Relationship between Metallophilic Interactions and Luminescent

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Relationship between Metallophilic Interaction and Luminescent Properties in Pt(II) Complexes: TD-DFT Guide for the Molecular Design of Light-Responsive Materials Julia R. Romanova, Malaviarachchige Rabel Ranga Prabhath , and Peter D. Jarowski J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12132 • Publication Date (Web): 23 Dec 2015 Downloaded from http://pubs.acs.org on December 29, 2015

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The Journal of Physical Chemistry

Relationship between Metallophilic Interaction and Luminescent Properties in Pt(II) Complexes: TD-DFT Guide for the Molecular Design of Light-Responsive Materials. Julia Romanova, M. R. Ranga Prabhath and Peter D. Jarowski* University of Surrey, Advanced Technology Institute Guildford, GU2 7XH, U.K. ABSTRACT: DFT/TD-DFT investigations have been performed on pyridyl-triazolato platinum(II) complexes with systematic variation of the donor/acceptor properties of the ligand in order to illuminate its effect on the metallophilic intermolecular interaction in ground- and excited-state. The π-electronic properties of the pyridyl-triazolate ligand were modified by variation in the pyridine substituent: –N(CH3) 2, –H, –CHO or –CHC(CN)2. The simulations reveal that the donor/acceptor strength of the substituent has strong impact on the metallophilic interaction in excited-state and affects the emission properties at the supramolecular level. The theoretically derived structure-property relationships are corroborated by experimental data. Finally, it is proposed that the modification of the π-electronic character of the substituent (ligand field) can be applied in the molecular design of smart luminescent materials with light-driven Pt-Pt interaction.

Introduction The molecular-scale response to stimuli is the basis for transferring optical and electrical information at the material level and in hierarchical devices. There are a greater and greater number of examples of complex molecular assemblies constructed by rational-design recently, where chemists aim to create materials whose properties can be altered in specific ways in order to attain a desired functionality.1-3 The state-of-the-art is now approaching natural examples, typified, perhaps, by biological examples.4 Such materials are called smart, intelligent or multi-stimuli responsive and are defined as designed materials that have one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as light, pressure, temperature, solvent, pH, electric and magnetic fields, etc. The increasing demand for smart materials and new avantgarde technologies has raised tremendous interest in the design of these systems.5-8A promising material class for the creation of new stimuli-responsive systems is that of the hybrid organic-inorganic and nanohybrid.9 While organic-based materials are lower in cost, processable, printable and flexible compared with existing inorganic materials10, it is also important to recognize that the most promising ‘organic’ materials have tended to owe their improved properties to the inclusion of some metal functionalities delivering beneficial redox and photoluminescence behaviour. Thus, many nanohybrid systems are under development with a large proportion in the metallopolymer area.11-15 A very intriguing class of metal-organic assembly is driven by metallophilic (metalmetal) interaction, where the metal-metal attraction is modulated by light. Since these systems assemble based on direct metal-metal interaction forming metallostrings

they maintain a high degree of metallic character evidenced by their high conductivity.16 At the same time, they can be tuned by organic ligand design both molecularly and at the material level by modulating π-π stacking strength and architecture, solubility (processability) and stability. The metallophilicity is usually found between metal centres with low coordination number and is also closely associated with relativistic effects.17-20 Consequently, it is encountered preferentially for the heaviest transition metals 21-23 such as Au, Pt, Pd and Hg but also to light metal atoms like Ag24 This unique property compensates for the extra cost in working with these expensive atoms. From the point of view of hybrid smart materials, the ability to modulate the metallophilic interactions would add a direct handle on controlling their fundamental properties in the solidstate and thus leading to a strong response to external stimuli. Stimuli such as light, pressure and voltage (redox state) have been shown to alter the metal-metal intra or intermolecular attraction and, in terms of responsiveness, the materials show significant change in luminescent and/or conducting properties.3 The light-driven modulation in the metallophilic interactions is associated with the photoluminescent properties of the metal-organic assemblies and an excimerization process,25-36 which usually results in strongly red-shifted emission. The first X-ray evidences on the metallophilic bonding in the excited-state was reported for the Pt(II)-pyrophosphate complex37 and the excimerization process can be explained in terms of electronic structure theory23,38,39. If one takes Pt(II) complexes as an example, Pt possesses a doubly occupied 5dz2 orbital. When metallophilic ground-state interaction occurs, the 5dz2 (a1g) orbitals of adjacent Pt-atoms (within

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one molecule or belonging to different molecules) interact and this results in the formation of bonding a1g and antibonding a2u molecular orbitals. Both molecular orbitals are doubly occupied and the electron couple on the antibonding one ‘destabilizes’ the dimer. Therefore, in the ground-state the Pt-Pt (intra or intermolecular) interaction is relatively weak and the metal-metal distances are usually ≥ 3.2Å. On the other hand, an excitation that leads to one-electron extraction from the antibonding a2u molecular orbital has positive impact on the stability of the Pt-Pt dimer. As a result, the attraction between the metals centres increases and the Pt-Pt bond approaches the covalent region ~2.7-2.8Å. We have recently synthesized in our laboratory four examples of light-responsive Pt(II) complexes based on a subsituted-pyridyl-1,2,3-triazolate ligand structure, where the metal-metal attraction is modulated by light.29,30 The complexes are structurally similar but differ in the donor/acceptor strength of the pyridine substituent. The substituents are -NMe2, -H, -CHO or -CHC(CN)2 and their π-electronic character shows wide variation in donor-acceptor range.40 Deprotonated ligands in ethanol at 10-5 M show a spectral range over 100 nm proceeding from 1a to 1d characterized by the expected charge transfer bands approaching a ca. 390 nm for the most red shifted example of 1d. Solid films of these exhibit only weak red shifts and their concentration dependant UV-vis spectroscopy are linear with respect to the Beer-Lambert law. Photolumiescence spectroscopy reveals single emissions in the near visible range from about 400 nm (1a) to about 469 nm (1d). Fluorescence for 1b was confirmed by lifetime measurements and all ligands show good overlap and symmetry with the corresponding absorption bands. Thus, a tunability was observed in the emission and absorption spectra. Platinum complexes of these ligands exhibited remarkably similar absorptions spectra in the UV range (ethanol, 10-5 M) and only became dissimilar when considering their very low intensity and broad visible absorptions, which suggested limited tunability in the series based on a smaller overall range of maxima from 342 nm (2a) to 370 nm (2d). No evidence of ground- state dimerization was found studying the concentration dependent absorption spectra. As a corollary, the photoluminescence spectra observed for the series also showed a lack of tunability and again, in contrast to the ligands themselves, 2a-d emit in a narrow range around 400 nm of 11 nm revealing also a blue shift of the emission band in the case of 2c and 2d, in particular. The molecular emission for 2b at low concentration had a clear singlet lifetime no greater than 15 ns. Thus, this outcome is opposite to that expected from the ligands and counterproductive from a control perspective and from a triplet generation and efficiency perspective. On the other hand, thin films of 2a-d complexes show a broad emission tunability of about 140 nm ranging from 460 to 600 nm. These changes were confirmed at high concentrations in ethanol suggesting

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that the effect has its origin in excimer formation in solution with these excimers representing the predominant geometrical arrangement in the solid-state. Moreover, linear dependence between the donor/acceptor strength of the substituents and the emission maxima was observed, which opens new horizons for the chemical fine tuning of supramolecular organization and design of smart materials. Intermediate bi-exponential lifetimes with a short ca. 42 ns and a longer ca. 153 ns were observed for 2b. Through theory we were able to identify these excimers as occurring through metallophilic interactions and assigned them as singlet emissions with the possibility of delayed fluorescence or phosphorescence. In this paper, we present density functional theory (DFT) and time-dependant (TD) DFT results on the ground- and excited-state structures and properties of the four bis-5-substituted-pyridyl-1,2,3-triazolato Pt(II) complexes (2a-d) and their dimers (2a-2a, 2b-2b, 2c-2c and 2d-2d) (Scheme 1), as well as on the ground-state charge distributions in the isolated ligands (1a-d). The absorption and emission properties of the monomers and dimers have been previously discussed in an experimental context with only supporting computation. Although there have been many theoretical investigations on metallophilicity27,41-51, to our knowledge this is the first systematic quantum-chemical study on the link between metallophilic bonding, electronic structure and emission properties in organometallic complexes. Moreover, the main goal of the present investigation is to employ the theoretically derived structure-property relationships in order to define simple guidelines for the molecular design of light responsive metal-organic assemblies and to answer two important questions: (1) why does platinum complexation attenuate the natural tunability of the ligands and (2) how is this tunability restored through metallophilic interactions?

