Directing Energy Transfer in Panchromatic Platinum Complexes for

Article pubs.acs.org/IC

Directing Energy Transfer in Panchromatic Platinum Complexes for Dual Vis−Near-IR or Dual Visible Emission from σ‑Bonded BODIPY Dyes Fabian Geist,† Andrej Jackel,† Peter Irmler,† Michael Linseis,† Sabine Malzkuhn,‡ Martin Kuss-Petermann,‡ Oliver S. Wenger,‡ and Rainer F. Winter*,† †

Fachbereich Chemie, Universität Konstanz, Universitätsstraße 10, D-78457 Konstanz, Germany Department of Chemistry, University of Basel, St.-Johanns-Ring 19, CH-4056 Basel, Switzerland

‡

S Supporting Information *

ABSTRACT: We report on the platinum complexes transPt(BODIPY)(8-ethynyl-BODIPY)(PEt 3) 2 (EtBPtB) and trans-Pt(BODIPY)(4-ethynyl-1,8-naphthalimide)(PR3)2 (R = Et, EtNIPtB-1; R = Ph, EtNIPtB-2), which all contain two different dye ligands that are connected to the platinum atom by a direct σ bond. The molecular structures of all complexes were established by X-ray crystallography and show that the different dye ligands are in either a coplanar or an orthogonal arrangement. π-stacking and several CH···F and short CH···π interactions involving protons at the phosphine substituents lead to interesting packing motifs in the crystal. The complexes feature several strong absorptions (ε = 3.2 × 105−5.5 × 105 M−1 cm−1) that cover the regime from 350 to 480 nm (EtNIPtB-1 and EtNIPtB-2) or from 350 to 580 nm (EtBPtB). Besides the typical absorption bands of both kinds of attached dyes, they also feature an intense band near 400−420 nm, which is assigned by time-dependent density functional theory calculations to a higher-energy transition within the ethynyl-BODIPY (EtB) ligand or to charge transfer between the BODIPY (B) and naphthalimide (NI) chromophores. All complexes show dual fluorescence and phosphorescence emission from either the B (EtNIPtB-1 and EtNIPtB-2) or EtB (EtBPtB) ligand with a maximum phosphorescence quantum yield of 41% for EtNIPtB-1. The latter seems to be the highest reported value for room temperature phosphorescence from a BODIPY dye. The complete quenching of the emission from the chromophore absorbing at the higher energy and the appearance of the corresponding absorption bands in the fluorescence and phosphorescence excitation spectra indicate complete and rapid energy transfer to the chromophore with the lower-energy excited state, i.e., EtNI → B in EtNIPtB-1 and EtNIPtB-2 and B → EtB in EtBPtB. The latter process was further investigated by transient absorption spectroscopy, indicating that energy transfer is complete within 0.6 ns. EtNIPtB-1 catalyzes the photooxidation of 1,5dihydroxynaphthalene with photogenerated 1O2 to Juglone at a much faster rate than methylene blue but with only modest quantum yields of 37% and with the onset of photodegradation after 60 min.

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INTRODUCTION Platinum diphosphine acetylide complexes trans-Pt(CCR)(PR3)2(X) and trans-Pt(CCR)2(PR3)2 are renowned because of their interesting photophysical properties. 1−5 Other favorable assets are the ease of synthesis from easily available PtCl2(PR3)2 precursors, high solubility imparted by the corresponding phosphine ligands (PEt3, PBu3, etc.), and chemical and thermal robustness. Platinum bis(acetylide) complexes with free ethynyl groups have also been used as building blocks for conjugated and often highly luminescent oligomers and polymers that can exhibit delocalization2−4,6−11 and energy transfer (ET) over considerable distances,2,3,8,10,12,13 whereas the first triplet excited state T1 tends to localize on just one platinum alkynyl subunit.14−17 The possibility to introducing two acetylide ligands in a stepwise fashion has also given access to unsymmetrically substituted © XXXX American Chemical Society

