Photophysics of Bis(thiocyanato)gold(I) Complexes: Intriguing

J. Phys. Chem. C 2007, 111, 10689-10699

10689

Photophysics of Bis(thiocyanato)gold(I) Complexes: Intriguing Structure-Luminescence Relationships Ravi K. Arvapally,† Pankaj Sinha,† Samanthika R. Hettiarachchi,‡,⊥ Nathan L. Coker,§ Charles E. Bedel,§ Howard H. Patterson,‡ R. C. Elder,§ Angela K. Wilson,† and Mohammad A. Omary*,† Department of Chemistry, UniVersity of North Texas, Denton, Texas, 76203, Department of Chemistry, UniVersity of Maine, Orono, Maine 04469, and Department of Chemistry, UniVersity of Cincinnati, Cincinnati, Ohio 45221-0172 ReceiVed: March 9, 2007; In Final Form: May 8, 2007

The electronic structure of bis(thiocyanato)gold(I) complexes is studied both experimentally and theoretically. Temperature-dependent photoluminescence studies for K[Au(SCN)2] reveal two unstructured luminescence bands: a strong green phosphorescence band (τ77K ) 45.4 µs) and a weak blue fluorescence band (τ77K ) 24.4 ns) that becomes well-resolved by cooling toward 4 K or by time-resolved measurements, representing a rare case for Au(I) compounds whereby both fluorescence and phosphorescence are observed simultaneously. Quantum mechanical calculations for dimeric models indicate Au-Au covalent bond formation in the T1 lowest triplet excited state (2.62 Å; υAu-Au ) 180 cm-1), compared to corresponding values of 2.95 Å and 84 cm-1, respectively, for the aurophilically bound S0 ground state. Intriguing structure-luminescence relations exist for bis(thiocyanato)gold(I) complexes with different cations such as K+, Rb+, n-Bu4N+, and Cs+ in which the salts with shorter Au‚‚‚Au nearest-neighbor separations show blue shifts in the phosphorescence emission energies as well as smaller Stokes’ shifts, contrary to the expected trends. We have also observed significantly red-shifted phosphorescence energies and larger Stokes shifts in frozen solutions of K[Au(SCN)]2 compared to those for the crystals. The computational data suggest that the emission energy is sensitive to the counterion, in support of the experimental photoluminescence data. Full optimizations of the T1 states for isolated dimeric models in vacuum predict a drastic rearrangement in the T1 states in contrast to the S0 ground state and provide a physical basis for understanding the experimental photophysical results for this class of compounds.

Introduction Gold(I) complexes have been among the most prominent luminescent transition-metal coordination compounds. The luminescence in most Au(I) compounds is attributed to the d10‚ ‚‚d10 closed-shell “aurophilic bonding” interactions,1 though ligand-centered emissions and Au-centered emissions in monomeric mononuclear complexes are also known.2 The aurophilic interactions between Au(I) centers are usually considered significant when the Au‚‚‚Au distance is less than 3.6 Å.3,4 These interactions between closed-shell Au(I) centers play an important role in determining the solid-state structural arrangement of Au(I) compounds.5,6 The luminescence exhibited by Au(I) compounds is useful in a variety of applications such as detection of volatile organic compounds,7 ion sensors,8 oxygen sensors,9 and molecular light-emitting devices.10 Several interesting luminescence phenomena associated with the alteration of Au‚ ‚‚Au interactions in Au(I) complexes have been reported, such as solvoluminescence,11 polymorph-selective luminescence,12 luminescence tribochromism,13 and exciplex tuning.14 Solvoluminescence has been reported by the Balch group for colorless crystals of [Au3(CH3NdCOCH3)3] previously irradiated with * Corresponding author. Telephone: (940) 565-2443. Fax: (940) 5654318. E-mail: [email protected]. † University of North Texas. ‡ University of Maine. § University of Cincinnati. ⊥ Present address: Dept. of Chemistry, The Open University, Sri Lanka.

near-UV light that exhibit solvent-stimulated spontaneous phosphorescence.11 The same group reported a few examples of polymorph-selective luminescence, including a recent report in which [µ3-S(AuCNC7H13)3](SbF6) undergoes a reversible phase change from orthorhombic to monoclinic upon cooling, affecting both the luminescence and aurophilic interactions.12 Luminescence tribochromism has been reported by the Eisenberg group for gold(I) thiouracilate complexes that exhibit bright blue or cyan emissions upon grinding;13 similar observations have been reported earlier by Fackler and co-workers for [(TPA)2Au][Au(CN)2] (TPA ) 1,3,5-triaza-7-phosphaadamantane).15 Some of us have shown that dicyano complexes of both Ag(I) and Au(I) exhibit multiple excimeric emissions that can be tuned across the UV and visible regions by wavelengthselective excitation, temperature, varying the concentration in solution or the solid state (alkali halide hosts), and/or controlled laser irradiation that leads to reversible “write/read/erase” changes.14 Despite the great attention given to luminescent Au(I) compounds over the past few decades, as illustrated above, establishing a clear structure-luminescence relationship remains an elusive goal in this area. What has been clearly established is the need to have intramolecular or intermolecular aurophilic bonding to observe Au-centered luminescence in two-coordinate complexes while three-coordinate complexes do not require such interactions.2 The difficulty lies in trying to relate the lumines-