Computational procedure The ground-state geometries and vibrational frequencies of the ligands, monomer complexes and their dimers (Scheme 1) were obtained with the PBE052 functional, Stuttgart/Dresden (SDD) pseudopotential and basis set for Pt, and Pople 6-31G* split-valence basis set for the ligands. Additionally, the polarizable continuum model (PCM) in its integral equation formalism variant was applied in order to take into account the electrostatic effects originating from the ethanol solvation environment (ε0=24.85, ε∞=1.85).53-55 The vibrational analysis confirmed that the optimized structures represent true minima on the potential energy surface and were used as the basis in their harmonic approximation to compute the zero-point energy (ZPE), thermal and free-energy (298 K) corrections used to compare molecular interaction energies. Finally, in order to take into account the effect of the dispersion correction to the dimers stability and geometry, the monomer and dimer ground-states were also simulated using PBE0 with

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Table 1. Optimized PBE0/PCM bond lengths [Å] in the ground state of 2a-d complexes (C2h symmetry) and their dimers (D2 symmetry). The atom labelling is according to Scheme 1. 2a S0

2a-2a S0

2b S0

2b-2b S0

2c S0

2c-2c S0

2d S0

2d-2d S0

Pt-N1py

2.058

2.059

2.055

2.055

2.054

2.055

2.055

2.054

N1py-C1py

1.359

1.358

1.362

1.362

1.365

1.368

1.366

1.366

C1py-C2py

1.392

1.392

1.392

1.392

1.397

1.394

1.396

1.396

C2py-C3py

1.381

1.381

1.388

1.387

1.381

1.384

1.380

1.379

C3py-C4py

1.412

1.412

1.392

1.393

1.400

1.397

1.409

1.409

C4py-C5py

1.412

1.411

1.386

1.386

1.390

1.394

1.402

1.402

C5py-N1py

1.337

1.337

1.343

1.342

1.338

1.334

1.333

1.332

Pt-N2tr

2.015

2.016

2.014

2.014

2.011

2.010

2.009

2.009

N2tr-N3tr

1.323

1.323

1.318

1.318

1.314

1.314

1.313

1.312

N3tr-N4tr

1.319

1.320

1.322

1.324

1.325

1.326

1.327

1.327

N4tr-C6tr

1.350

1.350

1.347

1.347

1.345

1.345

1.343

1.343

C6tr-C7tr

1.385

1.385

1.385

1.385

1.387

1.387

1.389

1.389

C7tr-N2tr

1.354

1.353

1.355

1.354

1.356

1.356

1.358

1.357

C7tr-C1py

1.443

1.433

1.442

1.442

1.437

1.437

1.433

1.433

Bond

Pt-Pt

3.754

D3 dispersion correction56 developed by Grimme et al. (PBE0-D3). Scheme 1. Chemical structure of the investigated ligands, Pt(II) monomer and dimer complexes. Atom labelling.

We note that counterpoise correction (to handle basis set superposition error) is not possible simultaneously with PCM solvation and the former was not included in our theoretical treatment as it can be expected that this correction will have the same order of magnitude for all

3.591

3.515

3.573

dimerization processes due to similar molecular size. Vertical electronic excitations for all compounds were obtained by the time dependent formulation of density functional theory with the same functional and basis sets as for geometry optimization. The TD-DFT approach was also applied in order to optimize the lowest-energy singlet (S1) and triplet (T1 and/or T2, for metal-containing compounds) excited-states and to estimate their vertical emission energies. Due to convergence problems with the TD-DFT optimization, an alternative computational procedure was used for T1 states of 2a-2a and 2d-2d dimers: DFT optimization of T1 followed by TD-DFT estimation of T1→S0 vertical emission energy. During TDDFT optimizations the symmetry optimized ground-state input structures were optimized without symmetry constraints (nosymm option), yielding the symmetric result in most cases. The vertical absorption and emission energies were obtained with linear response. The groundstate charges of the molecules were estimated by natural bond orbital (NBO) population analysis. All quantumchemical calculations were performed with the Gaussian 09 program package.57 The molecular orbitals were visualized with GaussView 5.58

Results and Discussion Properties of the monomers. The ground-state optimized geometries of monomer complexes 2a-d obtained with the PBE0 functional are listed in Table 1. In all complexes, the Pt-N2tr distances are shorter than PtN1py owing to a stronger interaction of the platinum ion

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Table 2. Vertical absorption and emission energies E [eV], wavelengths λ [nm] and oscillator strength f computed for monomer complexes 2a-d with TD-DFT/PBE0/PCM and linear response solvation in ethanol.

2a

2b

2c

2d

ES S1 S2 T1 T2 S1 T1 T2 S1 T1 T2 S1 T1 T2

Absorption E λ f 3.30 376 0.1407 3.30 376 0.0000 2.35 528 2.35 528 3.66 338 0.0337 3.00 413 3.06 405 3.35 370 0.0996 2.70 459 2.73 454 2.99 415 0.4189 2.07 600 2.08 597 -

ES S1 S2 T1 T2 S1 T1 T2 S1 T1 T2 S1 T1 T2

Emission E λ 2.89 428 2.98 416 1.81 686 * * 3.31 375 2.62 473 2.68 462 3.00 413 2.40 517 2.43 511 2.69 461 1.30 956 1.73 716

f 0.1491 0.2581 0.0616 0.1275 0.4771 -

*Above 528 nm-estimated on the basis of S0→T2 vertical excitations

with the five membered heterocycle due to the stronger donor ability of the triazolate ring, which is potentially σand π-donor. The pyridyl ring is also a σ-donor, but, simultaneously, potential a π-acceptor. Due to the electron deficient character of the pyridyl ring with respect to the triazolate cycle, the metal-ligand interaction results in relatively longer Pt-N1py bond length. The results indicate that there is a slight variation in the Pt-N2tr bond length as a function of the substituent. The length of the Pt-N2tr bond slightly decreases starting from 2.015 Å in 2a and -N(CH3)2 substituent to 2.009 Å in 2d and ̶ C=C(CN)2. On the other hand, the Pt-N1py bond length remains almost unaffected by the chemical substitution at the C4py position. The length of the Pt-N1py bond is 2.058 Å for 2a (-N(CH3)2) and a ca. 2.05 Å for the rest of the complexes. In addition, it is apparent that complexation increases the N1py-C1py and N1py-C5py bond lengths in respect to the isolated ligand (Table S1, SI). The results also demonstrate a slight variation in the C-C (pyridine) and N-N (triazole) conjugation path as a function of the substituent. The C-C and N-N bond length alternation (BLA) increases slightly in the order 1a = 1b < 1c < 1d and 2a ≤ 2b < 2c < 2d. It should be noted that for 2a-d monomer complexes PBE0D3 functional predicts identical equilibrium bond lengths as the PBE0 method (Table S2, SI). The calculated absorption properties of the 2a-d monomer complexes are listed in Table 2. The TDDFT/PBE0/PCM results reveal low-energy absorption bands at 376 nm (2a), 338 nm (2b), 370 nm (2c) and 416 nm (2d). It is also important to note that for the 2a complex the theory predicts degenerated S1 and S2 excited-states and S0→S2 as a forbidden electronic transition. According to the MO analysis all S0→S1 absorption bands for 2a-d monomers are associated with metal-to-ligand charge transfer transitions (MLCT) with HOMO localized on Pt and the triazolate ring and LUMO density concentrated in the pyridyl cycle (Figure 1). In the case of acceptor groups - 2c, 2d the substituent also participates in the LUMO, while in case of 2a the donor

substituent contributes to HOMO. The dark S2 in 2a is associated with HOMO-1 to LUMO transition. The results demonstrate also that each monomer is characterized with two nearly degenerated triplet excited-states, whose energies lie between 0.6 and 1 eV below the first singlet excited-state. Figure 1. Molecular orbitals in the leading electronic configurations associated with the lowest-energy S0 → S1, S2 transitions for 2a-d complexes. The MO surfaces are plotted at an isosurface value of 0.04 au.

