Complexes trans -Pt(BODIPY)X(PEt 3 ) 2 : excitation energy-dependent ﬂ uorescence and phosphorescence emissions, oxygen sensing and photocatalysis †

We report on ﬁ ve new complexes with the general formula trans -Pt(BODIPY)X(PEt 3 ) 2 ( Pt – X ), where the platinum( II ) ion is σ -bonded to a 4,4-di ﬂ uoro-4-bora-3a,4a-diaza-s -indacen-8-yl (BODIPY) and an anionic ligand X − (X − = Cl − , I − , NO 2 − , NCS − , CH 3 − ). All ﬁ ve complexes were characterized by multinuclear NMR, electronic absorption and luminescence spectroscopy and by X-ray di ﬀ raction analysis. Four of these complexes show e ﬃ cient intersystem crossing (ISC) from an excited singlet state to a BODIPY-centred T 1 state and exhibit dual ﬂ uorescence and phosphorescence emission from the BODIPY ligand. In Pt – I , the ﬂ uorescence is almost completely quenched, whereas the phosphorescence quantum yield reaches a value of 40%. The rate of ISC and the ratio of phosphorescence to ﬂ uorescence emissions depend on the excitation wavelength ( i.e. on which speci ﬁ c transition is excited). The performance of these complexes as one-component oxygen sensors and their photocatalytic activities were tested by Stern – Volmer quenching experiments and by monitoring the oxidation of 1,5-dihydroxynaphthalene with 1 O 2 generated from the long-lived triplet state of the sensitizer by triplet – triplet annihilation with 3 O 2 . Exceptionally high 1 O 2 generation quantum yields of up to near unity were obtained.


Introduction
][3][4][5][6][7][8][9][10] A particularly favourable asset of the BODIPY family of dyes is their modular construction from readily available building blocks, thus allowing for easy implementation of desirable properties or functionalities, e.g. for substrate binding, with important implications in the analytical sciences, or fine-tuning of the absorption and emis-sion wavelengths. 3,11,12][15][16] In particular, there are only a handful of phosphorescent metal-organic BODIPY derivatives, and until very recently, the phosphorescence quantum yields of such compounds did not exceed the rather modest value of 3.5%. 17In these complexes, the BODIPY dye(s) are either appended to a 2,2′-bipyridine ligand as in Ru-BDP or Ir-BDP or bonded to Pt(N^C^N) entities with cyclometalating bis(benzimidazol-2-yl)phenyl-derived ligands as in Pt 2 -BDP, Fig. 1. 3,14,[17][18][19] Much higher quantum yields of up to 31% for the PEt 3 derivative Pt-Br (Fig. 1) were achieved in complexes trans-Pt(BODIPY)Br(PR 3 ) 2 (R = Ph, Et) featuring a σ-bonded 4,4-difluoro-4-bora-3a,4a-diaza-s-indacenyl dye, which connects to the platinum(II) ion via its meso position. 20In the latter complexes, the Pt coordination centre acts as a remote heavy metal ion, as the HOMO and the LUMO are heavily biased to the BODIPY ligand and receive only very minor contributions from the coordination centre.As a consequence, the relevant excitation is adequately described as a BODIPY-based π → π* transition with essentially no chargetransfer contributions from the {PtBr(PR 3 ) 2 } fragment.Longlived excited triplet states of BODIPY dyes are of great interest for applications such as chemical sensing, 16,21,22 triplet-triplet † Electronic supplementary information (ESI) available: Multinuclear NMR spectra of the complexes, packing diagrams with short interatomic contacts, absorption, emission and excitation spectra recorded at different excitation wavelengths, figures displaying oxygen quenching of the phosphorescence emission, changes of the absorption spectra of DHN in the presence of catalytic amounts of MB or Pt-I and plots showing the absence of the reaction in the dark; table with the cell parameters and structure refinement data for the complexes; atomic positions for the geometry-optimized structures in the S 0 and the T 1 states and comparison with the experimental structure parameters.CCDC 1474955-1474959.5,24,25 For photodynamic therapy the ability of triplet emitters to transform triplet oxygen ( 3 O 2 ) to singlet oxygen ( 1 O 2 ) in a triplet-triplet annihilation process is of pivotal relevance. Its cell toicity makes the highly reactive 1 O 2 molecule a powerful weapon against cancer cells.[26][27][28][29] The different structural and electronic influences of a transition metal coligand entity on the photophysical properties of complexes, particularly the phosphorescence quantum yield Φ Ph , are not trivial, though.Decisive factors are the rate constant of the intersystem crossing (k ISC ), the ratio of the radiative and non-radiative decay rates, and the thermal accessibility of excited d-states, which typically provide nonradiative deactivation pathways. This energy separation largely depends on the ligand-field splitting. Tus, by introducing strong-field ligands, the d z 2 orbital can be pushed to higher energy, increasing the energy barrier for non-radiative decay via excited d-states.[30][31][32][33] In complexes of the type trans-Pt(Dye)X(PR 3 ) 2 (X − = Br − , Cl − , I − or CN − ), where Dye represents a σ-bonded thioxanthonyl or a BODIPY attached via its meso position, the ligand-field splitting can be modulated by the PR 3 ligand and the anionic ligand X − . Here we report our results on five new BODIPY complexes trans-Pt(BODIPY)X(PEt 3 ) 2 with anionic ligands X − that cover a wider range of the spectrochemical series and differ with respect to their trans-influence 35 and the results of our investigations into the performance of some representatives as one-component triplet sensors and sensitizers for the photocatalytic oxidation of 1,5-dihydroxynaphthalene (DHN) with molecular oxygen.

