Modulation of Intersystem Crossing Rate by Minor Ligand

Article pubs.acs.org/IC

Modulation of Intersystem Crossing Rate by Minor Ligand Modifications in Cyclometalated Platinum(II) Complexes Marsel Z. Shafikov,*,†,‡ Dmitry N. Kozhevnikov,*,† Michael Bodensteiner,§ Fabian Brandl,∥ and Rafał Czerwieniec*,∥ †

Ural Federal University, Mira 19, Ekaterinburg, 620002, Russia I. Postovsky Institute of Organic Synthesis, Ekaterinburg, 620041, Russia § Center for Chemical Analysis, Faculty of Chemistry and Pharmacy and ∥Institut für Physikalische und Theoretische Chemie, University Regensburg, Universitätsstrasse 31, D-93053 Regensburg, Germany ‡

S Supporting Information *

ABSTRACT: Photophysical properties of four new platinum(II) complexes comprising extended ppy (Hppy = 2-phenylpyridine) and thpy (Hthpy = 2-(2′thienyl)pyridine) cyclometalated ligands and acetylacetonate (acac) are reported. Substitution of the benzene ring of Pt−ppy complexes 1 and 2 with a more electron-rich thiophene of Pt−thpy complexes 3 and 4 leads to narrowing of the HOMO−LUMO gap and thus to a red shift of the lowest energy absorption band and phosphorescence band, as expected for low-energy excited states of the intraligand/metal-to-ligand charge transfer character. However, in addition to these conventional spectral shifts, another, at first unexpected, substitution effect occurs. Pt−thpy complexes 3 and 4 are dual emissive showing fluorescence about 6000 cm−1 (∼0.75 eV) higher in energy relative to the phosphorescence band, while for Pt−ppy complexes 1 and 2 only phosphorescence is observed. For dual-emissive complexes 3 and 4, ISC rates kISC are estimated to be in order of 109−1010 s−1, while kISC of Pt−ppy complexes 1 and 2 is much faster amounting to 1012 s−1 or more. The relative intensities of the fluorescence and phosphorescence signals of Pt−thpy complexes 3 and 4 depend on the excitation wavelength, showing that hyper-intersystem crossing (HISC) in these complexes is observably significant.

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INTRODUCTION Research on luminescent transition metal complexes has gained momentum since it was recognized that such compounds can be successfully applied as optical sensors,1−17 emitters in organic light-emitting diodes (OLEDs),18−40 photocatalysts,41−46 and dyes for bioimaging.47−55 Most of these applications make use of luminescence displayed by such materials. Therefore, understanding of the photophysical properties and the structure−function relationships is essential for engineering suitable materials. The third-row transition metal complexes usually show strong spin−orbit coupling (SOC), leading to efficient intersystem crossing (ISC) between singlet and triplet manifold.56−58 Thus, singlet excited states, initially populated by absorption of a photon, decay to the lowest triplet state T1 according to very fast ISC rates, usually in the order of 1012 s−1 or faster.57,59−62 As a consequence, singlet-state lifetimes are very short (from tens of femtoseconds to several picoseconds) and fluorescence quantum yields are very small, e.g., 10−4 or less.57,59,62 Thus, fluorescence is usually observed only using techniques of ultrafast luminescence spectroscopy.25,37,63−70 However, several groups have reported on fluorescence71−79 and dual luminescence of transition metal complexes11,67,71,72,80−87 in steady-state experiments, thus challeng© 2016 American Chemical Society

ing the traditional picture of the heavy atom effect on ISC and giving prospects for novel applications. In particular, Zhao et al. and Winter et al. reported on dualemissive platinum(II) complexes and proposed a use as ratiometric optical sensors for molecular oxygen.88,89 Yun Chi, Pi-Tai Chou, et al. described dual-emissive behavior of Os(II) and Ag(I) complexes comprising azanaphthalene chelating ligands.83 In their study, the efficiency of ISC populating the phosphorescent triplet state T1 depends on the photon energy of the light used for excitation. Thus, the fluorescence/phosphorescence intensity ratio was distinctly larger upon long-wavelength excitation (into the S1 ← S0 absorption band) than upon short-wavelength excitation (Sn ← S0). This relatively rare behavior was explained by an occurrence of “hyper-intersystem crossing” (HISC), i.e., distinctly faster ISC for certain higher singlet states (Sn) or higher vibrational levels of the S1 state as compared to the relaxed S1 state. The relatively slow “normal” S1 → T1 ISC was correlated with the ππ* character of the involved excited states that lack any significant metal contributions. On the contrary, some of the higher Sn and Tm states display more pronounced Received: March 21, 2016 Published: July 8, 2016 7457