The experimental measurement for the monomer complexes show low-energy absorption bands at ~377 nm for 2a-c and ~352 nm for 2d.29 With respect to the experimentally observed absorption maxima the PBE0 method has good performance with 0.24 eV mean absolute error (MAE). However, in both theory and experiment there is no direct indication about the possibility for the tuning of absorption properties as a function of the substituent π-acceptor strength. This is

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Table 3. NBO atomic and fragment charges in 2a-d complexes and 1a-d isolated ligands obtained with PBE0/PCM method. The atom labelling is according to Scheme 1. The fragments are denoted as R (substituent), Py (pyridine cycle), PyR (substituent and pyridine cycle), Tr (triazole cycle). The differences between the NBO charge of the fragments (Δ) in the complexes and in the isolated ligand are also presented. Atom

1a S0

2a S0

1b S0

2b S0

1c S0

2c S0

1d S0

2d S0

N1py

-0.470

-0.414

-0.488

-0.435

-0.485

-0.435

-0.478

-0.432

C1py

0.145

0.159

0.186

0.213

0.217

0.239

0.225

0.243

C2py

-0.258

-0.229

-0.279

-0.247

-0.275

-0.244

-0.267

-0.238

C3py

-0.270

-0.247

-0.214

-0.174

-0.164

-0.127

-0.165

-0.129

C4py

0.118

0.154

-0.316

-0.280

-0.252

-0.214

-0.190

-0.152

C5py

-0.042

0.003

0.008

0.063

0.066

0.110

0.090

0.128

N2tr

-0.370

-0.303

-0.355

-0.320

-0.337

-0.316

-0.327

-0.313

N3tr

-0.206

-0.133

-0.198

-0.120

-0.181

-0.111

-0.167

-0.106

N4tr

-0.399

-0.323

-0.396

-0.292

-0.390

-0.285

-0.384

-0.283

C6tr

-0.158

-0.106

-0.143

-0.085

-0.120

-0.069

-0.103

-0.062

C7tr

-0.019

0.051

-0.028

0.039

-0.034

0.033

-0.032

0.032

Pt

0.527

0.541

0.558

0.567

Fragment R

-0.027

0.051

0.257

0.278

-0.036

0.031

-0.206

-0.085

Py

-0.043

0.247

-0.362

-0.027

-0.132

0.177

-0.014

0.270

PyR

-0.070

0.298

-0.105

0.252

-0.168

0.208

-0.220

0.186

Tr

-0.930

-0.562

-0.895

-0.522

-0.832

-0.487

-0.780

-0.469

ΔR

0.079

0.022

0.068

0.122

ΔPy

0.290

0.335

0.309

0.284

ΔPyR

0.369

0.357

0.377

0.406

ΔTr

0.368

0.372

0.344

0.311

clear if one compares the excitation energies for the πdonor substituent –N(CH3)2 (2a) and relatively strong πacceptor group –CHO (2c): 376 nm and 370 nm in theory, as well as 377 nm and 377 nm in the experiment. The lack of tunability can be explained by the mild substituent effects on the molecular geometry and σdonor character behavior of all non-hydrogen substituents in the monomer complexes. Table 3 contains the NBO atomic charges computed for the ligands when isolated and within the complex. Comparison of the charge distribution in the ligands and in the corresponding complexes shows the complexation with Pt-atom results in more positive atomic charges of all atoms. The increased electrophilicity within the ligands is consistent with the fact that the metal-ligand bonds are formed due to donation of electrons from the anionic ligand to the platinum. The NBO data on the different fragments (R-substituent, Py-pyridine and Tr-triazole) in

1a-d ligands reveals that the negative charge of the anion is mainly localized in the triazole cycle. However, when the π-acceptor strength of the substituent increases the charge of the Tr fragment decreases: -0.930 (1a), -0.895 (1b), -0.832 (1c) and -0.780 (1d). The effect of the Ptligand bond formation can be extracted from the difference (Δ) in the charge of a given fragment in the complex and in the isolated ligand. The charge differences indicate that after complexation the Tr negative charge decreases with 0.368 (1a), 0.372 (1b), 0.344 (1c) and 0.311 (1d). This trend is directly related to the shortening of the Pt-N2tr bond going from –N(CH3)2 to –CHC(CN)2 group. As a result, the metal centre accepts less electronic density when the π-acceptor strength of the substituent increases: the charge of the Pt amounts to 0.527 (1a), 0.541 (1b), 0.558 (1c) and 0.567 (1d). Regarding the nitrogen atoms in the triazole ring, it can be seen that N3tr and N4tr donates even more electron density than the N2tr atom directly connected to platinum. On the other

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Table 4. Optimized TD-DFT/PBE0/PCM bond lengths [Å] in the singlet excited-states of 2a-2d complexes and their dimers. In case the ground-state symmetry is preserved in excited-state, the equivalent bond lengths from all four ligands are averaged and only the symmetrically unique bond lengths are listed. The atom labelling is according to Scheme 1. 2a

2a

2a-2a

2b

2b-2b

2c

2c-2c

2d

2d-2d

S1

S2

S1

S1

S1

S1

S1

S1

S1

Pt-N1py

2.044

2.031/2.025

2.054

2.042

2.052

2.051

2.056

2.063

2.064

N1py-C1py

1.378

1.376/1.401

1.367

1.386

1.371

1.385

1.377

1.377

1.369

C1py-C2py

1.388

1.390/1.401

1.391

1.395

1.393

1.399

1.397

1.397

1.398

C2py-C3py

1.383

1.382/1.374

1.383

1.383

1.386

1.376

1.380

1.377

1.377

C3py-C4py

1.422

1.420/1.428

1.416

1.408

1.399

1.415

1.407

1.420

1.416

C4py-C5py

1.401

1.401/1.408

1.407

1.378

1.382

1.388

1.390

1.407

1.403

C5py-N1py

1.357

1.355/1.347

1.346

1.355

1.349

1.338

1.336

1.327

1.331

Pt-N2tr

1.977

1.997/2.007

2.021

1.964

2.018

1.960

2.013

1.964

2.014

N2tr-N3tr

1.361

1.332/1.310

1.325

1.335

1.322

1.337

1.318

1.328

1.316

N3tr-N4tr

1.293

1.320/1.344

1.320

1.311

1.322

1.305

1.323

1.309

1.323

N4tr-C6tr

1.371

1.348/1.329

1.349

1.356

1.347

1.362

1.345

1.359

1.346

C6tr-C7tr

1.380

1.389/1.405

1.386

1.379

1.387

1.377

1.389

1.377

1.389

C7tr-N2tr

1.360

1.358/1.377

1.356

1.385

1.357

1.382

1.358

1.382

1.358

C7tr-C1py

1.435

1.435/1.406

1.437

1.425

1.435

1.426

1.430

1.427

1.432

Bond

Pt-Pt

2.797

hand, the charge of the PyR fragment increases with 0.369 (1a), 0.357 (1b), 0.377 (1c) and 0.406 (1d). This reveals that the donor ability of the PyR fragment in respect to Pt increases in the order 1b > 1a > 1c > 1d and is not in line with the π-acceptor strength of the substituent. On the other hand, the Py fragment donates most in the case of – H substituent (ΔPy 0.336/ΔR 0.022), followed by the case of –N(CH3)2 (ΔPy 0.290/ΔR 0.079), –CHO (ΔPy 0.309/ΔR 0.068) and –CHC(CN)2 group (ΔPy 0.284/ΔR 0.122), while opposite behaviour is observed for the substituent R. At first sight this result is a bit surprising because the –CHO and –CHC(CN)2 groups are good π-acceptors and donation from their side is not expected. However, regarding the inductive effects in the isolated ligands 1c and 1d, the –CHO and –CHC(CN)2 substituents attract electron density via the σ–skeleton, which potentially reduce the σ-donor ability of the pyridine cycle. Once the Pt center is introduced, it becomes the main attractor of electron density from the Py fragment, much stronger than the –CHO and –CHC(CN)2 groups. In return, in 2c and 2d the Py fragment tends to donate the same amount of electron density (ΔPy 0.284 and 0.309) but to attract back electron density from the substituent via the σ– skeleton (ΔR 0.068 and 0.122) and to deliver it to the metal. This resistant behaviour of the Py fragment in respect to the metal center is also obvious from the charge of the N1py atom: -0.470 (2a), -0.435 (2b), -0.435 (2c) and 0.432 (2d) and is related to the small difference in the geometry of the complexes in the pyridine cycles

