHCl3.

energy absorption in the unsubstituted PDI chromophore exhibits well-defined vibronic structure and with a maximum of 525 nm, both of which are entirely typical for simple PDIs;5 the low-energy bands of ortho-Pt-PDI, bay-Pt-PDI, and cyclo-PtPDI are similarly structured, but are increasingly red-shifted (by 805, 1390, and 3850 cm−1, respectively) and show decreasing peak absorptivities. The large red shift seen for cyclo-Pt-PDI, combined with the presence of another strong absorption band with a maximum at ca. 450 nm, results in a green, rather than red, appearance. The TDDFT (M06/LANL2DZ/6-31G(d,p)) calculations reveal some intriguing differences in the origins of the transitions of the PDI-based metal complexes. Table 2 provides select information on the low-lying singlet excited states (S0 → Sn). Overall, there is quite good agreement in terms of the transition energy trends for the low-lying S0 → Sn excitations in the series between the UV/vis experiments and the TDDFT calculations. For PDI, bay-Pt-PDI, cyclo-Pt-PDI, and bay-Br/ 13854


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Table 2. Energies (Wavelength), Oscillator Strength (f), and Electronic Configurations for Selected S0 → Sn Excitations As Determined via TDDFT at the M06/LANL2DZ/6-31G(d,p) Level of Theory molecule PDI ortho-Pd-PDI

ortho-Pt-PDI

bay-Pt-PDI cyclo-Pt-PDI bay-Br/cyclo-Pt-PDI a

transition

E (eV) (λ, nm)

f

electronic configurationsa

S0 → S1 S0 → S1

2.49 (497) 2.32 (535)

0.68 0.09

S0 → S2

2.40 (516)

0.20

S0 → S3

2.49 (498)

0.30

S0 → S1

2.24 (553)

0.03

S0 → S2

2.42 (513)

0.43

S0 → S1 S0 → S1 S0 → S1

2.32 (535) 1.97 (631) 1.95 (635)

0.37 0.23 0.21

HOMO → LUMO (99%) HOMO−2 → LUMO (44%) HOMO → LUMO (34%) HOMO−3 → LUMO (12%) HOMO → LUMO (51%) HOMO−1 → LUMO (35%) HOMO−1 → LUMO (54%) HOMO−2 → LUMO (31%) HOMO → LUMO (12%) HOMO−1 → LUMO (59%) HOMO → LUMO (31%) HOMO → LUMO (65%) HOMO−1 → LUMO (31%) HOMO → LUMO (95%) HOMO → LUMO (96%) HOMO → LUMO (97%)

Electronic configurations with greater than 10% contribution are listed.

Figure 4. Natural transition orbitals (NTOs) for select S0 → Sn excitations of ortho-Pt-PDI determined at the M06/LANL2DZ/6-31G(d,p) level of theory. λ is the fraction of the hole−particle contribution to the excitation.

PDI (and ortho-Pd-PDI), it is the S0 → S2 (and S0 → S2 and S0 → S3) excitation that corresponds to the first experimentally observed transition. In these transitions, the hole and electron NTO show significant PDI character, though the hole is strongly delocalized through the metal to the trans-Cl atom. To conclude this analysis, we briefly discuss the nature of the T1 states. The T1 state energies computed via ΔSCF (ET1 − ES0) and TDDFT at the M06/LANL2DZ/6-31G(d,p) level of theory are consistent. The T1 energies for PDI, bay-Pt-PDI, bay-2Pt-PDI, ortho-Pt-PDI, and ortho-Pd-PDI are ∼1.2 eV, while the formation of the cyclo structures in cyclo-Pt-PDI and bay-Br/cyclo-Pt-PDI reduces the T1 energy to 1.0 eV (Table 3). NTOs (see Supporting Information) for the S0 → T1 excitations reveal that the hole and electron reside mainly on the PDI moieties in all systems (3LC character), with only minor metal character. These results are confirmed by the T1 state spin densities shown in Figure 5, which for all systems show the unpaired spins to be present mostly on the PDI moieties; i.e., that T1 can be described as having predominantly 3 LC character. However, notably, there is more spin density

