Vitamin B5 copper conjugated triazine dendrimer improved the visible-light photocatalytic activity of TiO2 nanoparticles for aerobic homocoupling reactions

In this work, Cu-vitamin B5 (pantothenic acid) bonded to 2,4,6-trichloro-1,3,5-triazine produced a bioconjugated dendrimer giving rise to the visible-light photocatalytic activity of nanocrystalline TiO2. XPS spectra uncovered the coexistence of Cu(II)/Cu(I) oxidation states with a predominant contribution of Cu(I). The new heterogeneous bio-relevant Cu-photocatalyst (Cu(I) Cu(II) [PTAPA G2-B5] @TiO2) revealed a band gap value [Eg = (2.8 eV)] less than those of Cu free components [PTAPA G1-B5]@TiO2 (3.04) and [PTAPA G2-B5]@TiO2 (3.06) and particularly the bare TiO2 (3.15 eV). The reactions showed to be light-dependent with the best performance under room light bulbs. The photocatalytic efficiency of the as-prepared heterojunction photocatalyst was exploited in the aerobic Csp2–Csp2 homocoupling of phenylboronic acid and Csp–Csp homocoupling of phenyl acetylenes under visible-light irradiation to prepare structurally and electronically different biaryls. A radical pathway relying on the photogenerated e− and h+ and involving the Cu(I)–Cu(II) synergistic cooperation was postulated. The reusability and stability of the catalyst were verified by the recycling test, FT-IR spectra, and ICP-OES analysis.

The semiconductor-mediated solar-and visible-light-driven heterogeneous photocatalysis has been known as a low-cost high impact technology for energy, conversion, environmental remediation, and organic synthesis 1-4 .Among the available semiconductors, TiO 2 as an inert and safe material has been widely used in many applications including catalysis, antibacterial agents, civil as nano-paint (self-cleaning), and especially photocatalysis [5][6][7][8] .However, the relatively wide bandgap of TiO 2 (3.2 eV) limits its photocatalytic activity to harmful UV light i.e. just 5% of the solar energy.Further, it suffers from a low quantum efficiency resulting from the rather fast recombination of electron-hole pairs.To overcome these limitations, various innovative strategies have been developed to improve the photocatalytic properties of TiO 2 .The heterojunctions with other semiconductors, dye sensitization and metal ion implantation, noble metal deposition, elemental doping, and inorganic acids modification are some of these strategies [9][10][11][12][13] .Modification of TiO 2 with ascorbic acid (vitamin C) 14 and dendrimers [15][16][17] , are among our ongoing research activities in this area.
Vitamins are of great interest due to their important biochemical functions and biomedical applications.They have been used vastly in different chemical and biochemical processes inspired by their catalytic or regulatory nature to facilitate or control vital chemical reactions in the human body 18 .B vitamins are important cofactors of enzymatic reactions and among the important vitamins of this group [19][20][21] , pantothenic acid (vitamin B5), as the obligate precursor of coenzyme A (CoA) and the acyl carrier protein 22,23 .
On the other hand, the highly branched three-dimensional structures with a high affinity to encapsulate the transition metals onto the termini of dendritic tethers or at the dendrimer core have created an impressive position for the design of dendritic catalysts 24,25 .The dendritic-modified TiO 2 photocatalysts display a high-power conversion efficiency and a high absorption coefficient in visible light irradiation 26 .
