Spectrochimica Acta

Host-guest interaction studies of polycyclic aromatic hydrocarbons (PAHs) in alkoxy bridged binuclear rhenium (I) complexes

Veerasamy Sathish, Mani Murali Krishnan, Murugesan Velayudham, Pounraj Thanasekaran, Kuang-Lieh Lu, Seenivasan Rajagopal

Article Number:117160

Please cite this article as: V. Sathish, M.M. Krishnan, M. Velayudham, et al., Host- guest interaction studies of polycyclic aromatic hydrocarbons (PAHs) in alkoxy bridged binuclear rhenium (I) complexes, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117160

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Host-Guest Interaction Studies of Polycyclic Aromatic Hydrocarbons (PAHs) in Alkoxy Bridged Binuclear Rhenium (I) Complexes

Veerasamy Sathish,a Mani Murali Krishnana, Murugesan Velayudham,b Pounraj Thanasekaran,*c Kuang-Lieh Lu,*c and Seenivasan Rajagopal.*d

aDepartment of Chemistry, Bannari Amman Institute of Technology, Sathyamangalam – 638 401,
India

bDepartment of Chemistry, Thiagarajar College of Engineering, Madurai – 625 015, India
cInstitute of Chemistry, Academia Sinica, Taipei, 115 Taiwan
dSchool of Chemistry, Madurai Kamaraj University, Madurai – 625 021, India.

Abstract

The interaction of two neutral alkoxy bridged binuclear rhenium(I) complexes, 1 and 2

[{Re(CO)3(1,4-NVP)}2(μ2-OR)2] (1,R=C4H9;2,R=C10H21;1,4-NVP=4-(1-
naphthylvinyl)pyridine] with polycyclic aromatic hydrocarbons (PAH) is investigated. UV-vis absorption, emission, 1H NMR spectral titrations, TCSPC lifetime studies and DFT theoretical calculations were carried out to examine the binding responses of complexes 1 and 2 with various PAHs such as pyrene, naphthalene, anthracene and phenanthrene. The UV-Vis absorption spectra showed an increase in absorbance of the metal-to ligand charge-transfer (MLCT) and ligand centered (LC) bands upon addition of various PAH molecules to 1 and 2, whereas the emission behavior was found to show emission quenching, which might occur through energy transfer pathway. The binding constants (K) of complexes 1 and 2 for various PAHs are found to be in the order of 104 M with a 1:1 binding mode as determined from UV- vis absorption and emission spectral titration studies. 1H NMR spectral studies show that the chemical shifts of pyrene guest and the 1,4-NVP moiety of 2 are shifted up-field, whilst the alkoxy protons do not show any appreciable change in their chemical shifts. It is believed that the open cavities present in the Re(I) complexes may lead to the recognition of PAHs via CH···π interaction.

Introduction

There has been an upsurge of interest in the field of host–guest chemistry owing to a variety of applications in molecular recognition, [1-4] supramolecular catalysis, [5,6] luminescent or electrochemical sensors, [7-9] and biological systems[10,11], where the non- covalent interactions play a crucial role. Non-covalent interactions are ubiquitous in biological and chemical processes established by various intra- and inter-molecular interactions such as hydrogen bonding, π-π stacking, electrostatic interactions etc. [12-16]
Supramolecular host–guest chemistry based on inorganic and organometallic complexes has been flourished in the past two decades and is mainly developed by Fujita, [17-19] Stang, [20-22] Raymond, [23,24] Saalfrank, [25,26] and others [27-29] by using self-assembled or conventional strategy. Metal-containing molecular triangles, squares, and rectangles can act as hosts and their utilization as sensors will depend on the size of the host, the properties of the guests and the various interactions between them. In this connection, rhenium(I) metal complexes are excellent photophysical and photochemical behavior owing to their charge transfer states and their application as luminescence chemo and biosensors.[30-32] Re(I)- tricarbonyl diimine complexes have been used as corners of molecular rectangles where the spacers are alkoxy groups. [33,34] Earlier reports from our group and of Bala. Manimaran showed that the host-guest interaction of rhenium(I) complexes [7,35-39] with aromatic compounds is due to the combination of non-covalent CH- π interactions between soft acids (CH groups) and soft bases (π groups) and π–π interactions.
Polycyclic aromatic hydrocarbons (PAHs) possess condensed benzene structures. They are widespread organic environmental pollutants formed during the incomplete combustion of