DOI: 10.1021/acs.inorgchem.6b00704 Inorg. Chem. 2016, 55, 7457−7466

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Inorganic Chemistry

K2[PtCl4] to form platinum(II) dimers [Pt2(μ-Cl)2(L1-L4)2] that were finally cleaved with sodium acetylacetonate to afford in good yields the heteroleptic neutral complexes 1−4. X-ray quality crystals of Pt−thpy complexes 3 and 4 were obtained by slow convectional diffusion of methanol into dichloromethane solutions of 3 and 4, respectively. Perspective drawings of the molecules of 3 and 4 are shown in Figure 1. The platinum atoms in both complexes display a distorted square planar geometry typical for d8 transition metal complexes, with N−Pt−C and O−Pt−O angles of 81.8(5)° and 93.7(3)° in 3 and 79.8(4)° and 92.4(3)° in 4, respectively. Bond lengths to the metal center Pt−C, Pt−N, Pt−O1, and Pt−O2 are 1.971(15), 2.015(9), 2.006(7), and 2.079(9) Å in 3 and 1.989(11), 2.033(9), 2.008(8), and 2.081(9) Å in 4, respectively, which are unremarkable compared to analogous bond lengths found in [Pt(thpy)(acac)] and other Pt complexes with thpy-based ligands.90,97 The pendant aromatic fragments of the cyclometalating ligands, 4-fluorophenyl and 5ethylthienyl, are slightly twisted with respect to the plane of the metal-bound thienylpyridine core. In complex 3 the torsion angles between the 4-fluorophenyl and pyridine rings and between the two thienyl rings are 29° and 15°, respectively, whereby the thienyl groups are oriented trans to each other. In complex 4 the cyclopentene fragment, fused on pyridine ring, causes additional steric hindrance that enlarges the dihedral angle between 4-fluorophenyl and pyridine rings to 46°. Thus, the thpy central fragment is partly conjugated with the pendant aromatic rings, with the influence of the smaller thienyl ring being apparently larger than that of 4-fluorophenyl due to stricter steric requirements for the 6-membered ring. In complex 4, the orientation of the 4-fluorophenyl fragment is also influenced by steric repulsion with the cyclopentene ring reducing the conjugation. Electronic Absorption and Luminescence. Ambienttemperature absorption and emission spectra of 1 and 3 are shown in Figure 2, and pertinent photophysical data of complexes 1−4 and proligands HL1−HL4 are listed in Table 1 and Table S2 in the ESI, respectively. Proligands HL1−HL4 show intense absorption in the near UV region and moderately intense blue fluorescence centered at about 390 (HL1, HL2) and 440 nm (HL3, HL4). Compared to the proligands, the lowest energy absorptions of platinum complexes 1−4 occur at distinctly longer wavelengths. For instance, as shown in Figure 2, complex 1 in chloroform displays an intense long-wavelength band with two well-resolved maxima at λmax = 411 and 429 nm with molar absorption coefficients ε of 1.2 × 104 and 1.1 × 104 M−1 cm−1, respectively. Similarly, the lowest energy absorption band of complex 3 is centered at 462 nm (ε = 1.6 × 104 M−1 cm−1). The apparent strong red shift of these absorptions as compared to the proligands is explained by extension of the πconjugated systems onto the metal and acetylacetonate ligand, accompanied by planarization of the chromophoric ligand upon complexation. The partly resolved vibronic structure and large ε values for these absorptions point to the ligand-centered π−π* character of the respective electronic transitions. This conclusion is further supported by TD-DFT calculations (see below). Luminescence properties of complexes 1−4 were studied in poly(methyl methacrylate) (PMMA) films (Table 1, Figure 2, and Figure S2). Pt−ppy complexes 1 and 2 show moderately strong emissions at λem = 590 and 580 nm, respectively. The decay times τem are 15 μs (1) and 19.5 μs (2), indicating that

mixings of the metal-to-ligand charge-transfer character (MLCT), and thus, HISC involving these states is so fast that it can compete with internal conversion (IC) within the singlet manifold. In our previous study, we found that modification of the thiophene ring of the phosphorescent [Pt(thpy)(acac)] complex (with Hthpy = 2-(2′-thienyl)pyridine and Hacac = acetylacetone) with an additional thiophene or dithiphene unit leads to dual-emissive complexes [Pt(thpy-th)(acac)] and [Pt(thpy-th2)(acac)], respectively (with Hthpy-th = 5-(2pyridyl)-5′-dodecyl-2,2′-bithiophene and Hthpy-th2 = 5-(2pyridyl)-5″-dodecyl-2,2′:5′,2″-terthiophene).84 Such an extension of the π-aromatic system of the thpy ligand results, in particular, in less heavy-atom contributions to the lowest excited states of the resulting complexes, as compared to [Pt(thpy)(acac)]. Accordingly, the metal-induced spin−orbit coupling effects promoting intersystem crossing are significantly reduced. As a consequence, the ISC rates kISC(S1 → T1) are so low that the fluorescence S1 → S0 can efficiently compete with ISC, resulting in a dual emission. Herein we report a series of cyclometalated Pt(II) complexes with π-extended ppy and thpy type ligands (Chart 1). Chart 1. Chemical Structures of Complexes 1−4

