Using Room Temperature Phosphorescence of Gold(I) Complexes for PAHs Sensing

The synthesis of two new phosphane-gold(I)–napthalimide complexes has been performed and characterized. The compounds present luminescent properties with denoted room temperature phosphorescence (RTP) induced by the proximity of the gold(I) heavy atom that favors intersystem crossing and triplet state population. The emissive properties of the compounds together with the planarity of their chromophore were used to investigate their potential as hosts in the molecular recognition of different polycyclic aromatic hydrocarbons (PAHs). Naphthalene, anthracene, phenanthrene, and pyrene were chosen to evaluate how the size and electronic properties can affect the host:guest interactions. Stronger affinity has been detected through emission titrations for the PAHs with extended aromaticity (anthracene and pyrene) and the results have been supported by DFT calculation studies.

NI can be easily functionalized allowing not only the possibility to modulate their emission properties on a large scale [1] but also the design of ligands able to coordinate metal atoms giving rise to NI-metal complexes, though these have been less explored than purely organic NI-derivatives. They are also well known to establish π−π intermolecular contacts giving rise to the formation of aggregates, affecting their resulting luminescence in connection with the aggregation induced emission (AIE) and aggregation caused quenching (ACQ) concept [1,6,10,11]. Although fluorescence emission is mainly observed in diluted NI-solution samples, phosphorescence may also be achieved in some cases, such as NIaggregates or when included within a macrocyclic host [12][13][14]. The heavy atom effect is another way to induce room temperature phosphorescence (RTP) and, to the best of our knowledge, there are very few examples in the literature in the coordination of NI to heavy metals to induce RTP emission and all are platinum derivatives [15][16][17].
NI-fluorescence has been used in the molecular recognition of anions, cations and organic molecules [27][28][29][30][31][32]. Changes on the RTP emission band for sensing processes has not been explored in great detail and is a new subject for gold(I)-NI complexes. Hence, the present investigation focuses on the design and synthesis of new 1,8-naphthalimide based gold(I) complexes that display RTP and explores their use in molecular recognition processes. In particular, we are interested in sensing polycyclic aromatic hydrocarbons (PAHs) since they are well-known environmental contaminants with toxic, mutagenic, and carcinogenic properties [33,34].. They can be found in different natural sources such as water and soil and are produced from fossil fuels, industrial manufacturing, residential heating, food preparation, etc. In particular, low molecular weight PAHs (2-3 rings) usually show a higher concentration in water than high molecular weight PAHs (4-6 rings) since they present a higher vapor pressure and solubility in water [34]. Their planar aromatic structure makes them ideal candidates to interact with the NI-core, inducing changes on the emission properties of the gold(I) complexes used as hosts in an easy and fast process. Most importantly, the high sensitivity of luminescence will allow the detection of very low quantities of PAHs.

Synthesis and Characterization
The synthesis of the two new gold(I) complexes containing a naphthalimide chromophore was performed after previous derivatization of the napthalimide group with a terminal acetylene moiety with slight modifications of the procedure reported in the literature [35]. It is based on a three steps reaction where the 4-bromo-1,8-naphthalic anhydride gives the 6-bromo-1H-benzo [de].isoquinoline-1,3(2H)-dione (P1) that will lead to the final L compound through a Sonogashira reaction (Scheme S1).
The absence of the terminal alkynyl C≡C-H proton of L in the IR and NMR spectra of the complexes confirms the ligand bonds through its alkynyl motif (see Figures S1-S5). A ca. 0.3 ppm upfield shift of naphthalimide signals and the appearance of the expected pattern of the PTA/DAPTA protons in the corresponding 1 H-NMR spectra confirm the successful synthesis of the complexes. The 31 P-NMR spectra indicate the presence of a main peak at ca. −49 and −27 ppm for 1 and 2 respectively in accordance with P-donor coordination to the metal center [36,37]. HRESI-MS spectrometry gives the final evidence of the correct formation of the compounds with the corresponding molecular [M + H]. + peaks at m/z 575.0922 and 647.1134 for 1 and 2, respectively. The presence of broad signals at 1 H-NMR and additional peaks at 31 P-NMR spectra suggest possible intermolecular aggregation assemblies, which was confirmed through different methodologies (see below). The absence of the terminal alkynyl C≡C-H proton of L in the IR and NMR spectra of the complexes confirms the ligand bonds through its alkynyl motif (see Figures S1-S5). A ca. 0.3 ppm upfield shift of naphthalimide signals and the appearance of the expected pattern of the PTA/DAPTA protons in the corresponding 1 H-NMR spectra confirm the successful synthesis of the complexes. The 31 P-NMR spectra indicate the presence of a main peak at ca. −49 and −27 ppm for 1 and 2 respectively in accordance with P-donor coordination to the metal center [36,37]. HRESI-MS spectrometry gives the final evidence of the correct formation of the compounds with the corresponding molecular [M + H]. + peaks at m/z 575.0922 and 647.1134 for 1 and 2, respectively. The presence of broad signals at 1 H-NMR and additional peaks at 31 P-NMR spectra suggest possible intermolecular aggregation assemblies, which was confirmed through different methodologies (see below).

