Effect of Molecular Conformation Restriction on the Photophysical

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Effect of Molecular Conformation Restriction on the Photophysical Properties of N^N Platinum(II) Bis(ethynylnaphthalimide) Complexes Showing Close-Lying 3MLCT and 3LE Excited States Fangfang Zhong,†,⊥ Jianzhang Zhao,*,† Mustafa Hayvali,‡ Ayhan Elmali,§ and Ahmet Karatay*,§ †

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State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2 Ling Gong Road, Dalian 116024, P. R. China ‡ Department of Chemistry, Faculty of Science, and §Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100 Beşevler, Ankara, Turkey S Supporting Information *

ABSTRACT: Using naphthalimide (NI), complexes (Pt-PhNI and Pt-PhMeNI) based on the N^N platinum(II) bis(phenylacetylide) coordination framework were prepared, in which there are two close-lying triplet states, i.e., the metal-toligand-charge-transfer (3MLCT) and the NI localized emissive state (3LE). Pt-PhNI has better electronic communication between the Pt coordination center and the NI moiety, whereas in Pt-PhMeNI, they are more isolated by orthogonal geometry. For Pt-PhMeNI, the S0 → 1MLCT and S0 → 1LE absorption bands are separated by 5655 cm−1, while they are more overlapped in Pt-PhNI. The 3MLCT → S0 and 3LE → S0 dual phosphorescence emissions were observed for both Pt-PhNI (in toluene) and Pt-PhMeNI (in benzonitrile). The molecular conformation tunes the 3MLCT/3LE state population ratio, and the orthogonal geometry makes the 3LE state in Pt-PhMeNI basically a dark state (in toluene). Switching of the relative energy levels of the 3MLCT/3LE states by variation of the solvent polarity and temperature was achieved. For Pt-PhMeNI, the energy level of 3MLCT state is higher in a polar solvent; thus, the 3 MLCT emission decreases, while the phosphorescence lifetime is prolonged from 9.5 μs (in toluene) to 58 μs (in benzonitrile) because of the different equilibria with the nonemissive 3LE state. Conversely, increasing the temperature enhances the upward transition from the nonemissive 3LE state to the emissive 3MLCT state; as such, the phosphorescence of Pt-PhMeNI was intensified at higher temperature (which is unusual), and the phosphorescence lifetime decreased from 58 μs (298 K) to ca. 5 μs (348 K). The ultrafast intersystem crossing (ca. 0.5 ps) and intramolecular triplet−triplet energy transfer (3−11 ps) were studied by femtosecond transient absorption spectroscopy. These results are useful for an in-depth understanding of the photophysics of multichromophore transition-metal complexes and for the design of external stimuli-responsive sensing materials, for instance, temperature or microenvironment sensing materials.

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due to the S0 → 1MLCT transition.8 Efficient intersystem crossing (ISC) occurs for these complexes upon photoexcitation because of the heavy-atom effect of the Pt(II) coordination center. As a result, the 3MLCT state is populated, and this is a long-lived triplet state (the lifetime is on the microsecond scale). Recently, some acetylide ligands with strong visible-light-absorbing ability were attached to the Pt(II) center; in this case, the low-lying singlet and triplet states are the ligand localized states (1LE and 3LE states, respectively).2,7,21,22 Because of the reduced heavy-atom effect of the 3LE state compared to the 3MLCT state, the triplet-state lifetimes (3LE) of these complexes are much longer than the

INTRODUCTION Transition-metal complexes, for instance, N^N platinum(II) bis(acetylide) complexes, are of particular interest because of their visible-light absorption and long-lived triplet excited states.1−8 These complexes have been widely used in photocatalysis,9−11 photovoltaics,12,13 photocatalytic hydrogen production,12,14−16 phosphorescent molecular probes,17−20 and triplet−triplet-annihilation (TTA) photon upconversion.21−23 The study of the photophysical properties of these complexes is pivotal for the design of new Pt(II) complexes with improved application performance. The fundamental photophysical properties of the N^N platinum(II) bis(acetylide) complexes have been intensively studied. It is well accepted that the low-lying singlet excited state is a metal-to-ligand charge-transfer (1MLCT) state and the low-energy absorption in the UV−vis absorption spectra is © XXXX American Chemical Society

Received: September 11, 2018

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DOI: 10.1021/acs.inorgchem.8b02558 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Molecular Structures and Syntheses of the Pt(II) Complexesa

a The molecular structures of the ligands and reference complexes are also shown. (a) Pd(PPh3)4, K2CO3, toluene/EtOH/H2O, N2, reflux, 8 h, yield 64%. (b) TMSA, Pd(PPh3)2Cl2, PPh3, CuI, NEt3, reflux, 8 h. (c) K2CO3, THF/MeOH, RT, 1 h. (d) KOAc, Pd(dppf)Cl2, dioxane, N2, reflux, 11 h, yield 89%. (e) 5-Bromo-2-iodo-1,3-dimethylbenzene, Pd(PPh3)4, K2CO3, toluene/EtOH/H2O, N2: (1) RT, stirred for 15 min; (2) stirred at 78 °C for 22 h; (3) stirred at 110 °C for 12 h. Yield 60%. (f) CuI, (i-Pr)2NH, dry CH2Cl2, N2, RT, 12 h.

conventional 3 MLCT states, which are beneficial for intermolecular energy transfer or electron transfer. However, much room is left for photophysical studies of the N^N platinum(II) bis(acetylide) complexes, especially the multichromophore molecular systems. For instance, the above two kinds of complexes only represent the limits; i.e., either 3 MLCT or 3LE is the low-lying state, and there is no significant interaction between the two states. As a result, either a typical 3 MLCT state or an 3LE state was observed, by phosphorescence emission or by nanosecond transient absorption (ns TA) spectroscopies.4 Besides these two limits, N^N platinum(II) bis(acetylide) complexes with closely-lying 3MLCT and