2.805

2.886

2.845

(Table 1). The stronger the inductive effect of the substituent in the isolated ligand the higher the donation from the substituent in the complexes. In this sense, the case of the –N(CH3)2 is special since this group can donates both π– and σ–electron density. In other words, all non-hydrogen substituents act as donors in the complexes regardless of their π-acceptor strength in the isolated ligands. The optimized TD-DFT/PBE0/PCM geometries for the excited singlet states of 2a-d monomers can be found in Table 4. A comparison between S0 and S1 geometries reveals that more pronounced changes occur in the PtN2tr bond due to excitation. The ground-excited-state bond length differences (BLD) for the Pt-N2tr bond are 0.038 Å (2a), 0.050 Å (2b), 0.051 Å (2c) and 0.045 Å (2d). The prominent change in the Pt-N2tr bond length due to S0→S1 transition logically induces larger structural rearrangements, within the triazolate cycle. The N2tr-N3tr increases, while the N3tr-N4tr one decreases. In this respect, the changes are more pronounced for the 2a complex (BLD is -0.038 Å for N2tr-N3tr and 0.026 Å for N3tr-N4tr). Moreover, in 2b-d the excitation leads to redistribution in the N-N conjugation path: RN2tr-N3tr < RN3tr-N4tr in S0, while RN2tr-N3tr > RN3tr-N4tr in S1. The excitation affects also the C7tr-C1tr inter-ring bond that becomes shorter in the excited-state with BLDs of 0.008 Å (2a), 0.017 Å (2b), 0.011 Å (2c) and 0.006 Å (2d). On the other hand, the changes in the Pt-N1py distance due to S0→S1 transition are much smaller with BLDs of 0.014 Å (2a),

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0.013 Å (2b), 0.003 Å (2c) and -0.008 Å (2d). Although weakly pronounced, this trend can be related to the πacceptor strength of the substituent and the pyridine cycle, respectively. When the π-acceptor properties of the six-membered ring improve, the charge transfer from Pt to the pyridine cycle becomes more effective and the PtN1py bond in the excited-state elongates more noticeably. It also important to note that 2a is characterized with one additional low-energy singlet excited-states S2. The MOs associated with this second excited-state HOMO-1 and LUMO have electron density only on the ligands and hence S0→S2 represents an intra-ligand excitation (Figure 1). Although the triplet excited-states of the monomer were optimized, they will not be discussed in detail because it was shown experimentally by lifetime measurements that 2a-d complexes fluoresce at low concentration.29 The calculated emission properties of the 2a-d monomer complexes are presented in Table 2. For the monomer complexes, the S1→S0 emission energies are relatively redshifted, if only slightly, with respect to the absorption (shifts between 0.40 and 0.30 eV for all compounds). The predicted vertical emission energies are at 428 nm (2a), 375 nm (2b), 413 nm (2c) and 461 nm (2d), while the photoluminescent spectra in diluted solution reveal maxima at 397 nm (2a), 400 nm (2b), 405 nm (2c) and 408 nm (2d). In this respect, the MAE for the PBE0 functional is estimated to be 0.21 eV. Qualitatively there is an overall agreement between theory and experiment that at monomer level there is no strong tunability of emission wavelengths as a function of the substituent. For 2a there is a second excited-state, which lies just 0.009 eV above the first excited-state S1 and while in absorption the S0→S2 vertical transition is forbidden, the emission from S2 state to S0 is allowed. As can be expected, the theoretically predicted T1→S0 energies for the monomers are also red-shifted in respect to absorption but to a larger extent (shifts between 0.60 and 1.39 eV for all compounds). According to the TD-DFT results phosphorescence at the monomer can be expected at 686 nm (2a), 473 nm (2b), 517 nm (2c) and 956 nm (2d). Also, as it was discussed earlier in the absorption properties section, that for all monomers in addition to T1→S0 transition, phosphorescent emission with similar energy is also possible from the second excited triplet state T2. However, photoluminescent spectra at low concentrations do not show emission bands above 400 nm. Therefore, based on the TD-DFT results for S1→S0 and T1→S0 transitions (Table 2) it can be concluded that the observed emission for 2a-d monomer complexes are due to fluorescence. Moreover, the lifetime measurements at low concentrations also confirm the fluorescent origin of the emission maxima. At the monomer level, all S1 and T1 states have identical character and represent MLCT excitations with donating orbitals localized on the Pt atom and the triazole cycle with the

accepting orbitals having electron density mainly on the pyridyl ring (Figure 1). Properties of the dimers. The PBE0 ground-state geometries of the dimers of complexes 2a-d are listed in Table 1. The results show that in the ground-state the formation of dimers has negligible effect on the monomer geometry. The maximal absolute difference in the monomer-dimer bond lengths is 0.004 Å. The equilibrium Pt-Pt distances are equal to or larger than the sum of van der Waals radii of Pt-atoms (1.75 Å): 3.754 Å (2a-2a), 3.591 Å (2b-2b), 3.515 Å (2c-2c), and 3.573 Å (2d-2d). This suggests weak Pt-Pt bond and metallophilic interaction in the ground-state. As a results, the computed free energies of intermolecular interaction GInt, where GInt=Gelec.(dimer)-2Gelec. (monomer)) are slightly positive and vary between 5.97 and 11.37 kcal/mol (Table S3, SI). The observed trend in the intermolecular distances can not be directly related to the steric substituent effects because if they dominate one should observe shortest PtPt bonds in the case of H-substituent. However, our PBE0 results suggest that in the ground-state the Pt-Pt distance is sensitive to the electronic character of substituent and in general decreases when the acceptor strength of the substituent increases. This trend is even more pronounced with the PBE0-D3 method (Table S2, SI), which predicts the following Pt-Pt distances: 3.358 Å (2a2a), 3.311 Å (2b-2b), 3.191 Å (2c-2c), and 3.058 Å (2d-2d). Therefore, it can be concluded that the ligand effect on the ground-state intermolecular interaction is more accurately described when the dispersion correction was taken into account. In addition, as expected the inclusion of dispersion correction results in shorter Pt-Pt bond lengths, significantly more negative GInt energies of intermolecular interaction (between -19.70 and -13.03 kcal/mol) and hence stronger intermolecular interaction in respect to the PBE0 data (Table S4, SI). However, both PBE0 and PBE0-D3 methods estimate the ground-state metallophilic interaction as relatively weak. The absorption properties of the dimers of 2a-d complexes computed with TD-DFT/PBE0/PCM method are listed in Table 5. The results indicate that the formation of dimers in the ground-state causes only a small bathochromic shift of the low-energy absorption maxima: 0.10 eV (2a-2a), 0.24 (2b-2b, 2c-2c) and 0.26 eV (2d-2d). The latter is not surprising because the geometry of the complexes is almost unaffected by the ground-state aggregation and because the interaction between the monomers is relatively weak. These observations are in agreement with experimental results, where it was found that the 2a-d complexes are characterized by similar absorption profiles irrespective of concentration (monomer-dimer equilibrium).29 Based only on the experimental outcome, it is difficult to conclude whether this is due to a lack of dimer formation in the groundstate or to a lack of sensitivity in observing the shifts in the absorption spectra comparing monomer to dimer.

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Table 5. Vertical absorption and emission energies E [eV], wavelengths λ [nm] and oscillator strength f computed dimers of complexes 2a-d with TD-DFT/PBE0/PCM and linear response solvation in ethanol.

