Effect of solvent polarity on the spectroscopic properties of an alkynyl gold(I) gelator. The particular case of water

The spectroscopic properties of aggregates obtained from the hydrogelator [Au(4pyridylethynyl)(PTA)] were studied in solvents of different polarity. Inspection of the absorption and emission spectra of diluted solutions showed that the singlet ground state of the monomeric species is sensitive to polarity and is stabilized in more polar solvents whereas the triplet excited state is rather insensitive to changes in polarity. The study of relatively concentrated solutions revealed the presence of new emission and excitation bands at 77 K that were attributed to the presence of different kinds of aggregates. Particular interesting behaviour was observed in water where aggregation is observed to be more efficient. For this, absorption, emission quantum yields and luminescence lifetimes of water solutions at different concentrations were investigated in more detail. These data permitted to correlate the increase of non-radiative and radiative rate constants of the low lying triplet emissive state with concentration, and therefore with the low limit concentration for aggregation, due to the shortening of the Au···Au average distances in the aggregates and consequent enhancement of the spin-orbit coupling in the system.


Introduction
Luminescent materials based on transition-metal complexes receive increasing attention due to their wide application in several fields e.g. detection, sensing, biological labeling and displays. [1][2][3][4][5][6] In concrete, organometallic complexes represent a fascinating class of coordination compounds, which exhibit a particularly rich chemistry due to the formation of inter-and intramolecular metal-metal bonds (so-called metallophilic interactions). These interactions, together with metal-ligand coordination and additional supramolecular forces such as H-bonding, π-π interactions, and van der Waals forces among others, can originate a rich variety of metallogelators and coordination polymers. 7 Gold(I) complexes exhibit interesting emissive properties that usually are modulated by the presence of such aurophilic (Au···Au) interactions. 4,[8][9][10] They are a consequence of the strong relativistic effects displayed by gold atoms and their energy can range from 20-50 kJ/mol 11,12 which is comparable to that of strong hydrogen bonds. 13 Because of its strength, aurophilicity can play a key role in molecular aggregation in both the solid state and in solution. 8,14 Although aurophilic intermolecular bonding is well documented in the solid state, the number of publications reporting on the detection of this kind of bonding in solution is still low. Due to the influence of aurophilicity in the emission properties of gold(I) complexes, luminescence become an attractive tool to study the occurrence of intermolecular aurophilic interactions in solution. Well-known is the work of Patterson and co-workers with dicyano Au(I) complex which showed a progressive red-shift 15 in the excitation band energies when increasing concentration in aqueous solutions. More recently, Li and co-workers 16 reported on a trinuclear Au(I) pyrazolate cluster exhibiting aurophilic excimeric phosphorescence as a function of concentration. In other example, Balch et al. reported on a significant variation in the emission energy of an Au(I) diaminocarbene complex in frozen solutions of several solvents which was attributed to the occurrence of aggregation. 17 Alkynyl gold(I) complexes are an interesting family of gold(I) compounds due to their wide range of applications. The linearity imposed by the alkynyl group together with the preferential linear coordination of Au(I) and the occurrence of aurophilic contacts, make these compounds attractive building blocks for the design of emissive materials, NLO molecular materials, electronic conductors, bioimaging sensors, among others. 1,2,4,6 Moreover, they can also present interesting antiproliferative properties. 4,18 Luminescence is one of the most studied properties of these compounds and usually presents a triplet state parentage, because of the heavy-atom effect of the Au(I) center that increases the possibility to observe phosphorescence at room temperature. As a consequence, the radiative and non-radiative rate constants coupling the excited triplet and singlet ground states are enhanced. 19,20 In the past, several works reported on the luminescence originated in solution by metallophilic (Au···Au, Au···Ag or Au···Cu) contacts in heterometallic-alkynyl clusters, [21][22][23][24][25] showing that luminescence can be a powerful tool to detect intermolecular aurophilic contacts in solution.
Recently, we reported on the auto-association properties of the alkynyl gold(I) derivative [Au(4-pyridylethynyl)(PTA)] in water. 26 This compound is able to aggregate in this solvent up to the formation of very long fibers that originate, as a last resort, a gel structure. The driving force is mainly based on the establishment of intermolecular aurophilic interactions between gold(I) atoms, as supported by relativistic density functional theory computations (Scheme 1). 27 Related to this, it was recently found for a series of dinuclear alkynyl gold(I) derivatives ([(diphos)(AuC≡Cpy) 2 ]) (with diphosphine units containing different rigidity and chain length) a direct correlation between the Au(I)···Au(I) distance of the solids (X-ray crystal structure) and the emission quantum yields and decay times. The shortest distances gave rise to the highest radiative constants and emission quantum yields and this was attributed to the increase spin−orbit-coupling and the radiative rate constant of the lowest triplet state. Moreover, theoretical calculations predicted for these compounds a CT σ* Au···Au -π* transition which is related to the presence of aurophilic contacts. 28 In this work, the study of the aggregation of the hydrogelator [Au(4pyridylethynyl)(PTA)] is presented by means of the detection of the emission of potential aggregates in solvents of different polarity. Moreover, due to the particular aggregation effect observed in water, the effect of the concentration on the radiative and non-radiative deactivation channels of the excited triplet state and their connection with aggregation is also analyzed in more detail. These data will be of relevance in the analysis and understanding of other emissive supramolecular aggregates containing similar chromophoric units. Scheme 1. Self-association of [Au(4-pyridylethynyl)(PTA)] in water. 26,27 Page 7 of 32 Photochemical & Photobiological Sciences