bis(acetylide) congeners with the added benefit of vectorial ET from the chromophore absorbing at the higher energy to the one with the lower-energy excited state. Because of the direct connection of the arylacetylide ligands to the platinum heavy-metal ion, intersystem crossing (ISC) is usually competitive with fluorescence. As a result, many of these complexes show dual fluorescence and phosphorescence emission. Phosphorescence quantum yields at room temperature in solution are, however, usually rather modest but may be significantly enhanced upon lowering of the temperature or in solid films. The crucial frontier molecular orbitals (MOs) of these complexes are dominated by the acetylide ligands and only moderately perturbed by platinum bonding. As a Received: October 20, 2016

A

DOI: 10.1021/acs.inorgchem.6b02549 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthesis of the σ-Aryl-BODIPY Acetylide Complexes of This Study

center. Thus, σ-aryl complexes of extended π chromophores such as anthracene,28 perylene,29 corannulene,30,31 or dibenz[a,c]anthracene32 have been prepared via oxidative addition of the corresponding haloarene to a diphosphineplatinum(0) source such as Pt(PR3)4 or Pt(PR3)2(η2-olefin). Whereas such complexes with extended π-conjugated ligands are solely fluorescent,28,29,33 complexes of the type Pt(dye)(PR3)2(X) with σ-bonded thioxanthonyl or BODIPY dye ligands show dual fluorescence and phosphorescence emission with phosphorescence quantum yields of up to 40%, even though the parent dyes themselves are nonphosphorescent or only very weakly phosphorescent. Such a “turning-on” of the ligand phosphorescence is again effected by the platinum ion. Bringing the dye ligand closer to the platinum ion accelerates ISC not only from the 1ππ* excited state to the 3ππ* state but also from the 3ππ* state back to the ground state S0. The concurrent reduction of the phosphorescence lifetime by 3 orders of magnitude with respect to the parent dyes makes excited-state decay by phosphorescence emission much more competitive with other, nonradiative deactivation pathways, thereby increasing the quantum yield. A logical expansion of our previous work on platinum complexes with a σ-BODIPY ligand is to transfer the multichromophore approach of panchromatically absorbing sensitizers from bis(acetylide) complexes to mixed σ-aryl/ acetylide analogues. Depending on the energy ordering of the excited states of the two different kinds of dye ligands, the acetylide ligand will serve as either an antenna for the σBODIPY ligand or vice versa. Both of these scenarios have now been addressed by using either 4-ethynyl-1,8-naphthalimide (6-

consequence, the platinum ion merely acts as a remote heavy atom, such that the ISC efficiency scales with d−6 and thus decreases rapidly with increasing spatial separation d of the π chromophore from the platinum ion.18−20 As a further consequence of the π → π* nature of the underlying absorption, most of these complexes only absorb in the nearUV or high-energy region of the visible. One obvious way to overcome the latter limitation is to utilize dyes as ligands. Following this approach, strongly absorbing and highly emissive platinum acetylide complexes with perylenediimide,21 naphthalimide (NI) and coumarin,22 or BODIPY23−26 ligands have been prepared. More recently, the platinum bis(acetylide) platforms of unsymmetrically substituted complexes trans-Pt(CCR)(CCR′)(PR3)2 with two different dye-based ligands have been scrutinized with respect to excited-state ET from one dye to the other.21−25,27 Such complexes were shown to be highly useful photocatalysts, where multichromophoric visible light can be used to populate a single, well-defined excited state. Excitation of the higherenergy chromophore is followed by rapid ET, which funnels all excitation energy to the ultimate photoacceptor. Other applications include their utilization as sensors for triplet molecules or as sensitizers for triplet−triplet annihilation (TTA) upconversion.22,24,25 Nevertheless, the usually rather moderate quantum yields for room-temperature phosphorescence from these complexes remain a major drawback. Given that ISC in such complexes largely relies on the remote heavy atom effect, ISC can be accelerated by bringing the chromophore closer to the platinum ion. This can be achieved by directly attaching the dye to the coordination B

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orthorhombic space group Pmcn. Crystals of the PEt 3 complexes contain one molecule of CDCl3 (EtNIPTB-1) or a highly disordered molecule of n-pentane (EtBPtB) per complex molecule. Because all attempts to refine the atomic positions of the n-pentane molecules were unsuccessful, the corresponding reflections were removed from the original data set using the Squeeze procedure. The most important metric parameters derived from the structure solutions are provided in Table 1. Details of the data

EtNI) or 8-ethynyl-BODIPY (8-EtB) as the acetylide ligand. We herein discuss the manifestations and consequences of the two different directions of ET within such dyads.