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LE states were rarely reported, although the study of these systems is pivotal for an understanding of the fundamental photophysical properties of these complexes.4,24 Previously, the N^N platinum(II) bis(naphthalacetylide) complex was reported, showing the close-lying 3MLCT/3LE states. The energy level of the 3MLCT state is more dependent on the solvent polarity than the 3LE state; thus, the relative energy levels of the two states can be switched by using solvents with different polarities.4,25 This is an interesting example showing that two close-lying excited states may exist; even the π-conjugation framework of the ligand and the Pt(II) coordination center is connected via acetylide bonds. This is B


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Inorganic Chemistry against the general intuition that the fully π-conjugated ligand should be without distinct 3MLCT and 3LE states in the platinum(II) bis(acetylide) complexes. This scenario is different from the N^N platinum(II) bis(acetylide) ligands with heteroleptic acetylide ligands, for which the existence of two closel-lying triplet excited states is understandable because the two ligands are not in π conjugation.26−29 The examples showing this close-lying triplet excited state with fully πconjugated ligands are very limited. Herein we prepared the N^N platinum(II) bis(acetylide) complexes based on naphthalimide (NI)-derived acetylide ligands (Pt-PhNI and Pt-PhMeNI; Scheme 1), and we observed the close-lying 3MLCT and NI localized 3LE states, as well as the effect of the solvent polarity and temperature on the photophysical properties. In Pt-PhNI, the 1MLCT/1LE states share similar energy levels, whereas the 3MLCT/3LE states are separated by 1170 cm−1, based on the temperaturedependent phosphorescence studies. We also used the conformation restriction approach to fully separate the 3 MLCT state from the 3LE state, which is unprecedented in the related studies. The photophysical properties of the complexes were studied in detail with steady-state and timeresolved transient spectroscopies. We found that a triplet excited-state equilibrium exists in both Pt-PhNI and PtPhMeNI, and the relative energy levels of the two triplet excited states can be tuned by the solvent polarity and temperature. The results are useful for an in-depth study of the fundamental photophysical properties of the transition-metal complexes.

Figure 1. UV−vis absorption spectra of the complexes and ligands: (a) Pt-PhNI, PhNI-H, and Pt-Ph; (b) Pt-PhMeNI, PhMeNI-H, and Pt-Ph. c = 1.0 × 10−5 M in toluene and 25 °C.

MLCT transition.8 In a comparison of Pt-PhMeNI and PtPh, we can assign the shoulder absorption band in the range of 370−550 nm to the S0 → 1MLCT transition. The absorption band at 342 nm was observed in both Pt-PhMeNI and PhMeNI-H, which can be assigned to the S0 → π−π* transition of the NI moiety (Figure 1b).30 These results indicate that mixing of the 1MLCT and 1LE states is limited for Pt-PhMeNI. In other words, the platinum(II) phenylacetylide and NI units are well isolated (not in π conjugation). Very few transition-metal complexes were reported to show such wellseparated absorption bands. Previously, Pomestchenko and Castellano reported the N^N platinum(II) bis(naphthylacetylide) complex showing mixed 3MLCT/3LE states, but the separation of the 1MLCT and 1LE states is to a minor extent.4 Interestingly, for Pt-PhNI, a broad absorption band in the range of 330−550 nm in the visible region was observed, without any well-separated absorption bands (Figure 1a). A shoulder absorption band at ca. 450 nm is discernible. Thus, we propose more significant electronic interaction between the NI moiety and Pt(II) coordination center in Pt-PhNI compared to that in Pt-PhMeNI. This postulation is reasonable because there are no methyl groups on the ortho positions at the phenyl linker; thus, coplanarity is more likely for Pt-PhNI.26 In order to study the character of the absorption bands, the solvent-dependent UV−vis absorption spectra of the ligands and complexes were studied (Figures S21 and S22). The UV− vis absorption spectra of the NI acetylides do not vary in different solvents, indicating that the change in the dipole upon going from the ground state (S0) to 1LE is not significant (Figure S21). For Pt-Ph, the S0 → 1MLCT absorption band shows a blue shift in the polar solvent compared to that in nonpolar solvents. This is due to the significant charge-transfer character of the transition in Pt-Ph. For Pt-PhMeNI, the shoulder absorption band at 396 nm shows a blue shift, but the absorption band at 342 nm does not change in different solvents. However, no such clear change was observed for PtPhNI (Figure S22). These results indicate that the NI moiety and Pt(II) coordination center are well separated in PtPhMeNI but not in Pt-PhNI; i.e., the electronic coupling between the Pt(II) center and NI moiety at the ground state is more significant in Pt-PhNI than in Pt-PhMeNI. The phosphorescence spectra of the complexes were studied (Figure 2a). The phosphorescence emission band of PtPhMeNI (at 565 nm) is almost exactly the same as that of PtPh, which is structureless and can be assigned to the 3MLCT state. For Pt-PhNI, the NI localized emission (3LE) at 605 nm 1

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RESULTS AND DISCUSSION Molecule Design and Synthesis. Previously we reported the Pt(II) complex Pt-NI, in which Pt(II) is directly attached with the NI moiety and strong phosphorescence at 624 nm was observed.30 In this case, the 3LE state (1.99 eV) is well separated from the 3MLCT state (2.19 eV, estimated from the reference complex); thus, only the long-lived 3LE state is manifested in the phosphorescence and the ns TA spectrum (Kasha’s rule). Herein we designed complex Pt-PhNI (Scheme 1), in which the NI moiety and Pt(II) coordination center are separated by a phenyl moiety; the π conjugation between the Pt(II) coordination center and NI moiety is reduced compared to that of Pt-NI. Because the 3MLCT state (2.19 eV) and the 3LE state of NI (2.07 eV) are close, we thus expected that the photophysical properties of Pt-PhNI may be tuned by an external stimulus, such as the solvent polarity or temperature.4,24,25 In order to further separate the 3MLCT and 3LE state energy levels, i.e., to reduce the state mixing, we designed Pt-PhMeNI, in which there are two methyl groups at the ortho position of the NI substituent (Scheme 1). Because of the steric hindrance exerted by the methyl groups, the NI and phenyl moieties will assume an orthogonal geometry;26 thus, the coupling of the NI moiety and Pt(II) coordination center will be significantly reduced. To the best of our knowledge, no such molecular conformation restriction has been used for discrimination of the emissive 3MLCT and 3LE states in Pt(II) complexes. The parent complex Pt-Ph was used as a reference. UV−Vis Absorption and Luminescence Spectra. The absorption spectra of Pt-PhNI and Pt-PhMeNI were compared with the ligands and parent complex Pt-Ph (Figure 1 and Table 1). The broad and structureless absorption band in the visible region (ca. 424 nm) of Pt-Ph is the typical S0 → C