Single crystal X-ray diffraction
Single crystals suitable for X-ray diffraction analysis were obtained for all five Pt complexes.Fig. 3 displays the ORTEP representations of their molecular structures.Relevant bond lengths and angles can be taken from     In the present series of complexes the length of the C1-Pt σ-bond provides a measure for the trans-influence and consequently for the σ-donor strength of the anionic ligand X − , 35 which increases in the order Pt-NO 2 < Pt-Cl ≈ Pt-Br < Pt-I ≈ Pt-NCS < Pt-CH 3 .This ordering complies with that of a  related series of platinum complexes with a σ-bonded perylene or perylene monoimide dye. 37For Pt-CH 3 the difference between the Pt-C bond lengths to the methyl (2.127 (12) or 2.137(11) Å) and the BODIPY ligands (2.039 (11) or 2.053(12) Å for the two independent molecules of the unit cell) reflects the difference of the covalent radii of a sp 3 and a sp 2 carbon atom.Similar differences have e.g.been observed for trans-Pt(CH 3 ) (Ph)(PPh 3 ) 2 d(Pt−CH 3 ) = 2.226(4) Å, d(Pt−Ph = 2.058(4) Å). 38 The Pt-Me bond of Pt-CH 3 is expectedly longer than in complexes trans-Pt(CH 3 )Cl(PR 3 ) 2 owing to the opposite placement of two σ-carbyl ligands, which both exert a strong σ-trans-influence (cf.2.08(1) Å for R = Ph or 2.069(8) Å for R = C 6 H 4 F-4). 39,40ith deviations of 2.8°to 6.2°for the angle P1-Pt-P2 and 1.0°to 4.2°for bond angle C1-Pt-X (X = donor atom of the anionic ligand) and a maximum deviation of 4.0°for cis-angles X-Pt-P and C1-Pt-P from the ideal values and a coplanarity of all donor atoms with the Pt(II) ion the coordination centre exhibits a close to ideal square planar coordination geometry.This is also indicated by the summations of bond angles at the Pt(II) ion, which range from 359.94°to 360.20°.The P1-Pt-P2 angle opens to the side of the sterically demanding BODIPY ligand.
The various steric and electronic influences of a PtL 3 fragment for tipping the scale towards either κN or κS coordination of a thiocyanate ligand are textbook examples for the phenomenon of coordination isomerism. 41,42N coordination in spite of the soft character of the {Pt(BODIPY)(PEt 3 ) 2 } fragment is here favoured by the strong trans-influence of the opposite carbyl ligand, the light donor atom, and by steric effects.Thus, N coordination maintains a near coincidence of the NCS − axis with the C1-Pt-N vector Pt-N3-C22 = 162.2(4)°,S1-C22-N3 = 179.7(4)°,thus avoiding unfavourable steric interactions with the cis-disposed PEt 3 ligands (Fig. 3c).N coordination of the NCS − ligand has likewise been observed in the related perylene complex of Espinet and coworkers. 37acking diagrams of individual molecules in the crystal lattice are shown in Fig. S24-S28 of the ESI.† All structures exhibit several short intermolecular contacts.Most prevalent are hydrogen bonding interactions H⋯F-B between pyrrolic or methyl protons and the BF 2 − fluorine atoms.These latter contacts are in the range of 2.330 to 2.539 Å, which is by 0.330 to 0.131 Å shorter than the sum of the van der Waals radii.Most notably, H⋯F contacts to methyl hydrogens of the PEt 3 ligands are frequently shorter than those to the hydrogen atoms attached to the heterocycles.These hydrogen bonds are sometimes augmented by C-H⋯π interactions between methyl protons and a pyrrolic carbon atom ranging from 2.634 to 2.757 Å.In several cases, additional contacts exist between pyrrolic or methyl protons and heteroatoms of the anionic ligand X − , most importantly to the oxygen atoms of the nitrite ligand of Pt-NO 2 (2.378 to 2.487 Å with the shorter contacts again to PEt 3 methyl protons), the S atom of the κN-thiocyanate ligand in Pt-NCS (2.842 and 2.921 Å) or, very weakly, to the I − ligand in Pt-I (3.127 Å).The latter complex exhibits an interesting brick-wall packing in the ac plane, where individual molecules associate weakly along the c axis via C-H⋯I interactions and, more strongly so, along the a axis by CH⋯π interactions between the pyrrolic carbon atom C9 and a PEt 3 methyl proton of neighbouring molecules positioned above and below (C-H⋯C = 2.684 Å, see Fig. S25b of the ESI †).The structural relevance of CH⋯π interactions has recently been highlighted. 43