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RESULTS AND DISCUSSION Synthesis and Characterization. The three mixed acetylide/σ-BODIPY complexes shown in Scheme 1 were synthesized from cis-Pt(BODIPY)Br(PR3)2 precursors cis-1 (R = Et) or cis-2 (R = Ph)34 and the corresponding ethynylsubstituted BODIPY or NI dyes HEtB or HEtNI. Bromide abstraction with either AgOTf or AgBF4 in refluxing CDCl3 converted the precursor complexes to their more reactive BF4− or OTf− derivatives. Concomitant cis → trans isomerization was indicated by the appearance of just one sharp singlet resonance flanked by 195Pt satellites (JPtP = 2400−2700 Hz) in their 31P NMR spectra. Replacement of the weakly coordinated OTf− or BF4− ligands by the corresponding acetylide was then achieved under classical Sonogashira conditions. Formation of the PPh3 complex EtNIPtB-2 took considerably longer time or required higher reaction temperatures (24 h at 60 °C) compared to the PEt3 complexes EtNIPtB-1 (19 h at room temperature) and EtBPtB (1 h at 65 °C), most likely as a consequence of the larger steric shielding of the platinum coordination center. After the appropriate workup (see the Experimental Section), the target compounds were isolated in moderate yields of 37−53%. Their purity was confirmed by NMR spectroscopy [see Figures S1−S14 in the Supporting Information (SI)] and correct combustion analyses. σ bonding of the corresponding dye ligands to the transPt(PR3)2 fragment was confirmed by 2/3JPC coupling and the appropriate JPtC couplings between the phosphorus or platinum atom and the respective carbon atom(s) of the acetylide and the BODIPY ligands. The 13C NMR spectrum of EtBPtB shows platinum satellites for both types of BODIPY ligands, with a larger JPtC coupling of 870 Hz for the acetylide and a smaller one of 692 Hz for the sp2 carbon atom of the directly attached BODIPY ligand. This difference is due to the combined effects of the larger s character and the shorter Pt−C bond of the acetylide donor, as shown by crystallographically studied trans-ethynylarylbis(phosphine)platinum complexes trans-Pt(aryl)(CCR′)(PR 3)2.6,7,11,35−40 This trend is also reflected by the further decrease of JPtC to 413 Hz for Pt−CH 3 in the complex trans-Pt(BODIPY)(PEt3)2(CH3), which also has two different trans-disposed carbyl ligands.41 The presence of an aectylide ligand is further evidenced by the CC stretch at 2084 cm−1 for EtBPtB, 2093 cm−1 for EtNIPtB-1, or 2102 cm−1 for EtNIPtB-2. Upon platinum bonding, the CC triple-bond stretching vibration is redshifted, most probably as a result of π-back-donation from the Pt(PR3)2 fragment to a CC antibonding π* orbital (see Figure S15 in the SI). That shift amounts to 25 cm−1 in EtBPtB, 12 cm−1 in EtNIPtB-1, but only 3 cm−1 in EtNIPtB-2. We note, however, that acetylide ligands sometimes defy such simple interpretations of coordination-induced IR shifts as they were established for the paradigmatic carbonyl ligand.42−44 Single-Crystal X-ray Diffraction Analysis. Single crystals of all three complexes were obtained by the slow evaporation of concentrated CDCl3 solutions of the ethynylnaphthalimido complexes or by the slow diffusion of n-pentane into a concentrated CHCl3 solution of EtBPtB. The Pt(PEt3)2 complexes crystallize in the monoclinic space group P21/c, while the PPh3 complex EtNIPtB-2 crystallizes in the