10.1021/jp0719356 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/23/2007

10690 J. Phys. Chem. C, Vol. 111, No. 28, 2007 cence energy to the crystallographic Au‚‚‚Au ground-state distances as situations wherein direct correlation,16 inverse correlation,17,18 and even absence of correlation19 were reported to exist between the luminescence energy and the Au-Au distance. This is in contrast to the situation in luminescent systems of other metals that were studied earlier, such as the tetracyanoplatinates(II) in which a clear correlation has long been established between the luminescence energy and the PtPt distance;20 other recent reports reinforce this trend for other types of Pt(II) complexes that exhibit Pt‚‚‚Pt interactions.21 In this work, we examine the structure-luminescence relationship in an intriguing class of Au(I) complexes, [Au(SCN)2]-, in which varying the counterion leads to interesting and unexpected changes in both the supramolecular structure and the luminescence energy. Following a preliminary communication,17 here we report a detailed photophysical and computational study of M[Au(SCN)2] compounds (M ) K+, n-Bu4N+, Rb+, and Cs+) to assess the factors important for establishing structureluminescence relationships. A better understanding of these factors will aid in the design of tunable solid-state systems based on two-coordinate Au(I) complexes for photonic applications, in which phosphorescent materials are of much current interest.22,23 Experimental and Computational Methods Syntheses, Structures, and Analytical Characterization. These details are available in the main text and the Supporting Information of the preliminary communication by the Elder group.17 The synthesis of K[Au(SCN)2] was independently reproduced at the University of North Texas via a slight modification of the procedure published in ref 17. Thus, a solution of 0.0735 g (6.8 × 10-4 mol) of KSCN in acetonitrile was added to another solution composed of 0.0966 g (3.3 × 10-4 mol) of Au(Me2S)Cl in acetonitrile. The resultant solution was stirred for about 4 h. The white precipitate (potassium chloride) formed was filtered out, and the filtrate was subjected to evaporation under reduced pressure. The white product was washed with five 10-mL portions of pentane and then filtered and dried under reduced pressure. The yield obtained was 89%. The spectral results presented in this work were indifferent for the samples synthesized at the University of Cincinnati and those synthesized at the University of North Texas. Photophysical Measurements. The photoluminescence measurements were carried out for crystalline material. Steady-state photoluminescence spectra were acquired with PTI QuantaMaster models QM-4 and QM-1 scanning spectrofluorometers. The excitation and emission spectra were corrected for the wavelength-dependent xenon lamp intensity and detector response, respectively. Cooling in the steady-state temperaturedependent photoluminescence measurements for the crystals was achieved using a model LT-3-110 Heli-Tran cryostat from Air Products equipped with a temperature controller using liquid helium as the coolant. Lifetime and time-resolved data were acquired for crystalline K[Au(SCN)2] using fluorescence and phosphorescence subsystem add-ons to the aforementioned PTI QM-4 instrument. The pulsed excitation source was generated using the 337.1-nm line of the N2 laser. Frozen solution luminescence spectra were acquired for solutions of crystalline samples prepared in HPLC-grade acetonitrile; these solutions were transferred to 1-mm quartz cuvettes, and then luminescence spectra were run at 77 K using an Oxford optical cryostat using liquid nitrogen as coolant. Computational Details. Calculations were performed on [Au(SCN)2]-, M[Au(SCN)2], {[Au(SCN)2]-}2, and M2[Au-

Arvapally et al. (SCN)2]2 models in their S0 ground state and T1 lowest triplet excited state (where M ) Na+, K+, Rb+, or Cs+). The methods used included (1) full optimization using the second-order Møller-Plesset perturbation theory (MP2),24 (2) MP2 scan calculations by varying only the separation between the two monomeric units while keeping the geometry of each monomer as that in the crystal structure of K[Au(SCN)2], and (3) full optimization using the density functional B3PW91.25 The triplet states were spin-unrestricted. The absorption and emission energies were computed for the vertical transitions, per the Franck-Condon principle. Dissociation energies (De) were determined on the basis of optimized energies of the dimer with respect to its dissociated monomers. All calculations were performed using the Gaussian 03 suite of programs.26 Three designated basis set combinations are described below. For gold, the following three pseudopotential and basis set combinations were used with 19 valence electrons (5s25p65d106s1): the LANL2DZ combination described by Hay and Wadt,27 aug-cc-pVDZ-PP, and aug-cc-pVTZ-PP. The latter two are double- and triple-ζ correlation consistent basis sets recently developed by Peterson and Puzzarini28 and include a pseudopotential developed by the Stuttgart group.29 The correlation consistent basis sets are known for their systematic improvement in the description of molecular properties as basis set size is increased, thus eliminating inherent basis set errors in a given method.30,31 For sulfur, the following sets were utilized: a Huzinaga/Dunning basis set of a double-ζ quality with a polarization function (D95*),32 double- and triple-ζ correlation consistent basis sets augmented with a tight d function (augcc-pV(D+d)Z and aug-cc-pV(T+d)Z, respectively).31 Carbon and nitrogen were described with D95*,32 aug-cc-pVDZ, and aug-cc-pVTZ.30 The counterions were described by LANL2DZ.27 The theoretical methodologies employed for this study were selected according to the problem to be solved. For example, when comparing ground- vs excited-state bonding, MP2 was used instead of DFT because of the limitations of DFT to describe weak metallophillic bonding. In this situation, smaller basis sets were used because of the extensive computational cost associated with full optimizations using MP2 for large dimeric models. To isolate the effect of Au-Au interactions from other forces, MP2 scan calculations were used in conjunction with a Dunham analysis33 to determine the equilibrium AuAu distances, dissociation energies (De), and stretching frequencies (νe). On the other hand, when quantitative spectroscopic information is sought to evaluate phosphorescence energies in comparison with experiment, DFT was used because DFT can describe the excimeric covalent bonding. Large correlation consistent basis sets could then be used because of the lower computational cost of DFT compared to that of MP2. Results and Discussion 1. Solid-State Luminescence Behavior. When excited with UV light, colorless crystalline samples of the potassium salt show bright green emission at room temperature. As shown in Figure 1, temperature-dependent photoluminescence studies carried out from 4 to 295 K show two emission bands: an intense lower-energy (LE) green band at 535 nm and a weaker higher-energy (HE) blue band at 443 nm, which appears as a shoulder in the steady-state spectra. No significant changes in the peak maxima are observed upon decreasing the temperature from 295 to 4 K. However, the HE shoulder becomes more distinct, and its relative intensity increases at lower temperatures. Figure 2 shows the steady-state emission and excitation spectra of K[Au(SCN)2] at 77 K. The luminescence excitation spectra

Photophysics of Bis(thiocyanato)gold(I) Complexes

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10691

Figure 1. Temperature-dependent photoluminescence spectra for crystalline K[Au(SCN)2] using 365-nm excitation.