UV-vis spectroscopy, TD-DFT calculations and luminescence properties
The UV-Vis absorption spectra of complexes Pt-Cl to Pt-CH 3 are shown in Fig. 4.They are dominated by the sharp, vibrationally structured band of the attached BODIPY dye with extinction coefficients ε of 52 600 to 57 400 M −1 cm −1 .Peaking at a narrow range of 461 to 472 nm (Table 2), the position is almost invariant to the identity of the ligand X − .At higher energies in the near UV another weaker, asymmetric absorption is observed at λ = 370 to 300 nm with a maximum extinction coefficient of ca.11 000 M −1 cm −1 .In some cases that feature is resolved into two distinct bands which are separated a X represents the donor atom of the anionic ligand in trans-position to the dye at the Pt ion.b The molecule has a mirror plane which is defined by the plane of the dye's inner heterocycle.c Atom C1 could not be refined anisotropically.
by 20 to 30 nm.Time-dependent DFT (TD-DFT) calculations carried out on geometry optimized structures accordingly predict two separate absorptions in this energy range.The comparison of experimental and calculated TD-DFT data in Table 2 shows that our calculations reproduce the general absorption features well but overestimate the energy of the prominent BODIPY-based π → π* transition by ca.4200 cm −1 .The TD-DFT data reveal that the intense band at the lowest energy arises from the HOMO → LUMO transition.As it is evident from the graphical depictions of the relevant orbitals of Pt-NO 2 and Pt-I in Fig. 5 and the compilation in Tables 2 and 3, the latter is adequately described as a π → π* transition of the BODIPY ligand with only very small contributions of the {PtX(PEt 3 ) 2 } fragment.This also explains the negligible influence of the X − ligand on the transition energies.The absorption near 320 nm originates from two energetically close-lying transitions (HOMO−5 → LUMO, HOMO−6 → LUMO for Pt-I, HOMO−6 → LUMO, HOMO−7 → LUMO/HOMO−8 → LUMO for Pt-NO 2 , Table 2).One has distinct Pt(PEt 3 ) 2 → BODIPY charge-transfer (CT) character, while the second one involves another π → π* transition within the dye ligand.As we will see later, the more significant metal contribution to the higher energy transition has important implications on the intersystem crossing rate constants k ISC from the different excited states.Table S15 of the ESI † compares the calculated structure parameters of complexes Pt-Cl, Pt-I, Pt-NCS, and Pt-NO 2 to the experimental data from X-ray crystal diffraction and to those calculated for the T 1 state.Calculated bond parameters for the S 0 state retrace experimentally observed bond lengths and angles well.The only structural difference between the T 1 and the S 0 states is a slight elongation of the Pt-C1 bond by 2-3 pm while all other bond lengths and bond angles remain essentially unaffected.
Like the previously reported complex Pt-Br 20 all complexes exhibit dual fluorescence at λ ≈ 480 nm and phosphorescence at λ ≈ 640 nm when excited into their lowest energy absorption band.Emission spectra of the complexes and of Br-BODIPY are compared in Fig. 6 while relevant photophysical data are collected in Table 4.The small Stokes shifts of <500 cm −1 and luminescence decay rates in the subnanosecond range are typical assets of BODIPY-based fluorescence emissions.The congruence of electronic absorption and excitation spectra as documented in Fig. S29-S33 of the ESI † and the blue shift of the fluorescence peaks compared to that of the Br-BODIPY precursor (λ Fl = 517 nm) demonstrate, that both emissions originate from the complexes and not from impurities or unreacted Br-BODIPY.That blue shift has been traced to a preferential lifting of the BODIPY LUMO owing to a slightly larger contribution of the strongly electron-donating {Pt(PEt 3 ) 2 X} moiety 44 to the receptor orbital. 20The long lifetimes of 162 to 439 µs at r.t. and the large Stokes shifts of ca.5600 cm −1 characterize the low-energy emission band as phosphorescence Absorption data TD-DFT data  (Table 4).From the comparison of emission spectra in Fig. 6 and the data in Table 4 it becomes immediately apparent that larger phosphorescence quantum yields Φ Ph go along with a decrease of those of the fluorescence emission Φ Fl and vice versa.No luminescence data could be obtained for Pt-CH 3 , as this complex decomposed when irradiated in the fluorescence spectrometer.Fig. S34 of the ESI † illustrates that the decompo-sition product still constitutes a BODIPY dye.The distinct redshift of the fluorescence peak and its similar position to that of the Br-BODIPY precursor suggest that the BODIPY-ligand is detached from the Pt atom during photochemical degradation.Facile Pt-BODIPY bond breaking in this complex is likely caused by the strong σ-trans-influence of the methyl ligand and the concomitant weakening of the Pt-C(BODIPY) σ-bond, which is even amplified in the excited T 1 state (Tables 1 and  S15 of the ESI †).