Table 1. Selected Bond Lengths [Å] and Angles [deg] of EtBPtB, EtNIPtB-1, and EtNIPtB-2 bond parameter

EtBPtB

EtNIPtB-1

EtNIPtB-2

C1−Pt1 C10−Pt1 P1−Pt1 P2−Pt1 C10−C11

2.060(7) 2.009(6) 2.296(2) 2.3015(18) 1.212(8)

2.035(7) 2.006(8) 2.298(2) 2.297(2) 1.205(11)

2.029(6) 1.985(8) 2.3133(11)

C1−Pt1−P1 C1−Pt1−P2 C10−Pt1−P1 C10−Pt1−P2 P1−Pt−P2

90.87(19) 92.47(19) 88.84(18) 87.82(18) 176.66(6)

90.4(2) 94.1(2) 88.7(3) 87.1(3) 174.63(9)

91.85(3) 91.85(3) 88.24(3) 88.24(3) 174.20(6)

[B]−[Pt]a [EtB/EtNI]−[Pt]b [B]−[EtB/EtNI]c

89.5 88.9 3.2

89.3 9.8 89.6

90 90 0

1.197(11)

a Interplanar angle between the best plane through the σ-bonded BODIPY ligand and the coordination plane at platinum. bInterplanar angle between the plane through the acetylide ligand and the coordination plane at platinum. cInterplanar angle between the best plane through the σ-bonded BODIPY ligand and the acetylide ligand.

collection and structure refinement are collected in Table S1 in the SI. The molecular structures of the three complexes in Figure 1 reveal an almost ideal square-planar coordination geometry at the platinum atom with all cis bond angles at the coordination center close to 90°. Small deviations from an ideal square-planar coordination geometry result from the greater steric demand of the platinum-bonded BODIPY ligand relative to that of the respective acetylide ligand, which, because of the interspersed CC triple bond, is further away from the Pt(PR3)2 fragment. As a consequence, the phosphine ligands bend slightly toward the acetylide ligand, rendering the trans P−Pt−P angle slightly acute at 174.2° to 176.7°. The P−Pt1− C1 bond angles are correspondingly slightly obtuse and range from 90.9° to 94.1°, while the opposite angles P−Pt−C10 are somewhat smaller at 87.1−88.8°. When steric interactions are minimized, the plane of the σbonded BODIPY ligand is in a near-to-orthogonal orientation with respect to the PtP2C2 coordination plane defined by atoms Pt1, P1, P2 (or P1′), C1, and C10 (Table 1). The orientation of the other dye ligand, however, varies between the different complexes. For EtBPtB and EtNIPtB-2, the respective acetylide ligand is also rotated to orthogonality with the platinum coordination plane, whereas it is almost coplanar with the platinum coordination plane (and, hence, in an orthogonal arrangement with respect to the plane of the BODIPY ligand) in EtNIPtB-1. This finding already indicates that electronic π interactions between the two different trans-disposed dye ligands across the platinum ion are at best weak in the solid C