2a-2a

2b-2b

2c-2c

2d-2d

ES S1 S2 S4 a S9 T1 T2 T3 T4 S1 S2 T1 T2 T3 T4 S1 S4 T1 T2 T3 T4 S1 S3 T1 T2 T3 T4

Absorption E λ f 3.20 388 0.0012 3.22 386 0.0470 3.23 384 0.0806 3.70 335 0.0565 2.37 522 2.38 522 2.38 521 2.38 521 3.42 363 0.0566 3.58 347 0.0406 2.97 417 2.99 413 3.02 411 3.06 405 398 0.0402 3.11 3.36 369 0.1117 2.67 465 2.68 462 2.69 460 2.71 457 2.73 453 0.0180 2.93 423 0.4998 2.05 603 2.06 603 2.06 602 2.07 598 -

Emission λ

ES

E

S1 T1

2.37 2.12*

524 584*

0.1391 -

S1 T1

2.24 210/2.10*

554 591/591*

0.1290 -

S1 T1

2.11 2.02/2.02*

587 613/614*

0.0825 -

S1 T1

1.66 1.56*

745 796*

0.0309 -

f

*Estimated on DFT optimized T1geometry, i.e. by the alternative approach

Figure 2. Molecular orbitals involved in the leading electronic configurations associated with the lowest-energy S0 → S1, S9 transitions in the dimers of 2a-d monomer complexes. The MO surfaces are plotted at an isosurface value of 0.04 au.

The theoretical results also show that due to the aggregation process each low-energy absorption band (S0→S1) in the monomer splits in two low-energy transitions in the dimers (S0→S1 and S0→S2/3/4). Similar behavior is observed also for the monomer T1, T2 states

and therefore four closely lying triplet states can be found in the dimers. However, with respect to the monomers, the dimer triplet states are less bathochromically shifted ~0.02/0.03 eV than the singlet ones.

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The MO analysis (Figure 2) reveals that S1 states of dimers of 2b-d, as well as the S9 state of 2a-2a are associated with metal-to-metal-to-ligand charge transfer (MMLCT). For these excited-states the donating orbitals

The effect of the ligand on the metallophilic interaction in the ground- and excited-state can be qualitatively explained in terms of ligand field and substituent electronic properties. The ligand field determines the

represent a σ* combination of dz2 orbitals of Pt-atoms (HOMO in 2b-d and HOMO-4 in 2a), while the accepting once - LUMO are localized in the pyridine ring and on the substituents (2c-2c, 2d-2d). The second dipole allowed transition for 2b-2b (S2), 2c-2c (S4) and 2d-2d (S3) are also associated with MMLCT excitation. These excitations involve again LUMO as the accepting orbital but the donating one originates from dxz-dxz of the Pt-atoms and triazole rings. On the other hand for 2a-2a, there are several low-energy excited-states (S1, S2, S4, S7 and S8) that are intra-ligand electronic transitions.29

energy of the monomer dz2 orbitals: the stronger the πacceptor strength of the ligand (substituent), the lower

The optimized geometries for the first excited-state of the dimers obtained with TD-DFT/PBE0/PCM method are presented in Table 4. The excited-states of all dimers are associated with MMLCT transition, where the HOMO originated from σ* overlapping dz2 atomic orbitals of the metal atoms and LUMO is mainly localized on the pyridine cycle, as well as on the substituents in the case that they are acceptor groups. The results for the dimers clearly indicate that, going from S0 to S1, the bond lengths within the monomers do not change substantially and BLDs for the metal-metal bonds are less than 0.010 Å. However, after excitation one can observe strongly pronounced decreases in the Pt-Pt distance and BDLs are 0.957 Å (2a-2a), 0.786 Å (2b-2b), 0.629 Å (2c-2c) and 0.728 Å (2d-2d). The shortening of the metal-metal bonds in the excited-states of the dimers is so strong that the interaction is best described as a covalent Pt-Pt bond. Therefore, undoubtedly the S0→S1 electronic excitation is associated with excimer formation and is qualitatively different than in the case of 2a-d monomer complexes. In fact, the term ‘excimer’ must now be considered as loosely applied since the dimeric species can be considered as a transient species resulting from a photochemical reaction. It is also important to note that in general, the shortening in the Pt-Pt bond going from S0 to S1 transition depends on the π-acceptor strength of the substituents. The better the acceptor properties of the C4py attached group, the stronger the decrease in the metal-metal bond. However, it should be noted that seemingly the trend at PBE0 level is interrupted for the 2d-2d compound. This may be related only to the lack of dispersion correction in the S0 of 2d-2d and not in S1 because the PBE0 and PBE0-D3 S1 geometries for 2d-2d dimers are very similar and both methods predict the same Pt-Pt bond length ~2.84Å (Table S2, SI). The TD-DFT results with the PBE0 functional also indicate that the T1 and S1 states of the dimers are almost identical in geometry. In addition, resonance Raman spectroscopy is suggested as an elegant experimental technique to detect the excimer formation (Discussion D1, SI).

the energy of the monomer dz2 orbital (Table 6). In turn, the lower in energy monomer dz2 orbitals results in higher ΔEσ-σ* splitting and shorter Pt-Pt bond length. Therefore, although slightly, the metallophilic interaction in the ground-state increases in line with the π-acceptor strength of the ligand (substituent). The excitation of an electron from (dz2–dz2)* to π* orbital stabilizes the metallophilic interaction because reduce the number of electrons in an antibonding (dz2–dz2)* and the ΔEσ-σ* splitting in excited-state is again affected by the ligand field. The emission properties of the 2a-d dimers calculated with TD-DFT/PBE0/PCM method are presented in Table 5. For the dimers, results indicate well red-shifted S1→S0 emission energies in respect to the corresponding absorption bands (shifts between 0.83 to 1.22 eV). Taking into account that 1) the monomers and dimers possess very similar absorption bands and 2) the Stokes shifts at monomer level are relatively small, it can be concluded that the strongly red-shifted S1→S0 emission in dimers is a clear signature for intermolecular excited-state interaction (excimer formation). Moreover, due to the excimerization process, the effect of the substituent becomes highly pronounced and emission tunability is observed. The dimers emission maxima are predicted at 524 nm (2a-2a), 554 nm (2b-2b), 587 nm (2c-2c) and 745 nm (2d-2d) and are in line with the increase in the acceptor strength of the substituents. This trend is in agreement also with the experimental observations showing that emission maxima are found at 487 nm (2a2a), 541 nm (2b-2b), 602 nm (2c-2c) and 625 nm (2d-2d). The MAE for the PBE0/TD-DFT method with respect to the empirical measurements at high concentrations is 0.15 eV, which reveals good performance of the functional. On the other hand, the theory predicts the T1→S0 emissions at 584 nm (2a-2a), 591 nm (2b-2b), 613 nm (2c-2c) and 796 nm (2d-2d) and hence lower tunability in the case of phosphorescence. Due to the high tunability and good agreement between the calculated S1→S0 energies and observed photoluminescent maxima, it can be suggested that at high concentrations and in solid state the 2a-d complexes fluoresce. However, the phosphorescent emission cannot be excluded because for a given dimer the energy difference between S1 and T1 states are just 0.25 eV (2a-2a), 0.14 eV (2b-2b), 0.09 eV (2c-2c) and 0.1 eV (2d-2d). The calculated small S1-T1 energy differences are a consequence of the almost identical molecular geometries of S1 and T1 excimers (Table 4 and Table S5, SI) and represent a measure mainly of the spin-spin interactions in the excited-state. Also, the experimental measurements

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Table 6. Energy [eV] of dz2 orbitals in the monomers and of the dz2-dz2 bonding σ and antibonding σ* orbitals in the ground-state dimers as a function of the substituent. The corresponding energy splittings [eV] are also listed. The results are obtained at DFT/PCM level. 2a

2b

2c

2d

PBE0

PBE0-D3

PBE0

PBE0-D3

PBE0

PBE0-D3

PBE0

PBE0-D3

d z2

-7.05

-7.05

-7.16

-7.17

-7.31

-7.31

-7.40

-7.39

σ*

-6.51

-6.17

-6.57

-6.31

-6.66

-6.34

-6.86

-6.32

σ

-7.41

-7.71

-7.67

-7.86

-7.94

-8.22

-7.99

-8.42

ΔE (σ - σ*)