The ratio of phosphorescence to fluorescence intensities increases in the order Pt-NO 2 < Pt-NCS < Pt-Cl < Pt-I; Pt-Br    This ordering parallels an increasing trans-influence of the ligand X − , 35 but shows no clear correlation to its positioning within the spectroelectrochemical series.This indicates that thermal population of excited d-states is most probably not the dominant pathway for radiationless decay of the excited states, although the documented complexities of such processes still warrant caution. 30s the already very weak fluorescence of Pt-I was found to vanish altogether on excitation into the high-energy absorption band at 322 nm, the intensities of the phosphorescence and fluorescence emissions were monitored at different excitation wavelengths.Fig. 7a and b illustrate that, on irradiation into the higher energy absorption band(s), the phosphorescence quantum yield Φ Ph of Pt-NO 2 further increases at the expense of that of the fluorescence emission (Φ Fl ).6][47] This phenomenon relies on the different involvement of a heavy atom in the different excited states.In particular, a larger degree of charge-transfer between a metal/coligand entity and the emissive ligand (metal-toligand or ligand-to-metal charge-transfer) provides a more direct pathway for ISC, and hence a larger rate constant k ISC , than the remote heavy-metal effect alone. 45,46,48The efficiencies of the ISC from a higher-lying S n state (S n → T m → T 1 ) and from the S 1 state (S n → S 1 → T 1 ) may thus drastically differ if the initially populated states differ in character.
For the BODIPY-centred excited S 1 state, which is initially populated by irradiation into the prominent HOMO → LUMO π → π* absorption band, the coordination centre merely acts as a remote heavy metal atom, and the efficiency of ISC relies on the close proximity of the Pt ion to the dye (note that k ISC in that case relates to r −6 where r is the distance of the heavy metal atom to the midpoint of the dye). 47This is readily inferred from the spin density surfaces for the excited triplet states of Pt-Cl, Pt-I, Pt-NCS, and Pt-NO 2 in Fig. 8. Complying with the compositions of the HOMO and the LUMO, almost the entire spin density resides at the BODIPY ligand with only very modest contributions of 0.3% to 1.2% from the Pt ion.As it was already discussed, the higher energy absorption band,  populating (a) higher S n state(s), has more significant contributions from Pt(PEt 3 ) 2 → BODIPY charge-transfer (ML → L′CT, Fig. 5 and Tables 2 and 3).As is illustrated in Scheme 1, the faster k ISC,n from the higher-lying ML → L′CT excited state provides an even more competitive pathway for population of the phosphorescent T 1 state than ISC from S 1 .Excitation into (a) higher S n state(s) thus decreases the fluorescence quantum yield Φ Fl while further boosting Φ Ph .The highest phosphorescence quantum yields are found for the simple halogenido complexes.The values of Φ Ph of 36.4% or 39.7% for Pt-I on excitation at 467 or at 322 nm, respectively, are, to the best of our knowledge, the highest phosphorescence quantum yields of any BODIPY derivative, even surpassing those of Pt-Br. 200][51][52][53][54] We note here that a lower lifetime of the phosphorescence emission and hence lesser sensitivity towards O 2 quenching as it was observed for Pt-Br allows for O 2 detection in solution up to atmospheric concentration levels of the surrounding gas phase.The feasibility of using these complexes as sensitizers for 1 O 2 generation from 3 O 2 by triplet-triplet annihilation in productive chemical reactions 55,56 such as the oxidation of 1,5dihydroxynaphthalene (DHN) was investigated using the complexes Pt-Cl and Pt-I as catalysts.The catalytic cycle of the photocatalytic system consisting of the sensitizer, aereal O 2 and DHN is shown in Scheme 2. On the basis of this mechanism, the rate-law of DHN consumption can be written as At the initial stage of the reaction oxygen concentration can be treated as constant.The previous equation can therefore be simplified to ν i = k obs •[DHN] using a pseudo firstorder rate constant k obs .Rewriting this formula as ln(C t /C 0 ) = −k obs •t, where C t denotes the concentration of DHN at a certain reaction time t while C 0 is the initial concentration of DHN, allows for determining k obs from the slope of a plot of ln(C t /C 0 ) vs. reaction time t.The associated values of ν i and the number of photons absorbed by the sensitizer provide the 1 O 2 generation quantum yield (Φ Δ ) by using the relative method with methylene blue (MB) as a reference sensitizer. 57Details of these experiments are provided in the Experimental section.Fig. 11a depicts the changes of the absorption spectra of the reaction mixture with irradiation time t using Pt-Cl as a sensitizer, while Fig. 11b compares plots of ln(C t /C 0 ) as a function of t for Pt-Cl, Pt-I and the MB standard.The rate constants k obs , the rates ν i of DHN consumption, and quantum yields for the generation of 1 O 2 (Φ Δ ) in the photooxidation of DHN are summarized in Table 5.Both platinum complexes obey a linear relation between ln(C t /C 0 ) and the irradiation time t from which ν i was determined.This precludes side reactions and proves that the sensitizers are stable under these conditions.Control experiments in the absence of light showed that none of the sensitizers promotes oxidation of DHN to Juglone under dark conditions (see Fig. S38 and S39 in the ESI †).Both complexes show a significantly higher rate ν i of DHN consumption than MB.9][60] Contributing factors are the high ISC efficiencies and the long lifetimes of the triplet state (τ Ph = 277 μs for Pt-Cl, τ Ph = 297 μs for Pt-I).