The FT-IR spectrum of AP-TiO 2 showed broad peaks at 3414 and 1644 cm −1 , which are attributed to hydroxyl groups and the surface adsorbed water.Also, the bands at 500-750 cm −1 correspond to the stretching vibration of the Ti-O groups 29 .A strong peak located at 1121-1146 cm −1 is attributed to the Si-O bond (Fig. S1a, ESI) 30 .The peaks at 1579-1617 cm −1 (C=N) in the FT-IR spectra of b, d-g (Fig. S1b, S2d-e, S3f, g, ESI) confirm the presence of triazine units on the TiO 2 31 .Two index peaks at 1638, and 1586 cm −1 correspond to carbonyl groups of acid and amid groups, respectively (Fig. S1c, ESI).The remarkable spectral changes in Fig. S1d affirm the formation of the TA-B 5 ligand.The emergence of a new band at 1722 cm −1 with some shifting indicates the formation of ester groups conjugated with cyanuric rings.The formation of Cu-N and Cu-O bonds resulting from the complexation of Cu with [PTAPA G2-B5] coated TiO 2 nanoparticles were evidenced by the bands that appeared at 509 and 682 cm -1 respectively, in the FT-IR spectra depicted in Fig. S1g 32,33 .
Elemental mapping images (a-g) and EDX analysis by SEM as well as TEM images (h, i) of (Cu(I) Cu(II) [PTAPA G2-B5] @TiO 2 ) nanohybrid are shown in Fig. 3 which confirmed the presence of Cu, Ti, Si, N, O and C in the nanosphere heterostructure.The TEM images of the title catalyst revealed well-separated spherical nanoparticles with sizes ranging from 7 to 10 nm.The precise copper content was found to be 1.27 mmol g −1 based on the inductively coupled plasma optical emission spectrometry (ICP-OES).
XPS analysis was carried out to explore the chemical compositions and oxidation states of the composed elements of the title catalyst.The high-resolution XPS spectra of C 1s, N 1s, O 1s, Si 2p, Cu 2p, and Ti 2p are depicted in Fig. 4. The four signals of C 1s located at 285.05, 285.8, 287.21, and 288.5 eV correspond to C-C, C-O/C-N, C=N/C=O, and O=C-O bonds, respectively 34 .The N 1s spectrum was fitted into three peaks centered at 397.58, 399.05, and 400.5 eV, assigned to the sp 2 hybridized nitrogen (C=N-C), C-N-C, and C-N-H/N-C=O, respectively 35 .The O 1s spectra revealed three peaks at 530.9, 532.3, and 533.3 eV, attributed to Ti-O, C-O/ Ti-OH, and O-Si bonds, respectively 36 .In the high-resolution spectra of Cu 2p, two small peaks were observed at 934.7 and 958.19 eV corresponding to 2p 3/2 and 2p 1/2 of Cu 2+ .Further, two intense signals located at 932.8 (2p 3/2 ) and 952.9 eV (2p 1/2 ) testify to the main contribution of Cu(I) oxidation state featuring that the significant reduction of Cu(II) to Cu(I) occurred during the complexation of Cu(II) with dendrimer 37 .Ti 2p 3/2 and Ti 2p 1/2 of TiO 2 also appeared at the binding energies of 454.66 and 462.17 eV at Ti 2p spectra 38 .Further, deconvoluted    The thermal stability of [PTAPA G1-B5] and nanodendritic catalyst were studied by thermogravimetric analysis.The thermal decomposition curve of the nanodendritic catalyst showed a sequence of two decomposition steps, given in Fig. S2.The first weight loss up to 136 °C attributed to the dehydration of the samples and the second one in the range of 400-423 °C corresponds to the decomposition of organic parts (Fig. S2, ESI).

Optical properties
The diffuse reflectance UV-Vis spectra (DRS) of [PTAPA G1-B5]@TiO 2 , [PTAPA G2-B5] @TiO 2, and the final Cu-containing nanodendritic catalyst (Cu(I) Cu(II) [PTAPA G2-B5] @TiO 2 ) were recorded to assess the visible light absorption ability and band gap value (Fig. 5).Comparing the DRS of the bare TiO 2 possesses an absorbance threshold at less than 400 nm with a band gap of 3.2 eV 40 , our results in this work clearly show the modification roles of the bioconjugated dendrimer as well as Cu complexation on the optical properties of TiO 2 .The absorbance threshold of TiO 2 shifted to 400 and 440 nm after functionalization with bioconjugated dendrimer and Cu(I) Cu(II) complexation with vitamin B5, respectively (Fig. 5 DRS a-c).As determined by tauc plots 41 , the band gaps reduced to 3.0 and 2.8 eV respectively (Fig. 5, tauc plots a-c).The more important result is that the incorporation of copper increased significantly the amount of absorption of the final nanocomposite (Fig. 5 DRS c) in the wide range of visible regions (500-800 nm).