carbon containing fuels, apart from their occurrence in natural crude oil, fossil fuels, and coal deposits. [40] PAHs are known to be hazardous because of their carcinogenic and mutagenic properties. [41-45] Due to their lipophilic nature, their solubility in water is very poor and hence they persist in the environment for a longer period. [46,47] Therefore, it is important to develop an effective receptor for the detection and quantification of such molecules in solution. Several reports have appeared on the manganese rectangles, [48] ruthenium with β diketonates, [49] ruthenium-based rectangles [50] and alkyl platinum terpyridyl complexes. [51] These metal complexes have been employed as excellent receptors for aromatic hydrocarbons, as evidenced by NMR spectroscopic and X-ray crystallographic studies. Bachrach reported DFT studies on the host−guest interactions of aromatic hydrocarbons in both gas as well as solution phases. [52]
Herein, we report that the synthesized binuclear rhenium(I) complexes ( [{Re(CO)3(1,4- NVP)}2(μ2-OR)2] (1, R = C4H9; 2, R = C10H21; 1,4-NVP = 4-(1-naphthylvinyl)pyridine) have clip type structures with open cavities that are capable of serving as hosts for aromatic hydrocarbons by non-covalent interactions. These complexes also possess photoswitching, aggregation induced emission properties, and hence, they are excellent probes for sensing the biomolecules, amyloid, insulin fibrils as well as the nitroaromatics. [53-57] This is the first report on use of clip type structured binuclear rhenium(I) compound as sensor for aromatic hydrocarbons. With the unique features of these binuclear rhenium(I) complexes, we further developed that these can be potentially used as sensors of powerful environmental contaminants such as polycyclic aromatic hydrocarbons, and their host–guest interactions were investigated by UV-visible absorption, emission, NMR, TCSPC techniques as well as DFT calculations .
Experimental section

Materials and Instrumentation

The binuclear rhenium(I) complexes 1 and 2 were synthesized by one pot self-assembly method from Re2(CO)10 and 4-(1-naphthylvinyl)pyridine in 1-butanol or 1-decanol and characterized by various spectral techniques. [53] [Chart 1] Pyrene, anthracene, naphthalene and phenanthrene were procured from commercial sources and used as received. The solvents used in this study were of spectroscopic grade. Electronic absorption spectra were recorded on Analytik Jena Specord S100 spectrophotometer using 1 cm pathlength cuvette. Emission spectra were obtained using JASCO FP6300 spectrofluorimeter. Excitation and emission slits with a band pass of 2.5 nm were used for all measurements. HPLC grade dichloromethane solvent was employed in all photophysical and photochemical measurements. NMR titration experiments were performed by Bruker 300 MHz NMR spectrometer and a deutrized CDCl3 solvent was used.

Polycyclic aromatic hydrocarbons (PAH)s

Chart 1. The structure of two alkoxy bridged binuclear Re(I) complexes 1 and 2 and PAH guests.

Photophysical measurements

The stock solution for complexes 1, 2 and the aromatic hydrocarbons (1 ×10–3 M) were prepared in dichloromethane solution and necessary concentrations to be made from the stock solution. Fluorescence quenching experiments of all the aromatic hydrocarbons were carried out under aerated conditions.
Excited state lifetime measurement

Fluorescence decays were recorded using time correlated single photon counting (TCSPC) method using the following set up. A diode pumped millena CW laser (Spectra Physics) 532 nm was used to pump Ti:Sapphire rod in Tsunami picosecond mode locked laser system (Spectra Physics). The 750 nm (8 MHz) line was taken from the Ti:Sapphire laser and passed through a pulse picker (Spectra Physics, 3980 2s) to generate 80 kHz pulses. The second harmonic output (375 nm) was generated by a flexible harmonic generator (Spectra Physics, GWU 23 ps). The vertically polarised 375 nm laser was used to excite the sample. The fluorescence emission at the magic angle (54.7º) was dispersed in a monochromator (f/3 aperture), counted by a MCP PMT (Hamamatsu R 3809) and processed through CFD, time-to- amplitude converter (TAC) and multi channel analyzer (MCA). The instrument response function for this system is ≈ 52 ps and the fluorescence decay was analyzed by using the software provided by IBH (DAS-6) and PTI global analysis software.
Computational details

All the density functional calculations were performed by using the Gaussian 16 computational package. [58] Geometry optimizations of host-guest complexes were carried out at Density Functional Theory calculations using b3lyp and lanl2dz basis set.