Summary and conclusions
We report on the synthesis and the spectroscopic and photophysical properties of five new complexes trans-Pt(BODIPY)X ).All contain a σ-bonded BODIPY ligand that binds to the platinum ion via its meso position.With the exception of Pt-CH 3 , all complexes show dual fluorescence and phosphorescence emissions from the attached BODIPY dye at wavelengths that are largely invariant to the nature of the ligand X − .Phosphorescence quantum yields and Pt-C(BODIPY) bond lengths increase in the order Pt-NO 2 < Pt-NCS < Pt-Cl < Pt-I in parallel with the σ-transinfluence of the ligand X − .Most importantly, the ratio of phosphorescence to fluorescence intensities of each complex depends on the excitation wavelength.This is a direct consequence of the different natures of the initially populated excited states (BODIPY-based π → π* or a higher excited state with appreciable Pt(PEt 3 ) 2 → BODIPY π* ML → L′ charge-transfer character), which results in different rate constants k ISC .Thus, the higher-energy MLCT absorption offers a more direct pathway for Pt-triggered ISC than just the heavy atom effect.][47] Additional studies into phosphorescence quenching by 3 O 2 have yielded exceptionally large Stern-Volmer quenching constants of ca.2000 bar −1 and demonstrated that these complexes are excellent one-component sensors for triplet molecules.Moreover, they constitute highly efficient sensitizers for photocatalytic reactions involving 1 O 2 as the reactant, combining exceptionally high quantum efficiencies near unity for 1 O 2 generation with good photostabilities.These treats will be further explored in our future work.