cyclo-Pt-PDI, the S0 → S1 excitation has considerable oscillator strength and is well-approximated as a one-electron HOMO → LUMO transition. Natural transition orbitals (NTOs; see Supporting Information),101 which allow for examination of the nature of the electronic transitions through analysis of the transition density matrix, also reveal that the transitions are predominately PDI-centered with small contribution from the Pt. The S0 → S2 excitations are energetically well separated (by ∼1 eV) from the S0 → S1 transitions. A different picture emerges for ortho-Pt-PDI and ortho-PdPDI. As the metal-centered HOMO−1, HOMO−2, and HOMO−3 are close in energy to the HOMO, these orbitals are found to influence the nature of the low-lying S0 → Sn excitations. In both instances, the S0 → S1 excitations have minimal oscillator strength and are comprised of mixed electronic configurations. Further, the S0 → S1 NTOs reveal a strong 1MLCT character, which is also confirmed via natural population analysis that reveals large changes in the charges on the Pt and Pd centers on going from the S0 to the S1 states; see Figure 4 and the Supporting Information. Rather, for ortho-Pt13855


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The Journal of Physical Chemistry C Table 3. T1 Energies As Determined via TDDFT and ΔSCF at the M06/LANL2DZ/6-31G(d,p) Level of Theory molecule

TDDFT (eV)

ΔSCF (eV)

PDI ortho-Pd-PDI ortho-Pt-PDI bay-Pt-PDI cyclo-Pt-PDI bay-Br/cyclo-Pt-PDI

1.24 1.26 1.28 1.25 1.03 1.03

1.20 1.22 1.24 1.22 1.00 1.02

located on the Pt metal centers in the bay- and cyclosubstituted structures than either the Pt or Pd metal centers of the ortho-substituted structures. The fluorescence emission spectra of PDI, bay-Pt-PDI, and cyclo-Pt-PDI were measured in aerated chloroform at room temperature and are presented in Figure 6. The emission spectra of the model chromophore PDI is structured with a maximum of 535 nm. The fluorescence of ortho-Pt-PDI is completely quenched, resulting in no detectable photoluminescence, whereas that of ortho-Pd-PDI is very weak, but discernible; these results are consistent with the small oscillator strengths for the S0 → S1 excitations determined by the TDDFT calculations (Table 2) and/or ultrafast triplet formation (see below). The emission spectra of bay-Pt-PDI and cyclo-Pt-PDI have a similar profile as that of the model with both being red-shifted and the vibronic structure slightly less resolved. The quantum yields of all compounds were determined with an integrating sphere and the data are presented in Table 1. After collecting the fluorescence data in aerated solutions, the samples were thoroughly deaerated with N2 and no detectable long-lived phosphorescence was observed. The lack of phosphorescence was also observed in the previously studied Pt acetylene bonded PDIs so similar nonemissive long-lived 3LC excited states are likely formed.36,37 Transient Absorption Spectroscopy. Transient absorption experiments indicate that the rate of intersystem crossing

Figure 6. Normalized emission spectra of dyes PDI, bay-Pt-PDI, and cyclo-Pt-PDI in CHCl3.

differs for the three compounds studied, with the kinetic results from the global fit analysis presented in Table 4. As seen in Figure 7a, at 300 fs, ortho-Pt-PDI has a broad excited-state absorption feature centered at 700 nm that tails into the NIR region, a ground state bleach at 545 nm, and weaker peaks at 505 and 470 nm. The transient absorption profile initially observed is similar to the PDI model chromophore, albeit redshifted due to the platinum(II) being σ-bonded to the PDI core, indicating that the initial excited state is ligand-centered in character (Figure S9). The excited-state absorption from the initially populated excited state decreases in strength as new excited-state absorption features grow in at 490 and 525 nm, similar to the spectral profile of the triplet PDI observed in previous studies.37,71 Using global fit analysis, this transition between excited states is best fit to two time components of 345 ± 28 fs and 2.03 ± 0.38 ps (Figure S6). The fast time

Figure 5. T1 state spin densities as determine at the M06/LANL2DZ/6-31G(d,p) level of theory. The isosurface value is 0.006 Å−3. 13856