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displacement of the BF2 fragments out of the corresponding C3N2 planes, as indicated by the interplanar angle of 8.0° between the N2B and C3N2 planes in the case of the naphthalimidyl complex or of even 15.4° and 16.5° for EtBPtB, where the flaps of the BF2 envelopes point to opposite directions. The Pt−C10 [range of 1.985(8)−2.009(6) Å] and CC triple-bond lengths of 1.197(11)−1.212(8) Å fall within the usual range of previously reported trans-Pt(aryl)(aryl-C C)(PR3)2 complexes.6,7,11,35−40 The likewise strong trans influence of aryl and acetylide ligands is clearly reflected in the Pt1−C1 bond lengths. The latter range from 2.029(6) to 2.060(7) Å and are significantly longer than those in the parent bromo complexes trans-1 and trans-2 or the chloro, iodo, κNnitrito, or κN-thiocyanato complexes trans-Pt(BODIPY)(PEt3)2(X) (X = Cl, I, NO2, NCS), where they adopt values of 1.956(9)−1.994(10) Å.34,41 They, however, closely resemble those of 2.039(11) and 2.053(12) Å for the two crystallographically independent molecules of the corresponding methyl complex (X = CH3).41 All three complexes display interesting packing motifs in the crystal. The latter are supported by several short intermolecular contacts established by π stacking between the EtNI or EtB ligands and CH···F as well as CH···C contacts between the PR3 substituents and pyrrolic carbon atoms of the BODIPY ligands. The closest CH···C contacts involve hydrogen atoms of the PEt3 ligands and adopt remarkably small values of 2.554−2.624 Å, which is by 0.36−0.29 Å shorter than the sum of the van der Waals radii. In spite of the relatively modest energy of a single CH···π interaction of ca. 2.4−8.1 kJ/mol, the number of such interactions and the additivity of their energies have important implications for the organization of matter, in particular in the solid state.48−51 In the case of EtNIPtB-1 and EtNIPtB-2, the three-dimensional arrangement of interconnected molecules creates rhombic, prismatic channels with short and long diameters of ca. 5.8 and 7.4 Å or 11.4 and 12.2 Å, respectively, which are accessible to hydrophobic guests like CDCl3 or npentane solvent molecules. A detailed discussion of the individual packing motifs and interactions along with corresponding illustrations can be found in the Supporting Information (Figures S16−S18). Electronic Absorption Spectroscopy. Figure 2 compares the absorption spectra of the panchromatic Pt-BODIPY complexes and their precursors, while Table 2 lists all relevant data along with the time-dependent density functional theory (TD-DFT)-based band assignments and calculated transition energies. All complexes exhibit the characteristic sharp and intense (ε ≥ 50000 M−1 cm−1) π → π* absorption band of the σ-bonded BODIPY ligand at ca. 470 nm. The minor differences between the individual complexes with respect to the band position (see Figure 2a) seem to depend more on the identity of the PR3 coligands at platinum than on the trans-disposed acetylide ligand. This again points to only weak electronic interactions between the different dyes across the platinum ion. Previous TD-DFT studies on trans-1 and trans-2 have ascribed the subtle differences between PEt3- and PPh3-ligated congeners to stronger electron donation from the Pt(PEt3)2 fragment and its higher contribution to (and, consequently, preferential destabilization of) the corresponding π*-acceptor orbital, which is, nevertheless, strongly biased toward the BODIPY ligand.34 The same situation applies here (vide infra) and also explains the blue shift of the π → π* absorption band of the EtB ligand of 1080 cm−1 with respect to the free alkyne

Figure 1. ORTEP45 diagrams of (a) EtBPtB, (b) EtNIPtB-1, and (c) EtNIPtB-2. Ellipsoids are drawn at a 40% probability level. Hydrogen atoms and solvent molecules are omitted for reasons of clarity. Only one orientation of the rotationally disordered phenyl rings comprising atoms C33, C34 or C33′, C34′ of EtNIPt-B2 is shown.

state, although they have been implicated on several occasions in explaining the electronic and photophysical properties of platinum diphosphinebis(acetylide) oligomers and polymers.4,8−11,15,36,46 Such a lack of electronic interactions in complexes with two different dye ligands is not without precedence22,47 and results from the inherently nonidentical energies of the corresponding dye-based MOs. This is also in accordance with the results of our quantum-chemical calculations on the full models of complexes EtBPtB and EtNIPtB-2, where, despite the coplanarity of the dye ligands, all crucial MOs are largely localized at the BODIPY or acetylide ligand with only modest delocalization between them (vide infra). In complexes EtBPtB and EtNIPtB-1, the BODIPY ligands display some slight folding of 2.5−5° about the Pt1−C1−B1 or Pt−C10−C11−C12−B2 vector. This, in turn, causes a D

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Figure 2. (a) UV−vis spectra of EtBPtB (orange), EtNIPtB-1 (violet), and EtNIPtB-2 (green) in toluene. (b) UV−vis spectra of EtBPtB (green), HEtB (violet), and trans-1 (orange) in CH2Cl2. (c) UV−vis spectra of EtNIPtB-1 (green), HEtNI (violet), and trans-1 (orange) in CH2Cl2. (d) UV−vis spectra of EtNIPtB-2 (green), HEtNI (violet), and trans-2 (orange) in CH2Cl2.