Figure 2. Photoluminescence spectra of crystalline K[Au(SCN)2] at 77 K.

for crystalline K[Au(SCN)2] monitoring either the HE or LE emission bands give a similar profile. Two excitation bands are obtained: one at 315 nm and the other at 365 nm. Both excitation peaks likely represent oligomeric species because they are significantly red shifted from the solution absorption bands, which occur at very short wavelengths below 300 nm.17 Time-resolved luminescence spectra shown in Figure 3 for crystalline K[Au(SCN)2] at 77 K give rise to a complete resolution of the HE blue band from the more dominant LE green band. The lifetime data show that the HE emission represents fluorescence with a lifetime of 24.4 ns, while the LE emission represents phosphorescence with a lifetime of 45.4 µs. Thus, the blue and green emissions are assignable to radiative transitions that are formally S1 f S0 and T1 f S0 of oligomeric {K[Au(SCN)2]}n species.34 This is somewhat unusual because the presence of the gold heavy atom is associated with rather strong spin-orbit coupling (ξ5d ) 5.1 × 103 cm-1),35 which usually leads to unity intersystem crossing and thus observation

of only phosphorescence bands in Au(I) compounds.2 However, a precedent wherein both fluorescence and phosphorescence are exhibited simultaneously in an Au(I) complex has been reported by Eisenberg and co-workers for dimeric dithiophosphate complexes.36 Given the assignment of the HE and LE emissions in K[Au(SCN)2] to fluorescence and phosphorescence, respectively, we now assign the corresponding excitation feature near 315 nm to an S0 f S1 transition while the feature near 365 nm is assigned to an S0 f T1 transition. Photoluminescence spectra for other salts are shown in Figure 4, illustrating the tuning of the emission energy across the visible region by changing the counterion. A discussion of the factors that govern the origin of this tuning is provided in later sections. To represent the non-alkali metal salts, we have selected (nBu4)N[Au(SCN)2] to carry out detailed luminescence studies similar to the above ones for the potassium salt. These photophysical data for (n-Bu4)N[Au(SCN)2] are available in the Supporting Information (Figures S1 and S2). Similar to the

10692 J. Phys. Chem. C, Vol. 111, No. 28, 2007

Arvapally et al.

Figure 3. Time-resolved spectra showing the resolution of the fluorescence and phosphorescence bands for crystalline K[Au(SCN)2].

potassium salt, two luminescence bands are observed. The LE band appears in the green region and is more intense than the HE blue band. No significant change in peak maximum is observed in either band upon decreasing the temperature from 295 to 4 K. However, the two notable changes from the luminescence results for the potassium salt are (1) a greater reduction in the relative intensity of the LE band upon heating and (2) a more structured profile in the emission spectra. The reduction in the intensity of both the fluorescence and phosphorescence bands in K[Au(SCN)2] and (n-Bu4)N[Au(SCN)2] upon increasing the temperature from 4 to 295 K is due to multiphonon relaxation processes to the ground state, leading to faster nonradiative decay at higher temperatures. It is not unusual for this decay to be different for multiple emission bands in the same material or for different materials. The above luminescence data illustrate broad emission bands and large energy gaps between the emission and excitation maxima for all compounds examined. These are typical characteristics of Au-centered emissions in associating Au(I) complexes. The solid-state luminescence data for various [Au(SCN)2]- salts examined herein are consistent with what is established in the literature in that two-coordinate Au(I) complexes exhibit Au-centered emissions only in the presence of aurophilic bonding between adjacent complexes.2 The R4P[Au(SCN)2] salts are the only samples that do not exhibit such emissions, which is consistent with the lack of Au‚‚‚Au

interactions only in these phosphonium salts. Crystals of all other salts exhibit Au‚‚‚Au interactions and thus Au-centered emissions. It is interesting to note that the observation of luminescence and Au-Au interactions in R4N[Au(SCN)2] crystals is dissimilar to the situation for the analogous dicyano complexes, wherein [Au(CN)2]- units are essentially isolated from one another in salts such as (n-Bu4)N[Au(CN)2],37 for which no Aucentered emissions have been reported. 2. Aurophilic Au‚‚‚Au and Excimeric Au-Au Bonding. Since the spectroscopic data suggest that oligomeric species are responsible for the luminescence bands in [Au(SCN)2]- salts, we have carried out quantum mechanical calculations for the S0 ground state and T1 lowest excited state of dimeric models. Table 1 summarizes the results that assess the aurophilic Au‚ ‚‚Au bonding vs the excimeric Au-Au bonding in {[Au(SCN)2]-}2 in comparison to analogous bonding parameters previously reported14a for {[Au(CN)2]-}2. Figure 5 shows the computed potential surfaces for the S0 ground state and T1 excited state along the major distortion axis based on MP2 calculations. The bonding parameters in Table 1 and Figure 5 for the S0 dimeric and T1 excimeric states of the {[Au(SCN)2]-}2 model are generally comparable to the corresponding values reported for the {[Au(CN)2]-}2 dicyano analogue (S0: De ) 3.574 × 103 cm-1 and νe ) 90 cm-1; T1: De ) 11.426 × 103 cm-1 and νe ) 166 cm-1).14a



Figure 5. Potential energy surfaces for the T1 and S0 states of {[Au(SCN)2]-}2 calculated with scan calculations using MP2/Au: LANL2DZ; SCN: D95*.