Results and discussion

The interesting structural features of compounds 1 and 2, including clip type structures with open cavities, provide the opportunity for utilization as molecular receptors for recognizing aromatic planar molecules. To investigate the binding characteristic of these receptors with aromatic compounds, electronic absorption and emission spectral titration studies were carried out in CH2Cl2 at room temperature. When a dichloromethane solution of rhenium complexes 1 and 2 are added to anthracene, which is used as a probe, the absorbance of anthracene (guest) is enhanced with an increase in the concentration of the Re(I) complexes (host), revealing a strong host-guest interaction between the Re(I) complex and anthracence. (Fig.1). A similar trend is observed when other hydrocarbons are added to the Re(I) complex 1 and 2. (Fig. S1-S3)

Fig. 1. Absorption spectral changes of anthracene (2 × 10-5 M) with an increase in the concentration of hosts 1 and 2 (0 to 2.2 × 10-5 M) in CH2Cl2.
It is presumed that the electron density of the Re(I) coordinated pyridyl group of 1,4-NVP is reduced as a result of metal coordination. Therefore, the electron rich anthracene

likely forms a charge transfer complex with the 1,4-NVP unit of 1 and 2, thus producing an adduct that is stabilized by donor-acceptor complexation, which is in good agreement with previous results. [33] The binding constants for the donor-acceptor complex formation between the binuclear rhenium(I) complexes and anthracene were evaluated using the Benesi -Hildebrand relationship (eq 1). [59,60]
1/ ΔA = 1/ Δε [G] + (1 + ΔεK[H][G]) (1)

Here ΔA is the change in the absorbance of the guest upon the addition of the host, Δε denotes the difference in the molar extinction coefficient between the bound and free guest molecule, and K is the binding constant, while [H] and [G] are the total concentrations of the host and guest molecules, respectively. A double-reciprocal plot of the change in the intensity of the absorption of the guest with a change in the concentration of the host yields a linear correlation, indicating 1:1 host-guest complex formation [61] (Fig. 2 and Fig. S4) and the binding constant (K) values were estimated to be in the range of 104 M-1 and are given in Table 1.

Fig. 2. Benesi- Hildebrand plot for the binding of anthracene with host 2 in CH2Cl2.

As compounds 1 and 2 are weakly emissive, we follow the emission quenching of guests upon the addition of rhenium complexes 1 and 2. On increasing the concentration of Re(I) complex the emission intensity of anthracene (Fig.3) and other aromatic hydrocarbons decreases without any considerable shift in the λmax values. (Fig. S5-S7)

Fig. 3. Emission spectral changes of anthracene (2 × 10-5 M) with an increase in the concentration of host 1 and 2 in CH2Cl2. (0 to 2.2 × 10-5 M).
We propose that the emission quenching is observed as a result of an intermolecular energy transfer from the emitting π-π* state of the guest to the low-lying CT excited state, which returns to the ground state via radiationless decay. These studies were corroborated with other reports. [62-64] Moreover, both the hosts and guests are neutral species, we propose that the CT is observed as a result of CH···π interaction. [65] The luminescence quenching was analyzed in terms of the Stern-Volmer equation. [66] (eq. 2)
I0/I = 1 + KSV[Q] = 1 + kqτ0[Q] (2)
I0 and I are the emission intensities in the absence and presence of quencher and [Q] is the quencher concentration and τ0 is the lifetime of fluorophore in the absence of quencher. The quenching rate constants can be obtained from the Stern-Volmer constant, KSV, and the fluorescence lifetime, τ. The quadratic relationship between I0 /I and [Q] predict from a plot of I0/I vs [Q] is non-linear, leading to upward curvature. This indicates that binding takes place along with efficient quenching. A nonlinear plot suggests the presence of a static component in the quenching process along with dynamic quenching. To explain the non-linearity of the curve, the modified Stern-Volmer equation (eq 3) is used. [67,68]
I0/I = (1 + KD[host])(1 + KS[host]) (3)