0.91

1.54

1.11

1.55

1.28

1.88

1.13

2.10

ΔE (σ*- dz2)

0.54

-0.88

0.60

-0.86

0.65

-0.98

0.53

-1.07

ΔE (σ - dz2)

0.37

-0.66

0.51

-0.70

0.63

-0.91

0.60

-1.03

on emission lifetimes in concentrated solution for 2b-2b show two-exponential decays with a 42 ns component and 153 ns component. 29 Recently, it has been demonstrated that molecules with small S1-T1 differences represent promising candidates for OLED materials. The small S1-T1 differences (smaller than 0.2 eV) suggest complex photodynamics at room temperatures due to the possibility for simultaneous population of S1 and T1 states and can have positive effect on the emission quantum yields. 59-60 The design of such molecules usually targets metal containing or metal free donor/acceptor dyes and exciplexes (intermolecular charge transfer complexes in excited-state). Here, we demonstrate that due to the excimer formation in Pt(II) complexes the S1-T1 difference is also small and it can be tuned as a function of the ligand: the better accepting properties of the substituent the smaller S1-T1 gap. The possibility to tune the S1-T1 difference and the emission wavelength by chemical modification in Pt(II) complexes could represent a promising approach for the discovery of new highly efficient luminescent61 and singlet fission materials62. Figure 3. Qualitative perturbation molecular orbital diagrams for the interaction of the anionic ligands and a d-shell platinum atom and between the monomers (arrow).

Qualitative Model. The theoretical context of these effects is most simply found within a basic

perturbation molecular orbital (PMO) approach, which begins with the assumption that blue shifting in the emission spectra is likely due to a selective effect on the HOMO level of the ligands through strongly stabilizing interaction with the platinum (II) d-shell. In particular, since we are dealing with anionic ligands the HOMO levels of the isolated ligands will be comparably high and close to the potential of the d-shell (Figure 3). In Figure 3 energy levels of the π-system of the ligands 1a and 1d, their interaction in the complex where the metal has been removed ((21a)2- or (21d)2-, the orbital configuration of the complexes themselves and the isolated platinum atom are shown and are based on, and interpreted through, the discussion above. We neglect the strongly stabilizing effect of the electrostatic interaction between the anionic ligands and the metal atom, which will lower the whole orbital manifold in the final complex. This effect obscures the nature of the relevant orbitals, since the HOMO levels in the complex are actually antibonding (Figure 1) and arise from the interaction of the HOMO-1 level of (2L)2-. (L = 1a to 1d). For (2L)2- the combination of molecular ligand orbitals from L- leads to an energy level splitting based on symmetric and antisymetric combinations of the ligand orbitals. The HOMO-1 level has the correct planar symmetry and parity to affectively mix with the dxz atomic orbital (where x bisects the complex along the ligand molecular axis and through the platinum atom). On the other hand, the HOMO level of (2L)2- has the wrong parity and symmetry and can not effectively mix with metal-centered d-shell orbitals, this interaction can be considered having a null effect. Similarly, the LUMO level is involved in a null interaction with platinum. It is noted that this is the primary reason for the lack of metal density in the LUMO level and the alternating pattern of bonding/anti-bonding and non-bonded arrangements. In Figure 3 two cases are presented. On the left is the case of complex 2a where the HOMO-1 level of (2L)2-, having the highest energy of the HOMO-1 levels of 2a-2d, is set arbitrarily close to the dshell on platinum. In the second case of 2d (right side), the increasing accepting power of the ligand has lowered

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the HOMO-1 level to be far away in energy from the dshell. In the first case we assume a large interaction between the d-shell and the HOMO-1, in the second, we have assigned an arbitrarily small extent of interaction. In line with PMO theory, the energy gap between HOMO-1 and the d-shell will be inversely proportional to the splitting of the levels in the corresponding complex. This is supported by the decreasing bond lengths from 2a to 2d between the triazolate moiety and platinum atom; the anti-bonding orbital should extend the bond proportionally to its destabilizing contribution. This is also in agreement with the decreasing charge transferred to platinum comparing 2a to 2d as the anti-bond will place more charge on the ligand side. It is shown pictorially that the extent to which the optical gap of the ligands lowers as a function of the substituent this is compensated for by the equally contributing effect of decreased energy level splitting. The final result is an attenuation of the substitution effect, as observed experimentally where ΔE2a and ΔE2d are equal, while ΔE1a is greater than ΔE1d. Thus, it is the apparent close correspondence in the d-shell platinum level with the least accepting 1a orbital that causes the experimentally observed trend. A change in the nature of the HOMO level in the excimer state to a Pt-Pt bond will strongly affect the fundamentals of the photophysics of these systems changing both the symmetry and energy of the metal centered orbitals. The predominantly dz2 orbital is also shown in Figure 3. Its energy in the complex is set by interactions with the sigma bonding orbitals of (2L)2-. Upon excimerization this orbital experiences a large splitting from interaction with the same orbital in the additional monomer with increasing affect (Table 6) from 2a to 2d (Figure 3 (shown as an arrow for simplicity)) where the final position of the (dz2-dz2)* restores the tunability of these systems by adding a new contributing factor without a compensating one. The rest of the energy levels remain mostly unperturbed.

Conclusions In summary, the absorption and emission properties of four recently designed bis-5-subsituted-pyridyl-1,2,3triazolato Pt(II) complexes and their dimers have been simulated with the PBE0 functional in an implicit solvent environment. For this purpose, the ground-, first excitedsinglet and triplet states of the monomers and dimers were fully optimized at the DFT and TD-DFT levels. Within the series of complexes, the intermolecular interaction in the ground-state is estimated as relatively week at both PBE0 and PBE0-D3 level and has been demonstrated to originate from metallophilic association. On the other hand, the electronic excitation strongly enhances the metal-metal attraction and the emission is observed from excimeric states, where the Pt-Pt distance is within the covalent region. We have pointed out that

the excimeric species may be understood as a transient ‘hot’ photochemical product. Our investigation gives an insight into the role of the ligand (substituent) electronic structure in the metallophilic interaction in ground- and excited-state. Despite the broad range of the donor/acceptor strength of the substituents (ligands), the series of monomer complexes is characterized with reduced absorption and emission tunability. The lowest in energy singlet and triplet states of the monomers have MLCT character. The metallophilic interaction in ground-state leads only to a small bathochromic shift of the absorption spectrum with respect to the monomers and does not induce tunability. Although weak, the metallophilic interaction in groundstate depends on the ligand field (substituent) and slightly improves when the acceptor strength of the substituent increases. At the dimer level, the emission maxima are associated with the formation of excimer and are red-shifted with respect to the monomers. The singlet and triplet excimers have MMLCT character. Due to the excimers formation the emission tunability in the series of complexes is turned-on and is directly related to the donor/acceptor properties of the substituent. In all the complexes, the geometries of the S1 and T1 states are similar and differ from the S0 structures mainly in the shorter Pt-Pt bond lengths. Moreover, the simulations reveal that due to the excimerization process the S1-T1 energy difference decreases and varies as a function of the donor/acceptor strength of the substituent (from 0.25 to 0.1 eV). This result suggests that the substituents modification could affect the intersystem crossing rate constants and quantum yield at the supramolecular level and hence represents a promising approach for the molecular design of luminescence and singlet fission materials. Finally, we have shown that there is a lack of correspondence between molecular, concentrated solution and solid-state spectroscopic properties in system able to undergoes metallophilic bonding due to a fundamental change in orbital configuration. This is an important conclusion that will open up molecular design space in this field. The difference between the molecular and ‘supramolecular’ properties is particularly highlighted in the present series of molecules since the orbital energy matching between the metal and ligand is close. In terms of molecular design, this indicates that tunability and control of emission energies will be better obtained when the ligand energy levels are farther below those of the metal. This can be achieved by increased aromaticity in the ligand, for example, or by a change in the metal center.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Structural information about the ligands, monomer and dimer complexes in ground- and excited-state, their

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absorption and emission properties, molecular orbital analysis, ∆Eσ-σ* splittings, and free energies of the dimerization process; discussions on the charge distribution, the structural identification of the excimers and the ligand field effect on the metallophilic interaction (PDF).