Photophysical Characterization
Absorption, emission and excitation spectra of L and complexes 1 and 2 were recorded in DMSO at 2 × 10 −5 M and the results are summarized in Table 1. The absorption spectra of the compounds display a vibronically structured band with spacings of 1368 cm −1 corresponding to π−π* intraligand transition of the naphthalimide ligand [9,22,24,[38][39][40][41]. This band is ca. 50 nm red-shifted upon coordination to the metal atom, as commonly observed for other chromophores coordinated to the Au(I) unit (Figure 1) [36,37]. The emission of 1 and 2 display two bands, one at ca. 455 nm due to the intraligand fluorescence emission of the naphthalimide group and another at longer

Photophysical Characterization
Absorption, emission and excitation spectra of L and complexes 1 and 2 were recorded in DMSO at 2 × 10 −5 M and the results are summarized in Table 1. Table 1. Absorption and emission (λ exc = 418 nm) spectral data of L, 1 and 2 in DMSO.

Compound
Absorption λ max (nm) (  The absorption spectra of the compounds display a vibronically structured band with spacings of 1368 cm −1 corresponding to π−π* intraligand transition of the naphthalimide ligand [9,22,24,[38][39][40][41]. This band is ca. 50 nm red-shifted upon coordination to the metal atom, as commonly observed for other chromophores coordinated to the Au(I) unit ( Figure 1) [36,37]. The emission of 1 and 2 display two bands, one at ca. 455 nm due to the intraligand fluorescence emission of the naphthalimide group and another at longer wavelengths (ca. 620 nm) attributed to the phosphorescence of the chromophore [12,22,23,39,41]. The phosphorescence assignment is evidenced by the quenching of the intensity of this emission band in the presence of oxygen ( Figure S10) and can be achieved thanks to the presence of the gold(I) heavy atom that favors the intersystem crossing and population of the emissive triplet state [42]. The heavy atom effect on room temperature phosphorescence of gold(I) naphthalimide complexes is reported herein for the first time although it has previously been observed in platinum complexes, with the similar vibronically structured shape [39]. Previous luminescent studies with other gold(I)-napthalimide complexes contain N-substituted napthalimide chromophores and display pure fluorescence emission [22][23][24][25][26]. In our cases, the naphthalimide group does not present any substitution, being thus less bulky. Hence, the resulting higher planarity of the ligand in our systems may favor the approach between different molecules being the chromophore more affected by the presence of the heavy atom of the neighbor molecules, affecting the resulting phosphorescence emission. No significant emission of L was recorded in DMSO although it is emissive in CH 2 Cl 2 ( Figure S11).
tons of the molecules, the phosphane moieties, as usually happens in this type of gold(I) supramolecular assemblies. Thus this serves as direct evidence of the presence of these aggregation motif [43].
In fact, intermolecular contacts are expected to happen even at lower concentrations than those used for 1 H-NMR and have been verified to start at concentrations of ca. 2 × 10 −5 M and 5 × 10 −5 M (critical aggregation concentrations, c.a.c.) for 1 and 2, respectively, by recording emission spectra at increasing concentrations (Figures S13 and S14). The lower c.a.c. value of 1 is due to the well-known lower solubility of the PTA phosphane compared to DAPTA. Small well-organized aggregates have been also detected by optical microscopy with cross polarizers ( Figure S15).  The room temperature phosphorescence (RTP) of the gold(I) complexes may be observed thanks to the presence of the gold(I) atom in the proximity of the chromophore through intermolecular packing. This is in agreement with the corresponding 1 H-NMR of freshly prepared samples of 1 and 2 (Figures 2 and S12) which display very low intensity of the aromatic protons, with lower integration values with respect to the rest of the protons of the molecules, the phosphane moieties, as usually happens in this type of gold(I) supramolecular assemblies. Thus this serves as direct evidence of the presence of these aggregation motif [43]. The molecular electrostatic potential (MEP) of compounds 1 and 2 have been computed through DFT, to investigate the most electron rich and poor regions of the molecule. The MEP plots are shown in Figure 3, indicating that the most nucleophilic region corresponds to the O-atoms of the naphthalimide unit and the most electrophilic one to the H- In fact, intermolecular contacts are expected to happen even at lower concentrations than those used for 1 H-NMR and have been verified to start at concentrations of ca. 2 × 10 −5 M and 5 × 10 −5 M (critical aggregation concentrations, c.a.c.) for 1 and 2, respectively, by recording emission spectra at increasing concentrations (Figures S13 and S14). The lower c.a.c. value of 1 is due to the well-known lower solubility of the PTA phosphane compared to DAPTA. Small well-organized aggregates have been also detected by optical microscopy with cross polarizers ( Figure S15).
The molecular electrostatic potential (MEP) of compounds 1 and 2 have been computed through DFT, to investigate the most electron rich and poor regions of the molecule. The MEP plots are shown in Figure 3, indicating that the most nucleophilic region corresponds to the O-atoms of the naphthalimide unit and the most electrophilic one to the H-atoms of the phosphine ligands. The molecular electrostatic potential (MEP) of compounds 1 and 2 have been computed through DFT, to investigate the most electron rich and poor regions of the molecule. The MEP plots are shown in Figure 3, indicating that the most nucleophilic region corresponds to the O-atoms of the naphthalimide unit and the most electrophilic one to the Hatoms of the phosphine ligands.
The MEP is also negative above and below the triple bond and slightly positive at the Au atom in case of compound 2. The MEP values are small over the naphthalimide unit.  The MEP is also negative above and below the triple bond and slightly positive at the Au atom in case of compound 2. The MEP values are small over the naphthalimide unit.
The ability for self-assembly and aggregation of compounds 1 and 2 has been computed from the possible self-assembled dimers. The obtained geometries are given in Figure 4 and they adopt an antiparallel arrangement where the naphthalimide unit approaches the phosphane ligand in order to maximize the electrostatic attraction between the negative O-atoms of naphthalimide and the positive H-atoms of the Au-coordinated phosphine, in line with the MEP surface analysis. The dimerization energy is larger in compound 1, in agreement with the larger ability of 1 to form aggregates in DMSO. In the "on-top" representation of the assemblies, it can be observed that there is not π-overlap in compound 1 and, contrariwise, a small overlap between the π-systems of compound 2 is observed.
proaches the phosphane ligand in order to maximize the electrostatic attraction between the negative O-atoms of naphthalimide and the positive H-atoms of the Au-coordinated phosphine, in line with the MEP surface analysis. The dimerization energy is larger in compound 1, in agreement with the larger ability of 1 to form aggregates in DMSO. In the "on-top" representation of the assemblies, it can be observed that there is not π-overlap in compound 1 and, contrariwise, a small overlap between the π-systems of compound 2 is observed.