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Inorganic Chemistry Table 1. Photophysical Parameters of the Acetylide Ligands and Pt(II) Complexesa λabs/nm NI-H PhNI-H PhMeNI-H Pt-PhNI Pt-PhMeNI Pt-NI Pt-Ph

350 352 341 384 342, 396 414 424

εb 2.70 1.81 1.29 3.53 3.62, 1.39 4.84 0.89

λem/nm

ΦΔc d

400 419 400 605 563 624 565

0.16 0.10d 0.12d 0.71, 0.85e 0.55, 0.38e 0.74, 0.89e 0.87, 0.38e

τeme

τTg

Φemi

0.7 ns 1.4 ns 0.9 ns 64 μsg 9.3 μsg 69 μsg 0.98 μsg

h h i 46 μs 7.9 μs 64 μs 0.94 μs

0.15 0.82 0.31 0.030j 0.16k 0.18k 0.55k

In toluene, 20 °C. bMolar extinction coefficient at the absorption maxima. ε: 104 M−1 cm−1. cQuantum yield of singlet oxygen (1O2) in toluene. Anthracene (ΦΔ = 0.61 in ethanol) as the standard. λex = 356 nm. eIn acetonitrile, Ru(bpy)32+ (ΦΔ = 0.57 in CH2Cl2) as the standard. λex = 470 nm. fLuminescence lifetimes at RT. gPhosphorescence lifetimes. hTriplet-state lifetimes, determined with nanosecond transient difference absorption spectroscopy, λex = 425 nm, and 3.0 × 10−5 M in deaerated toluene. iNot observed. jEmission quantum yields with 9,10diphenylanthracene (ΦF = 97% in cyclohexane) as the standard.43 kPhosphorescence quantum yields, [Ru(dmb)3]PF6 (ΦP = 7.3% in acetonitrile) as the standard.44 a

d

Figure 2. (a) Normalized phosphorescence emission spectra of PtPhNI, Pt-PhMeNI, Pt-NI, and Pt-Ph. c = 1.0 × 10−5 M in deaerated toluene, λex = 410 nm, and 25 °C. (b) Gaussian peak-fitting results of the emission spectrum of Pt-PhNI.

is dominant, while a shoulder band (3MLCT) at ca. 550 nm (simulated by Gaussian peak fitting) is also observed (Figure 2b). The dual phosphorescence emission indicates the existence of mixed 3MLCT/3LE states in Pt-PhNI. Although the emission bands of Pt-PhMeNI and Pt-Ph are the same, the phosphorescence quantum yield of Pt-PhMeNI (ΦP = 16%) is much lower than that of Pt-Ph (ΦP = 55%). Interestingly, the phosphorescence lifetime of Pt-PhMeNI (9.3 μs) is much longer than that of Pt-Ph (0.98 μs). These results indicate that the emissive triplet state of Pt-PhMeNI is perturbed by another dark triplet state, most likely the 3LE state localized on the NI moiety. In order to study the effect of solvent on phosphorescence, the emissions of the complexes in different solvents were compared (Figure 3). For Pt-Ph, we observed a blue shift of the phosphorescence band in polar solvents as compared to that in nonpolar solvents (Figure 3d). However, the emission band of Pt-NI hardly changed in different solvents (Figure 3c). The luminescences of Pt-Ph and Pt-NI exhibit typical 3MLCT and 3LE features, respectively. For Pt-PhMeNI, the phosphorescence intensity is highly dependent on the solvent polarity, which is significantly quenched in polar solvents (Figure 3b). For Pt-PhNI, interestingly, the shoulder emission band (ca. 550 nm) in toluene disappeared in polar solvents. We proposed that, in general, the energy of the 3MLCT state was promoted to higher levels in polar solvents. As a result, the energy gap between the 3MLCT (at higher energy level) and 3LE (NI localized state, a dark state in toluene) states in Pt-PhMeNI increases; thus, the 3MLCT emission intensity (565 nm) is decreased. For Pt-PhNI, however, the 3LE state is also an

Figure 3. Phosphorescence emission spectra of (a) Pt-PhNI (absorbance of the solution A = 0.26), (b) Pt-PhMeNI (A = 0.10), (c) Pt-NI (A = 0.48), and (d) Pt-Ph (A = 0.08). Optically matched solutions were used. c = ca. 1.0 × 10−5 M, λex = 410 nm, and 25 °C.

emissive state (because of the strong interaction with Pt); as such, the shoulder 3MLCT emission band (ca. 550 nm) disappeared in polar solvents, but the 3LE emission (ca. 605 nm) band persisted. We also studied the effect of the solvent polarity on the emission lifetime (Figure S24). The phosphorescence lifetime of Pt-PhNI in the polar solvent acetonitrile (67 μs) is similar to that in the nonpolar solvent toluene (64 μs), while significantly a prolonged phosphorescence lifetime was observed for Pt-PhMeNI in acetonitrile (38 μs) than in toluene (9.3 μs). Both complexes show much longer emission lifetimes in benzonitrile (103 and 58 μs for Pt-PhNI and PtPhMeNI, respectively). We attribute this result to the promotion of the 3MLCT state energy level in polar solvents, which results in a poor triplet energy equilibrium with the 3LE state. As a result, the lifetime is prolonged by the energy reservoir effect of the 3LE state.31−38 The phosphorescence emission spectra at 77 K (in a solid matrix, toluene) were compared with that at room temperature (RT; Figure 4). The emission band of Pt-NI hardly changed upon a decrease in the temperature, as a feature of the 3LE state (Figure 4c). The emission band of Pt-Ph at 77 K is D


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Inorganic Chemistry

Figure 4. Normalized emission spectra of (a) Pt-PhNI, (b) Pt-PhMeNI, (c) Pt-NI, (d) and Pt-Ph in deaerated toluene at RT and 77 K. c = 1.0 × 10−5 M, and λex = 410 nm.