Experimental section
Materials and general methods DHN was bought form Acros Organics and purified by sublimation ( p = 4 × 10 −3 mbar, 160°C oil bath).cis-Pt(BODIPY)Br-(PEt 3 ) 2 was prepared as described elsewhere. 20All manipulations where conducted under air except for reactions involving MeMgI, MgMe 2 and MeLi, which were performed under N 2 atmosphere by standard Schlenk techniques.Solvents for the reactions under inert gas atmosphere were distilled over adequate drying agents and stored under N 2 atmosphere.All other solvents were used as received from the suppliers.
NMR experiments were carried out on a Bruker Avance III DRX 400 or a Bruker Avance DRX 600 spectrometer. 1H and 13 C NMR spectra were referenced to the solvent signal, while 31 P and 195 Pt NMR spectra were referenced using the Absolute Reference tool in the MestReNova software.NMR data are given as follows: chemical shift (δ in ppm), multiplicity (br, broad; d, doublet; dd, doublet of doublets; m, multiplet; s, singlet; t, triplet), integration, coupling constant (Hz).Unequivocal signal assignments were achieved by 2D NMR experiments.The numbering of the nuclei follows that of the crystal structures in Table 5 Parameters of the pseudo first-order kinetics, 1 O 2 generation quantum yields of the photooxidation of DHN using the complexes Pt-Cl, Pt-I (λ exc = 460 nm) and MB (λ exc = 655 nm) as sensitizers, and turnover frequency, as well as Stern-Volmer constants K SV for the phosphorescence quenching of Pt-Cl and Pt-I by 3 Pt-Cl Fig. 3. Combustion analysis was conducted with an Elementar vario MICRO cube CHN-analyzer from Heraeus.X-ray diffraction analysis of single crystals was performed at 100 K on a STOE IPDS-II diffractometer equipped with a graphitemonochromated radiation source (λ = 0.71073 Å) and an image plate detection system.A crystal mounted on a fine glass fiber with silicon grease was employed.If not indicated otherwise, the selection, integration, and averaging procedure of the measured reflex intensities, the determination of the unit cell dimensions and a least-squares fit of the 2θ values as well as data reduction, LP-correction, and space group determination were performed using the X-Area software package delivered with the diffractometer.A semiempirical absorption correction was performed. 61All structures were solved by the heavy-atom methods (SHELXS-97, SHELXS-2013, or SHELXS-2014). 62,63Structure solutions were completed with difference Fourier syntheses and full-matrix lastsquares refinements using SHELXL-97, SHELXS-2013, or SHELXS-2014, 63 minimizing ω(F o 2 − F c 2 ) 2 .The weighted R factor (wR 2 ) and the goodness of the fit GOF are based on F 2 .All non-hydrogen atoms were refined with anisotropic displacement parameters, while hydrogen atoms were treated in a riding model.Molecular structures in this work are plotted with ORTEP 32 64,65 or Mercury. 66 UV-Vis absorption spectra were recorded on a TIDAS fiberoptic diode array spectrometer MCS from j&m in HELLMA quartz cuvettes with 1 cm optical path length at room temperature.