The possibility of photoinduced electron transfer (PET) between TiO 2 and denderimer in the resulting hybrids was assessed by PL spectroscopy.The significant decrease in the PL intensity of TiO 2 after functionalization with PTAPA G1-B5, PTAPA G2-B5, and Cu(I) Cu(II) [PTAPA G2-B5] shows an efficient separation of the carriers resulting from charge transfer between TiO 2 and dendrimer parts (Fig. 6).The superiority of G2 over G1 can be assigned to the more extensive π-conjugated bonds making it more efficient for both visible light absorption and charge separation.Based on the DRS and PL results, modification of the photoelectronic properties of TiO 2 seems obvious, which is expected to improve its photocatalytic activity.

Catalytic activities
The catalytic activity of the as-prepared catalyst was assessed in the homocoupling of aryl boronic acids as well as terminal alkynes to biaryl compounds and 1,3-diynes, respectively.Initially, the reaction conditions such as solvent type and amount, catalyst loading, and temperature under the fluorescent lamp (room light lamps) were optimized using phenylboronic acid as a model substrate (Fig. S3, ESI).The homocoupling of phenylboronic When the catalytic potential of the title catalyst was examined in the homocoupling of phenylacetylenes new optimization conditions were needed.According to the results presented in Fig. S4, for efficient homocoupling of 0.125 mmol phenylacetylene, the reaction needs 5 mol% (0.006 mmol) catalyst and 0.1 ml DMF containing 0.125 mmol Et 3 N and should be heated up to 100 °C (Fig. S4i-vi).
With the optimal reaction conditions in hand, the generality of the method was evaluated using phenylboronic acid and phenylacetylene derivatives.According to the results presented in Table 1, biaryls were generally produced in good to high yields within 2 h, although molecules bearing electron-deficient groups (entries 5, 6), as well as heteroaryl derivatives (entry 7), exhibited less activity.The homocoupling of phenylacetylenes (entries 8, 9) took about 3 h and the pertinent 1,3-diynes produced in moderate to high yields (45-85%), nevertheless, alkyl counterparts were actually inactive under these conditions (entries 10, 11) even after 12 h.
To show the photocatalytic superiority of the as-prepared Cu(I) Cu(II) [PTAPA G2-B5]@TiO 2 composite some control experiments for coupling of phenylboronic acid (Table 1, entry 1) and phenylacetylene (Table 1, entry 8) in the presence of the relevant catalysts were performed (Table 2).Under optimized conditions, the bare TiO 2 was incompetent as a catalyst for homocoupling reaction, and Cu(OAc) 2 exhibited low to moderate activity to produce 35 and 45% yield of the pertinent biaryl and 1,3-diyne respectively (Table 2, entry 1).Replacement of G2 dendrimer with G1 in the catalyst reduced the homocoupling products to 45 and 40%, respectively (Table 2, entry 3).Given the same effect of the G1 and G2 dendrimers on the band gap of TiO 2 (Fig. 3a, b), the promoting effect of growing dendritic branches on the photocatalytic activity caused by more effective carriers' separation resulting from a more extensive system of conjugated π bonds as evidenced by PL spectra (Fig. 6).We also replaced the TiO 2 core in the Cu(I) Cu(II) [PTAPA G2-B5] @TiO 2 with MoO 3 and silica-coated γ-Fe 2 O 3 nanoparticles which resulted in a significant reduction in the homocoupling performance (25 and 15% for biphenyl and 1,4-diphenylbuta-1,3-diyne, respectively) (Table 2, entries 6, 8).Thus, the presence of TiO 2 core is inevitable for the photocatalytic activity of the as-prepared (Cu(I) Cu(II) [PTAPA G2-B5] @TiO 2 nanocomposite.
