Anna
Pintus
a,
M. Carla
Aragoni
a,
Simon J.
Coles
b,
Susanne L.
Coles (née Huth)
b,
Francesco
Isaia
a,
Vito
Lippolis
a,
Ana-Daniela
Musteti
c,
Francesc
Teixidor
c,
Clara
Viñas
c and
Massimiliano
Arca
*a
aDipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Cagliari, S.S. 554 bivio per Sestu, 09042 Monserrato, CA, Italy. E-mail: marca@unica.it
bDepartment of Chemistry, Faculty of Natural and Environmental Sciences, University of Southampton, Highfield, Southampton SO17 1BJ, UK
cInstitut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Spain
First published on 7th August 2014
Five new [Pt(N^N)(dtoc)] complexes (1–5; N^N = diimine: 2,2′-bipyridine and its 4,4′-alkyl/aryl-substituted derivatives or 1,10-phenanthroline; dtoc2− = 1,2-dithiolate-1,2-closo-dicarbadodecaborane) have been synthesized and characterized by spectroscopic and electrochemical methods, and by means of X-ray diffraction in the case of complexes 1 and 4. Hybrid DFT and time-dependent (TD) DFT calculations were performed on complexes 1–5 and the previously reported complex [Pt(Ph2phen)(dtoc)] (6; Ph2phen = 4,7-diphenyl-1,10-phenanthroline) both in the gas phase and in the presence of several solvents (CH2Cl2, CHCl3, CH3CN, acetone, THF, DMF, DMSO, and toluene) to gain an insight into the electronic structure of the complexes and explain their experimental features. Theoretical calculations allowed for the determination of structure–property relationships within the series of the six complexes considered, and the prediction of their second order nonlinear optical (SONLO) properties by evaluating their first static hyperpolarizabilities (βtot).
Starting from the pioneering work carried out by Eisenberg and co-workers in the 1990s,26,27 a large number of diimine–dithiolate complexes have been reported, mainly featuring PtII as a central metal ion. The possibility of tuning both the electrochemical and photophysical properties of these chromophores by systematically varying both the central metal ion28 and the S^S and N^N ligand29 was distinctly proved. In this context, some of the authors recently reported on a series of new PtII diimine–dithiolate complexes featuring differently functionalized 1,2-dithiolato ligands belonging to the classes of R-edt2− (monosubstituted ethylene-1,2-dithiolates; R = phenyl, 2-naphtyl, 1-pyrenyl)30,31 and R-dmet2− (N-substituted 2-thioxo-1,3-thiazoline-4,5-dithiolates; R = methyl, ethyl, phenyl),32,33 along with variously alkyl- and aryl-substituted 2,2′-bipyridine and 1,10-phenanthroline ligands, thus showing the possibility of fine-tuning the linear and nonlinear optical properties of these systems.34
The twelve-vertex 1,2-closo-C2B10H12 icosahedral carborane has been widely studied.35 The unique stabilities and geometrical properties of the isomeric closo C2B10H12 carboranes suggested this species as a building block for stereo precise structural platforms with novel reaction centres having properties that cannot be achieved with organic hydrocarbon compounds.36 In addition, the 1,2-closo-C2B10H12 cluster is electron-withdrawing through the carbon atoms37 and electron donating through the boron atoms. Consequently, when sulphur atoms are bonded to the carbon cluster atoms, as in the dithiol 1,2-(SH)2-closo-C2B10H10 (H2dtoc), the electron density will be depleted on them, thus varying the potentials required for the electrochemical reduction of the corresponding complexes.38 In particular, as far as diimine–dithiolate systems are concerned, it is reasonable to assume that more strong electron-deficient39 moieties such as the 1,2-closo-C2B10H12 cluster could affect the electrochemical and optical properties of these complexes with respect to those obtained with organic dithiolates.