TABLE 2: Comparison of Calculated Bond Distances (Å) and Angles (deg) in the S0 State of the {[Au(SCN)2]-}2 Model with Crystallographic Parameters

Au···Au Au-S (av) S-C (av) C-N (av) S-Au-S (av) Au-S-C (av) Figure 4. Photoluminescence spectra at 77 K for crystalline samples of various [Au(SCN)2]- salts, as identified by the counterion. Emission and excitation spectra are indicated by thick and thin lines, respectively.

TABLE 1: Aurophilic vs Excimeric Au-Au Bonding for {[AuX2]-}2 with X ) SCN in Comparison to Reported14a Values for X ) CN X SCN

method MP2 MP2 MP2 scana MP2 MP2 MP2 scana B3PW91 B3PW91

CN

MP2 MP2

basis

state

re/Å

Au: LANL2DZ(Au); SCN: D95* Au: aug-cc-pVDZ-PP; S: aug-cc-pV(d+D)Z; CN: aug-cc-pVDZ Au: LANL2DZ; SCN: D95* Au: LANL2DZ; SCN: D95* Au: aug-cc-pVDZ-PP; S: aug-cc-pV(d+D)Z; CN: aug-cc-pVDZ Au: LANL2DZ; SCN: D95* Au: aug-cc-pVDZ-PP; S: aug-cc-pV(d+D)Z; CN: aug-cc-pVDZ Au: aug-cc-pVTZ-PP; S: aug-cc-pV(d+T)Z; CN: aug-cc-pVTZ Au: LANL2DZ; CN: D95* Au: MP2/LANL2DZ; CN: D95*

S0

2.945

S0

2.993

S0

2.946

T1

2.723

T1

2.692

T1

2.625

T1

2.745

T1

2.743

S0

2.960

T1

2.664

a Equilibrium geometry was obtained by single-point scan calculations, where only the distance between the two complexes was varied, coupled with a Dunham analysis33 to obtain De and νAu-Au.

A point to recognize from a theoretical consideration is that the underestimation of re and subsequent overestimation of De are likely due to basis set superposition error (BSSE),38,39 which is usually significant when using double-ζ level basis sets such

MP2/Aa

MP2/Bb

expt, K+ salt17

expt, n-Bu4N+ salt17

2.945 2.372 1.695 1.201 176.7 101.0

2.993 2.290 1.693 1.200 179.4 102.6

3.007 2.311 1.680 1.148 173.5 102.1

3.070 2.290 1.663 1.148 174.3 102.0

a Au: LANL2DZ; SCN: D95*. b Au: aug-cc-pVDZ-PP; S: augcc-pV(d+D)Z; CN: aug-cc-pVDZ.

as LANL2DZ. This is manifested by the fact that the crystallographic Au‚‚‚Au distances for M[Au(SCN)2] salts in ref 17 are somewhat longer than the computed values; Table 2 provides a comparison between the computed geometry for [Au(SCN)2]22with representative experimental geometries for dimeric units in the structures of K[Au(SCN)2] and (n-Bu4)N[Au(SCN)2]. Along with BSSE, underestimation of the bond distance and the consequent overestimation of the dissociation energy values in Table 1 likely result from the fact that the computations do not account for further packing interactions with counterions (vide infra) and other complexes. Accounting for BSSE and further packing interactions is not tractable with MP2 for these large dimeric models. There is a general agreement between the computed and experimental bond distances and angles within each monomeric complex, as shown in Table 2, suggesting that the bare [Au(SCN)2]22- model is reasonable for the description of the dimer ground state. For the description of the T1 triplet state of dimeric models, we have utilized both MP2 and DFT to treat {[Au(SCN)2]-}2 and only DFT to treat the larger {M[Au(SCN)2]}2 models (where M ) Na, K, Rb, and Cs). DFT is sufficient to describe the excimeric covalent bonding40 (but not the weak metallophilic bonding)40 and is much less demanding computationally than MP2. Hence, we have been able to utilize DFT calculations with large basis sets to treat the counterion-containing dimeric models (while treating such models has not been feasible with MP2). DFT data for the T1 state of all dimeric models show significant Au-Au bonding, as shown in Table 1. Figure 6 shows contour plots of the Kohn-Sham frontier orbitals of {[Au(SCN)2]-}2 and {K[Au(SCN)2]}2. Plotted are the two singly occupied Kohn-Sham orbitals (SOKSOs) for the optimized T1 state of each system. In each case, it is clear that the higher SOKSO contains a significant orbital-density overlap in


Figure 6. Contours of the frontier orbitals for the optimized T1 states of {[Au(SCN)2]-}2 (left) and {[KAu(SCN)2]}2 (right) based on DFT calculations (B3PW91/basis set: Au: aug-cc-pVDZ-PP; S: aug-ccpV(d+D)Z; CN: aug-cc-pVDZ; K: LANL2DZ). Note the Au-Au bonding character in the higher SOKSO in both cases.