Interestingly, Pt−ppy complexes 1 and 2 display “typical” phosphorescence with the emission decay times in the order of a few tens of microseconds, while the steady-state luminescence spectra of Pt−thpy complexes 3 and 4 consist of two different signals: a short-lived fluorescence with subnanosecond decay times and longer lived phosphorescence. Thus, substitution of the benzene ring with the thiophene leads to a drastic, and unexpected, change of the photophysical behavior. Moreover, dual-emissive complexes 3 and 4 show more efficient population of the triplet state T1 when excited to higher excited singlet states Sn as compared with excitation to S1 state, due to HISC.

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RESULTS AND DISCUSSION Syntheses and Structures. The ligands were synthesized utilizing the 1,2,4-triazine method.90−92 Thus, a condensation of 2-bromo-1-(4-fluorophenyl)-ethanone with 4-bromobenzoic hydrazide (HL1 and HL2) or 2-thienoylhydrazide (HL3 and HL4) afforded the corresponding 1,2,4-triazines (Scheme 1), which were then coupled with an additional thienyl ring in Stille reactions.93,94 These π-expanded 1,2,4-triazine derivatives were eventually transformed into the differently substituted pyridines (proligands) by elimination of N2 in inverse electrondemand Diels−Alder reactions95,96 either with 2,5-norbornadiene (HL1 and HL3) or with 1-(4-morpholin)-cyclopentene (HL2 and HL4). The proligands were cyclometalated with 7458


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Inorganic Chemistry Scheme 1. Syntheses of Phenylpyridine and Thienylpyridine Ligands HL1−HL4 and Platinum Complex 1a

a Complexes 2−4 were prepared analogously from the corresponding ligands. 1:2-Bromo-1-(4-fluorophenyl)ethanone, sodium acetate, ethanol/ acetic acid (3/1), 100 °C, 8 h. 1a: N-bromosuccinimide (NBS), dimethylformamide, 100 °C, 5 h. 2:2-(Tri-n-butyltin)-5-ethylthiophene, tetrakis(triphenylphosphine)palladium(0), dimethylformamide, 100 °C, 12 h. 3:2,5-Norbornadiene, o-xylene, 200 °C, 12 h. 4:1-(4-Morpholin)-1cyclopentene, 180 °C, 2 h. 5: K2[PtCl4], acetic acid/water, 120 °C, 24 h. 6: Sodium acetylacetonate, acetone, 60 °C, 48 h.

Figure 2. UV−vis absorption (blue) and luminescence spectra (red) of complexes 1 (top) and 3 (bottom) and respective proligands HL1 and HL3 (short dashed lines). Absorption spectra were recorded in CHCl3 solution, whereas emission spectra were measured in poly(methyl methacrylate) (PMMA) films at ambient temperature upon excitation at λexc = 360 nm.

the emissions represent spin-forbidden phosphorescence. The emission bands show well-resolved vibronic structures suggesting that the triplet state T1 is a ligand-centered state (3LC) of 3 ππ* character.20,21,84,90,97,99−102 The emission of complex 2 is slightly blue shifted with respect to that of complex 1, owing to the degradation of conjugation within the ligand because of

Figure 1. Perspective view (OLEX-298 plots with 50% probability thermal ellipsoids) of molecules 3 (top) and 4 (bottom). Hydrogen atoms are omitted for clarity.

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can be unequivocally distinguished from the proligand’s emission. The absence of any emission at λ < 450 nm in the spectra recorded for 3 and 4 further points to the analytical purity of the investigated samples. Moreover, the high-energy emissions of 3 and 4 occur at different wavelengths, excluding an origin from a common fluorescent impurity. Excitation spectra of 3 and 4 recorded with detections at the longer wavelength (T1 → S0) and shorter wavelength (S1 → S0) emission maxima match well with the absorption spectra, signifying that both phosphorescence and fluorescence come from the same compound (Figure 3). However, these excitation spectra are not identical. In particular, the intensities of the high-energy features at λ < 400 nm relative to the low-energy excitations observed between 400 and 500 nm for both complexes are higher when the spectra are detected at λdet = 715 (3) and 710 nm (4) than with a shorter wavelength detection at λdet = 525 (3) and 515 nm (4). In parallel, the phosphorescence/fluorescence intensity ratios also depend on the excitation wavelength (Figure 4). For example, for complex 3 measured in PMMA, intensity share of phosphorescence out of total emission intensity I(phos.)/I(total) rises from 18% to 25% when the excitation wavelength λexc is changed from λexc = 440 nm to λexc = 360 nm, respectively. For complex 4, a similar trend is observed, with an increase of I(phos.)/I(total) from 22% (λexc = 430 nm) to 41% (λexc = 350 nm). These wavelength dependencies of the spectroscopic properties probably result from ultrafast ISC occurring from higher singlet excited states.60,83 When some of the Sn → Tm ISC transitions (Sn and Tm represent the nth singlet and mth triplet excited state, respectively) are fast enough to compete with the internal conversion Sn → S1, the T1 and S1 states may be populated with different efficiency when the compound is excited into different initial Sn states. This ultrafast ISC between higher excited states was recently described by Pi-Tai Chou and Yun Chi et al. in dual-emissive Os(II) and Ag(I) complexes and denoted as hyper-intersystem crossing (HISC).83 Thus, apparently in the studied Pt−thpy complexes 3 and 4, the T1 state is populated more efficiently when higher singlet states are involved than upon excitation into the lowest singlet state S1. This HISC may be related to high metal contributions in some of the higher singlet and triplet states, in contrast to the S1 and T1 states being ligand-centered (LC) ππ* states. The occurrence of dual emission in Pt−thpy complexes 3 and 4 is a result of relatively slow ISC unable to deplete completely the singlet manifold within the fluorescence lifetime. Thus, using the τ and ϕPL data from Table 1, the ISC rates kISC, can be estimated. Apparently, according to ϕPL