Figure 5. Emission comparison of Pt-PhNI and Pt-PhMeNI with Pt-Ph in deaerated (a and b) toluene and (c and d) benzonitrile. Optically matched solutions were used (λex = 420 nm, A = 0.20, and 25 °C).

structured and blue-shifted by 2709 cm−1 (thermally induced Stokes shift) compared to that at RT, which is typical for the 3 MLCT state (Figure 4d).1,2,39−41 For Pt-PhNI, the shoulder emission band at 565 nm disappeared, while the emission band at 605 nm became more structured at 77 K (Figure 4a). For

Pt-PhMeNI, the emission band is narrower and structured at 77 K than at RT (Figure 4b). These results demonstrate that the emission states of both Pt-PhNI and Pt-PhMeNI are 3LE states (558 nm, 2.22 eV) at 77 K. This is reasonable because the 3MLCT state cannot be well solvated or stabilized in a E


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Inorganic Chemistry

Figure 6. Temperature-dependent photoluminescence spectra of (a and b) Pt-PhNI, (c and d) Pt-PhMeNI, (e and f) Pt-NI, and (g and h) Pt-Ph in deaerated (a, c, e, and g) toluene and (b, d, f, and h) benzonitrile, respectively. Optically matched solutions were used for every compound (λex = 420 nm, and A = 0.20).

is normal for luminescence because vibration-coupled nonradiative decay is accelerated at high temperature compared to that at low temperature. Similar results were observed for PtNI (Figure 6e,f). For Pt-PhNI in toluene, with the 3LE emission at 605 nm decreased at higher temperature, interestingly, the 3MLCT emission at ca. 565 nm is intensified concurrently (Figure 6a). This result infers that the 3MLCT state is slightly above the 3LE state in toluene, and the larger thermal energy at higher temperature makes the 3MLCT state more populated; thus, the 3MLCT emission is enhanced.24 The luminescence lifetimes at 605 and 540 nm were determined as 64 and 65 μs at 298 K, respectively. These similar lifetimes indicate a good equilibrium between the two states.42 The energy gap of the 3MLCT/3LE states was calculated as 0.15 eV, according to the phosphorescence emission wavelength. In benzonitrile, the phosphorescence of Pt-PhNI at 620 nm decreased at higher temperature, but the emission intensity at ca. 536 nm increased only slightly (Figure 6b). This result is reasonable because the 3MLCT energy increased in benzonitrile compared to that in toluene, but the 3LE energy level does not change, as a result the 3MLCT/3LE state energy gap increases (0.31 eV, approximated with the emission maxima) in benzonitrile. The thermal energy (0.026 eV at RT) is insufficient to populate the 3MLCT state significantly. For Pt-PhMeNI, the emission decreased at elevated temperature (Figure 6c in toluene). Interestingly, the phosphorescence increased at higher temperature in benzonitrile (Figure 6d), which is in contrast to most normal phosphorescent complexes, and it is a clear indication of the 3 MLCT/3LE excited-state equilibrium (or state mixing).24 In toluene, the 3MLCT state is proposed to be isoenergetic or slightly lower than the 3LE state; thus, increasing temperature will reduce the phosphorescence intensity because of the enhanced nonradiative decay at higher temperature. In

frozen solution because of the restricted solvent reorganization; thus, the 3MLCT state energy level will rise in a frozen matrix. As a result, the triplet-state equilibrium collapses, and the 3LE state becomes the emissive state. It should be pointed out that the energy level of the 3LE state differs between these complexes because of different π conjugations. Because the energy level of the 3MLCT state is more dependent on the solvent polarity than that of the 3LE state, we propose that the solvent polarity can affect the relative energy level of the 3MLCT and 3LE states; as a result, the emission properties of the complexes should be able to be tuned by the solvent polarity.4 The phosphorescence emission spectra of the complexes in toluene and benzonitrile were compared (Figure 5). In toluene, the emission of Pt-PhMeNI is weaker than that of Pt-Ph; this is due to the triplet−triplet energy transfer (TTET) from the 3MLCT state to the dark 3 LE state and the establishment of an 3MLCT/3LE state equilibrium (Figure 5b). This postulation is supported by the prolonged phosphorescence lifetime of Pt-PhMeNI (9.3 μs) compared to Pt-Ph (0.98 μs). In benzonitrile, the 3MLCT energy level will increase; as a result, we expect a reduced phosphorescence intensity. This postulation was supported by the experimental results (Figure 5d). For Pt-PhNI, the emission band at 605 nm and a shoulder band at ca. 565 nm indicate an 3MLCT/3LE state equilibrium (Figure 5a in toluene). Similarly, dual phosphorescence emission of PtPhMeNI indicates 3MLCT/3LE state equilibrium (Figure 5d in benzonitrile); this result is in accordance with the prolonged emission lifetime in benzonitrile. Temperature-Dependent Emission. In order to further confirm the existence of excited-state mixing and/or equilibrium in the complexes, we studied the temperaturedependent emission (Figure 6). For the reference complex PtPh in toluene and benzonitrile (Figure 6g,h), the phosphorescence intensity decreased at higher temperature. This result F


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Inorganic Chemistry benzonitrile, the 3MLCT state energy level becomes higher than that of the dark 3LE state; as a result, the emission from the 3MLCT state will be enhanced at higher temperature because of the upward transformation from the 3LE state to the 3 MLCT state. The temperature-dependent emission decay traces were studied (Figures S25 and S26). The temperature-dependent curves of the phosphorescence lifetimes of the complexes in toluene and benzonitrile were studied (Figure 7). For Pt-Ph,