In this context, we report on the synthesis, characterization and DFT investigation of five new [Pt(N^N)(S^S)] complexes featuring the 1,2-dithiolato-1,2-closo-dicarbadodecaborane ligand (dtoc2−),40 in combination with differently substituted 2,2′-bipyridine and 1,10-phenanthroline derivatives. The properties of the newly synthesized complexes were compared with those based on organic ethylene-1,2-dithiolates30–33 and with [Pt(Ph2phen)(dtoc)] (Ph2phen = 4,7-diphenyl-1,10-phenanthroline), the only example of a [Pt(N^N)(S^S)] complex featuring the dtoc2− ligand reported so far.41–43
Scheme 1 General pathway for the synthesis of [Pt(N^N)(dtoc)] complexes 1–6. N^N = bipy (1), phen (2), Me2bipy (3), tBu2bipy (4), Ph2bipy (5), and Ph2phen (6). |
Analogously, C–S and C–C bond lengths within the dtoc2− ligand in complexes 1 and 4 are very close to the mean values found in the Cambridge Structural Database (CSD) for complexes featuring dtoc2− as a bidentate ligand coordinated to a single transition metal ion [1.782(9) and 1.652(20) Å, respectively],48 and only slightly different from the Ccarborane–thiol and the skeletal C–C bond distances found in the carboranyl sulphide anion [1,2-S(Ph)C2B10H10]−.49 The dithiol-o-carborane forms in both cases a five membered chelate ring, with S–Pt–S/N–Pt–N angles of 92.35(6)/79.8(2) and 92.19(7)°/78.6(2)° for complexes 1 and 4, respectively, which fall within the expected ranges.50
In complex 4 the bipyridyl ligand lies in the PtN2S2 coordination plane, and a mirror plane bisects the molecule by passing through the bond joining the two pyridine rings, the platinum ion, and the C–C bond of carborane. Complex 1 displays a similar arrangement of the bipyridyl ligand, which however slightly deviates from planarity [dihedral angle N(1)–Pt–S(2)–C(2) = 173.92°; Fig. 1]. Both complexes 1 and 4 are packed in a head-to-tail fashion to form columnar structures along axis a and b respectively, but they differ in the nature of the interactions involved. In complex 1 short intermolecular Pt⋯C contacts align the complex molecules in pillars developing along the direction of the a-axis with alternating Pt⋯C distances of 3.395(8) and 3.530(8) Å (Fig. 2).51 Similar short contacts have been explained with a weak axial interaction between the filled dz2 orbital of platinum and the π* antibonding orbital of the involved C atoms in the bipyridyl ring.52 Weak intermolecular interactions involving the B atoms of the dtoc2− ligand and the H atoms of both ligands hold the stacks together [B(6)⋯H(11) 3.180, B(10)⋯H(7A) 3.163, B(7)⋯H(6) 2.884, B(3)⋯H(6) 3.179, B(2)⋯H(6) 3.039, B(9)⋯H(11) 3.134 Å].
In complex 4 the molecules are aligned into mono-dimensional columns running along the crystallographic a-axis through extensive face-to-face π–π interactions involving both rings of bipy moieties with stacking distances of 3.89 Å, though no interactions involving the platinum ions can be evidenced. Weak interactions involving the H atoms of the tert-butyl substituents of the 2,2′-bipyridines and the B atoms of the dtoc2− ligand [B(1)⋯H(14B) 3.034, B(3)⋯H(14B) 3.138, B(4)⋯H(14B) 3.137 Å] are responsible for the 3D packing of the columns. It is noteworthy, probably due to the bulky substituents at the pyridine rings, that the structure of 4 contains solvent accessible channels running along the b-axis (376.16 Å3, 12.6% of the cell volume, Fig. 3).53
Fig. 4 Cyclic voltammogram recorded at a platinum electrode in an anhydrous DMSO solution of 1 [298 K; scan rate 100 mV s−1; supporting electrolyte [NBu4][PF6] 0.1 M]. |
E I1/2 | |EIpc–EIpa| | i Ipc/iIpa | E II1/2 | |EIIpc–EIIpa| | i IIpc/iIIpa | |
---|---|---|---|---|---|---|