Table 1. Photophysical Data for Platinum Complexes 1−4 at Ambient Temperature absorptiona

emissionb

λmax/nm (ε/M−1·cm−1)

λemc/nm

τ/ns

ϕPL/ %

590

15 000

11

580c

19 500

18

3

259 (23 540), 339 (2 2850), 411 (12 050) , 429 (11 200) 328 (22 200), 357 (23 100), 403 (14 250) , 423 (12 350) 261 (15 350), 354 (21 300), 462 (15 700) 256 (20 324), 349 (20 887), 450 (16 130)

0.13d 1900 0.13d 2500

1.5e

4

502 720 492 710

1 2

2.1e

a

Measured in CHCl3 at ambient temperature. bMeasured in poly(methyl methacrylate) (PMMA) film with 1 mass % of compound at ambient temperature. cPosition of the highest energy maximum in the observed vibronic progression. λem corresponds roughly to the energy of the emitting state (“purely electronic” 0−0 line). dMeasured in diluted CH2Cl2 solution after pulsed excitation at λexc = 455 nm. e Measured at the excitation wavelength of λexc = 450 nm. Shorter wavelength excitations (λexc = 350 nm) produced smaller ϕPL values of about 1%, probably due to hyper-intersystem crossing (HISC) between higher singlet excited states and appropriate triplet states.

spatial hindrances and electron-donor influence on LUMO, both caused by the fused cyclopentene ring.90 Pt−thpy complexes 3 and 4 display two weak emissions with total quantum yields of about 2% as measured in PMMA films at ambient temperature (Table 1). The steady-state spectra of both compounds are composed of green short-lived bands at λem = 502 (3) and 492 nm (4) and red−near-infrared long-lived bands at λem = 720 (3) and 710 nm (4), respectively. Thus, the two signals are separated by about 6000 cm−1. The low-energy emissions, with decay times of 1.9 (3) and 2.5 μs (4), respectively, are assigned as phosphorescence stemming from the lowest triplet state T1. These emissions are significantly red shifted relative to the phosphorescence of Pt−ppy congeners 1 and 2, owing to higher HOMO energy of thpy compared to ppy. The higher energy emissions, according to subnanosecond decay times of 0.13 ns found for both complexes, are assigned as fluorescence stemming from the lowest singlet excited state S1. These emissions display characteristic mirror-image relationships with the partly overlapped lowest energy absorption bands, as expected for absorption and emission originating from the same electronic transition S1 ↔ S0. Moreover, fluorescence of the Pt−thpy complexes 3 and 4 occurs about 3000 cm−1 lower in energy than fluorescence of the proligands HL3 and HL4. Thus, the fluorescence of complexes 3 and 4

Figure 3. Excitation spectra of complexes 3 and 4 measured in PMMA films at ambient temperature with detection wavelengths corresponding to the fluorescence signal (λdet = 525 and 515 nm, respectively) and the phosphorescence signal (λdet = 715 and 710 nm, respectively). 7460


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Figure 4. Luminescence spectra of complexes 3 and 4 recorded in PMMA films at ambient temperature upon excitation at the absorption bands corresponding to the lowest energy excitations S1 ← S0 (at λexc = 440 and 430 nm, respectively) and at shorter excitation wavelength (λexc = 360 and 350 nm, respectively) corresponding to higher energy transitions Sn ← S0. Spectral regions corresponding to the fast decaying fluorescence and the longer lived phosphorescence are indicated.