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes The authors declare no competing financial interest

ACKNOWLEDGMENTS The authors would like to thank Dr. David Carey for the helpful discussion and the Leverhulme Trust (RPG-2014-006) for funding as well as the National Service for Computational Chemistry Software (NSCCS) at Imperial College London MRRP would like to thank the Southeast Physics Network (SEPnet) for student funding.

REFERENCES 1.

2.

3.

4.

5.

6.

7. 8.

9.

10. 11. 12.

13.

Schattling, P.; Jochum, F. D.; Theato, P. Multi–Stimuli Responsive Polymers – the all–in–one Talents. Polym. Chem. 2014, 5, 25–36. Zhuang, J.; Gordon, M. R.; Ventura, J.; Li, L.; Thayumanavan, S. Multi–Stimuli Responsive Macromolecules and Their Assemblies. Chem. Soc. Rev. 2013, 42, 7421–7435. McConnell, A. J.; Wood, C. S.; Neelakandan, P. P.; Nitschke, J. R. Stimuli–Responsive Metal–Ligand Assemblies. Chem. Rev. 2015, doi:10.1021/cr500632f. Jeong B.; Gutowska A. Lessons from nature: stimuliresponsive polymers and their biomedical applications. Trends Biotechnol. 2002, 20, 305–311. Cohen Stuart, M. A.; Huck, W. T. S.; Genzer, J.; Müller, M.; Ober, C.; Stamm, M.; Sukhorukov, G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M.; Winnik, F.; Zauscher, S.; Luzinov, I.; Minko, S. Emerging Applications of Stimuli–Responsive Polymer Materials. Nat. Mater. 2010, 9, 101–113. Thompson, M. P.; Chien, M.–P.; Ku, T.–H.; Rush, A. M.; Gianneschi, N. C. Smart Lipids for Programmable Nanomaterials. Nano Lett. 2010, 10, 2690–2693. Kojima, C. Smart Materials for Drug Delivery: Volume 1, Royal Society of Chemistry 2013. Dai., L. Intelligent Macromolecules for Smart Devices. From Materials Synthesis to Device Applications, Springer, New York, 2004. Liu, L.–H.; Métivier, R.; Wang, S.; Wang, H. Advanced Nanohybrid Materials: Surface Modification and Applications. J. Nanomat. 2012, vol. 2012, Article ID 536405. Wong, W. S.; Salleo, A. Flexible Electronics: Materials and Application, Springer, New York, 2009. Whittel, G. R.; Manners, I. Metallopolymers: New Multifunctional Materials. Adv. Mater. 2007, 19, 3439–3468. Cameron, C. G.; MacLean, B. J.; Pickup, P. G. Electron transport in conjugated metallopolymers. Macromol. Symp. 2003, 196, 165–171. Ho, C. –L.; Wong, W. –Y. Metal–Containing Polymers: Facile Tuning of Photophysical Traits and Emerging Applications in Organic Electronics and Photonics. Coord. Chem. Rev. 2011, 255, 2469–2502.

Page 12 of 14

14. Plenio, H.; Hermann, J.; Sehring, A. Optically and Redox– Active Ferroceneacetylene Polymers and Oligomers. Chem. Eur. J. 2000, 6, 1820–1829. 15. Pickup, P.G. Conjugated metallopolymers. Redox polymers with interacting metal based redox sites. J. Mat. Chem. 1999, 9, 1641–1653. 16. Campbell, M. G.; Powers, D. C.; Raynaud, J.; Graham, M. J.; Xie, P.; Lee, E.; Ritter, T. Synthesis and structure of solution– stable one–dimensional palladium wires. Nat. Chem. 2011, 3, 949–953. 17. Pyykkö, P. Strong Closed–Shell Interactions in Inorganic Chemistry. . Rev. 1997, 97, 597−636. 18. Schmidbaur, H.; Schier, A. A Briefing on Aurophilicity. Chem. Soc. Rev. 2008, 37, 1931−1951. 19. Schmidbaur, H.; Schier, A. Aurophilic Interactions as a Subject of Current Research: an Up–Date. Chem. Soc. Rev. 2012, 41, 370−412. 20. Kaupp, M.; von Schnering, H. G. Dominance of Linear 2– Coordination in Mercury Chemistry: Quasirelativistic and Nonrelativistic ab Initio Pseudopotential Study of (HgX2)2 (X = F, Cl, Br, I, H). Inorg. Chem. 1994, 33, 2555. 21. Schmidbaur, H.; Schier, A. Mercurophilic Interactions. Organometallics 2015, doi:10.1021/om501125c. 22. Jobbágy, C.; Deák, A. Stimuli–Responsive Dynamic Gold Complexes. Eur. J. Inorg. Chem. 2014, 2014, 4434–4449. 23. Krogmann, K. Planar Complexes Containing Metal–Metal Bonds. Angew. Chem. Int. Ed. 1969, 8, 35–42. 24. Schmidbaur, H.; Schier, A. Argentophilic Interactions, Angew. Chem. Int. Ed. 2015, 54, 746 – 784. 25. Han, M.; Tian, Y.; Yuan, Z.; Zhu, L.; Ma, B. Phosphorescent Molecular "Butterfly" that undergoes a Photoinduced Structural Change allowing Temperature Sensing and White Emission. Angew. Chem. Int. Ed. 2014, 53, 10908–10912. 26. Tsai, J. L.–L.; Zou, T.; Liu, J.; Chen, T.; Chan, A. O.–Y.; Yang, C.; Lok, C.–N.; Che, C.–M. (2015) Luminescent Platinum(II) Complexes with Self–Assembly and Anti–Cancer Properties: Hydrogel, pH Dependent Emission Color and Sustained– Release Properties Under Physiological Conditions. Chem. Sci. 2015, 6, 3823–3830. 27. Kim, D.; Brédas, J–. L–. Triplet Excimer Formation in Platinum–Based Phosphors: a Theoretical Study of the Roles of Pt–Pt Bimetallic Interactions and Interligand π–π Interactions. J. Am. Chem. Soc. 2009, 131, 11371–11380. 28. Wan, K. T.; Che, C. M.; Cho, K. C. Inorganic excimer. Spectroscopy, Photoredox Properties and Excimeric Emission of Dicyano(4,4′–di–tert–butyl–2,2′– bipyridine)Platinum(II). J. Chem. Soc. Dalton Trans. 1991, 1077 – 1080; 29. Ranga Prabhath, M. R.; Romanova, J.; Curry, R. J.; Silva, S. Ravi P.; Jarowski, P. D. The Role of Substituent Effects in Tuning Metallophilic Interactions and Emission Energy of Bis–4–(2–pyridyl)–1,2,3–Triazolatoplatinum(II) Complexes. Angew. Chemie. Int. Ed. 2015, 54, 7949–7953. 30. Ranga Prabhath, M. R.; Curry, R. J.; Silva, S. Ravi P.; Jarowski, P. D. Role of metallophilic interactions in “turning on” substituent effects and emission tunability of novel bis–4–(2– pyridyl)–1,2,3–triazolatoplatinum(II) complexes. In: Abstracts of Papers of the American Chemical Society 2014, 248. 248th National Meeting of the American-ChemicalSociety (ACS), San Francisco, CA. 31. Ma, B. W.; Djurovich, P. I.; Thompson, M. E. Excimer and Electron Transfer Quenching Studies of a Cyclometalated Platinum Complex. Coord. Chem. Rev. 2005, 249, 1501 – 1510.