Molecular Recognition of Polycyclic Aromatic Hydrocarbons, PAHs
The emissive properties of the gold(I) compounds together with the planarity of the organic chromophore encouraged us to use them as hosts for the molecular recognition of PAHs, being suitable for detection at very low concentrations. With this goal in mind, naphthalene, anthracene, phenanthrene, and pyrene, which differ by the number and position of aromatic rings with an effect on the corresponding size and shape were chosen as target PAHs. Absorption and emission titrations were performed and the changes on the spectroscopic properties of the hosts, 1 and 2, were followed upon addition of different amounts of the corresponding guest. No significant changes were recorded in any case in the absorption spectra of the host, besides the expected presence of increasing amounts of guest molecules. More interesting data were retrieved from emission titrations. In general trends we can observe the formation of 1:2 host:guest adducts and stronger interactions between the host molecules and PAHs with extended aromaticity present: antracene ≥ pyrene > phenanthrene > naphthalene. Additionally, the resulting adducts with compound 2 are expected to interact more strongly in agreement with the major decrease in the phosphorescence emission intensity recorded ( Table 2, Figures 5 and S16-S23). It must be stressed that titrations show, in general, two different tendencies: (i) a (fast) first decrease of the phosphorescence emission (between 0 and 1 equivalent of guest added) and (ii) a recovery of some of the quenched emission reaching a plateau at 2 equivalents of the corresponding guest. Considering that the phosphorescence is expected to be enhanced by the proximity of the chromophore to the gold(I) atom, due to heavy atom effect, the observed variations let us suggest that the interaction with PAHs, especially with the anthracene and pyrene, promotes the disruption of homo-aggregates at low concentration where π···gold or gold···gold interactions are present. At higher concentrations, these interactions are restored to some extent with the formation of hetero-aggregates. The different behaviour in the process between host 1 and anthracene may be ascribed to the phosphorescence recovery at the end of the titration where heteroaggregates are formed from