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The Journal of Physical Chemistry C Table 4. Transient Absorption Global Fit Analysisa molecule PDI ortho-Pt-PDI bay-Pt-PDI cyclo-Pt-PDI

τ1 (ps)

τ2

τ3 (ns)

± ± ± ± ±

3.83 ± 0.2 ns 2.029 ± 0.38 ps 2.67 nsb 1.29 nsb

− − − 10.0 ± 3.3d

324 0.345 261 166 331

28 0.028 36 43 48c

nanosecond ISC time scales found in other perylene chromophores with Pd or Pt atoms directly attached. As discussed below, ISC for bay-Pt-PDI and cyclo-Pt-PDI occur with time constants of a few nanoseconds. In previously reported Pd and Pt perylene derivatives there is some quenching of the initially populated 1LC state, but moderate fluorescence quantum yields indicate relatively slow ISC,71,72 and ISC time constants (determined from decay of the 1LC state) for two previously reported bay-substituted Pd(II) PDI complexes are 5.7 ± 1.3 ns and 2.5 ± 0.06 ns.71 As shown by the NTO analysis for ortho-Pt-PDI, the S0 → S2 excitation corresponding to the experimentally observed absorption corresponds to formation of an essentially PDI-based 1LC state, with comparable metal character to that seen for the S1 states of bay-Pt-PDI and cyclo-Pt-PDI. However, the NTO analysis also indicates that S1 for ortho-Pt-PDI is essentially a MLCT state with considerable Pt contributions to the hole wave function; moreover, it is only a little lower in energy than S2, meaning there is likely significant energetic overlap of the corresponding vibrational manifolds, thus suggesting triplet formation in ortho-Pt-PDI likely occurs via ultrafast decay of S2 to S1, followed by ultrafast ISC (either to T1 or to a higher-lying triplet) from this S1 state, which has considerably more heavymetal character than that of other perylene Pd and Pt derivatives. The bay-Pt-PDI has a transient absorption profile that appears to be very similar to that of ortho-Pt-PDI and PDI (Figure 7b); however, it is shifted to lower energy. The excitedstate absorbance at 5 ps has a broad double-top feature with peaks at 725 and 940 nm. The ground-state bleach can be seen at 525 and 565 nm. Over the course of 6000 ps, these excitedstate features decrease in intensity and new features appear at 500, 535, and 600 nm. These features likely represent the intersystem crossing between the 1LC and 3LC states; however, this happens much more slowly than in ortho-Pt-PDI. After analyzing the transients at 525 and 725 nm, the transition can be fit to two time components, 333 ± 18 ps and 2.56 ± 0.1 ns (Figure S7). The fast time component likely represents the vibrational cooling of the initially populated excited state and/ or solvent reorganization around the molecule, similar to what is observed in the model PDI. The slow time component represents the intersystem crossing as this also correlates to the fluorescence decay time constant (2.67 ± 0.02 ns). Global fit analysis shows similar kinetic parameters when fixing the second time constant to the value determined by the fluorescence decay. This technique also reveals that the broad 700 nm to NIR excited-state feature associated with the 1LC state is observed in τ2 spectra, further connecting this process to intersystem crossing. The transient absorption spectra of cyclo-Pt-PDI is different than that of the previous two compounds (Figure 7c). The ground-state bleach is observed at 655 nm, which is significantly red-shifted in energy relative to the PDI and the other two complexes studied. At 5 ps, the excited-state features appear as two broad bands, one located at 515 nm and another at 915 nm. Over the time period of the experiment, the excitedstate absorption in the NIR decreases to the baseline (τ1 = 370 ± 28 ps, τ2 = 1.25 ± 0.05 ns) while the signal located at 515 nm initially increases in intensity while becoming more structured (τ1 = 1.46 ± 0.2 ns), and then begins to decrease in intensity through the end of the delay stage (τ2 = 6.7 ± 3.6 ns). The kinetic data from the multiple transients show that there are likely three major processes taking place in the ultrafast time

a

The time constants displayed in this table are obtained from the global fit analysis of the ultrafast transient absorption data from 450 to 775 nm, located in the Supporting Information. bThese time constants were fixed to the fluorescence decay values obtained by TCSPC. cThe first value (166 ps) was determined from global fit analysis of visible TA data and the second value (331 ps) was determined from global fit analysis of NIR TA data. dThis time constant has a large error due to the relatively small change of the transient absorption signal through the end of the 6 ns delay stage.