Computational details
The ground sate electronic structures were calculated by density functional theory (DFT) methods using the Gaussian 09 67 program packages.Quantum chemical studies were performed without any symmetry constraints.Open shell systems were calculated by the unrestricted Kohn-Sham approach (UKS). 68Geometry optimization followed by vibrational analysis was made either in vacuum or in solvent media.4][75] Solvent effects were accounted for by the Polarizable Conductor Continuum Model (PCM) [76][77][78] with standard parameters for dichloromethane.Absorption spectra and orbital energies were calculated using time-dependent DFT (TD-DFT) 79 with the same functional/basis set combination as mentioned above.For easier comparison with the experiment, the obtained absorption and emission energies were converted into wavelengths and broadened by a Gaussian distribution (full width at half-maximum = 3000 cm −1 ) using the programm GaussSum. 80Molecular orbitals were visualized with the GaussView programm 81 or with Avogadro. 82

Luminescence spectroscopy and quenching experiments
All luminescence spectra and excited state lifetimes were recorded for ca. 10 −6 M solutions in CH 2 Cl 2 or toluene with a PicoQuant FluoTime 300 spectrometer at room temperature, if not stated otherwise.Luminescence experiments under inert gas atmosphere and defined O 2 concentrations were conducted in a quartz cuvette modified with an angle valve from Normag.Defined O 2 concentrations were adjusted by completely degassing the sample and subsequent injection of adequate volumes of air and nitrogen by syringe.while Juglone production was monitored by an increase of the absorption at 427 nm (ε = 3811 M −1 cm −1 ). 57The yield of Juglone was calculated from the concentration of Juglone and the initial concentration of DHN.The singlet oxygen quantum yield (Φ Δ ) was determined using eqn (1), 57,83 Φ Δ ¼ Φ Δ;std ðν i ÁI std =ν i;std ÁIÞ ð 1Þ where Φ Δ,std is the singlet oxygen quantum yield of the standard sensitizer MB (Φ Δ = 0.57), [58][59][60] ν i is the rate of DHN consumption and I and I std are the number of photons absorbed by the sensitizer and the standard, respectively.I was estimated from eqn (2) using the λ interval 455 to 465 nm for Pt-Cl and Pt-I, and 650 to 660 nm for MB, where I f (λ) is the wavelength dependence of the intensity of the incident light evaluated with a photometer (for values vide supra), ε(λ) is the extinction coefficient of the respective sensitizer recorded in CH 2 Cl 2 /MeOH (9/1), C s is the concentration of the sensitizer, and l is the length of the optical cell.

Notes
The authors declare no competing financial interest.
3 ) 2 is then transformed by AgOTf to trans-Pt(BODIPY)(OTf )(PEt 3 ) 2 .Subsequent treatment with NaX (X − = Cl − , NO 2 − , NCS − ) resulted in the replacement of the weakly coordinated OTf − by the respective counter ion and provided complexes Pt-Cl, Pt-NO 2 and Pt-NCS (see Fig. 2) in moderate to good yields.Our attempts to introduce a methyl ligand by transmetalation using the Grignard reagent MeMgI failed and the complex trans-Pt(BODIPY)I(PEt 3 ) 2 (Pt-I) was formed instead.The use of MgMe 2 as a transmetalating agent was likewise unsuccessful.Reaction of Pt-OTf with MeLi finally afforded Pt-CH 3 (Fig. 2). 195Pt NMR spectra of the trans-complexes show a triplet with a coupling constant J PtP in the range of 2692 Hz to 2450 Hz.Correspondingly, the 31 P NMR spectra give a singlet for the two trans-disposed P donors, which is flanked by the 195 Pt satellite doublet with the same J PtP coupling constant.The formation of a direct Pt-C σ-bond is confirmed by the observation of platinum satellites in the 13 C NMR spectra, which range from 492 Hz to 409 Hz for J PtC and from 25 Hz to 17 Hz for 2 J PtC and 3 J PtC couplings, respectively.Some couplings could, however, not be detected due to a low signal-to-noise ratio.The NMR spectra can be found in the ESI, Fig. S1-S23.†

Fig. 2
Fig. 2 Synthesis of the complexes Pt-X.

Fig. 3
Fig. 3 ORTEP representations of the molecular structures of (a) Pt-Cl, (b) Pt-I, (c) Pt-NCS, (d) Pt-NO 2 , and (e) Pt-CH 3 .For Pt-Cl and Pt-CH 3 only one of the independent molecules per unit cell are shown.The ellipsoids are drawn at a 40% probability level.Hydrogen atoms are omitted for reasons of clarity.Atom C1 of Pt-NO 2 remained isotropic and could not be refined further.

a
Percent contributions of the given fragments.b X − represents the anionic ligand in trans-position to the dye at the Pt ion.c Spin density contribution of the respective fragment to the spin density surface.

Fig. 6
Fig. 6 Emission spectra of Pt-Cl, Pt-I, Pt-NO 2 Pt-NCS and Br-BODIPY in degassed CH 2 Cl 2 solutions at concentrations of ca. 10 −6 M upon irradiation into the lowest energy absorption band of the complexes.