a All potentials E1/2, Epc, and Epa are referred to the Fc+/Fc couple. Scan rate 100 mV s−1. | ||||||
1 | −1.593 | 0.088 | 1.0 | −2.251 | 0.100 | 1.1 |
2 | −1.604 | 0.081 | 1.3 | −2.229 | 0.047 | 1.2 |
3 | −1.709 | 0.135 | 0.8 | — | — | — |
4 | −1.698 | 0.044 | 1.1 | — | — | — |
5 | −1.536 | 0.071 | 1.0 | −2.090 | 0.091 | 0.4 |
6 | −1.541 | 0.071 | 1.1 | −2.100 | 0.091 | 0.9 |
Complexes 1, 2, 5, and 6 exhibit a further quasi-reversible reduction (iIIpc/iIIpa = 0.4–1.2) at about −2.1/−2.2 V vs. Fc+/Fc (EII1/2, Table 1 and Fig. 4 for complex 1), which leads from monoanionic to dianionic species. The second reduction process was previously reported for [Pt(Ph2Phen)(dtoc)] (6; Ph2Phen = 4,7-diphenyl-1,10-phenanthroline; Scheme 1) in DMF (E1/2 = −2.14 V vs. Fc+/Fc).43 Weinstein and co-authors supposed this process to be localized on the diimine,43 although in analogy to what was reported for different [Pt(bipy)L2] (L = py, Me2N-py, Cl), [Pt(bipy)(en)], and [Pt(phen)(py)2] complexes,55 this reduction could be attributed to the formation of a dianionic PtI radical species [PtI(N^N˙)(S^S)]2−.56
An examination of the reduction potentials EI1/2 and EII1/2 listed in Table 1 shows that no significant effect is observed on passing from N^N = bipy to N^N = phen in complexes 1 and 2, respectively. The introduction of alkyl substituents at the diimine (3 and 4) induces a potential shift towards more negative values, while phenyl substituents (5 and 6) shift both EI1/2 and EII1/2 towards more positive values with respect to the complexes 1 and 2 featuring unsubstituted diimines. Given the predominant localization of the LUMO on the diimine in [Pt(N^N)(S^S)] systems,23 this trend suggests that aryl substituents at the N^N ligands induce a stabilization of the LUMO, while alkyl substituents cause an opposite effect.34 This could also account for the absence of the reduction to dianionic forms in the case of complexes 3 and 4, whose EII1/2 potential would be too negative to be electrochemically explored.
Although a further oxidation process at about 0.95 V vs. the Fc+/Fc couple was reported in DMF for 6,43 this process was not observed in DMSO within the explored potential range. The lack of oxidation processes to cationic species suggests a remarkable stabilization of the HOMO in 1–6 as compared to different diimine–dithiolate complexes, possibly ascribed to the electron-withdrawing effect of the 1,2-closo-C2B10H12 cluster.38
Fig. 5 UV–vis absorption spectra (255–655 nm) recorded for complexes 1 (blue line) and 2 (red line) in DMSO. |
CH2Cl2 | CHCl3 | CH3CN | Acetone | THF | DMF | DMSO | Toluene | |
---|---|---|---|---|---|---|---|---|
a If not reported, the molar extinction coefficients were not determined for solubility reasons. | ||||||||
1 | 447 (6300) | 452 (6500) | 425 (5700) | 434 (5600) | 452 (6300) | 428 (5800) | 424 (5800) | 481 |
2 | 444 (6200) | 453 (6600) | 426 (4400) | 436 (5800) | 453 (6100) | 430 (6000) | 427 (6100) | 476 |
3 | 437 (5200) | 434 (4300) | 445 | 428 (5000) | 445 (4900) | 425 (4400) | 420 (4700) | 454 |
4 | 438 (7300) | 447 (7200) | 418 (6700) | 428 (7200) | 446 (7600) | 423 (6700) | 419 (6000) | 470 (8600) |
5 | 458 (9300) | 467 (7000) | 438 (8900) | 448 (8100) | 466 (7700) | 444 (8400) | 439 (8200) | 492 |
6 | 452 (8000) | 460 (8200) | 438 (7300) | 448 (8700) | 463 (8400) | 444 (7000) | 441 (6700) | 484 (8000) |
In particular, both the broad absorption featured by the bipyridine-containing complexes and the two bands shown in the case of complexes 2 and 6 display a negative solvatochromism, and a dependence of their ε values on the nature of the solvent (Table 2 and Fig. 6 for complex 2).