Here KD and KS are the dynamic and static Stern –Volmer constants, respectively. Both the static (KS) and dynamic (KD) quenching rate constants were found, and a good agreement between binding constants (K) (obtained from absorption measurement) and KS (obtained from emission measurement) was found. The values of KD and KS calculated from least-square fitting are given in Table 1. No significant difference in binding constants was observed even on the use of the short or long alkoxy chain in 1 and 2, respectively. Furthermore, the high value of the quenching rate constant, kq, 1012 M-1s-1 shows that the kq value is almost three orders greater than the diffusion controlled rate constant. It indicates efficient bimolecular quenching between the compounds and aromatic hydro carbons along with binding. [69] (Fig.4 & Fig. S8) The binding constant is obtained from the emission measurement and it provides the additional evidence for the binding between the host and guest. [39]

Fig. 4. Stern-Volmer plot for anthracene with host 2 in CH2Cl2.

Table. 1 Ground-State Binding Constants (K), Excited-State Dynamic(KD), Static( KS), Stern – Volmer Constants, and Quenching Rate Constants (kq) of compounds 1 and 2 with aromatic hydrocarbons at 298 K
1 2
Guest K KD KS kq K KD KS kq
×104 M-1 ×104 M-1 (M-1s- ×104 M- ×104 M-1 (M-1s-
1) 1 1)
×1012 ×1012
Pyrene 1.3 6.2 3.4 2.2 3.5 5.8 4.3 2.07
Naphthalene 1.4 2.8 1.8 - 2.3 3.1 1.9 -
Anthracene 2.1 8.3 2.3 - 2.3 6.2 2.4 -
Phenanthrene 1.8 2.4 3.1 - 2.5 2.7 2.5 -

TCSPC studies also showed that the excited-state lifetimes (τ) of the pyrene is efficiently quenched in the presence of rhenium(I) complexes in dichloromethane (Fig. 5). In the absence of rhenium complex, the excited state lifetime of pyrene is 28 ns but its lifetime is decreased to 14
ns after the addition of 0.2 mM complex 2. Two fold decrease in the lifetime is observed when Re(I) complex 2 (0.02 mM to 0.2 mM) is incrementally added to the solution of pyrene. Since the lifetime of other aromatic hydrocarbons is less than 2 ns, it is not possible to carry out lifetime measurements using the available instrument.

Fig. 5. Lifetime decay curve of pyrene (20μM) in the presence of complex 2 (0.02 mM to 0.2 mM)
The host-guest interaction of molecular clip 2 with pyrene was investigated by monitoring the chemical shift of clip 2 as a function of different concentration of pyrene guest in CDCl3 (Fig. 6).

Fig.6. Partial 1H NMR spectra of (300MHz, CDCl3) spectral changes of (A) 2 and the addition of pyrene 1:1 and 1:2 ratios. (B&C).

The 1H NMR spectral studies show that all the chemical shifts of protons of pyrene and the protons of the 1,4-NVP ligand are shifted up field. The protons are shifted to the tune of 0.15 ppm from 8.35 to 8.20 ppm. The 1,4-NVP ligand’s proton signals are affected, due to shielded π ring of the guest molecule. However, there is no change in chemical shift of alkoxy protons upon addition of the guests and similar shifts are observed in several other report [70] and also consistent with our previous report. [33] Hence these observations reveal the strong interaction between protons of the 1,4-NVP ligand and aromatic hydrocarbons.
To confirm the binding of host-guest interactions and structural features, we try to make crystals for host-guest complexes. But, unfortunately we could not make the good quality of crystals. Therefore, we try to solve this problem theoretical aspects using DFT (density

functional theory calculations. [58] The electronic structure of Re(I) complex investigated using b3lyp/lanl2dz basis set.
Optimized geometry of complex 2 with pyrene is shown in Fig.7a. The distance between two Re(I) metal center is 3.64Å and 3.46Å. The pyrene molecule lies between two naphthalene moieties and slightly tilting away. The guest pyrene interacts with complex 2 via CH···π interactions and the distance between C-H atoms and the π- bond is in the range of 2.9 to 3.5 Å. (Fig. 7b) Therefore, this evidence is buttressed for the interaction between the host and guest through the CH··· π interactions.
(a) (b)

Fig. 7. (a) Optimized geometry of 2 with pyrene (b) C-H···π interaction between (host) 2 and (guest) pyrene.