Fig. 5
Fig. 5 Energies and graphical representations of the relevant molecular orbitals along with calculated electronic transitions of (a) Pt-NO 2 and (b) Pt-I.

Fig. 7
Fig. 7 (a) Emission spectra of Pt-NO 2 on excitation at λ = 326 nm and λ = 472 nm, respectively.(b) Absorption and excitation spectra of Pt-NO 2 .The excitation spectra were recorded for the fluorescence band at 483 nm and the phosphorescence band at 646 nm.Measurements were performed on degassed CH 2 Cl 2 solutions at concentrations of ca. 10 −6 M.
O 2 and 1 O 2 generation The very long lifetimes of the excited triplet states of up to 439 μs make these compounds interesting candidates for applications such as triplet molecule sensing and photocatalysis.Their capabilities to act as one-component sensors for triplet molecules were tested by Stern-Volmer quenching experiments using 3 O 2 as the quencher.Fig. 9 and as Fig. S35 and S37 of the ESI † illustrate the results of such experiments for Pt-I, Pt-Cl and Pt-NO 2 .The Stern-Volmer equation is given asI 0 /I = 1 + K SV [O 2 ], where I 0 is the luminescence intensity under exclusion of oxygen, I is the luminescence intensity at a specific oxygen concentration, and K SV is the Stern-Volmer quenching constant, which is a measure for the sensitivity of the sensor.Fig. 10 displays plots of (I 0 /I) − 1 and (τ 0 /τ) − 1 as a function of the partial oxygen pressure ( p(O 2 )).The quenching constants of K SV = 2380 ± 170 bar −1 for Pt-Cl and K SV = 2580 ± 70 bar −1 for Pt-I are identical within the experimental error limits.As expected from the longer triplet state lifetime, Pt-NO 2 has an even larger K SV of 2810 ± 110 bar −1 .Quenching constants evaluated by the ratios of lifetimes are somewhat smaller but still reach values of close to 2000 to 2200 bar −1 .All complexes show high sensitivities for small partial oxygen pressures.Above p(O 2 ) = 0.1 bar the plots start to deviate from linearity which relates to the low intensity of the residual signal.Our results render complexes Pt-Cl, Pt-I and Pt-NO 2 particularly efficient oxygen sensors when compared to other

Fig. 9
Fig. 9 Stacked luminescence spectra of Pt-I in CH 2 Cl 2 solution at different oxygen concentration levels.

Fig. 11 (
Fig. 11 (a) Spectral change in the UV-Vis region for the photooxidation of DHN using Pt-Cl as the sensitizer.(b) Plots of ln(C t /C 0 ) vs. irradiation time for the photooxidation using complexes Pt-Cl, Pt-I and MB.

a
Pseudo first-order rate constant for DHN consumption.b Rate of DHN consumption.c Relative value of the number of photons absorbed by the sensitizer (I = 1 for the standard sensitizer MB).d Corrected 1 O 2 generation quantum yield using the value of MB (Φ Δ = 0.57) 58-60 as a reference.e Yield of Juglone after a reaction time of 180 min.f Turnover frequency.g In CH 2 Cl 2 solution.

Table 2
Absorption data of complexes Pt-Cl, Pt-I, Pt-NCS, Pt-NO 2 and Pt-CH 3 in ca. 10 −5 M CH 2 Cl 2 solutions at 298 K and TD-DFT calculations in CH 2 Cl 2

Table 3
Calculated Mulliken parameters of Pt-Cl, Pt-I, Pt-NCS and Pt-NO 2

Table 4
20minescence data of Pt-Br,20Pt-Cl, Pt-I, Pt-NO 2 and Pt-NCS in N 2 saturated CH 2 Cl 2 and toluene solutions, respectively, at concentrations of ca. 10 −6 M. If not stated otherwise the samples were excited into the lowest energy absorption bandλ max,Fl [nm] (Stokes shift [cm −1 ]) λ max,Ph [nm] (Stokes shift [cm −1 ]) Fluorescence and phosphorescence quantum yields measured at an excitation wavelength of 467 nm or 322 nm, respectively.
a b Not determined.c Measured in toluene solution at r.t.d Measured in a toluene glass at 77 K.