Fig. 6 Normalized absorption spectra in the Vis region (330–600 nm) recorded for complex 2 in eight selected solvents (Table 2). |
The observed solvatochromic behaviour is in agreement with the empirical scale previously proposed by Eisenberg and co-workers for platinum diimine–dithiolate complexes,20 thus suggesting a CT character for the electronic transitions involved (Fig. S1† for complex 2).
The λmax values recorded in DMSO for the solvatochromic absorption at the lowest energy of complexes 1–6 strongly depend on the nature of the diimine, analogously to what was previously observed for [Pt(N^N)(S^S)] complexes featuring R-dmet2− 1,2-dithiolato ligands.34 In particular on passing from complexes 1 and 2, bearing unsubstituted diimines, to complexes 5 and 6, where the N^N ligands are phenyl-substituted, a red-shift can be observed. On the other hand, alkyl substituents (complexes 3 and 4) induce a blue-shift of the λmax value of the corresponding complexes. Since the solvatochromic band is attributed to the HOMO–LUMO transition, its wavelength can be correlated to the LUMO energy, and thus spectroscopic and electrochemical CV data have been cross-compared. In fact, a linear correlation holds between the λmax values and the E1/2 potentials (Fig. 7 and S2† for EI1/2 and EII1/2, respectively; R2 = 0.82 and 0.98). This correlation suggests that the variation of the λmax values should be attributed mainly to a variation in the energies of the LUMO, localized in the diimine, while the uppermost filled MOs would not be affected, and should therefore be localized on the dtoc2− ligand and/or the central metal ion, in agreement with what was reported for different [Pt(N^N)(S^S)] complexes.25
Fig. 7 Correlation between the reduction potentials EI1/2 (V vs. Fc+/Fc) and the λmax of the solvatochromic absorption (nm) in DMSO solutions of complexes 1–6 (R2 = 0.82; Tables 1 and 2). N^N = unsubstituted (rhombs) 2,2′-bipyridine (empty) or 1,10-phenathroline (filled), and alkyl (circles) or phenyl (triangles) derivatives. |
The structural features optimized both in the gas phase and in DMSO are summarized in Table 3. Very similar sets of data were found for the optimized geometries of complexes 1–6 in the gas phase, the Pt–S and Pt–N distances falling in the ranges 2.284–2.289 and 2.050–2.059 Å, respectively.