values of 1.5% (3) and 2.1% (4), relaxation to the ground state is dominated by nonradiative processes. In particular, for the S1 state, two paths exist: (i) direct internal conversion (IC) to the ground state S1 → S0 and (ii) ISC to a triplet state (S1 → Tn) followed by IC to T1 within the triplet manifold and, finally, nonradiative ISC relaxation T1 → S0. When ISC S1 → Tn is slower than the path (i), then the latter dominates. Thus, for complex 3, and assuming phosphorescence quantum yield of 100%, we find θISC ≈ ϕPL × I(phos.)/I(total) = 0.015 × 18% = 0.27 % and the lower limit for kISC of about 106−107 s−1. On the other hand (path (ii)), assuming that combined efficiency of ISC and fluorescence is close to 100% (no losses due to IC S1 → S0), the upper limit for kISC ≈ 1/0.13 ns = 8 × 109 s−1 is obtained. It is mentioned that for Pt−thpy complexes usually phosphorescence quantum yields of several percent are observed.84,90 Thus, an assessment of kISC as being in the range of 109−1010 s−1 is realistic. The HISC rates kHISC from higher singlet states are apparently much faster, but its efficiency is still not high enough for complete depletion of the singlet manifold due to fast competing IC within the singlet manifold. It is remarked, even though ISC in Pt−thpy complexes 3 and 4 is relatively slow, it is probably a barrierless process, since the phosphorescence is not frozen out even at cryogenic temperatures (Figure 5). Electronic Structures and Transitions. Given the unusual photophysical behavior of Pt−thpy complexes 3 and 4, a combined DFT and TD-DFT study was performed in an attempt to elucidate the electronic structures in more detail. Calculations were carried out at the def2-TZVP103,104/ B3LYP105 level of theory, which before was successfully applied for theoretical investigations of Pt(II) complexes.106,107 The UV−vis absorption spectra were then modeled using TD-DFT computations. Vertical excitation energies and oscillator strengths, calculated at optimized ground-state geometries, are in good agreement with experimental absorption spectra of the complexes (Figure S2), which gives credit to the simulations of electronic structures from the ground-state perspective. An energy level diagram of the molecular orbitals obtained for the

Figure 5. Emission spectra of complex 4 recorded in a PMMA film at cryogenic temperatures with excitation λexc = 430 nm.

relaxed ground-state geometries is presented in Figure 6. It is remarked though that relativistic effects and vibrational relaxation can affect the character and energy of excited states,56,61,108−110 and the calculation provided in this work only gives an assessment of nature and energy of the excited states without claiming full accuracy. According to the calculations, in all complexes 1−4 the HOMO and LUMO are localized on the cyclometalating ligands and HOMO-1 and HOMO-2 have a mixed ligand character with significant contributions from the arylpyridine and acac ligands as well as from the metal. The 5d contributions amount to about 30% and 40%, respectively. The HOMO-3s are mainly based on Pt (contribution of over 90%, Tables S4 and S5 and Figure S3). Thus, the heavy-atom contribution increases in a row from HOMO to HOMO-3. The LUMOs and several higher unoccupied orbitals are of π* character. The LUMOs are dominantly localized on the arylpyridine ligand, whereas LUMO+1s are centered on pyridine and the acac ligand. The calculated energies of spectrally relevant singlet Sn ← S0 and triplet Tn ← S0 excitations are summarized in Figure 6 and Table S6 in the ESI. Thus, the lowest transitions, arising from HOMO → LUMO excitations within the chromophoric 7461


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Figure 6. Energy level diagram for selected Kohn−Sham molecular orbitals (left) and energy level diagram of the lowest energy excited states resulting from TD-DFT calculations (right) of complexes 1−4.

ligands, are predicted to be essentially of π → π* character (Figure 7).

kISC =

∑ kISC(S1 → Tn)

(1)

n

Each ISC rate kISC(S1 → Tn) can be expressed in terms of the Fermi golden rule as a combination of electronic and nuclear factors as kISC(S1 → Tn) ∝ ⟨S1|Ĥ SO|Tn⟩2 × exp(−ΔE(S1 − Tn))2 (2)