Table 2. Absorption Data of HEtB and HEtNI and Complexes EtBPtB, EtNIPtB-1, EtNIPtB-2, trans-1, and trans-2 Recorded on ca. 5 × 10−6 M CH2Cl2 Solutions at 298 K and Results from TD-DFT Calculations in DCEa absorption data compound

solvent

EtBPtB

toluene CH2Cl2

λmax/nm (ε/10−3 M−1 cm−1) 394 (32.1), 473 (54.5), 552 (51.2) 255 (15.6), 287 (13.5), 312 (9.5), 397 (39.4), 471 (63.3), 547 (59.1)

TD-DFT data λ/ nm

major contribution

f

HOMO → LUMO (87%) HOMO−1 → LUMO+1 (75%)/HOMO−1 → LUMO (22%) HOMO−2 → LUMO (85%)

0.55 0.44

316

HOMO−5 → LUMO (79%)/HOMO−5 → LUMO+1 (10%) n.c.b

0.18

413 402

HOMO−1 → LUMO (97%) HOMO → LUMO (91%)

0.74 0.083

π → π* (EtNI) B → EtNI CT

392

HOMO → LUMO+1 (65%)/HOMO−1 → LUMO+1 (26%) HOMO−1 → LUMO+3 (76%)/HOMO−1 → LUMO+5 (10%) n.c.b HOMO → LUMO (97%) HOMO−4 → LUMO HOMO−5 → LUMO HOMO−6 → LUMO

0.18

π → π* (B)/EtNI → B CT B → Pt CT/B → PPh3 CT

448 390 336

EtNIPtB-1

toluene CH2Cl2

EtNIPtB-2

toluene CH2Cl2

416 (31.5), 470 (55.4) 287 (19.4), 321 (11.8), 332 (11.2), 349 (10.8), 422 (37.2), 467 (57.4) 421 (31.7), 478 (49.3) 292 (11.9), 311 (11.4), 333 (9.9), 350 (9.6), 424 (35.0), 476 (49.3)

295 trans-1 trans-2

CH2Cl2 CH2Cl2

254 (7.5), 322 (11.7), 359 (5.9), 468 (52.6) 253 (31.2), 320 (9.6), 355 (6.1), 477 (44.5)

HEtB HEtNI

CH2Cl2 CH2Cl2

352 (7.7), 407 (5.8), 438 (4.5), 587 (54.3) 367 (16.2)

402 339 333 320

0.78

0.12

0.32 0.031 0.033 0.033

assignment π → π* (EtB) π → π* (B)/B → EtB CT π → π* (EtB)/ MLCT π → π* (EtB)/EtB → B CT

π → π* (B) Pt(PPh3)2 → B CT π → π* (B) π → π* (B)/phenyl → B CT

n.c.b n.c.b

a f = oscillator strength; EtB = ethynylated BODIPY ligand; B = σ-bonded BODIPY ligand; EtNI = ethynylnaphthalimide ligand; CT = charge transfer; DCE = 1,2-dichloroethane. bNot calculated.

(λ = 587 nm for HEtB and 552 nm in EtBPtB). The sharpness and vibrational structuring also suggest that the π → π* bands

of the BODIPYs are only slightly perturbed upon platinum binding. Further experimental evidence for the π → π* E

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Figure 3. Graphical representation of the relevant MOs and TD-DFT energies of EtBPtB. Arrows symbolize the main contributor to the respective transitions.