the Au-Au center, which manifests the strong excimeric AuAu covalent bonding with very short Au-Au distances (Table 1). The orbital contours also explain the large Stokes’ shifts seen experimentally since they suggest a large excited-state distortion. An unusual result illustrated in Figure 6 is that the lower SOKSO has the electron density essentially localized only on one complex. This is in sharp contrast to the situation reported for the analogous dicyano complexes in ref 14a as well as other known excimeric closed-shell transition-metal systems,2,41 wherein the electron density has been reported to be delocalized onto both metal centers with symmetrically antibonding and bonding interactions in the lower and higher molecular orbitals, respectively. The differences in the electronic and steric factors between the thiocyanate and cyanide ligands are likely responsible for this contrast. The situation here seems related to that known for organic excimers, which are perceived to have localized excitation of one monomeric unit that then interacts with another ground-state unit to form the excimer.42 The DFT-computed geometries for the optimized T1 states of [Au(SCN)2]22- and {K[Au(SCN)2]}2 models are shown in Figure 7, while further DFT computational data for dimeric models with different basis sets and other M+ counterions are shown in the Supporting Information (Figures S3 and S4). All models exhibit rather short Au-Au distances, consistent with an excimeric luminescent species, and these Au-Au distances are noticeably shorter in the presence of the counterion than those in its absence with some variation between one counterion and another. Nevertheless, the counterion remains nonbonded to the complex. Thus, the situation herein is somewhat different from that described by Che and co-workers for [Au2(diphosphine)2]2+ dimeric complexes, for which it was suggested that the counterion forms an exciplex bond with the Au(I) atoms in their emissive triplet states.43 The data here, in contrast, suggest that the counterion interacts more strongly with the thiocyanate ligands than with Au(I) but the Au-Au bonding and the triplet energy are affected indirectly but significantly as a result of this interaction with the counterion. The geometry of the T1 state in each of the dimeric models computed via DFT shows that the S-Au-S coordination in one of the two complexes deviates more drastically from linearity than the other complex (Figures 7, S3, and S4). This is likely a consequence of the unsymmetrical orbital character in some of the frontier molecular orbitals, as discussed earlier.

Arvapally et al. Interestingly, MP2 calculations show that the two complexes in the dimeric dianionic unit remain symmetrical in both the S0 and T1 states. While the DFT data suggest that photoexcitation significantly affects the geometry of only one complex in the dimer, the MP2 data show that both complexes in the dimer are distorted identically. Figure 8 shows the MP2-calculated geometry of fully optimized models of [Au(SCN)2]22- in both the S0 and T1 electronic states; further metric parameters for the computed structure of the T1 state are in Table S1 of the Supporting Information. The drastic rearrangement in the relaxed geometry of the T1 state entails not merely the formation of an Au-Au bond and deviation from planarity, but also the coordination of one thiocyanate N atom from each complex to the Au atom in the adjacent complex so that each complex unit exhibits a T-shaped structure. Rearrangements to such T-shaped structures have been invoked in the excited states of other Au(I) complexes investigated by some of us22i,44 and others.45 The computed structures of the T1 states of dimeric models correspond to true minima because no imaginary frequencies are found in frequency calculations for the structures optimized using both theoretical treatments (MP2 and DFT). The bearing of these data on the experimental luminescence data, on the other hand, will be discussed in the following section. Further experimental and computational evidence has been gathered to shed more light onto the structure of the excitons that are responsible for the luminescence bands and explain the puzzling trends in the luminescence energies of the various bis(thiocyanato)gold(I) salts above. With respect to the computational data, it is important to establish the context with regard to quantitative aspects of the theoretical treatment when modeling the spectroscopy of transition-metal complexes. A recent study by some of us40 on excimeric Hg clusters, for example, has illustrated that, to describe the photophysics with a reasonable accuracy, a triple-ξ correlation consistent basis set must be used while widely used basis sets such as LANL2DZ yielded errors as high as 8000 cm-1 (∼1 eV) for electronic transitions. For *Hg2 and *Hg3 species, CCSD(T) and B3PW91 yielded much more accurate transition energies than MP2 in conjunction with the aforementioned triple-ξ correlation consistent basis sets.40 To establish a benchmark for the bis(thiocyanato)gold(I) complexes herein, we have carried out calculations to evaluate the atomic 1S T 3D transition energy for the Au(I) free ion. Thus, Table 3 summarizes these results using the same methods and Au basis sets used for the [Au(SCN)2]22- and M2[Au(SCN)2]2 models. From Table 3, it can be seen that B3PW91 calculations using a LANL2DZ basis set for Au overestimates the S0 T T1 Au(I) atomic transition by >6000 cm-1 (∼35% error). Even upon augmenting LANL2DZ with polarization functions, the error remains very large as it decreases only by a few hundred cm-1 (i.e., 32% error with respect to the experimental atomic transition energy). On the other hand, with the use of correlation consistent basis sets, the error decreases to 7 and 2% for double-ξ and triple-ξ, respectively, with the latter being only a few hundred cm-1 from the experimental value. The MP2 results are unsatisfactory with all basis sets examined, which is in line with the conclusions made in ref 40 for the Hg monomer and clusters. Overall, it can be seen from Table 3 that using the DFT functional B3PW91 along with the correlation consistent basis sets leads to a reasonable description of the Au(I) atom, and hence, this DFT treatment is adopted for quantitative determination of the electronic transition energies for monomeric and dimeric models of the bis(thiocyanato)gold(I) species.



Figure 7. Computed rearrangement of dimeric {[Au(SCN)2]-}2 (top) and {[KAu(SCN)2]}2 (bottom) models taken from the crystal structure of K[Au(SCN)2] (left) upon photoexcitation and relaxation to the T1 state (right) based on DFT calculations (B3PW91/basis set: Au: aug-cc-pVDZPP; S: aug-cc-pV(d+D)Z; CN: aug-cc-pVDZ; K: LANL2DZ). Note the excimeric contraction in the Au-Au distances and lack of Au-K exciplex bond formation.

Figure 8. Computed structure of the {[Au(SCN)2]-}2 dimer in the S0 state (left) and the T1 state (right) based on full ab initio optimizations using MP2/Au: aug-cc-pVTZ-PP; S: aug-cc-pV(d+T)Z; CN: aug-cc-pVTZ.