where Ĥ SO and ΔE(S1 − Tn) are the spin−orbit coupling operator and the energy difference between the interacting states at their equilibrium geometries, respectively. The first term in eq 2 is the electronic spin−orbit coupling matrix element squared. The second term is related to the Franck− Condon weighted density of vibronic states that ensures energy conservation. This exponential term modulates the ISC rate according to specific interactions between the vibronic states of the initial electronic state S1 and the final state Tn.111−113 Results of the TD-DFT computations, summarized in Figure 6 and Table S6, predict that (i) the lowest triplet states T1 of all studied Pt complexes are several thousand cm−1 below the S1 level and (ii) the S1 state and a second triplet state T2 are close in energy. Therefore, in order to rationalize the observed differences in ISC rates, two ISC paths need to be considered: (i) a direct ISC from the S1 state to the lowest triplet state T1 and (ii) ISC from S1 to T2 followed by internal conversion within the triplet manifold from T2 to T1. However, the transition energies, calculated for the ground-state geometry, represent nonadiabatic states. Therefore, geometry relaxations are expected to stabilize the S1 state relative to the initially reached Franc−Condon (nonrelaxed) state. Thus, in complexes 3 and 4 the actual energy difference between the equilibrated S1 state and the nearest triplet state (T2) is probably higher than calculated, consequently resulting in a higher barrier between them. Indeed, according to experimental data the S1 → Tn ISC path(s) (with n > 1) in 3 and 4 seem to be not effective. The ISC rates in 3 and 4 are particularly low, yet the T1 state population is barrierless, which points to a forbidden ISC character, as opposed to the S1 → Tn transitions being El-Sayed allowed. Thus, in the following discussion of ISC rates only the direct ISC from S1 to the lowest triplet state T1 is considered for complexes 3 and 4. Since the spin−orbit coupling constant for a heavy transition metal is significantly larger than that of lighter elements, one can restrict the discussion of the SOC strength to contributions from d electrons of the metal. Thus, the SOC integral in eq 2 can be simplified to113,114

Figure 7. Isosurface contour plots (isovalue 0.05) of HOMO and LUMO calculated for complex 3.

The calculated S1 and T1 energies of Pt−thpy complexes 3 and 4 are distinctly lower than the corresponding energy levels of Pt−ppy complexes 1 and 2 (Figure 6). For instance, the S1 state of 3 is stabilized by 0.17 eV and T1 is stabilized by 0.44 eV with respect to 1, which perfectly reproduces the spectroscopic behavior. The lowest excited states, S1 and T1, indeed originate from ligand-centered (LC) transitions, which are confined to the phenylpyridine (1, 2) or thienylpyridine (3, 4) fragment and the pendant thienyl group of the ligands. The next transitions, in both the singlet and the triplet manifold, are assigned a mixed 1LC/MLCT character and display significant electron density shifts from acac, platinum ion, and the thienylsubstituted benzene/thiophene moiety of the ppy/thpy-core to the pyridine part of the cyclometalating ligand. It is remarked that states S2 and S3 of dual-emissive complexes 3 and 4, respectively, originate from transitions HOMO-3 → LUMO and have almost pure 1MLCT character (Figures S3 and S4, Tables S4−S6). The S1 − T1 energy separation is calculated to be about 0.5 eV in compounds 1 and 2 and about 0.8 eV in compounds 3 and 4. Larger ΔE(S1 − T1) values obtained for 3 and 4 point to a stronger localization (less charge transfer character) of S1 and T1. In all platinum complexes, 1−4, the second triplet state T2 and S1 are close one to each other. However, from a groundstate perspective, in Pt−ppy complexes 1 and 2 state T2 is below S1, whereas in Pt−thpy complexes 3 and 4, oppositely, the energy level of state T2 is above S1. This difference also might contribute to different ISC rates in Pt−ppy (1 and 2; ISC fast) and Pt−thpy (3 and 4; ISC slow) complexes. Intersystem Crossing in Platinum Complexes. The decay rate of a state S1 due to ISC is given as a sum of rates of ISC to each individual triplet state Tn 7462


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Inorganic Chemistry ⟨S1|Ĥ SO|Tn⟩ =

∑ aS1aTnckck′⟨1d kπ *|ĤSO|3d k′π *′⟩ k,k ′

giving rise to the slowly decaying phosphorescence, whereas in complexes 3 and 4 ISC is relatively slow and dual emission is observed. It is mentioned that similarly to Pt−thpy complexes 3 and 4, small ISC rates were correlated with large ΔE(S1 − T1) energy separations between the ligand-centered S1 (1ππ*) and T1 (3ππ*) states in cyclopentadienyl Rh(III) and Ir(III)complexes.116 However, for the latter compounds, only fluorescence was observed due to very small energies of the T1 states in the order of 1 eV (infrared region). Also, our calculations suggest that in phosphorescent molecules of 1 and 2 the T2 excited state is energetically accessible from the lowest singlet state S1 (Figure 6 and Table S6). Since the S1 and T2 states are of different origin (Table S6), the ⟨S1|Ĥ SO|T2⟩ term coupling states with different metal d contributions is large. If the relaxed state S1 is still higher in energy than state T2 then a small energy gap ΔE(S1 − T2) (Table S6) also favors fast ISC from S1 to T2 as discussed above. In contrast to the ligand-centered lowest singlet and triplet excited states S1 and T1, higher excited states of the Pt complexes 1−4, beginning with S2 and T2, display larger contributions of the Pt d character. Moreover, it was shown above that state S2 of complex 3 and state S3 of complex 4 have pure 1MLCT character with charge transfer from platinum to cyclometalating ligand, originated from HOMO-3 → LUMO transitions (Figures S3 and S4, Table S6). On top of that, each of the Sn states (n > 1) experiences a significantly larger density of isoenergetic vibronic sublevels of the triplet manifold than the S1 excited state. Thus, conditions for efficient HISC, e.g., large d-orbital coefficients for the SOC interacting states, different d character in the singlet and triplet state, small energy separation, and large overlap of the vibronic wave functions, are met; therefore, very high kHISC rates for higher excited singlet states are possible. In fact, HISC from some Sn states is fast enough even to compete with the internal conversion within the singlet manifold. The latter case of hyper-intersystem crossing (HISC) was concluded for Pt−thpy complexes 3 and 4, based on the different phosphorescence and fluorescence contributions to the total emission intensity upon excitation using light of different photon energy and the related variations of the excitation spectra.