Figure 7. Excited-state absorption difference spectra of ortho-Pt-PDI (A) in CHCl3 following 550 nm pulsed excitation, bay-Pt-PDI (B) in CHCl3 following 550 nm pulsed excitation, and cyclo-Pt-PDI (C) in CHCl3 following 650 nm pulsed excitation (105 fs fwhm).

component likely represents the intersystem crossing rate with the second time constant representing the vibrational cooling of the triplet excited state, similar to what is seen in other Pt−PDI compounds.37 After 5 ps, the excited-state difference spectra observed persist throughout the time scale of the experiment (∼6 ns) demonstrating that, regardless of the assignment of the two time components, the long-lived excited state (presumably an essentially 3LC state) is obtained very rapidly through efficient intersystem crossing, likely as a result of significant spin−orbit coupling from the Pt. In particular, the subpicosecond time scale of triplet formation can be contrasted to 13857


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The Journal of Physical Chemistry C scale. As observed previously with the bay-Pt-PDI, the fast process τ = ∼ 150−350 ps) likely represents the vibrational cooling of the initial excited state and/or solvent reorganization around the molecule. The intermediate process is likely the intersystem crossing to the triplet state as this time constant correlates well with the fluorescence decay rate (1.29 ± 0.02 ns). The slow process is the repopulation of the ground state as there is no change in excited-state features, only a decrease in intensity through the end of the delay stage (6 ns). Using a global-fit analysis of the visible transient absorption spectra, the data can be best fit to an exponential decay with three time constants, 166 ± 43 ps, 1.29 ns (fixed), and 10.0 ± 3.3 ns, correlating well with the single wavelength kinetic analysis (Figure S8). The transient spectra observed at 3 ns for cyclo-PtPDI begins to decrease in intensity through the end of the delay stage (6 ns) with no change in of features detected (not shown), indicating that the most long-lived excited state in this chromophore is very short-lived relative to that of the other two complexes. The three compounds studied with ultrafast transient absorption were all also studied in the nanosecond time domain. Both ortho-Pt-PDI and bay-Pt-PDI exhibit transient features consistent with the formation of the triplet LC state, in agreement with the spin-density distributions from the DFT calculations. In the ortho-Pt-PDI transient absorption difference spectra (Figure 8a), a ground-state bleach is formed at 540 nm while excited-state absorption features are observed at 485, 520, and 565 nm. The features decay in intensity with a time

constant of 112 ns. In the case of the bay-Pt-PDI transient absorption difference spectra (Figure 8b), the features are comparable to that of ortho-Pt-PDI, albeit red-shifted by about 20 nm. This suggests that a similar excited state is populated on this time scale. However, this species is much longer lived (828 ns), so the 3LC state likely has less coupling with the metal orbitals on the platinum when compared to ortho-Pt-PDI and, therefore, less efficient intersystem crossing to the ground state and, hence, a longer excited-state lifetime. The cyclo-Pt-PDI transient absorption difference spectra (Figure 8c) has significantly different spectroscopic features, characterized by a broad ground-state bleach at 650 nm and a broad but structured excited-state absorption with a peak at 515 nm. The excited state is also much shorter lived, with a decay time constant somewhere between 10 and 13.2 ns, as obtained from ultrafast and nanosecond transient absorption experiments, respectively. Singlet Oxygen Sensitization. The three Pt chromophores in this study did not display phosphorescence, which would give direct evidence of the triplet nature of the long-lived excited state, at room temperature. However, using aerated solutions of toluene, all three complexes sensitized the phosphorescence of singlet oxygen at ∼1270 nm (Figure 9),