Molecular Recognition of Polycyclic Aromatic Hydrocarbons, PAHs
The emissive properties of the gold(I) compounds together with the planarity of the organic chromophore encouraged us to use them as hosts for the molecular recognition of PAHs, being suitable for detection at very low concentrations. With this goal in mind, naphthalene, anthracene, phenanthrene, and pyrene, which differ by the number and position of aromatic rings with an effect on the corresponding size and shape were chosen as target PAHs. Absorption and emission titrations were performed and the changes on the spectroscopic properties of the hosts, 1 and 2, were followed upon addition of different amounts of the corresponding guest. No significant changes were recorded in any case in the absorption spectra of the host, besides the expected presence of increasing amounts of guest molecules. More interesting data were retrieved from emission titrations. In general trends we can observe the formation of 1:2 host:guest adducts and stronger interactions between the host molecules and PAHs with extended aromaticity present: antracene ≥ pyrene > phenanthrene > naphthalene. Additionally, the resulting adducts with compound 2 are expected to interact more strongly in agreement with the major decrease in the phosphorescence emission intensity recorded (Table 2, Figures 5 and S16-S23). It must be stressed that titrations show, in general, two different tendencies: (i) a (fast) first decrease of the phosphorescence emission (between 0 and 1 equivalent of guest added) and (ii) a recovery of some of the quenched emission reaching a plateau at 2 equivalents of the corresponding guest. Considering that the phosphorescence is expected to be enhanced by the proximity of the chromophore to the gold(I) atom, due to heavy atom effect, the observed variations let us suggest that the interaction with PAHs, especially with the anthracene and pyrene, promotes the disruption of homo-aggregates at low concentration where π···gold or gold···gold interactions are present. At higher concentrations, these interactions are restored to some extent with the formation of hetero-aggregates. The different behaviour in the process between host 1 and anthracene may be ascribed to the phosphorescence recovery at the end of the titration where heteroaggregates are formed from the interaction of the 1:2 adducts as exemplified in Scheme S2. In particular, it may be due to the type of the resulting intermolecular contacts. That is, heteroaggregates containing gold···π or gold···gold interactions in the vicinity of the chromophore are expected to induce the resulting RTP of the naphthalimide. Thus, either anthracene:1 heterodimers do not form larger heteroaggregates in the concentration range used or the resulting aggregates' geometry do not contain this type of interactions at this concentration range. the interaction of the 1:2 adducts as exemplified in Scheme S2. In particular, it may be due to the type of the resulting intermolecular contacts. That is, heteroaggregates containing gold···π or gold···gold interactions in the vicinity of the chromophore are expected to induce the resulting RTP of the naphthalimide. Thus, either anthracene:1 heterodimers do not form larger heteroaggregates in the concentration range used or the resulting aggregates' geometry do not contain this type of interactions at this concentration range.   (Figures S24-S31).
The calculated geometric and energetic features of 1:1 and 1:2 adducts between hosts 1 and 2 and naphthalene (N), anthracene (A), phenanthrene (Ph), and pyrene (P) are shown in Figures 6 and 7, respectively. In all cases the binding site is the naphthalimide that forms stacking interactions with the PAHs. The 1:1 complexes 1···Ph, 1···A, and 1···P exhibit similar interaction energies (−14.6, −15.0, and −15.8 kcal/mol, respectively), which are larger (in absolute value) than that of complex 1···N (−11.7 kcal/mol, respectively). A similar trend is observed in the 1:2 complexes, where Ph, A and P exhibit stronger binding than N. The 1:2 complexes are energetically more favorable than the homodimer 1···1 for Ph, A, and P supporting the disaggregation of the host molecules in the presence of these PAHs, as displayed through emission titrations. The opposite is observed for N, in agreement with the absence of interaction detected experimentally for this PAH.  (Figures S24-S31).
The calculated geometric and energetic features of 1:1 and 1:2 adducts between hosts 1 and 2 and naphthalene (N), anthracene (A), phenanthrene (Ph), and pyrene (P) are shown in Figures 6 and 7, respectively. In all cases the binding site is the naphthalimide that forms stacking interactions with the PAHs. The 1:1 complexes 1···Ph, 1···A, and 1···P exhibit similar interaction energies (−14.6, −15.0, and −15.8 kcal/mol, respectively), which are larger (in absolute value) than that of complex 1···N (−11.7 kcal/mol, respectively). A similar trend is observed in the 1:2 complexes, where Ph, A and P exhibit stronger binding than N. The 1:2 complexes are energetically more favorable than the homodimer 1···1 for Ph, A, and P supporting the disaggregation of the host molecules in the presence of these PAHs, as displayed through emission titrations. The opposite is observed for N, in agreement with the absence of interaction detected experimentally for this PAH.
The energies gathered in Figure 6 for the 1:1 and 1:2 complexes suggest that the binding of the first molecule of PAH has little influence on the binding of the second molecule of PAH, thus indicating a lack of cooperativity effects.
The optimized geometries of 1:1 and 1:2 complexes of compound 2 with the PAHs are displayed in Figure 7 and they are similar to those detailed above for compound 1. The energies gathered in Figure 6 for the 1:1 and 1:2 complexes suggest that the binding of the first molecule of PAH has little influence on the binding of the second molecule of PAH, thus indicating a lack of cooperativity effects.
The optimized geometries of 1:1 and 1:2 complexes of compound 2 with the PAHs are displayed in Figure 7 and they are similar to those detailed above for compound 1. The largest 1:1 binding energies are expected for complexes 2···P, 2···A, and 2···Ph with the calculated energies of −16.3, −15.0, and −14.6 kcal/mol, respectively. Again the naphthalene complex is the weakest (−11.9 kcal/mol). A similar trend is observed in the 1:2 complexes.  The reaction energies of the following hypothetical transformations (H···H) + 2 × PAH → 2 × (H···PAH), where H stands for hosts 1 and 2 and PAH = N, A, Ph and P, have also been computed. The corresponding reaction energies for such transformations, namely ∆E d1 (for 1) and ∆E d2 (for 2), are gathered in Table 2. That is, assuming that the dimer is the predominant specie in solution, the ∆E d values give hints about the feasibility to transform this strong homodimer (1···1 or 2···2) into two heterodimers upon the addition of one equivalent of PAH (Table 3). It can be observed that in the case of 1 the ∆E 1d values suggest that the three larger A, Ph and P are able to form the 1:1 hetero-dimers (exothermic process). In the case of 2, the ∆E 2d values are in all cases negative, thus suggesting that all PAHs should be able to form the host-guest complexes. However, the value is very small for N compared to the others, which is in line with the experimental findings that detect a weak binding for N. In addition, we have also calculated the formation of the heterotrimers using similar transformations: (H···H) + 4 × PAH → 2 × (PAH···H···PAH). The corresponding reaction energies for such transformations, where two equivalents of PAH are added, namely, ∆E t1 (for 1) and ∆E t2 (for 2), are also summarized in Table 2. All reactions are exothermic, thus suggesting than the addition of an excess of PAH should provoke the transformation of the homodimer into the heterotrimer (PAH···H···PAH). Inspection of Table 3 lets us to observe a direct trend between ∆E and the number of aromatic rings, with more negative ∆E values as the number of rings increases (2 for N, 3 for Ph and A with similar ∆E, and 4 for P) for both the heterodimers and heterotrimers.