(3)

Herein, aS1 and aTn are the normalized configuration interaction (CI) coefficients of the S1 ← S0 and Tn ← S0 excitations, respectively, and c k and ck′ represent the normalized contributions of the dk and dk′ atomic orbitals to the molecular orbitals involved in the S1 and Tn states, respectively. The matrix elements ⟨1dkπ*|Ĥ SO|3dk′π*′⟩ are not vanishing only when dk ≠ dk′ and π* = π*′.115 The S1 ← S0 and T1 ← S0 transitions stem dominantly from the HOMO → LUMO excitations (Table S6). For instance, in complex 4, the HOMO → LUMO contribution amounts to 89% in S1 and 94% in T1. Since both frontier orbitals are localized on the cyclometalating ligand, the lowest excited states are assigned to be ligand-centered states 1(ππ*) and 3(ππ*), respectively. Owing to the fact that in these S1 and T1 states dk = dk′ and due to the overall very small metal contribution to the S1 and T1 states, the SOC element ⟨S1|Ĥ SO|T1⟩ is also very small. A similar situation is found for complex 3. The TD-DFT calculations performed for Pt−ppy complexes 1 and 2 that display relatively high ISC rates reveal more significant CI mixings in the S1 states as compared to Pt−thpy complexes 3 and 4. For instance, in 1, the S1 ← S0 transition consists of HOMO → LUMO, HOMO-1 → LUMO, and HOMO-2 → LUMO excitations, with CI coefficients of 76%, 11%, and 7%, respectively, whereas the T1 state is largely of HOMO → LUMO origin (Table S6). Since HOMO-1 and HOMO-2 display different d-orbital contributions from that in the HOMO (dHOMO ≠ dHOMO‑1; dHOMO ≠ dHOMO‑2) the SOC matrix element becomes larger than in the case of SOCnoninteracting “pure” HOMO → LUMO derived states. Thus, the configuration interactions in the S1 state of 1 relaxes the ElSayed forbidenness of SOC that to some extent enhances ISC from the S1 state. Similar conclusions are also valid for complex 2, in which the CI coefficients for the HOMO-1 → LUMO and HOMO-2 → LUMO contributions into S1 amount to 9% and 8%, respectively. This effect can, thus, explain, on the basis of simple qualitative considerations, the observed faster ISC in the case of phosphorescent Pt−ppy complexes 1 and 2 and slower ISC in dual-emissive Pt−thpy complexes 3 and 4. Emission spectra of Pt−thpy complexes 3 and 4 display large spectral separations between the fluorescence and the phosphorescence signals. For both compounds, ΔE(S1 − T1) values of about 6000 cm−1 are found, as determined from energy differences between the offsets (high energy flanks) of the two emissions (Figure 2 and Figure S2). The large S1 − T1 energy separations are further substantiated by TD-DFT computations which reveal ΔE(S1 − T1) values (computed as the difference between the vertical S1 ← S0 and T1 ← S0 excitations; Table S6) of 0.82 and 0.84 eV for complexes 3 and 4, respectively. The S1 − T1 energy separations in complexes 1 and 2 are significantly smaller and amount to 0.55 and 0.53 eV, respectively. Given the exponential dependence of kISC on ΔE(S1 − T1)2 (eq 2), the consequences of these distinctly different energy gaps for the ISC rates and spectral behavior of the platinum complexes are straightforward. ISC from S1 to T1 in molecules with smaller ΔE(S1 − T1) gaps (complexes 1 and 2) is expected to be faster than in molecules with a larger gap (complexes 3 and 4). As a result, in complexes 1 and 2 ISC is fast enough to efficiently depopulate the singlet manifold (no detectable fluorescence) and populate the lowest triplet state,