1 | 2 | 3 | 4 | 5 | 6 | |
---|---|---|---|---|---|---|
a Atom labelling scheme as reported in Fig. 1 for complex 1. | ||||||
Pt–S(1) | 2.287 (2.305) | 2.284 (2.302) | 2.289 (2.306) | 2.289 (2.306) | 2.288 (2.306) | 2.286 (2.303) |
Pt–S(2) | 2.287 (2.305) | 2.284 (2.302) | 2.289 (2.306) | 2.289 (2.306) | 2.288 (2.306) | 2.286 (2.303) |
Pt–N(1) | 2.053 (2.052) | 2.059 (2.059) | 2.053 (2.051) | 2.052 (2.051) | 2.050 (2.050) | 2.054 (2.053) |
Pt–N(2) | 2.053 (2.052) | 2.059 (2.059) | 2.053 (2.051) | 2.052 (2.051) | 2.050 (2.050) | 2.054 (2.053) |
S(1)–C(1) | 1.795 (1.793) | 1.795 (1.794) | 1.795 (1.793) | 1.795 (1.793) | 1.795 (1.793) | 1.795 (1.794) |
S(2)–C(2) | 1.795 (1.793) | 1.795 (1.794) | 1.795 (1.793) | 1.795 (1.793) | 1.795 (1.793) | 1.795 (1.794) |
C(1)–C(2) | 1.617 (1.621) | 1.618 (1.622) | 1.618 (1.623) | 1.618 (1.624) | 1.618 (1.623) | 1.619 (1.624) |
S(1)–Pt–S(2) | 90.36 (90.31) | 90.99 (90.72) | 90.39 (90.26) | 90.46 (90.22) | 90.47 (90.17) | 90.89 (90.59) |
N(1)–Pt–N(2) | 79.22 (79.49) | 80.08 (80.30) | 79.10 (79.31) | 80.00 (79.37) | 79.11 (79.34) | 79.64 (79.82) |
N(1)–Pt–S(2)–C(2) | 170.88 (175.31) | 169.37 (174.10) | 170.09 (174.01) | 169.87 (174.93) | 170.04 (175.19) | 169.78 (174.97) |
For all the complexes, almost no changes in the optimized bond distances and angles were observed upon considering solvation in the calculations, the principal bond distances and angles varying by less than 0.1 Å and 1°, respectively. The most significant difference regards the N(1)–Pt–S(2)–C(2) dihedral angle, which is increased for all complexes by about 5° towards planarity on passing from the gas phase to the implicitly solvated species. The calculations performed in the case of complex 1 taking into account eight different solvents confirm that the optimized parameters vary only slightly upon changing the solvent (Table S2†).65 A very good agreement was found between the parameters optimized in the gas phase and the corresponding experimental structural data for complexes 1 and 4, with all of the bond lengths and angles differing by less than 0.03 Å and 2°, respectively.
In Fig. 8 and S3† the ground-state (GS) bonding scheme is shown in terms of the frontier Kohn–Sham (KS) molecular orbital (MO) scheme for complexes 1 and 2, respectively. Analogously to what was reported for related complexes,66,67 KS-HOMO and KS-HOMO−1 (MOs 102 and 101, respectively, in Fig. 8) are π-in-nature MOs mainly localized on the sulphur atoms of the dtoc2− ligand. The stabilization effect induced by the electron-withdrawing dtoc2− ligand allows for a larger extent of mixing between the ligand fragment molecular orbitals (FMOs) and the atomic orbitals (AOs) of the metal ion in the title complexes. In fact, KS-HOMOs show a remarkable contribution from 5d AOs of the Pt atom, which increases when the solvent is taken into account (17–18 and 24–25% in the gas phase and in DMSO, respectively; Fig. 9 for complex 1). Accordingly, KS-HOMO eigenvalues calculated for complexes 1–6 in the gas phase68 are remarkably more negative than those calculated at the same level of theory for different [Pt(N^N)(S^S)] complexes reported previously.34,69 The stabilization of the eigenvalues calculated for HOMOs and HOMO−1s for the complexes featuring the dtoc2− ligand is reflected in the absence of oxidation processes in the voltammograms recorded for complexes 1–6 in DMSO solution (see above).
Fig. 8 KS-MO scheme and isosurface drawings calculated for complex 1 in DMSO (IEF-PCM SCRF model; Cs point group; contour value = 0.05 e). |
Fig. 9 Frontier molecular orbital (KS-MOs 95–105; KS-HOMO = 102, KS-LUMO = 103) composition calculated for complex 1 in DMSO [fragments: platinum atom, green; dtoc2− ligand, red; N^N ligand (blue)]. |
The KS-LUMO is almost exclusively centred on the diimine ligand both in the gas phase and in DMSO (92–95%, Table S3†). The localization of the KS-LUMO on the N^N ligand is in agreement with what was hypothesized based on the analysis of the electrochemical and spectroscopic data discussed above. Accordingly, the potentials of the two reduction processes EI1/2 and EII1/2 observed for the complexes are linearly correlated with the KS-eigenvalues calculated for LUMOs of 1–6 in DMSO solution (R2 = 0.84 and 0.61 for the two reductions; Fig. 10). No significant changes were observed in the composition of the frontier MOs upon varying the solvents considered for the IEF-PCM SCRF calculations in the case of complex 1 (Table S4†).