3. Structure-Luminescence Correlation. Table 4 summarizes the calculated T1 f S0 phosphorescence and S0 f T1 excitation energies for the dimeric complexes. The experimental data in Figure 4 clearly show that each complex luminesces in

the visible region. However, calculations for the dimeric models give rise to luminescence in the near-IR region even when using theoretical treatments that have been validated, as described earlier. Indeed, these treatments give rise to a reasonable


Arvapally et al.

TABLE 3: Calculated 1S T 3D Transition Energy (cm-1) in the Au+ Ion B3PW91/Aa B3PW91/Bb B3PW91/Cc MP2/Bb MP2/Cc 23 756

18 921

18 061

a

19 063

19 245

expt48 18 451

b

Au: LANL2DZ; SCN: D95*. Au: aug-cc-pVDZ-PP; S: augcc-pV(d+D)Z; CN: aug-cc-pVDZ. c Au: aug-cc-pVTZ-PP; S: augcc-pV(d+T)Z; CN: aug-cc-pVTZ.

TABLE 4: Calculated Spectroscopic Parameters for {[MAu(SCN)2]}2 Dimeric and Excimeric Models M+

λem/nmc

λex/nmd

SS/cm-1 e

dAu-Au(T1)/Å

nonea noneb K+ a Rb+ a Cs+ a

1229 1241 1389 1222 1158

348 361 398 395 391

20 585 19 642 17 942 17 133 16 940

2.745 2.743 2.667 2.698 2.701

a Method/basis: B3PW91/aug-cc-pVDZ-PP - Au, aug-cc-pV(d+D)Z - S, aug-cc-pVDZ - C,N, and LANL2DZ - counterion. b Method/ basis: B3PW91/aug-cc-pVTZ-PP - Au, aug-cc-pV(d+T)Z - S, and aug-cc-pVTZ - C,N. c T1 f S0 excitation at the experimental geometry of a dimeric unit from the structure of K[Au(SCN)2]. d S0 f T1 at the T1 optimized geometry. e Stokes’ shift based on the computed λem and λex values listed.

Figure 9. Luminescence spectra of K[Au(SCN)2] in acetonitrile frozen glassy solutions (77 K) at different concentrations.

agreement with the experimental data for the excitation energies. Table 4 shows that the vertical excitations for dimeric models at their experimental crystallographic geometries give rise to wavelengths that are well within the 350-400-nm experimental range when correlation consistent basis sets are used in conjunction with B3PW91. Because monomeric models give rise to significantly higher S0 f T1 excitation energies while dimeric models are sufficient to shift these excitation energies to the experimentally observed range, this identifies dimeric clusters to be responsible for the luminescence in M[Au(SCN)2] solids. This conclusion is further supported by the fact that the calculated T1 f S0 phosphorescence energies for dimeric models are already further red shifted from the experimental energies, so further modeling of trimeric or larger oligomers is not warranted because such models would further red shift the Au cluster-centered excimeric emissions, as known for multiple other classes of two-coordinate Au(I) complexes that associate beyond dimers.2,7,11,14,18 The problem is that, indeed, the fully optimized geometries of dimeric models in their T1 state give rise to drastically red-shifted phosphorescence energies versus the experimental values. This is surprising particularly because literature work suggests that Au-Au excimer formation in dimeric Au(I) complexes leads to UV phosphorescence. While Patterson and co-workers have shown that *[Au(CN)2]22excimers and even trimeric analogues emit in the UV region,14a Che and co-workers have shown that intramolecular excimer formation in dinuclear Au(I) phosphine complexes leads to UV phosphorescence that is shifted to the visible region only upon exciplex formation with a counterion or solvent molecule.43 Indeed, when we restrict the excited-state distortion to excimeric Au-Au bonding by carrying out scan calculations in which only the distance between the two complexes is varied, we obtain UV phosphorescence and a Stokes’ shift of only ∼5 × 103 cm-1 with this partial optimization (Figure 5). Full optimizations by a computational treatment that has been validated to attain accurate transition energies give rise to NIR phosphorescence and huge Stokes’ shifts (∼17-20 × 103 cm-1; Table 4) computed for the [Au(SCN)2]22- and M2[Au(SCN)2]2 dimeric models herein. Thus, we conclude that these energies are related to the fact that the solid-state lattice does not allow for the drastic rearrangements in the excimeric T1 states shown in Figures 7 and 8. Thus, the constrained environment in the crystalline solid

state would not allow such distortions to the extent predicted by gas-phase calculations for dimeric models in vacuum, which lack matrix effects. A similar conclusion was reached by Coppens and co-workers by time-resolved X-ray diffraction studies that experimentally determined the crystal structures of phosphorescent molecular states. These elegant studies provided “calibrations” for computed structures, and it was clearly demonstrated that the discrepancy between the experimental and computed structures is not always related to shortcomings of the theoretical treatments.46 In an effort to provide experimental evidence of the aforementioned hypothesis that the predicted T1 structures and NIR emissions are due to lack of matrix effects, we have performed frozen solution studies for K[Au(SCN)2] in acetonitrile. Figure 9 shows that these solutions exhibit phosphorescence (τ77K ) 55 ( 2 µs) in the yellow-orange region, significantly red shifted from the bluish-green phosphorescence exhibited by the solid compound. For example, the 10-3 M frozen solution exhibits an emission maximum of 582 nm and a Stokes’ shift of 14.2 × 103 cm-1, compared to analogous values of 525 nm and 8.6 × 103 cm-1 for the crystals. The 10-2 M frozen solution exhibits a Stokes’ shift of 13.5 × 103 cm-1, slightly lower than that for the more dilute 10-3 M solution but still significantly higher than that for the crystals. These trends clearly support the aforementioned hypothesis since the frozen solution matrix is less constrained and thus allows for a greater distortion in the phosphorescent state of dimeric units compared to the solid-state environment. We now turn our attention to explaining the puzzling trend of the emission energy with different counterions (Figure 4). Figure 10 shows that there is a general counterintuitive correlation between the emission energy (as well as the Stokes’ shift) and the Au‚‚‚Au distance in different salts whereby blue instead of red shifts are observed for Au-centered emissions in salts with shorter aurophilic bonds. Compounds that have lower emission energies, such as the red-emitting Cs+ salt (λmax ) 688 nm; Stokes’ shift ) 15.7 × 103 cm-1), exhibit significantly longer Au‚‚‚Au distances as well as longer cation‚‚‚complex distances than those in compounds that exhibit much higher emission energies, such as the blue-green emitting Rb+ (λmax ) 513 nm; Stokes’ shift ) 9.5 × 103 cm-1) and K+ salts (λmax ) 525 nm; Stokes’ shift ) 8.6 × 103 cm-1). The shortest Au‚ ‚‚Au distances in the Cs+ salt are 3.213 and 3.240 Å, much