Figure 4. Graphical representation of the relevant MOs and TD-DFT energies of EtNIPtB-2. Arrows symbolize the main contributors to the respective transitions.

that band is again devoid of any notable solvatochromism, which makes larger charge-transfer (CT) contributions to the underlying transition unlikely. For the other two complexes featuring the EtNI ligand, some negative solvatochromism (hypsochromic shift with increasing solvent polarity) is observed (see Figure S19b,c in the SI). TD-DFT calculations were performed on the full model complexes in order to aid interpretation of the experimental spectra. Comparisons of calculated and experimental spectra can be found as Figure S20 in the SI. Figures 3 and 4 display the energy ordering and graphical representations of the crucial MOs in the frontier orbital region that are relevant for the calculated transitions. Overall, our calculations provide only a

parentage of the electronic transitions comes from the near invariance of the band position with solvent polarity, as shown by the negligible shift of 150 cm−1 between CH3CN and tetrahydrofuran solutions (see Figure S19 in the SI), similar to what has been observed for purely organic meso-substituted BODIPY dyes.52,53 A comparison of the spectra of the three complexes with those of the parent dyes HEtB and HEtNI and complexes trans-1 and trans-2 in Figures 2b−d reveals that they are not merely the sum of those of their constituents. Thus, every complex shows one additional intense absorption band (ε ≥ 35000 M−1 cm−1) at ca. 400 or 420 nm, which has no direct equivalent in any of the corresponding precursors. For EtBPtB, F

DOI: 10.1021/acs.inorgchem.6b02549 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry qualitative level of agreement between calculated and experimental spectra (see Table 2). In line with the known tendency of DFT methods to overestimate the energies of electronic transitions of BODIPY and related cyanine dyes,24,54 the calculated BODIPY (B)-based π → π* transitions are by 4200 cm−1 (EtB) or 3900 to 4500 cm−1 (B) higher than the experimental values. As a consequence, the predicted energy ordering of transitions in EtNIPtB-2 differs from that indicated by our experimental data. Thus, the B-based π → π* transition HOMO → LUMO+1 of the latter complex is calculated at slightly higher energy than the HOMO−1 → LUMO transition within the EtNI ligand. The computational results, nevertheless, fully agree with our experimental findings in predicting that all frontier MOs are strongly biased to one dye ligand with only minor involvement of the platinum ion or the other dye ligand. Some calculated transitions, nevertheless, have minor contributions involving CT from one dye to the other, such as, e.g., the HOMO → LUMO transition of EtBPtB. On the basis of our calculations, the band at 400 nm of EtBPtB is most probably due to the HOMO−2 → LUMO transition, which essentially constitutes a higher-energy π → π* type transition within the ethynylated BODIPY ligand with a minor CT component to the BODIPY ligand. A similar band,55 albeit of considerably smaller absorptivity, appears in the free 8EtB ligand HEtB at 352 nm. For complexes EtNIPtB-1 and EtNIPtB-2, the expected π → π* band of the EtNI ligand, which corresponds to the HOMO−1 → LUMO transition, seems to be overlapped with the two close-lying HOMO → LUMO and HOMO−1 → LUMO+1 transitions. Both of these excitations are best described as ligand-to-ligand charge transfer (LL′CT) with CT either from the BODIPY ligand to the EtNI ligand or vice versa. That assignment is also in line with the experimentally observed solvatochromism. Luminescence Spectroscopy. The presence of two kinds of emissive ligands with different π/π* energy levels, held in close proximity by the platinum ion, and the nearly unperturbed π → π* transitions with or without CT between the individual dyes that are largely confined to one individual chromophore hold the promise of interesting emission properties of the complexes. One issue of particular relevance is that of the efficacy of ET between the different dyes, i.e., the prospect of using the dye with the higher-energy excited state to sensitize the emission of the other, as has been observed in closely related platinum bipyridine or diphosphine complexes with two different kinds of ethynylated BODIPY ligands.24,25,47 Such behavior can be advantageously used to funnel polychromatic irradiation onto one single acceptor and to achieve vectorial ET in multichromophoric arrays. The ordering of the excited-state energy levels, 4-EtNi > σ-BODIPY (B) > 8EtB, as derived from UV−vis spectroscopy already suggests that the direction of ET, if occurring, should differ between the EtNI- and EtB-containing complexes, i.e., EtNI → B in EtNIPtB-1 and EtNIPtB-2 but B → EtB in EtBPtB. This is indeed found to be the case. A direct comparison of the emission spectra of complexes trans-1 and EtNIPtB-1 or trans-2 and EtNIPtB-2 in Figure 5a,b and the associated photophysical data in Table 3 demonstrate a striking level of similarity between these complexes but profound differences from those of the parent alkynes HEtNI and HEtB. Thus, all complexes exhibit dual emission at ca. 480 and 640 nm with almost no effect of bromo substitution by the EtNI ligand on the emission wavelengths, band structuring, and Stokes shifts (for luminescence decay curves, see Figures S21−