Fig. 10 Correlation between the experimental half-wave potentials of the EI1/2 (top) and EII1/2 (bottom) reduction processes (V vs. Fc+/Fc) and the IEF-PCM TD-DFT calculated KS-LUMO eigenvalues (eV) for complexes 1–6 in DMSO (R2 = 0.84 and 0.61, respectively; Tables 1 and S3†). N^N = unsubstituted (rhombs) 2,2′-bipyridine (empty) or 1,10-phenathroline (filled), and alkyl (circles) or phenyl (triangles) derivatives. |
An analysis of the natural atomic charges70 calculated on complexes 1–6 shows that a charge separation of about 1.0 and 1.2 e exists between the two ligands in the gas phase and in DMSO, respectively (Table S5†). This polarization arises from the partial negative charge carried by the dtoc2− ligand (ranging between −0.54 and −0.71 e in the gas phase and between −0.57 and −0.73 e in DMSO solution) and the partial positive charge shown by the N^N ligands (in the ranges 0.42–0.46 e in the gas phase and 0.57–0.59 e in DMSO), the remaining positive charge of the complex being carried by the platinum ion (0.13–0.14 and e in DMSO). The calculated charge separation, which confirms what was previously reported for PtII diimine–dithiolate systems,34 also shows a small but not negligible dependence on the nature of the solvent (Table S6† for complex 1).
Time-dependent DFT (TD-DFT) calculations were carried out on complexes 1–6. Simulated UV–vis absorption spectra of the complexes are in very good agreement with the experimental ones, in particular as far as the solvation calculations are concerned (Tables 4 and S7,† and Fig. 11 for complex 2). For all complexes, the S0→S2 electronic transition was calculated to be responsible for the lowest energy solvatochromic absorption band observed experimentally in the 410–500 nm region (see above). This transition is mainly assigned to an almost pure (97% for complex 1, Table 4) monoelectronic excitation between the KS-HOMO−1 and KS-LUMO (KS-MOs 101 and 103, respectively, for complex 1; Table 4 and Fig. 8), in contrast with the previous DFT calculations reported on complex 6, where the transitions involving the frontier MOs were calculated to have negligible oscillator strengths.67 In all complexes, KS-HOMO−1, analogous to KS-HOMO, is a π-in-nature orbital predominantly localized on the sulfur atoms of the dtoc2− ligand, with a significant contribution from the Pt atom (27–28% in DMSO; Fig. 8, 9 and S3† and Table S3†). Thus, given the localization of KS-LUMO described above for complexes 1–6, the transition could be considered a mixed-metal/ligand-to-ligand CT (MMLL'CT), analogous to most of the [Pt(N^N)(S^S)] complexes reported so far (where it arises from the HOMO→LUMO one-electron excitation).25
Fig. 11 TD-DFT simulated UV–vis absorption spectrum (230–630 nm) of complex 2 in DMSO (IEF-PCM SCRF model; Table 4). |
Exc. state | E | λ | f | Comp.a | % | Assignment |
---|---|---|---|---|---|---|
a MOs are labelled according to Fig. 8. | ||||||
S2 | 2.553 | 443.6 | 0.103 | 101→103 | 97 | dtoc2−(65%) + Pt(27%)→bipy(95%) |
S3 | 3.174 | 390.6 | 0.007 | 102→106 | 76 | dtoc2−(73%) + Pt(24%)→dtoc2−(43%) + Pt(35%) |
S9 | 3.893 | 318.5 | 0.111 | 102→105 | 66 | dtoc2−(73%) + Pt(24%)→bipy(93%) |
S10 | 3.929 | 315.5 | 0.008 | 101→105 | 79 | dtoc2−(65%) + Pt(27%)→bipy(93%) |
S11 | 4.385 | 282.8 | 0.303 | 99→103 | 75 | bipy(95%)→bipy(95%) |
S13 | 4.448 | 278.8 | 0.043 | 98→103 | 90 | Pt(53%) + dtoc2−(37%)→bipy(95%) |
S14 | 4.515 | 274.6 | 0.132 | 97→103 | 79 | dtoc2−(39%) + Pt(38%)→bipy(95%) |
S15 | 4.524 | 274.1 | 0.035 | 100→105 | 83 | Pt(90%)→bipy(93%) |
The CT character of this transition leads to a lowering in the charge separation in the excited state as compared to the GS that accounts for the negative solvatochromism observed.