Figure 10. Correlation of the luminescence excitation and emission energies and the Stokes’ shifts with the shortest Au‚‚‚Au distance in the six [Au(SCN)2]- salts whose crystal structures have been determined. Each salt is identified by the counterion.

longer than the analogous distances in the Rb+ salt (3.082 and 3.114 Å) or the K+ salt (3.006 and 3.043 Å). Likewise, the shortest counterion‚‚‚N (thiocyanate) distances in the Cs+ salt (3.124, 3.179, 3.210, and 3.219 Å) are much longer than the analogous distances in the Rb+ salt (2.958, 3.011, 3.014, and 3.056 Å) or the K+ salt (2.832, 2.863, 2.866, 2.889 Å). These crystallographic distances are consistent with the aforementioned structure of the emitting exciton because there is more freedom for photoinduced rearrangement in the lattices of crystals exhibiting long Au‚‚‚Au and counterion‚‚‚complex separations such that the geometry of the emitting exciton becomes closer to the predicted distortion of the excimeric T1 state. Thus, such compounds will emit at lower energies compared to compounds whose lattices allow for significantly smaller excited-state distortions and thus smaller Stokes’ shifts and higher emission energies. In contrast, the excitation energies in different salts lead to an intuitive correlation with the aurophilic distances (Figure 10). This is because the excitation process occurs at the crystallographic geometry at which shorter Au‚‚‚Au distances lead to smaller HOMO-LUMO gaps and thus lower transition energies, as indeed observed. The correlations in Figure 10 are unique in the photophysics of Au(I) complexes and suggest predictive information about the structures of complexes based on their luminescence energies in the absence of direct crystallographic data. Finally, we would like to contrast the photophysical behavior suggested earlier for M[Au(SCN)2] species with that of haloisonitrilegold(I) complexes. It has been suggested that the contrast in the Au-centered phosphorescence energies of these RNCAuX neutral complexes is based on the association mode so that extended-chain structures with longer Au‚‚‚Au distances emit at lower energies and exhibit larger Stokes’ shifts than those in crystals whose structures entail crossed dimers with shorter Au‚‚‚Au distances.18,19 This is not the case here for M[Au(SCN)2] crystals because the emitting species is a dimeric unit (vide supra) even for crystals in which the complexes pack in infinite chains, such as the three alkali metal salts whose structures exhibit extended chains according to ref 17. Thus, it is possible that crystals in which complexes pack as dimers or oligomers emit at lower energies than those for crystals in which complexes stack as extended chains. Consistent with this argument is that both ammonium salts exhibit a lesser extent

of clustering of their aurophilically interacting bis(thiocyanato)gold(I) complexes (dimers and loosely connected trimers for the n-Bu4N+ and MeN4+ salts, respectively); yet, both emit at lower energies than the K+ and Rb+ salts in which the complexes stack as chains. Compression of extended structures of interacting d10 complexes into dimeric units, like we are suggesting here, is not unreasonable. Indeed, a recent study by Coppens and co-workers has shown that a trinuclear Cu(I) pyrazolate complex whose molecules stack in zigzag infinite chains in the ground state exhibits discrete excimeric units in its phosphorescent state.47 While a drastic contraction by 0.58 Å of the intermolecular Cu‚‚‚Cu distances takes place within the excimer, the excimeric units are well separated from one another (dCu···Cu ) 4.33 Å, further separated by 0.55 Å from the ground-state structure). Concluding Remarks The photophysics, ground-state Au‚‚‚Au aurophilic bonding, and excited-state Au-Au excimeric bonding of bis(thiocyanto)gold(I) complexes have been studied both experimentally and computationally. A rare phenomenon of simultaneous existence of fluorescence and phosphorescence is observed in these complexes, and the two components have been resolved in the K[Au(SCN)2] salt by time-resolved luminescence spectroscopy. Unstructured emission bands with large Stokes’ shifts are exhibited by all five compounds whose crystal structures show Au‚‚‚Au interactions, including salts of some bulky counterions such as n-Bu4N+, while this luminescence is undetectable in the one structurally characterized salt that does not exhibit any Au‚‚‚Au interactions. The luminescence from these gold complexes is assigned to gold-centered emissions in which excimeric Au-Au bonding plays a major role but ligand rearrangement also takes place. The anomalous trend in the emission energies in which higher-energy phosphorescence is obtained in crystals that exhibit shorter Au‚‚‚Au interactions has been related to a more constrained photoinduced rearrangement in matrices that do not allow freedom for a drastic structural change in the two complexes comprising the excimer. This has been substantiated by observing significantly redshifted phosphorescence and larger Stokes’ shifts in frozen solutions of K[Au(SCN)2] compared to crystals.