Figure 9. Near-infrared emission spectra (assigned to 1O2 emission) of optically matched (∼0.1 OD at λex) ortho-Pt-PDI, bay-Pt-PDI, and cyclo-Pt-PDI in oxygenated toluene at room temperature.

providing indirect evidence for assignment of the long-lived excited states to triplets. All three Pt-PDI complexes had reasonable 1O2 sensitization quantum yields (ΦΔ) in toluene with the values being 32%, 36%, and 14% for ortho-Pt-PDI, bay-Pt-PDI, and cyclo-Pt-PDI, respectively. The magnitude of ΦΔ depends on both the ΦISC (quantum yield of triplet states produced from excitation) and the triplet lifetime (the ability to properly sensitize O2 before returning to the ground state). The ΦΔ values correlate well with the triplet excited-state lifetimes determined by the nanosecond transient absorption experiment showing that the excited-state lifetime is the dominant factor in determining ΦΔ. It is worth noting that ΦΔ values for ortho-PtPDI (trans phosphines) and bay-Pt-PDI (cis phosphines) indicate in both cases that the ΦISC values are larger than those previously determined for either 1,7-bis(trans-Pd(PPh3)2Br)N,N′-bis(cyclohexyl)-PDI (6%) or 1,7-bis(cis-Pd(dppe)Br)N,N′-bis(cyclohexyl)-PDI (20%).71

Figure 8. Excited-state absorption difference spectra of ortho-Pt-PDI (A) in CHCl3 following 540 nm pulsed excitation, bay-Pt-PDI (B) in CHCl3 following 560 nm pulsed excitation, and cyclo-Pt-PDI (C) in CHCl3 following 650 nm pulsed excitation (7 ns fwhm). The sample was deaerated using the freeze−pump−thaw method. 13858


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The Journal of Physical Chemistry C Table 5. Time-Resolved Photophysical Data of the PDI-containing Chromophoresa LC τ

molecule

1

ortho-Pt-PDI cyclo-Pt-PDI bay-Pt-PDI PDI

± ± ± ±

0.35 1.29 2.67 4.21

0.03 0.02 0.02 0.03

d

ps nse nse nse

ΦISC (%)b

ΦΔ (%)c

∼100 66 40 −

32 14 36 −

LC τTA (ns)

3

112 ± 1 10.0 ± 3.3 < τ < 13.2 ± 0.2f 828 ± 4 −

a

Chromophores measured in chloroform at room temperature unless otherwise noted. bISC quantum yield obtained by kISC/kobs, using the observed singlet state time constant and assuming no change to krad and knr from the model PDI. cSinglet oxygen quantum yields measured in oxygenated toluene using ZnTPP as reference (ΦΔ = 0.93 in air) and are estimated to have ∼15% error.102 dDecay time constant obtained from ultrafast transient absorption measurement. eDecay time constants obtained from TCSPC measurement. fDecay values obtained from ultrafast and nanosecond transient absorption experiments where the actual value lies somewhere between the two due to experimental limitations.

Triplet Formation and Decay Rates. The rates of formation for the triplet state varies widely across this series of Pt-PDI complexes with the data presented in Table 5. The NIR excited state absorbance decay time constant seen using the ultrafast TA correlates well with the fluorescence decay time constant obtained from the TCSPC experiment for the emissive bay-Pt-PDI and cyclo-Pt-PDI molecules, suggesting these time constants correspond to the depletion of the singlet state (1LC). Assuming no change in the radiative and nonradiative rate constants from the PDI model (kr and knr), the faster decay of the singlet state in the three Pt complexes can be attributed to intersystem crossing (kISC). Using the decay rate constant of the model PDI (effectively kr + knr), kISC can then be estimated for each Pt complex, and thus the intersystem crossing quantum yields (ΦISC) can also be estimated, as compared in Table 5. [Footnote: We note that this is an approximation; in particular kr for ortho-Pt-PDI would expected to be much lower than for the other derivatives due to the much lower calculated S0−S1 oscillator strength (see Table 2). On the other hand, kISC is so rapid for this compound that the approximation has no effect on the estimated value of ΦISC.] When comparing the calculated intersystem quantum yields to the singlet oxygen quantum yields, bay-Pt-PDI has the best match between values, but there is a large difference with both ortho-Pt-PDI and cyclo-Pt-PDI. These two complexes have short triplet excited-state lifetimes, so likely they cannot be fully quenched by the oxygen in solution. The ΦISC values have an inverse correlation with the photoluminescence quantum yields, supporting the notion that the decrease in fluorescence is primarily due to intersystem crossing. There is no clear relation between the intersystem crossing rates and the triplet decay rates for the three compounds. The decay and intersystem crossing rates for ortho-Pt-PDI are both larger than for bay-Pt-PDI, but the difference in decay rates is much less dramatic, consistent with the discussion above in which the ultrafast intersystem crossing for the former compound orthoPt-PDI is attributed to a MLCT S1 state, whereas T1 is thought to be primarily LC. The cyclo-Pt-PDI decay rate is significantly faster than the other two complexes, suggesting that factors besides metal d-orbital contributions in the relevant states, such as additional vibrational overlap between the ground and excited states associated with the lower-lying excited state (i.e., via the energy gap law44) or with square planar geometry distortion around the Pt atom may contribute to additional nonradiative decay pathways to shorten the excited state lifetime.