The KS-LUMO composition also accounts for the linear correlation observed experimentally between the λmax of the corresponding absorption and the reduction potentials EI1/2 and EII1/2 of complexes 1–6, and for the dependence of the lowest energy solvatochromic band on the nature of the diimine observed experimentally. Accordingly, the experimental λmax values of the lowest energy solvatochromic absorption can be directly correlated to the eigenvalues of the KS-LUMO calculated for complexes 1–6 (Fig. S4†).
A very good agreement was also found between the experimental and calculated λmax values of the lowest energy absorption in different solvents in the case of complex 1 (R2 = 0.94; Fig. S5†).71,72
As regards complexes 2 and 6 bearing a 1,10-phenanthroline ligand, the solvatochromic absorption observed experimentally in the region 370–540 nm was calculated to derive mainly from the S0→S3 vertical transition (Table S7†). This transition is mainly (73% for complex 2) due to the KS-HOMO→KS-LUMO+1 monoelectronic excitation (108→110 for complex 2, Table S7 and Fig. S3†). Also in this case, the excitation can be considered a MMLL'CT, since KS-LUMO+1 is almost exclusively localized on the N^N ligand (99% for complex 2), thus accounting for its solvatochromism.
Finally, since complexes 1–6 could be exploited as SONLO materials,41 in analogy with different PtII complexes,73 static dipole moments μ and static first hyperpolarizabilities βtot were calculated both in the gas phase and in DMSO (Table 5). The calculated values of βtot (75–136 × 10−30 esu in the gas phase) are remarkably lower than those previously calculated for [Pt(N^N)(S^S)] complexes featuring different 1,2-dithiolato ligands (150–650 × 10−30 esu in the gas phase),34 thus confirming the experimental observations reported by Grinstaff and co-workers on the SONLO properties of [Pt(N^N)(S^S)] complexes with S^S = dithiolate o-carborane and different 1,2-dithiolates.42 No significant differences were observed in this case between the hyperpolarizability values calculated in the gas phase and in DMSO, while a clear trend was observed upon varying the N^N ligand. The largest values of βtot were calculated for complexes 5 and 6, featuring phenyl-substituted diimines, thus confirming that aryl pendants at the N^N ligand could improve the SONLO properties of [Pt(N^N)(S^S)] systems.34
Gas phase | DMSO | |||||
---|---|---|---|---|---|---|
β tot × 104 (a.u.) | β tot × 10−30 (esu) | |μ| | β tot × 104 (a.u.) | β tot × 10−30 (esu) | |μ| | |
1 | 0.87 | 75 | 7.43 | 0.82 | 71 | 1.02 |
2 | 1.04 | 90 | 7.09 | 0.94 | 81 | 9.56 |
3 | 0.89 | 77 | 7.60 | 0.76 | 66 | 1.00 |
4 | 0.99 | 85 | 8.26 | 0.74 | 64 | 1.09 |
5 | 1.57 | 136 | 8.04 | 1.03 | 89 | 1.05 |
6 | 1.57 | 136 | 8.15 | 1.07 | 92 | 1.05 |
As already observed for different PtII diimine–dithiolate complexes, solvatochromic absorptions are displayed in the visible region of the complexes’ spectra, and in particular a single broad absorption was observed in the case of the complexes featuring a 2,2′-bipyridine derivative (λmax = 420–440 nm in DMSO). Based on TD-DFT calculations, the broad absorption was attributed to the HOMO−1/LUMO electronic excitation, whose MMLL'CT character accounts for its solvatochromism. On the other hand, two different bands were shown in the case of complexes bearing 1,10-phenanthroline derivatives (λmax = 427–441 nm in DMSO), both assigned to excitations having a MMLL'CT character as well. The DFT calculations reported in this work thus allowed for the first persuasive interpretation of the peculiar spectroscopic features of [Pt(N^N)(dtoc)] complexes, in particular those containing the 1,10-phenanthroline moiety.