Results and Discussion
Solvent dependence of the spectroscopic properties of compound 1.
The absorption spectra of 1x10 -5 M solutions of [Au(4-pyridylethynyl)(PTA)] in several solvents of different polarity were recorded and the results are shown in Figure 1 and summarized in Table 1. It can be seen that the more polar solvents induce a progressive blue shift of the absorption band. This is compatible with the existence of some charge transfer character in the intraligand ππ* transition, previously assigned to the ethynylpyridine unit. 26,29 This transition shows a vibronically resolved structure with a space between maxima around 1800 cm -1 attributed to υ(C≡C) stretching frequencies in the excited state (Table   1). 29 In diluted solutions it is not expected significant aggregation and the recorded spectra correspond basically to the monomer. Nevertheless, it can be observed in Table   1 that the molar extinction coefficient in water is significantly lower than in the rest of solvents, which can be indicative that some aggregation already occurs in diluted solutions in this solvent. This is in agreement with the observed broadening and lower definition of the vibronic absorption band. gives rise to the population of a less energetic triplet excited state from the more energetic singlet excited state. The emission is assigned to an intraligand 3 [ππ*(alkynyl)] origin ( Figure 2A) and no fluorescence from the singlet excited state is observed. 26,28 The bands present a vibronic structure in all the solvents and are indicative of the ligand (C≡Cpy) orbitals participation in the emission process. In this case there is not a clear dependence of the emission maximum with the solvent nature.
Interestingly, a linear dependence appears when ∆E (which is defined as the energy difference between the wavelength corresponding to the absorption maximum (E π→π* ) and the wavelength corresponding to the emission maximum (E (π*) 3 →π )) is represented as a function of the solvent permitivity constant ( Figure 2B). The data reveal the increase of ∆E with the permitivity constant, which is related to the solvent polarity.
These results indicate that the triplet state is rather insensitive to the polarity of the solvent whereas the ground state has a polar character and is stabilized with solvent polarity. Relatively concentrated solutions (ca. 1 x10 -4 M) were also studied, in order to investigate the effect of potential intermolecular contacts in the spectroscopic properties of the compound in these solvents, due to aggregation. As a general trend, a decrease on the molar extinction coefficient at the maximum of the ethynylpyridine chromophore was observed (compare values in Table 1 and 2), and also a slight decrease in the ratio between the lowest energy and highest energy vibronic peaks (due to broadening As commented above, calculations based on similar alkynyl dinuclear gold(I) complexes predicted the existence of a charge transfer band above 300 nm assigned to a σ* Au···Au -π* transition and related to the presence of aurophilic interactions. 28 In the present case, the solutions did not present significant absorption above 300 nm in any solvent, and only presented residual emission when excited above this wavelength at room temperature. Interesting data were recorded when the samples were excited at 340 nm as frozen solutions at 77 K ( Figure 3A), using front-face detection. As expected, the non-radiative channels due to motions of the molecules or interactions solvent-solute are restricted at this temperature and the diffusion of oxygen is much slower, allowing the detection of triplet states quenched at room temperature. All the samples presented several emission bands above 400 nm: i) a higher energy and narrower band in the range 408-414 nm, comparable to that we previously assigned to an intraligand 3 [ππ*(alkynyl)] origin; ii) one or two broader bands at lower energies in the range 450-555 nm. The corresponding excitation spectra at 77 K show several absorption bands above 300 nm not detectable in the UV-vis absorption at room temperature (see example in Figure 3B), which are in agreement with the calculated σ* Au···Au -π* transition for analogous compounds 28   The case of water is rather opposite to that observed with CH 3 CN where the 475 nm band is detected only in aged samples ( Figure S3). This observation seems to indicate that there is an equilibrium between species emitting at different wavelengths (different aggregates). These transitions, only observable at very low temperatures, should be originated on triplet states (see Table 2 and Figures S7-S10 for emission and excitation spectra corresponding to CH 3 15 In our case, the most important red-shifted contribution to the emission is found in water and, for this, we assume that the occurrence of the strongest or more quantitative aurophilic interactions, and therefore the strongest aggregation, is found in water. These results are not particularly unexpected, since aurophilic contacts result from van der Waals type forces 8,11 and therefore will contribute to the hydrophobicity of the molecules in strongly polar solvents like water, leading to extensive aggregation. The absorption spectra of the solutions are shown in Figure 6A. The broadening of the main band of the chromophore due to aggregation is clear when the ratio between the two maxima at 277 and 264 nm is plotted as a function of concentration ( Figure 6B).
This broadening is associated to the exciton coupling or coupling of transition moments of the ethynylpyridine chromophores when they approach to each other. The decrease in the ratio tends to stabilize for the higher concentrations and at ca. 3x10 -5 M practically reaches a plateau. This means that at this concentration all the chromophores are "seeing" the same environment, i.e. additional aggregation is not changing the average environment detected by each chromophore. Emission quantum yields at room temperature were also measured for the same solutions and the results are plotted in Figure 7A and summarized in Table 3. It can be seen that in this concentration range the emission quantum yield does not change significantly with the concentration.  Figure 7B and summarized in Table 3.  Figure 6B) and τ becomes shorter at increasing concentrations. Thus, the decay time decreases with Au···Au average distance ( Figure   7B), as previously observed for solids where aurophilic contacts (Au···Au distances below 3.5 Å) are present. 28 The emission quantum yields and decay times relate to the radiative and nonradiative rate constants for the deactivation of the triplet state through Eq. (1), in which the first term of the right side is the intersystem-crossing efficiency and the second term is the triplet emission efficiency (k r and k nr stands for radiative and non-radiative rate constants of the triplet, k' isc stands for inter-system crossing rate constant and k' r and k' nr stands for the singlet state deactivation constants).