drain channel for the excited states. The 3MLCT/3LE states have a smaller energy gap in toluene than in benzonitrile because of the dependence of the energy level of the 3MLCT state on the solvent polarity, which shows hyperchromatic shifts. Thus, the phosphorescence lifetime becomes longer in benzonitrile than in toluene. Similar results were observed with Pt-PhNI (Figure 7). At higher temperature, the luminescence lifetime of PtPhMeNI decreased from 9.5 μs (298 K) to 3.3 μs (348 K). In benzonitrile, the 3MLCT state energy increases and becomes higher than the 3LE state energy (which is a dark state, nonemissive). Thus, at elevated temperature, the thermal energy will make the 3MLCT state more populated (the nonemission 3LE state acts as an energy reservoir), and the phosphorescence originating from the 3MLCT state will become stronger; as a result, the phosphorescence lifetime is reduced. To the best of our knowledge, only one Ru(II) complex with a pyrene−acetylide ligand with such an enhanced emission at elevated temperature has been reported.24 The redox properties of Pt-PhNI and Pt-PhMeNI were studied by cyclic voltammetry (Figure S27). For both Pt-PhNI and Pt-PhMeNI, a reversible reduction wave at ca. −1.7 V and an irreversible oxidation wave at ca. +0.9 V have been detected, which are assigned as the NI reduction and platinum acetylide oxidation, respectively.45 The energy levels of charge-separated states (ECS) have been calculated for both Pt-PhNI and PtPhMeNI in different solvents46 and are higher than the energy levels of the 3MLCT and 3LE states (Table S1). Thus, the effect of electron transfer on the 3MLCT/3LE state equilibrium is negligible. ns TA Spectroscopy. The triplet states of the complexes were studied with ns TA spectroscopy in toluene (Figure 8) and benzonitrile (Figure S28). For Pt-Ph, the excited-state absorption (ESA) band is at 380 nm, which is in agreement with the previous observations.39 A broad ESA band centered at 650 nm was also observed for Pt-Ph. This spectrum was obtained with the phosphorescence background correction; otherwise, the ESA band may be masked by the strong emission band (Figure 8g).39 The triplet-state lifetime of Pt-Ph is determined as 0.94 μs (Figure 8h), similar to the previously

Figure 7. Temperature dependence of the phosphorescence lifetimes of Pt-PhNI, Pt-PhMeNI, Pt-NI, and Pt-Ph in deaerated (a) toluene and (b) benzonitrile. The temperature varied from 298 to 348 K. c = 1.0 × 10−5 M.

the phosphorescence lifetime at different temperatures does not change significantly; for example, variation of the phosphorescence lifetime is from 0.96 μs (298 K) to 0.74 μs (348 K) in toluene and from 1.0 μs (298 K) to 0.57 μs (348 K) in benzonitrile. For Pt-NI, the phosphorescence lifetime increased in benzonitrile (105 μs) compared to that in toluene (69 μs). For both Pt-PhNI and Pt-PhMeNI, the phosphorescence lifetimes increased in benzonitrile compared to those in toluene. For instance, the phosphorescence lifetime of PtPhMeNI in toluene is 9.5 μs (298 K), and it increased to 58 μs in benzonitrile. The 3MLCT state is relatively short-lived compared to the 3LE state; thus, the 3MLCT state acts as a

Figure 8. ns TA spectra of (a) Pt-PhNI, (c) Pt-PhMeNI, (e) Pt-NI, and (g) Pt-Ph. Decay traces of (b) Pt-PhNI at 490 nm, (d) Pt-PhMeNI at 470 nm, (f) Pt-NI at 420 nm, and (h) Pt-Ph at 380 nm in deaerated toluene. λex = 425 nm, c = 3.0 × 10−5 M except for Pt-NI (c = 1.5 × 10−5 M), and 20 °C. G


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Inorganic Chemistry

Figure 9. fs TA spectra of Pt-PhMeNI: absorption spectra with different time delays in (a) toluene and (c) benzonitrile. Time evolution at different probe wavelengths in (b) toluene and (d) benzonitrile. λex = 400 nm, c = 1.0 × 10−4 M, and 20 °C.

reported value of 1.2 μs (in CH2Cl2).47 The triplet-state lifetime of Pt-NI is 64 μs (Figure 8f), much longer than the 3 MLCT state of Pt-Ph. The lowest triplet excited state (T1 state) of Pt-NI can be assigned as the 3LE state. For both Pt-PhNI and Pt-PhMeNI, the ESA features are different from that of Pt-Ph. Moreover, the triplet-state lifetimes of Pt-PhNI (46 μs) and Pt-PhMeNI (7.3 μs) are much longer than that of Pt-Ph but shorter than that of Pt-NI. Thus, we propose that the T1 states of Pt-PhNI and PtPhMeNI are different from those of Pt-Ph and Pt-NI, namely, a mixed 3MLCT/3LE state. This is consistent with the phosphorescence lifetime. Note that the triplet-state lifetime is determined at higher concentration; thus, its value is different from that of the phosphorescence lifetime. In benzonitrile, no significant changes were observed for PtPhNI, Pt-NI, and Pt-Ph compared to those in toluene (Figure S28). For Pt-PhMeNI, however, the ESA feature changed; for instance, the ESA at 800 nm decreased in benzonitrile. Moreover, the triplet-state lifetime of Pt-PhMeNI significantly prolonged to 47 μs in benzonitrile compared to that in toluene (7.3 μs). This prolonged triplet-state lifetime of Pt-PhMeNI in benzonitrile can be rationalized with the increased 3MLCT state energy levels compared to those in toluene; as a result, the 3MLCT/3LE state energy gap is reduced, and better excited-state equilibrium is achieved. We propose that the 3LE state is the major component of the excited state of PtPhMeNI observed by the ns TA in benzonitrile. Femtosecond Transient Absorption (fs TA) Spectroscopy. In order to investigate the kinetics of the intramolecular energy transfer and ISC processes, ultrafast fs TA (pump− probe technique) spectroscopy was performed in both benzonitrile and toluene.