Conclusions
The self-assembly of phosphane-gold(I)-naphthalimide complexes gives rise to strong aggregates in solution even at very low concentrations (~10 −5 M). Nevertheless, the compounds show a higher affinity to interact with long PAHs compounds, such as anthracene and pyrene. This interaction induces a disassembly of the gold(I) complexes to form very sTable 1:2 (host:guest) adducts. The smaller naphthalene PAH induces little effect on the spectroscopic properties of naphthalimide-gold(I) complexes, indicating that the established π-π interactions with naphthalimide are not strong enough to promote disassembly of these gold(I) complexes.
DFT calculations suggest intermolecular contacts in a head to tail disposition of the gold(I) complexes with Au···π or Au···N-H interactions. The proximity of the gold(I) heavy atom to the naphthalimide chromophore must be the responsible for the observed room temperature phosphorescence emission of the hosts.

Theoretical Methods
The calculations of the dimers and trimers were performed at the DFT level of theory using PB86 functional [44] the def2-TZVP basis set [45] and the D3 dispersion correction e4we [46] with the help of the Turbomole 7.0 program package [47]. Solvent effects were considered using COSMO (solvent DMSO) during the optimization [48]. The MEP surfaces were computed at the PB86-D3/def2-TZVP level of theory and visualized using the Gaussview [49] and using the cubes generated by the Multiwfn software [50] and the wavefunction obtained using Turbomole 7.0 (TURBOMOLE GmbH, Karlsruhe, Germany).

Absorption and Emission Titrations
Absorption and emission spectra were recorded using a quartz cuvette with a path length of 10 × 10 mm and a volume of 3 mL. All solutions were prepared with dry DMSO, diluted from a more concentrated stock solution, and used on the same day they were prepared.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.