A B A B
As stated above, the intersystem crossing for the monomer emission at room temperature must be very efficient, since only triplet emission can be observed.
From equations 2 and 3, it is straightforward to calculate both radiative and nonradiative rate constants for the triplet deactivation, and the calculated values are also summarized in Table 1 and plotted in Figure 9. Inspection of Figure 9 (and Table 3) shows that both radiative and non-radiative pathways for triplet decay increase with concentration. This trend is expected due to the decrease in the average distances between the heavy gold atoms during aggregation which will increase the intersystem crossing between the triplet excited state and the singlet ground state. As a consequence of the mixing of the gold orbitals, the T1→S0 transition becomes more allowed, as previously observed for solids where aurophilic contacts are present. 28 In this case, because of the non-radiative channel increases significantly there is no overall variation in the quantum yield.

Conclusions
The In the particular case of water, aggregation produces a noticeable increase in the intersystem-crossing to the singlet ground state and increases both radiative and nonradiative rate constants. This is directly related to the approaching of gold atoms and establishment of aurophilic interactions. These interactions seem to be already established at 3x10 -5 M where the increase in the radiative and non-radiative constants reaches a plateau.
Solutions were prepared with Millipore water and spectroscopic grade solvents.
Absorption spectra were acquired on a Varian Cary 100 Bio spectrophotometer.
Emission and excitation spectra were recorded on a Horiba−Jobin−Yvon SPEX     is also shown.