For Pt-PhMeNI, upon excitation at 400 nm, a broad ESA band between 500 and 750 nm (λmax ≈ 665 nm) was observed (Figure 9a), which is the T1 → Tn absorption of the 3MLCT excited state.48 The ISC rates for Pt-PhMeNI were extracted from the growth of the signal at 735 nm, determined as 0.27 ps (3.7 × 1012 s−1) and 0.23 ps (4.3 × 1012 s−1) in benzonitrile and toluene, respectively (Table S2). This is faster than the reported platinum(II) diimine complex bearing a naphthalene−diimide electron acceptor, with an ISC rate constant of 1.1 × 1012 s−1 (0.9 ps).49 Following the ISC, intramolecular energy transfer was observed in Pt-PhMeNI. With the ESA band of 3MLCT (around 650 nm) decreasing, a long-lived ESA band at ca. 470 nm grows, which is due to the T1 → Tn transition of the NI moiety (3LE).50 The isoabsorptive point at 497 nm (after 0.53 ps delay) and the same evolution kinetics of the 3MLCT (decreasing at 670 nm) and 3NI (increasing at 473 nm) ESA signals indicate the TTET process in Pt-PhMeNI (Figure 9a). Neither of the triplet excited states decays to the ground state at 3 ns delay, which suggested the triplet equilibrium in PtPhMeNI. From the fast-growth part in the kinetic curve at 480 nm, we can conclude that the TTET rate is faster in benzonitrile (3 ps, Figure 9d) than in toluene (11 ps, Figure 9b). Previously, a faster intramolecular TTET from the 3MLCT/ LLCT state to the 3C60 state in the platinum(II) bipyridine complex with phenylacetylene ligands has been reported with a rate constant of 4 ps in dichloromethane.51 A much slower intramolecular TTET process (within 15 ns) from the 3MLCT excited state to the 3LC (3PNI) state has also been reported in a carbonylrhenium(I) phenanthroline complex with a naphthalene chromophore.42 The TTET rates may be affected by the energy gap between these two triplet excited states and the H


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Inorganic Chemistry

Scheme 2. Simplified Jablonski Diagram Showing the Energy Levels of (a) Pt-PhNI, (b) Pt-PhMeNI, (c) Pt-NI, and (d) Pt-Ph

a

The energy levels of the T5 states in Pt-PhNI and Pt-PhMeNI, the T3 state in Pt-NI, and the T2 and 3LE states in Pt-Ph are obtained by timedependent density functional theory calculations. The energy levels of the other excited states are derived from the spectral data. The lifetimes of the excited states are detected by luminescence decays. BZN and Tol stand for benzonitrile and toluene, respectively.

of the C−C linker between the NI and phenyl moieties were studied (Figure S38). Interestingly, for Pt-PhNI and PtPhMeNI, we observed drastically different PES changes. For Pt-PhNI, there are two energy minima, with dihedral angles of 50° and 120° between the NI and phenyl moieties, respectively. The thermal energy at RT is kT = 0.026 eV (k is the Boltzmann constant, and T is the temperature); thus, the dihedral angles accessible at RT are in the ranges of 40−71° and 107−138°. Therefore, it is a mixture of different conformers for Pt-PhNI in solution at RT. For Pt-PhMeNI, however, there is only one energy minimum on the PES of the torsion of the C−C bond, which has a dihedral angle of 90°. This is due to the steric hindrance exerted by the methyl groups on the phenyl bridge between the NI and phenyl moieties. The conformations accessible at RT for Pt-PhMeNI have dihedral angles of 74−97°. Thus, the geometry of PtPhNI is more coplanar than that of Pt-PhMeNI. The energy diagrams of the complexes are shown in Scheme 2. The low-lying triplet states of Pt-Ph and Pt-NI are typical 3 MLCT and 3LE states, respectively. Because of the large energy gap between the 3MLCT and 3LE states in Pt-Ph and Pt-NI, neither the solvent polarity nor the temperature can switch the emission properties of the two complexes.4 For PtPhNI and Pt-PhMeNI, however, two nearly degenerated triplet states were observed. For Pt-PhNI, both the 3MLCT and 3LE states are emissive (the energy gap is 0.14 eV in toluene), and a significant temperature effect on the phosphorescence emission was observed. However, for PtPhMeNI, only one emission band was observed by variation of the temperature in toluene (Figure 6c). In benzonitrile, the

distance between the donor and acceptor moieties, as well as the different linkers. The fs TA spectrum of Pt-PhNI is not very different from that of Pt-PhMeNI (Figure S29). Generally, there are two ESA signals of the 3NI state in the visible range, i.e., a band centered at ca. 475 nm and a broad band spread over 600−750 nm.50 In Pt-PhMeNI, the band at ca. 470 nm is dominant, while the broad band in the red portion surpassed it in Pt-PhNI; we attribute this to the different configurations of the NI ligands in these two complexes, which results in different 3MLCT/3LE state population ratios. The ISC rate of Pt-PhNI was detected as ca. 0.48 ps, slower than that of Pt-PhMeNI (ca. 0.25 ps). The fs TA spectra of Pt-NI and Pt-Ph have also been studied (Figures S30 and S31). Pt-Ph gives a broad ESA peak above 500 nm.48 The ISC rates of Pt-NI and Pt-Ph were determined as 0.3 and 0.4 ps, respectively (Table S2). The ESA band decay kinetics of the 3MLCT state in Pt-PhMeNI, Pt-PhNI, Pt-NI, and Pt-Ph were compared (Figure S32). These results indicate that, although the acetylide ligands in Pt-PhMeNI and Pt-PhNI are bulky, their ISC kinetics is similar to that of Pt-NI and Pt-Ph. To visualize localization of the lowest triplet states, the spindensity surfaces of Pt-PhNI, Pt-PhMeNI, Pt-NI, and Pt-Ph were calculated based on the optimized ground-state geometry (Figure S37). For Pt-PhNI, the spin-density surface is confined on the ligand with an 3EL state feature, while typical 3MLCT state surfaces were obtained for Pt-PhMeNI, which are in good agreement with the emissive features of the complexes. In order to study the molecular conformation restriction in Pt-PhNI and Pt-PhMeNI, the PES curves about the rotation I