Figure 5. (a) Emission spectra of EtNIPtB-1 (green), trans-1 (blue), and HEtNI (violet) in an ca. 10−6 M argon/nitrogen-saturated CH2Cl2 solution. (b) Emission spectra of EtNIPtB-2 in an argon-saturated toluene solution and of trans-2 and HEtNI in nitrogen-saturated CH2Cl2 solutions at concentrations of ca. 10−6 M. The emission intensities of the complexes are normalized to the fluorescence band. The compounds were excited into their HOMO−LUMO band.

S24 in the SI). This allows us conclude that the emissions of the present complexes emanate from the same BODIPYcentered 1ππ* and 3ππ* states as those for their bromo precursors. A further indication comes from the TD-DFTcalculated first singlet excited state (vide supra) and the calculated spin density surface of EtNIPtB-2 in its T1 state, as shown in Figure 6. In agreement with the above assignment, the lower-energy emission is almost completely quenched under aerobic conditions, while the higher-energy emission remains unaffected. Lifetime measurements indicate that the fluorescence emission decays completely within 200 ps and thus at a faster rate, as can be resolved with our equipment. The fluorescence lifetime is thus appreciably shorter than that of 8-phenylBODIPY (5.1 ns) and several other derivatives,52,55−60 including 8-bromo-BODIPY (4.6 ns),34 HEtB (5.1 ns),56 and mono-, di-, tetra-, or hexabrominated BODIPYs (3.5−1.2 ns), where quantum yields for triplet formation of up to 66% are obtained.61 That shortening of the fluorescence lifetime is traced to an enhanced ISC rate on platinum coordination. The phosphorescence lifetimes of 107 ± 1 μs for EtNIPtB-1 and 135 ± 1 μs for EtNIPtB-2 are somewhat shorter than those measured for their bromo precursors and other complexes trans-Pt(BODIPY)(PEt3)2(X) with simple halogeno or pseudohalogeno ligands X.41 Nevertheless, the phosphorescence lifetimes are by at least 1 order of magnitude longer than those expected for a triplet state with a more significant platinum contribution. Rapid forward ISC, a relatively rapid phosphorescence decay to the ground state (when compared to the free G

DOI: 10.1021/acs.inorgchem.6b02549 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Table 3. Luminescence Data of Alkynes HEtNI and HEtB, of Complexes trans-1 and trans-2 in ca. 10−6 M CH2Cl2 Solution, and of the BODIPY Acetylide Complexes EtNIPtB-1, EtNIPtB-2, and EtBPtB in Toluene λmax,Fl/nm (Stokes shift/cm−1) HEtNI HEtB EtNIPtB-1 EtNIPtB-2 EtBPtB trans-1 trans-2 a

381 589 480 490 568 479 490

(1000) (774) (444) (513) (511) (491) (556)

λmax,Ph/nm (Stokes shift/cm−1)

635 653 798 637 657

(5529) (5607) (5605) (5669) (5743)

ΦFl

ΦPh

τFl/ns

τPh/μs

0.091 0.4a 0.012 0.014 0.04 0.011 0.072

0.41 0.26