In the title complexes, the electron-withdrawing effect of the dtoc2− ligand determines a remarkable stabilization of the HOMOs and HOMO−1s with respect to [Pt(N^N)(S^S)] systems bearing different sulfur ligands. Thus, no oxidation processes were observed by CV in DMSO solution within the explored range of potentials (−2.5–1.5 V vs. Fc+/Fc). As a consequence, the energy gap between HOMO/HOMO−1 and LUMO is significantly increased with respect to other [Pt(N^N)(S^S)] complexes, thus shifting the typical solvatochromic absorption to higher energies.
It is worth underlining that the peculiar electronic structure of [Pt(N^N)(S^S)] complexes allows the fine-tuning of the energy of their frontier orbitals by a proper choice of the ligands, and hence the tailoring of both the electrochemical stability of the neutral complexes and the energy of the solvatochromic visible absorption. In this context:
(1) Donor S^S ligands raise HOMOs eigenvalues, while electron-withdrawing dithiolates, such as dtoc2−, exert the opposite effect.
(2) Diimines with donor (alkyl) substituents feature the LUMO at higher energy as compared to phenyl-substituted ones.
(3) As a consequence, on passing from complexes featuring a donor dithiolate and a phenyl-substituted diimine, such as [Pt(Ph2bipy)(Me-dmet)], to complexes featuring electron-withdrawing dithiolates and alkyl-substituted diimines, such as [Pt(Me2bipy)(dtoc)], the wavelength of the solvatochromic band can be varied by more than 150 nm in DMSO (λmax = 437 and 602 nm for [Pt(Me2bipy)(dtoc)] and [Pt(Ph2bipy)(Me-dmet)], respectively).
Finally, the calculated βtot values show that [Pt(N^N)(dtoc)] complexes have a less pronounced SONLO behaviour in comparison with [Pt(N^N)(S^S)] systems bearing 1,2-dithiolene ligands. The static βtot values reported in this work are in agreement with the trend previously observed with regard to the influence of the N^N ligand on the hyperpolarizability values, which showed that the presence of phenyl substituents increases βtot values.
The insight into the electrochemical and linear and nonlinear spectroscopic features of [Pt(N^N)(dtoc)] complexes given in the present work represents therefore a significant step forward in the understanding of the electronic features of this class of compounds, and opens the way to their employment in devices of interest in optical processing.
By using Kleinman symmetry relationships,79 the final equation was obtained:
The programs Gaussview 5.080 and Molden 5.081 were used to investigate the charge distributions and molecular orbital shapes. The software GaussSum 2.182 was used to calculate the molecular orbital contributions (MOC) from groups of atoms, along with the contribution of singly excited configurations to each electronic transition, and to generate all the necessary data to simulate absorption spectra.
Footnote |
† Electronic supplementary information (ESI) available: Details on the characterization of complexes 2–5, crystallographic data for complexes 1 and 4, metric parameters optimized in the gas phase and in selected solvents, calculated eigenvalues and MO compositions, NPA charges, details on vertical transitions calculated at the TD-DFT level, correlations between UV–vis absorption wavelengths in selected solvents and empirical parameters, correlation between CV data and UV–vis absorption wavelengths, KS-MO scheme calculated for complex 2 in DMSO, correlation between experimental and calculated absorption wavelengths in DMSO, correlation between experimental and calculated solvatochromic absorption wavelengths. CCDC 1010434 and 1010435. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt01929f |
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