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Inorganic Chemistry 3

MLCT energy level increased, and the large 3MLCT/3LE energy gap makes the upper state (3MLCT state) much less populated than the 3LE state. As a result, the phosphorescence intensity (originating from the 3MLCT state) is enhanced at higher temperature because this upper state is more populated at higher temperature.24 Note that for Pt-PhMeNI the 3LE state is a dark state (nonemissive state) in toluene. In some cases, we suggested an intermediate triplet state for the ISC; it is known that the ISC is dependent on the energy gap of the two states, and a smaller energy gap is beneficial for ISC. It should be pointed out that it is not easy to confirm the involvement of the intermediate-high triplet state in the ISC because the internal conversion is most likely faster than the ISC; thus, the high triplet state will not contribute to the ultrafast transient absorption spectral features. The energy gap between the 3MLCT and 3LE states of PtPhNI is larger in benzonitrile than in toluene; thus, at elevated temperature in toluene, the emission at 605 nm decreased and a distinct new emission band at 565 nm intensified. In benzonitrile, the 3MLCT state energy level increased, and the larger energy gap between the 3MLCT and 3LE states makes the appearance of the 565 nm emission band unlikely. The excited-state equilibrium constant (Keq) of Pt-PhNI and Pt-PhMeNI in toluene and benzonitrile was calculated by the reported methods (eqs 1 and 2)52 1 1 1 =α + (1 − α) τ τLE τMLCT (1)

3

MLCT-featured. This can be explained by the much higher emission quantum yield of the 3MLCT state (ca. 55%) than the 3LE state (ca. 18%).

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CONCLUSIONS In summary, with acetylide naphthalimide ligands, we prepared two N^N platinum(II) bis(acetylide) complexes (Pt-PhNI and Pt-PhMeNI) showing degenerate 3MLCT/3LE states. This is in contrast from the normal N^N platinum(II) bis(acetylide) complexes showing the T1 state dominant in either the 3 MLCT or 3LE state feature. For Pt-PhNI, the π conjugation between the NI moiety and Pt(II) coordination center is strong via the phenyl bridge; as a result, both the 3MLCT and 3 LE states are phosphorescent. For Pt-PhMeNI, however, the NI moiety and Pt(II) coordination center are more separated by the orthogonal geometry of the NI and phenyl bridge, achieved by the conformation restriction exerted by the two omethyl substituents on the phenyl bridge. As a result, the 3 MLCT state is phosphorescent, but the 3LE state is a nonemissive dark state (in toluene). The phosphorescence of both complexes is dependent on the solvent polarity, as well as the temperature. In toluene at elevated temperature, Pt-PhNI shows a blue-shifted emission band due to the increased 3 MLCT population. On the other hand, intensified phosphorescence emission was observed for Pt-PhMeNI at elevated temperature in benzonitrile, which is unusual, because of the more populated 3MLCT state (higher than the nonemissive 3 LE state). The triplet-state lifetime of Pt-PhMeNI is extended to 58 μs in benzonitrile compared to that in toluene (9.5 μs). The molecular conformation distributions of the complexes were studied with the PES curves generated by rotation about the C−C linker between the NI moiety and Pt(II) coordination center. The ns TA spectra indicate that the lowest triplet excited states of Pt-PhNI and Pt-PhMeNI are mixed 3MLCT/3LE states. fs TA spectroscopy show ultrafast ISC (0.50 ps) and intramolecular TTET (3−11 ps) processes in both Pt-PhNI and Pt-PhMeNI. These external stimuliresponsive Pt(II) complexes may be useful for the study of novel molecular materials that are responsive to solvent polarity and temperature and also an understanding of the fundamental photophysical properties of the N^N platinum(II) bis(acetylide) complexes.

α (2) 1−α where τ is the observed phosphorescence lifetime of the complexes, τLE and τMLCT correspond to the time constants for the decays of the excited NI and platinum moieties, respectively, α corresponds to the fraction of excited NI triplets, and 1 − α corresponds to the fraction of excited MLCT triplets in the equilibrated population. The excited-state equilibrium constants of Pt-PhNI and PtPhMeNI in toluene and benzonitrile are shown in Table 2. For Keq =

Table 2. Calculated Excited-State Equilibrium Constants of Pt-PhNI and Pt-PhMeNI in Toluene (Tol) and Benzonitrile (BZN) Pt-PhNI Pt-PhMeNI

Tol BZN Tol BZN

τ/μsa

τLEb

τMLCTc

1−α

Keq

64 103 9.3 58

69 105 69 105

0.98 1.02 0.98 1.02

0.0011 0.00020 0.092 0.0080

8.9 × 102 5.2 × 103 9.8 1.3 × 102

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EXPERIMENTAL SECTION

Analytical Measurements. 1H NMR spectra were recorded by a Bruker 400 MHz spectrometer with CDCl3 as the solvent and tetramethylsilane (0.00 ppm) as the internal standard. Highresolution mass spectrometry (HRMS) spectra were determined in a TOF MALDI-HR MS system (U.K.). Elemental analyses (C, H, and N) were performed on a PerkinElmer model 240C elemental analyzer. Fluorescence spectra were measured on a RF5301 PC spectrofluorometer (Shimadzu, Japan), while the absorption spectra were recorded on a UV2550 UV−vis spectrophotometer (Shimadzu, Japan). ns TA Spectroscopy. ns TA spectra and decay curves were recorded on an LP980 laser flash photolysis spectrometer (Edinburgh Instruments, U.K.), under the excitation of a wavelength-tunable nanosecond pulsed laser (Opolette 355II+UV, OPOTEK, USA). The samples were purged with N2 for 20 min before measurements. The transient signal was digitized by a Tektronix TDS 3012B oscilloscope, and the data were transferred to the L900 software (Edinburgh, U.K.). fs TA Spectroscopy. The ultrafast pump−probe spectroscopy measurements were performed on a Ti:sapphire laser amplifier− optical parametric amplifier system (Spectra Physics, Spitfire Pro XP,

a Phosphorescence lifetimes of the complexes. bThe time constants for the decays of the 3LE state, approximated with the phosphorescence lifetime of Pt-NI. cThe time constants for the decays of the 3MLCT state, approximated with the phosphorescence lifetime of Pt-Ph.

Pt-PhNI, α was thus estimated to be almost unity in both solvents, which means that the equilibrated population is predominantly in favor of the NI chromophore in toluene and benzonitrile. This is in agreement with the emission spectra, which give a 3LE phosphorescence peak in both solvents. For Pt-PhMeNI, the Keq value is much smaller than that for PtPhNI, although the equilibrium is also NI-dominant. It should be noted here that, although the fraction of excited MLCT triplets is less than 10% in toluene, the emission spectrum is J


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Inorganic Chemistry TOPAS) and a commercial setup ofan ultrafast transient absorption spectrometer (Spectra Physics, Helios). All of the spectroscopy experiments were pumped at 400 nm wavelength, which corresponds to the 1MLCT state. The synthesis and characterization data of the compounds are presented in the Supporting Information.