or the bay position of the PDI, or in which a Pt(PPh3)2 moiety bridges the bay 1,12-positions forming a cyclo PDI were synthesized and thoroughly characterized in terms of structure, electrochemistry, and photophysical properties. These molecules all display visible absorption bands beyond 500 nm, characteristic of PDIs, with the cyclo-Pt-PDI having the lowest energy absorbance of the series at 650 nm. The fluorescence quantum yields and the quenching rates of the initially populated excited state observed by ultrafast transient absorption point to significantly different ISC rates across the series, varying by almost 4 orders of magnitude as a result of where the Pt is attached to the PDI chromophore. Nanosecond transient absorption experiments and quantum-chemical calculations suggests that the long-lived triplet excited states are ligand centered in character, similar what has been seen for previously studied Pt acetylene PDI complexes. All three Pt− PDI complexes sensitized the phosphorescence of singlet oxygen at ∼1270 nm, further supporting the assignment of the long-lived excited states to triplets.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b01003.

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All synthetic details and structural characterization data for the complexes in this study, as well as additional static and time-resolved spectra (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(F.N.C.) E-mail: [email protected]. *(S.R.M.) E-mail: [email protected]. *(C.R.) E-mail: [email protected]. ORCID

Victor N. Khrustalev: 0000-0001-8806-2975 Tatiana V. Timofeeva: 0000-0001-7475-3206 Felix N. Castellano: 0000-0001-7546-8618 Seth R. Marder: 0000-0001-6921-2536 Chad Risko: 0000-0001-9838-5233 Stephen Barlow: 0000-0001-9059-9974 Author Contributions ¶

These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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CONCLUSIONS In summary, a series of new Pt(II) chromophores in which a Pt(PPh3)2X (X = Cl, Br) moiety is attached in either the ortho

Notes

The authors declare no competing financial interest. 13859


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ACKNOWLEDGMENTS Work at Georgia Tech was supported by the Air Force Office of Scientific Research through the COMAS MURI Program (Agreement FA9550-10-1-0558). The crystallographic work was supported in part by the RUDN University Program “5100”. Synchrotron radiation-based single-crystal X-ray diffraction measurements were performed at the unique scientific facility Kurchatov Synchrotron Radiation Source supported by the Ministry of Education and Science of the Russian Federation (Project Code RFMEFI61917X0007). Funding for this research was partially provided by National Science Foundation (Grant PREM DMR-1523611). The work at the University of Kentucky was supported by the Department of the Navy, Office of Naval Research, ONR Award No. N0001416-1-2985. Supercomputing resources on the Lipscomb High Performance Computing Cluster were provided by the University of Kentucky Information Technology Department and Center for Computational Sciences (CCS). Transient absorption kinetics, singlet O2 quantum yields, and fluorescence intensity decay kinetics measured at NC State was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant Number DE-SC0011979. J.E.Y. was supported by the Air Force Institute of Technology (AFIT).

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