The light dependent catalytic activity of the Cu(I) Cu(II) [PTAPA G2-B5]@TiO 2 was also assessed under various light sources such as Reptile lamp, LT NARVA (18 W, full range visible light + 4% UV), Actinic BL TL-D Philips (15 W, λ = 366-400 nm), and blue LED, AC86, Z.F.R (12 W, λmax = 505 nm), UV light (λ = 200-290 nm, 15 W), room light lamps (Fluorescent lamp, λ = 400-650 nm, 40 W).The light contributions of homocoupling reaction of phenylboronic acid and phenylacetylene under the aforementioned lamps were depicted in Fig. 7a,  b.In both reactions, the room light lamp exhibited the greatest irradiation contribution to the overall conversion rate.Different light contributions under light sources with different emission wavelength ranges clearly show that the reactions are light dependent.

Mechanism study
Given the results of the photocatalytic assessment mentioned in the previous section and previous studies 42 , a photocatalytic process is definite for Cu(I) Cu(II) [PTAPA G2-B5] @ TiO 2 .Irradiating the catalyst excites an electron from the HOMO of dendrimer to its LUMO leaving holes (h+) in the HOMO, forming electron-hole pairs (Fig. 8).For the CB of TiO 2 matched the LUMO level of dendrimer well for the charge transfer, this excited state dendrimer species can be converted to a semi-oxidized radical cation (D •+ ) by the injection of an electron into the CB of TiO 2 .In line with this hypothesis, G2 with a more extensive system of conjugated π bonds than G1 is expected to absorb light more efficiently in the visible range and also facilitate electron transfer providing more effective charge separation.Those electrons of the TiO 2 conduction band were captured by the O 2 preadsorbed on the TiO 2 surface to form a superoxide anion radical (O •−2 ) capable of reducing Cu(II) to Cu(I).At the end of the reaction, on the other hand, the Cu(I) can be reoxidized to Cu(II) by photogenerated holes on the VB of TiO 2 to complete the photocatalytic cycle.As the redox reaction goes on, the number of electrons  injected and the holes produced in TiO 2 gradually increases, yielding more reactive radical species on the surface of TiO 2 , thus increasing the photocatalytic activity of the composite photocatalyst.To prove this claim, the homocoupling reaction of phenylacetylene (and phenylboronic acid) in the presence of benzoquinone as superoxide radical scavenger and ammonium oxalate (AO) and formic acid (FA) as hole scavengers as well as 2,6-Di-tert-butyl-4-methylphenol (BHT) and TEMPO as common radical scavengers under light irradiation were performed (Fig. S5).The conversion of phenylacetylene reduced to 34, 22, 15, 35 and 31% respectively (28, 17,10, 20, and 25 for phenylboronic acid respectively), much close to the results obtained in darkness attributed to the contribution of thermal effect.Based on the above results and previous reports, a radical mechanism relying on the photogenerated e− and h+ assisted by Cu(I)-Cu(II) synergistic cooperation is proposed 43 .As displayed in Fig. 8, the superoxide radicals produced by photogenerated electrons on CB of TiO , reduce Cu(II) to Cu(I) followed by attachment to terminal alkyne molecules to generate a coordination adduct intermediate (A).By this step, the inactive C−H bond could be activated (A) to be deprotonated by NEt  8).By inspection of the aforementioned mechanism steps, the catalytic role of Cu(I) seems more dominant than Cu(II) matching well with the high activity of the title catalyst possessing mainly Cu(I) evidenced by XPS analysis (Fig. 4).The Cu(I) coordinates with alkyne molecules to activate C-H bond followed by a rapid electron-transfer step from Cu(II) leading to the homocoupling product.The overall reaction is driven by involving Cu(I)-Cu(II) synergistic cooperation.(See proposed mechanism as Fig. S6 for homo coupling reaction of phenylboronic acid in SI) 44 .