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CONCLUDING REMARKS The Pt(II) complexes investigated in this study differ in the photophysical behavior. The complexes having a benzene as the metalating part of the chromophoric ligand (Pt−ppy derivatives 1 and 2) display typical phosphorescence, while the complexes having thiophene instead of benzene (Pt−thpy derivatives 3 and 4) display an unusual property of dual emission. Our previous study on Pt−thpy compounds revealed that dual luminescence can be achieved when the cyclometalating thpy ligand is extended with an additional thiophene unit. In such compounds, the lowest excited states obtain distinct LC character and metal contributions to S1 and T1 become small relative to the nonextended phosphorescent compounds. Such a minimized mixing of the metal character into S1 and T1 seems to promote an occurrence of dual emission by reducing of the metal-induced ISC.84 Results of this work are in line with our previous conclusions and show that minor changes in cyclometalating ligand’s structure, more so than even shown before with addition of a thiophene ring, can modulate the ISC rate in a complex and alter the balance between phosphorescence and fluorescence. 7463


Article

Inorganic Chemistry However, dual luminescence occurs also in d8−d8 dimeric complexes, in particular, [Pt2(μ-P2O5H2)4]4− and its borofluorinated derivative [Pt2(μ-P2O5(BF2)2)4]4−.81,87,117 In the latter systems the lowest singlet and triplet excited states result from 5dσ* → 6pσ excitations that display large metal character. Thus, small metal d contributions to the lowest excited states are not the only factor leading to dual emission. Also, the energy separation between the lowest (fluorescent) singlet excited state and the ISC interacting triplet state seems to play an important role at determining the kISC rates. When this gap is large, e.g., ΔE(S1 − T1) of about 6000 cm−1 as determined for [Pt2(μ-P2O5(BF2)2)4]4− 87 and Pt−thpy complexes 3 and 4, or even larger, like in the fluorescent cyclopentadienyl Rh(III) and Ir(III) complexes,76,116 the ISC from S1 may become so slow as being unable to deplete the S1 completely in the time scale of fluorescence, enabling the latter state to decay radiatively in the form of fluorescence. In complexes 1 and 2, with the energy difference ΔE(S1 − T1) being relatively small, ISC from S1 to T1 (directly or via another triplet state close to S 1 ) is fast and leads to efficient quenching of the S 1 fluorescence. On the contrary, complexes 3 and 4, characterized by large ΔE(S1 − T1) values, display relatively low kISC rates in the order of 109−1010 s−1, being low enough for an occurrence of dual emission. Moreover, the comparison of the photophysical properties of Pt−ppy compounds 1 and 2 and Pt−thpy compounds 3 and 4 demonstrates that even modest changes in the ligand structure can lead to impressive variations in the emission properties, other than the conventional substitution effects like spectral shifts and variations of the nonradiative rates. It is remarked, however, that the situation is complicated, especially in terms of high density and relative order of higher excited states of different origin, and may change from complex to complex. In particular, a thorough analysis of the ISC paths and rates requires more sophisticated quantum mechanical computation endeavors, including calculations of the potential energy surfaces and vibronic structures of the singlet and triplet excited states (S1, S2, S3... and T1, T2, T3...) and treating the spin−orbit interactions explicitly.56,61,108−110 Nevertheless, even the relatively simple TD-DFT approach, used in this work, gives valuable information about the character of the relevant excited states and helps to rationalize the photophysical effects observed experimentally. Thus, the calculations performed for the ground-state-optimized geometries reproduce the trend observed for the energy separations ΔE(S1 − T1), with the ΔE(S1 − T1) values for dual-emissive Pt−thpy complexes 3 and 4 being approximately 0.3 eV larger than the values obtained for phosphorescent Pt−ppy complexes 1 and 2. In addition, large energy separations between the T1 and the higher triplet states have been revealed, in particular, for compounds 3 and 4, suggesting that at least in the latter complexes the ISC from S1 represents direct S1 → T1 transition. This assumption is further supported by the fact that the emission spectra recorded between 2 and 300 K are similar in terms of the phosphorescence/fluorescence intensity ratio. Signs of HISC between the higher lying excited states are clearly seen for dual-emissive Pt−thpy complexes 3 and 4. This is related to the high MLCT character admixtures in some of the higher excited singlet states, El-Sayed allowedness of ISC transitions between states originating from transitions involving nonequal metal d orbitals, and possibly large overlaps of the relevant vibronic substates, in contrast to the situation found for the S1 → T1 transition. It is noteworthy that combination of

HISC with ISC within a dual-emissive complex allows one to modulate the efficiency of the population of the triplet manifold after photoexcitation. This, in the future, might open prospects for new applications based on the dependence of emission color on excitation wavelength.

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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.inorgchem.6b00704. Experimental details, compound characterization, X-ray data, absorption and emission data for the proligands, computational details, DFT ground-state geometries, excited-state energies, dominant orbital excitations from TD-DFT calculations, molecular orbitals and excitedstate density difference plots, molecular orbital compositions in the ground states for the represented Pt(II) complexes (PDF) (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: shafikoff@gmail.com. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.Z.S. is grateful to the Russian Foundation for Basic Research for financial support, project No. 12-03-31318. M.Z.S. also gratefully acknowledges Professor Burkhard Kö nig and Professor Hartmut Yersin (Regensburg) for kind help and hospitality.

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