It has been observed that, there are two possible orientations (perpendicular and parallel) of aromatic hydrocarbons binding with the pyridine ring of host complex. The bond distances of CH···π interaction between Re-complex 2 and aromatic hydrocarbons in both parallel and

perpendicular orientations are given in table S1. The shortest bond length (2.906 Å) is observed between perpendicular orientation of phenanthrene C(85) with pyridine ring of complex 2 C(26)
– H(27) and longest bond length (3.530 Å) between parallel orientation of phenanthrene C(86) with pyridine ring of complex 2 C(50) – H(56). While comparing bond distance of parallel and perpendicular orientation, perpendicular orientations have shorter bond length. (Fig.S9) However, the bond distances of both orienations reveals that, the strong C-H···π interaction between host and guest.

Fig. 8. Optimized geometry of complex 2 with guest molecules (aromatic hydrocarbons) by using B3LYP/LANL2DZ basis set. 1-4: Guest molecules parallel to the pyridine ring, 5-8: Guest molecules perpendicular to pyridine ring.

Table .1 Complexation energy 2 with aromatic hydrocarbons in Kcal/mol.

Complex 2
with guest Energy of host molecule Energy of guest molecule Complex energy by
Parallel orientation Complex energy by
Perpendicular orientation Complexation energy
Parallel orientation Perpendicular orientation
2-
Pyrene -1512691.352 -386342.67 -1899035.715 -1899040.170 -1.695 -6.150
2-
Phenantharene -1512691.352 -338513.13 -1851204.925 -1851208.752 -0.441 -4.268
2-
Naphthalene -1512691.352 -242113.98 -1754807.401 -1754808.844 -2.072 -3.515
2-
Anthracene -1512691.352 -338507.86 -1851200.783 -1851203.481 -1.570 -4.268

The complexation energy of both perpendicular and parallel orientation of guest aromatic hydrocarbons with the host complex 2 are listed in the table 1. [71-74] In above table, it was observed that the negative complexation energy for host-guest interaction indicates that interactions are more favorable. [75] In parallel orientation, pyrene, anthracene and naphthalene interactions are more favorable than phenanthrene. In perpendicular, the complexation energy of pyrene interactions is more favorable than others. From the experimental and theoretical results, we concluded that the aromatic hydrocarbons are strongly binds through CH···π interactions with the rhenium(I) complexes efficiently.
Conclusion

The study of interaction of the rhenium(I) complexes 1 and 2 with various aromatic hydrocarbons investigated by, UV-visible absorption, emission and 1H NMR spectroscopic titration show strong interaction between the host 1 and 2 and aromatic hydrocarbons with high binding constants (K) and quenching rate constant (kq). It is proposed that these changes are attributed to the formation of ground state complex via charge transfer mechanism and stabilized by CH···π interactions. The structural features and the binding of guest molecules through the
CH···π interactions are illustrated by DFT calculations. Thus, these rhenium based molecular clips can be employed as a potential probe for the molecular recognition of polycyclic NVP-2 aromatic hydrocarbons. This could be opens up a new pathway to the future researchers in the area of host-guest chemistry of metallosupramolecules.

Acknowledgment

The authors acknowledge the Department of Science and Technology (DST)- National Science Council (NSC) for Joint research project funding under Indo-Taiwan S&T Programme. Prof. S.R thanks UGC-BSR Faculty Fellowship, and UGC- Emeritus Fellowship New Delhi. Dr.V.S is the recipient of SERB-IUSSTF Indo –US Postdoctoral fellowship. Prof. Lu acknowledges the financial support from Academia Sinica and the NSC of Taiwan. The authors thank Dr. M.M. Balakrishna Rajan, Dr. P. D. Pancharatna, Pondicherry University for theoretical calculations and Prof. P. Ramamurthy, National Centre for Ultrafast Processes, University of Madras, Chennai for lifetime measurements.
Appendix A. Supplementary data

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Graphical Abstract

Alkoxy bridged binuclear rhenium(I) complexes as a host for polycyclic aromatic hydrocarbons (PAHs) and the formation of Host-Guest complexes through CH···π interactions.

Highlights

Alkoxy bridged binuclear rhenium(I) complexes as a host for polycyclic aromatic hydrocarbons (PAHs)
Aromatic hydrocarbons quenches the luminescence of Re(I) complexes.
Host-Guest complex formation between aromatic hydrocarbons with Re(I) complexes through CH···π interactions