Therefore, by coupling the TiO 2 semiconductor with a dendrimer, a greater photocatalytic performance under visible light irradiation can be achieved as the dendrimer increase the efficiency of sunlight utilization in the visible light range and lower the rate of electron-hole pair recombination in TiO 2 .

Recycling test
The promising results for the catalytic activity of Cu(I)Cu(II)[PTAPAG2-B5]@TiO 2 nanocomposite encouraged us to assess its reusability in sequential reactions.As can be seen in Fig. S7 the catalyst preserved its activity during five runs for homocoupling reactions of both phenylboronic acid and phenylacetylene under optimized reaction conditions.Further, the FT-IR spectra of the reused nanocatalyst depicted in Figs.S8 and S9 revealed that the catalyst maintained its structural integrity during the reaction.However, to confirm the bonding between the catalyst's components, a leaching experiment (hot filtration test) was operated for the homocoupling reaction of phenylboronic acid under optimized conditions (Table 1, entry 1).The catalyst was quickly removed after 15 min in which the conversion of substrate reached 40%, and the filtrate was allowed to stir at 50 °C for a further 45 min.No progress in the reaction was detected confirming that the catalyst's components are held tightly and the Cu(I)Cu(II)[PTAPAG2-B5]@TiO 2 did act as a heterogeneous catalyst (Figure S10, SI).The ICP-OES analysis of both the filtrate and the used catalyst also excluded any leaking of Cu providing further evidence for the heterogeneous nature of the title photocatalytic system.No Cu was detected in the filtrate and the precise Cu content of the used catalyst was 1.25 mmol g −1 corresponding to a 1.6% loss of Cu in comparison with the fresh catalyst (1.27 mmol g −1 ) that is ignorable in the range of analysis error.

Conclusion
In conclusion, combining 2,4,6-trichloro-1,3,5-triazine with vitamin B5 followed by complexation with copper rendered a new bioconjugated dendrimer to improve effectively the visible-light photocatalytic activity of TiO 2 nanoparticles.The coexistence of Cu(II)/Cu(I) oxidation states with a predominant contribution of Cu(I) was uncovered by XPS analysis.The DRS and PL spectra showed that the copper-containing dendrimer reduced the band gap value as well as increased the charge separation resulting in the promotion of the visible-light photocatalytic activity of TiO 2 .Moreover, the incorporation of copper into the bioconjugated dendrimer increased significantly the absorption amount of the final nanocomposite in the wide range of visible regions (500-800 nm).The heterogeneous aerobic homocoupling of phenylboronic acid and phenylacetylene was successfully driven by the as-prepared bio-nanocatalyst under the visible light and the catalyst retained its activity and structural integrity after several recyclings.The reactions showed light dependency and the room light lamp exhibited the greatest irradiation contribution to the overall conversion rate.The improved photoactivity of the as-prepared nanohybrid predominantly benefits from the synergistic effects of Cu(I) Cu(II) [PTAPA G2-B5] and TiO 2 nanoparticles relying on the extensive conjugated π bonds of dendrimer in a heterojunction structure.A radical mechanism based on the photogenerated e− and h+ and involving the Cu(I)-Cu(II) synergistic cooperation was proposed.The present system that employs an air-stable, very active, robust, and recyclable photocatalyst under a visible light source qualifies the significant conditions for implementation in the industry.This strategy will open up a new outlook for better use of semiconducting materials in photochemical applications and sequential organic transformations.
3 forming C−Cu(II) bond to generate intermediate B. The next step is the dimerization of B to generate intermediate C. Finally, C is involved in the electron transfer and C−C bond formation step to produce a homo-coupled product and regenerate the reduced Cu(I) species to drive a new cycle by cooperation of Cu(II) (Fig.

Figure 8 .
Figure 8. Proposed mechanism for homo coupling reaction of phenylacetylene.