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(7) Castellano, F. N.; Pomestchenko, I. E.; Shikhova, E.; Hua, F.; Muro, M. L.; Rajapakse, N. Photophysics in Bipyridyl and Terpyridyl Platinum(II) Acetylides. Coord. Chem. Rev. 2006, 250, 1819−1828. (8) Williams, J. A. G. Photochemistry and Photophysics of Coordination Compounds: Platinum. Top. Curr. Chem. 2007, 281, 205−268. (9) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis. Chem. Rev. 2013, 113, 5322−5363. (10) Hari, D. P.; König, B. The Photocatalyzed Meerwein Arylation: Classic Reaction of Aryl Diazonium Salts in a New Light. Angew. Chem., Int. Ed. 2013, 52, 4734−4743. (11) Shi, L.; Xia, W. Photoredox Functionalization of C-H Bonds Adjacent to a Nitrogen Atom. Chem. Soc. Rev. 2012, 41, 7687−7697. (12) Dai, F.-R.; Zhan, H.-M.; Liu, Q.; Fu, Y.-Y.; Li, J.-H.; Wang, Q.W.; Xie, Z.; Wang, L.; Yan, F.; Wong, W.-Y. Platinum(II)Bis(aryleneethynylene) Complexes for Solution-Processible Molecular Bulk Heterojunction Solar Cells. Chem. - Eur. J. 2012, 18, 1502− 1511. (13) Zhou, G.; Wong, W.-Y.; Poon, S.-Y.; Ye, C.; Lin, Z. Symmetric Versus Unsymmetric Platinum(II) Bis(aryleneethynylene)s with Distinct Electronic Structures for Optical Power Limiting/Optical Transparency Trade-off Optimization. Adv. Funct. Mater. 2009, 19, 531−544. (14) DiSalle, B. F.; Bernhard, S. Orchestrated Photocatalytic Water Reduction Using Surface-Adsorbing Iridium Photosensitizers. J. Am. Chem. Soc. 2011, 133, 11819−11821. (15) Gärtner, F.; Cozzula, D.; Losse, S.; Boddien, A.; Anilkumar, G.; Junge, H.; Schulz, T.; Marquet, N.; Spannenberg, A.; Gladiali, S.; Beller, M. Synthesis, Characterisation and Application of Iridium(III) Photosensitisers for Catalytic Water Reduction. Chem. - Eur. J. 2011, 17, 6998−7006. (16) Goldsmith, J. I.; Hudson, W. R.; Lowry, M. S.; Anderson, T. H.; Bernhard, S. Discovery and High-Throughput Screening of Heteroleptic Iridium Complexes for Photoinduced Hydrogen Production. J. Am. Chem. Soc. 2005, 127, 7502−7510. (17) Lanoe, P.-H.; Fillaut, J.-L.; Toupet, L.; Williams, J. A. G.; Bozec, H. L.; Guerchais, V. Cyclometallated Platinum(II) Complexes Incorporating Ethynyl-Flavone Ligands: Switching between Triplet and Singlet Emission Induced by Selective Binding of Pb2+ Ions. Chem. Commun. 2008, 4333−4335. (18) Coogan, M. P.; Fernandez-Moreira, V. Progress with, and Prospects for, Metal Complexes in Cell Imaging. Chem. Commun. 2014, 50, 384−399. (19) Zhao, Q.; Li, F.; Huang, C. Phosphorescent Chemosensors Based on Heavy-Metal Complexes. Chem. Soc. Rev. 2010, 39, 3007− 3030. (20) Xiang, H.; Zhou, L.; Feng, Y.; Cheng, J.; Wu, D.; Zhou, X. Tunable Fluorescent/Phosphorescent Platinum(II) Porphyrin−Fluorene Copolymers for Ratiometric Dual Emissive Oxygen Sensing. Inorg. Chem. 2012, 51, 5208−5212. (21) Wu, W.; Wu, X.; Zhao, J.; Wu, M. Synergetic Effect of C^N^N/ C^N^N Coordination and the Arylacetylide Ligands on the Photophysical Properties of Cyclometalated Platinum Complexes. J. Mater. Chem. C 2015, 3, 2291−2301. (22) Liu, L.; Zhang, C.; Zhao, J. The Effect of the Regioisomeric Naphthalimide Acetylide Ligands on the Photophysical Properties of N^N Pt(II) Bisacetylide Complexes. Dalton Trans. 2014, 43, 13434− 13444. (23) Singh-Rachford, T. N.; Castellano, F. N. Photon Upconversion Based on Sensitized Triplet−Triplet Annihilation. Coord. Chem. Rev. 2010, 254, 2560−2573. (24) Harriman, A.; Khatyr, A.; Ziessel, R. Extending the Luminescence Lifetime of Ruthenium(II) Poly(Pyridine) Complexes in Solution at Ambient Temperature. Dalton Trans. 2003, 2061− 2068. (25) Goeb, S.; Rachford, A. A.; Castellano, F. N. Solvent-Induced Configuration Mixing and Triplet Excited State Inversion Exemplified in a Pt(II) Complex. Chem. Commun. 2008, 814−816.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02558. Synthesis, experimental details, 1H and 13C NMR spectra, HRMS data, extra photophysical spectra, and electrochemical and density functional theory (DFT)/ time-dependent DFT calculation details of the complexes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jianzhang Zhao: 0000-0002-5405-6398 Ahmet Karatay: 0000-0001-9373-801X Present Address ⊥

F.Z.: College of Materials Science and Engineering, Changsha University of Science and Technology, Changsha 410114, P. R. China.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the NSFC (Grants 21473020, 21673031, 21761142005, 21273028, 21603021, and 21421005), the Fundamental Research Funds for the Central Universities (Grants DUT16TD25, DUT15ZD224, and DUT2016TB12), and the State Key Laboratory of Fine Chemicals (Grant ZYTS201801) for financial support.

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REFERENCES

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