12


Page 13 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


32. Abe, T.; Itakura, T.; Ikeda, N.; Shinozaki, K. Luminescence Color Change of a Platinum(II) Complex Solid Upon Mechanical Grinding. Dalton Trans. 2009, 711 – 715. 33. Wang, K. W.; Chen, J. L.; Cheng, Y. M.; Chung, M.W.; Hsieh, C. C.; Lee, G. H.; Chou, P. T.; Chen, K.; Chi, Y. Mono– versus Dinuclear Pt(II) 6–(5–Trifluoromethyl–Pyrazol–3–yl)–2,2′– Bipyridine Complexes: Synthesis, Characterization, and Remarkable Difference in Luminescent Properties, Inorg. Chem. 2010, 49, 1372 – 1383. 34. Wang, Y. F.; Liu, Y.; Li, X. S.; Qi, H. R.; Zhu, M. X.; Wang, L.; Lei, G. T.; Wei, Q. L.; Zhu, W. G.; Peng, J. B.; Cao, Y. Novel Cyclometalated Platinum (II) Complex Containing Alkyl– Trifluorene Picolinic Acid as Emitter for Single–Layer White PLEDs. Org. Electron. 2010, 11, 1954 – 1959. 35. Fleetham, T.; Ecton, J.; Wang, Z. X.; Bakken, N.; Li, J. Single– Doped White Organic Light–Emitting Device with an External Quantum Efficiency Over 20%. Adv. Mater. 2013, 25, 2573 – 2576. 36. Po, C.; Ke, Z. H.; Tam, A. Y. Y.; Chow, H. F.; Yam, V.W. W. A Platinum(II) Terpyridine Metallogel with an L–Valine– Modified Alkynyl Ligand: Interplay of Pt⋅⋅⋅Pt, π–π and Hydrogen–Bonding Interactions. Chem. Eur. J. 2013, 19, 15735 – 15744. 37. Kim, C. D.; Pillet, S.; Wu, G.; Fullagar, W. K.; Coppens, P. Excited-state structure by timeresolved X-ray diffraction. Acta Crystallogr. A. 2002, 58,133–137. 38. Underhill, A. E.; Watkins, D. M. One-dimensional Metallic Complexes. Chem. Soc. Rev. 1980, 9, 429–448. 39. Reis, A. H.; Peterson, S. W. Correlation between the formal partial oxidation states observed in the one-dimensional platinum metal complexes and the resonating valence bond theory of metals. Inorg. Chem. 1976, 15, 3186–3187. 40. Hansch, C.; Leo, A.; Taft, R. W. A Survey of Hammett Substituent Constants and Resonance and Field Parameters Chem. Rev. 1991, 91, 165-195. 41. Ilie A.; Raţ C. I.; Scheutzow S.; Kiske C.; Lux K.; Klapötke T.M.; Silvestru C.; Karaghiosoff, K. Metallophilic bonding and agostic interactions in gold(I) and silver(I) complexes bearing a thiotetrazole unit. Inorg Chem. 2011, 50, 2675– 2684. 42. Záliš, S.; Lam, Y-. C.; Gray, H. B.; Vlček, A. Spin–Orbit TDDFT Electronic Structure of Diplatinum(II,II) Complexes, Inorg. Chem. 2015, 54, 3491-3500. 43. Seki, T.; Sakurada, K.; Muromoto, M.; Ito, H. Photoinduced single–crystal–to–single–crystal phase transition and photosalient effect of a gold(I) isocyanide complex with shortening of intermolecular aurophilic bonds. Chem. Sci. 2015, 6, 1491−1497. 44. Aullon, G.; Alemany, P.; Alvarez, S. On the existence of a pyramidality effect in d8···d8 contacts. Theoretical study and structural correlation. Inorg. Chem. 1996, 35, 5061−5067. 45. Díez, Á.; Forniés, J.; Larraz, C.; Lalinde, E.; López, J. A.; Martín, A.; Moreno, M. T.; Sicilia, V. Structural and Luminescence Studies on π···π and Pt···Pt Interactions in Mixed Chloro–Isocyanide Cyclometalated Platinum(II) Complexes. Inorg. Chem. 2010, 49, 3239–3251. 46. Amar, A.; Meghezzi, H.; Boixel, J.; Le Bozec, H.; Guerchais, V.; Jacquemin, D.; Boucekkine, A. Aggregation Effect on the Luminescence Properties of Phenylbipyridine Pt(II) Acetylide Complexes: a Theoretical Prediction with Experimental Evidence. J. Phys. Chem. A 2014, 118, 6278– 6286.

47. Lockard, J. V.; Rachford, A. A.; Smolentsev, G.; Stickrath, A. B.; Wang, X.; Zhang, X.; Atenkoffer, K.; Jennings, G.; Soldatov, A.; Rheingold, A. L.; Castellano, F. N.; Chen, L. X. Triplet Excited State Distortions in a Pyrazolate Bridged Platinum Dimer Measured by X–ray Transient Absorption Spectroscopy. J. Phys. Chem. A 2010, 114, 12780–12787. 48. Zhou, X.; Zhang, H. –X.; Pan, Q. –J.; Li, M. –X.; Wang, Y.; Che, C. –M. Electronic Structures and Spectroscopic Properties of [Pt(CNMe) 2(CN)2]n (n = 1–4): a Theoretical Exploration of Promising Phosphorescent Materials. Eur. J. Inorg. Chem. 2007, 2181–2188. 49. Stoyanov, S. R.; Villegas, J. M.; Rillema, D. P. Time– Dependent Density Functional Theory Study of the Spectroscopic Properties Related to Aggregation in the Platinum(II) Biphenyl Dicarbonyl Complex. Inorg. Chem. 2003, 42, 7852–7860. 50. Georgieva, I.; Trendafilova, N.; Dodoff, N.I. Effect of the Metal· · ·Metal Interactions on the Absorption Properties of Pt(II) and Pd(II) Complexes of Glyoxilic Acid Oxime in Solution and Solid state: Theoretical and Experimental study. J. Photochem. Photobiol. A: Chem. 2013, 267, 35–48. 51. Mayoral, M.J.; Rest, C.; Stepanenko, V.; Schellheimer, J.; Albuquerque, R.Q.; Fernández, G. Cooperative Supramolecular Polymerization driven by Metallophilic PdPd Interactions, J. Am. Chem. Soc. 2013, 135, 2148-2151. 52. Adamo, C.; Barone, V. Toward Reliable Density Functional Methods without Adjustable Parameters: the PBE0 Model. J. Chem. Phys. 1999, 110, 6158-6169. 53. Miertus, S.; Scrocco, E.; Tomasi, J. Electrostatic Interaction of a Solute with a Continuum. A Direct Utilization of ab Initio Molecular Potentials for the Prevision of Solvent Effects. Chem. Phys. 1981, 55, 117-129. 54. Cammi, R.; Mennucci, B. Linear Response Theory for the Polarizable Continuum Model. J. Chem. Phys. 1999, 110, 98779886. 55. Tomasi, J.; Mennucci, B.; Cammi, R. Quantum Mechanical Continuum Solvation Models. Chem. Rev. 2005, 105, 29993093. 56. Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parameterization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu,” J. Chem. Phys. 2010, 132, 154104. 57. Gaussian 09, Revision A.1, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al., Gaussian, Inc., Wallingford CT, 2009. 58. GaussView, Version 5, Dennington, Roy; Keith, Todd; Millam, John. Semichem Inc., Shawnee Mission, KS, 2009. 59. Tao, Y.; Yuan, K.; Chen, T.; Xu, P.; Li, H.; Chen, R.; Zheng, C.; Zhang, L.; Huang, W. Thermally Activated Delayed Fluorescence Materials Towards the Breakthrough of Organoelectronics. Adv. Mater. 2014, 26, 7931–7958. 60. Valchanov, G.; Ivanova, A.; Tadjer, A.; Chercka, D.; Baumgarten, M. Tuning the Optical Absorption of Potential Blue Emitters. Org. Electronics 2013, 14, 2727–2736. 61. Wang, X.; Gong, S.-L.; Song, D.; Lu, Z.-H; Wang, S. Highly Efficient and Robust Blue Phosphorescent Pt(II) Compounds with a Phenyl-1,2,3-triazolyl and a Pyridyl-1,2,4-triazolyl Chelate Core. Adv. Funct. Mater. 2014, 24, 7257-7271. 62. Smith, M. B.; Michl, J. Recent Advances in Singlet Fission, Ann. Rev. Phys. Chem. 2013, 64, 361-386.

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Table of Contents Relationship between Metallophilic Interaction and Luminescent Properties in Pt(II) Complexes: TDDFT Guide for the Molecular Design of Light-Responsive Materials. Julia Romanova, M. R. Ranga Prabhath and Peter D. Jarowski


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Relationship between Metallophilic Interactions and Luminescent

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