10698 J. Phys. Chem. C, Vol. 111, No. 28, 2007 Acknowledgment. M.A.O. acknowledges support from the National Science Foundation (CAREER Award, CHE-0349313), the Robert A. Welch Foundation (B-1542), and the U.S. Department of Energy (Award DE-FC26-06NT42859). H.H.P. and S.R.H. thank the National Science Foundation (CHE0315877) for support of their contribution. R.C.E. thanks W. B. Connick for helpful discussions and use of his spectrofluorometric apparatus for preliminary measurements. N.L.C. and C.E.B. acknowledge support from the University of Cincinnati Foundation. A.K.W. acknowledges support from the National Science Foundation (CAREER Award, CHE-0239555) and the U.S. Department of Education (P116Z050070 for the Center for Advanced Scientific Computing and Modeling (CASCaM)), and for computational resources provided via the National Science Foundation (CHE-0342824), and by the National Computational Science Alliance; this latter support was for the NCSA IBM p690 under #CHE010021. Additional computational support was provided by the University of North Texas Academic Computing Services for the use of the UNT Research Cluster. Supporting Information Available: Further spectral and computational data referred to in the article. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) For reviews, see: (a) Pyykko¨, P. Chem. ReV. 1997, 97, 597-636. (b) Pyykko¨, P. Angew. Chem., Int. Ed. 2004, 43, 4412-4456. (c) Pyykko¨, P. Inorg. Chim. Acta 2005, 358, 4113-4130. (2) For a review, see: Forward, J. M.; Fackler, J. P., Jr.; Assefa, Z. Photophysical and Photochemical Properties of Gold(I) Complexes. In Optoelectronic Properties of Inorganic Compounds; Roundhill, D. M., Fackler, J. P., Jr., Eds.; Plenum: New York 1999; Chapter 6. (3) Schmidbaur, H. Gold: Progress in Chemistry, Biochemistry and Technology; Wiley: New York, 1999. (4) Grohman, A.; Schmidbaur, H. In ComprehensiVe Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Elsevier: Oxford, 1995. (5) Jones, P. G. Gold Bull. 1986, 19, 46-54; 1983, 16, 114-120; 1981, 14, 159-168; 1981, 14, 102-118. (6) Pathaneni, S. S.; Desiraju, G. R. J. Chem. Soc., Dalton Trans. 1993, 2, 319-322. (7) Mansour, M. A.; Connick, W. B.; Lachicotte, R. J.; Gysling, H. J.; Eisenberg, R. J. Am. Chem. Soc. 1998, 120, 1329-1330. (8) Yam, V. W.-W.; Li, C.-K.; Chan, C.-L. Angew. Chem., Int. Ed. 1998, 37, 2857-2859. (9) Mills, A.; Lepre, A.; Theobald, B. R. C.; Slade, E.; Murrer, B. A. Anal. Chem. 1997, 69, 2842-2847. (10) (a) Ma, Y.; Zhou, X.; Shen, J.; Chao, H.-Y.; Che, C.-M. Appl. Phys. Lett. 1999, 74, 1361-1363. (b) Ma, Y.; Che, C.-M.; Chao, H.-Y.; Zhou, X.; Chan, W.-H.; Shen, J. AdV. Mater. 1999, 11, 852-857. (c) Yam, V. W.-W.; Chan, C.-L.; Choi, S. W.-K.; Wong, K. M.-C.; Cheng, E. C.-C.; Yu, S.-C.; Ng, P.-K.; Chan, W.-K.; Cheung, K.-K. Chem. Commun. 2000, 1, 53-54. (d) Lai, S.-W.; Che, C.-M. Top. Curr. Chem. 2004, 241, 27-63. (11) (a) Fung, E. Y.; Olmstead, M. M.; Vickery, J. C.; Balch, A. L. Coord. Chem. ReV. 1998, 171, 151-159. (b) Vickery, J. C.; Olmstead, M. M.; Fung, E. Y.; Balch, A. L. Angew. Chem., Int. Ed. 1997, 36, 11791181. (12) Gussenhoven, E. M.; Fettinger, J. C.; Pham, D. M.; Malwitz, M. M.; Balch, A. L. J. Am. Chem. Soc. 2005, 127, 10838-10839. (13) Lee, Y.-A.; Eisenberg, R. J. Am. Chem. Soc. 2003, 125, 77787779. (14) (a) Rawashdeh-Omary, M. A.; Omary, M. A.; Patterson, H. H.; Fackler, J. P., Jr. J. Am. Chem. Soc. 2001, 123, 11237-11247. (b) Omary, M. A.; Patterson, H. H. J. Am. Chem. Soc. 1998, 120, 7696-7705. (c) Omary, M. A.; Hall, D. R.; Shankle, G. E.; Siemiarczuck, A.; Patterson, H. H. J. Phys. Chem. B 1999, 103, 3845-3853. (d) Rawashdeh-Omary, M. A.; Omary, M. A.; Shankle, G. E.; Patterson, H. H. J. Phys. Chem. B 2000, 104, 6143-6151. (e) Hettiarachchi, S. R.; Rawashdeh-Omary, M. A.; Kanan, S. M.; Omary, M. A.; Patterson, H. H.; Tripp, C. P. J. Phys. Chem. B 2002, 106, 10058-10064. (f) Hettiarachchi, S. R.; Patterson, H. H.; Omary, M. A. J. Phys. Chem. B 2003, 107, 14249-14254. (g) Omary, M. A.; Colis, J. C. F.; Larochelle, C. L.; Patterson, H. H. Inorg. Chem. 2007, 46, 3798-3800.

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Photophysics of Bis(thiocyanato)gold(I) Complexes: Intriguing

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