Invited feature article
Photo-induced electron transfer of carotenoids in mesoporous sieves (MCM-41) and surface modified MCM-41: The role of hydrogen bonds on the electron transfer

https://doi.org/10.1016/j.jphotochem.2017.03.013Get rights and content

Highlights

  • Carotenoids can form two types of hydrogen bonds with –SiOH on MCM-41 surface, H-bond 1 and H-bond 2.

  • H-bond 1 disfavors electron transfer (ET) from the carotenoid to MCM-41, and the opposite is true for H-bond 2.

  • The ET efficiency of a carotenoid physisorbed on the surface of MCM-41 is very low.

  • The conjugation length of a carotenoid has little effect on the H-bonding properties.

Abstract

Two types of hydrogen bonds (H-bond) can be formed when a carotenoid containing a hydroxy group, such as retinol, interacts with a silanol group on the surface of MCM-41. H-bond 1, the oxygen atom of the carotenoid interacts with the proton of the single bondSiOH group on the surface of MCM-41, and H-bond 2, the proton of the single bondOH group of the carotenoid interacts with the oxygen atom of single bondSiOH group on the surface of MCM-41. DFT calculations show that the formation of the H-bond 1 decreases the LUMO of the carotenoid and stabilizes the neutral species more than the radical cation, and thus disfavors the photo-induced electron transfer (ET) from the carotenoid to MCM-41. The opposite is true for the formation of H-bond 2. This conclusion is confirmed by the EPR study of the photo-induced ET of retinol imbedded in MCM-41 and the surface modified MCM-41. Although the formation of H-bond 1 results in low ET efficiency, the efficiency is much higher than that of the same carotenoid physisorbed on the surface of MCM-41. These results are relevant for improving the low solar-light-to-energy conversion efficiency in dye-sensitized solar cells.

Introduction

Dye-sensitized solar cells (DSCs) are attractive as an alternate class of solar cells, and extensive studies have been carried out in recent years [1]. The use of DSCs have gained attention because they have some advantages over other solar cells. These are low cost, flexibility and lightweight, potential for indoor application, and color (with choice of different colors of dyes) availability [2]. However, DSCs have not been widely used owing to their low solar-light-to-energy conversion efficiency and long-term stability.

The performance of DSCs can be improved by increasing the incident photon-to-current conversion efficiency (IPCE). IPCE is determined by four factors: (1) the light-harvesting efficiency, (2) the electron injection efficiency (Φinj) from the dye to the semiconductor, (3) the efficiency of the regeneration of the oxidized dye by the redox system, and (4) the charge collection efficiency from the device to the external circuit [1]. Of all these processes, the mechanism for the electron injection is the least understood. Three important factors are found to control Φinj: the free energy change (−ΔGinj) for electron injection, the surface geometry and adsorption characteristics of the adsorbed dyes on the semiconductor surface, and the presence of fast charge recombination (electron back transfer) [2]. −ΔGinj can be evaluated from the energy difference between the lowest unoccupied molecular orbital (LUMO) of the dye and the conduction band edge of the semiconductor. A positive value of −ΔGinj is necessary, but not sufficient, to achieve high Φinj. Surface treatment of the semiconductor and a more detailed understanding of fast charge recombination are also significantly important to realize high-performance devices.

The importance of investigating the type of anchoring between the sensitizer molecules and the surface of the semiconductor is emphasized by the fact that the bonding mechanism and the electron coupling between semiconductor and dye directly impacts electron transfer and the performance of the dye-sensitized photoanode [3]. A complete understanding of the interaction between the dye and the semiconductor is essential for the design of a DSC. When dyes adsorb on the surface of a semiconductor, chemical bonds, such as coordination bonds and hydrogen bonds, can form. It is important to know how the bond formation affects Φinj. A previous study[4] has shown that the formation of H-bonds between the carotenoid canthaxanthin (CAN) (see Scheme 1) and the silanol groups on the surface of MCM-41 decreases the HOMO and LUMO energies of CAN, and stabilizes the neutral species more than its radical cation. This results in lower electron transfer (ET) efficiency for CAN versus that for β-carotene.

Carotenoids as natural dyes have been used in the construction of DSCs [5], [6], [7]. The use of natural dyes has become of interest recently because of their ease of preparation, low cost, biodegradability, availability, purity, environmental friendliness, and most importantly, significant reduction of noble metal and chemical synthesis cost [8], [9], [10]. Kispert [7] and co-workers fabricated a DSC, for the first time, using β-apo-8′-carotenoic acid as a sensitizer and hydroquinone as a reductant. It exhibited a reasonably high IPCE of 34% at the absorption maximum at 426 nm, the open-circuit voltage (Voc) of 0.15 V, and no bleaching of the dye even after 12 h. In order to further understand the electron transfer (ET) process of carotenoids, Kispert’s group has studied the ET reactions of carotenoids imbedded in mesoporous molecular sieves MCM-41 and metal ion substituted MCM-41 (Ni-MCM-41 [11], Fe-MCM-41 [12], Ti-MCM-41 [13], and Cu-MCM-41 [14]). Understanding the mechanisms of these ET reactions are useful in the design of DSCs. MCM-41 is a mesoporous silica containing a regular array of uniform cylindrical pores. The pore size ranges from 15 to 100 Å depending on the chain length of the template used in the synthesis [15]. Previous studies [11], [12], [13], [14], [16], [17] have shown that such material provides a microenvironment appropriate for retarding back ET and thus increase the lifetime of photo-produced radical ions.

When all trans carotenoids are adsorbed on the MCM-41 surfaces, electron transfer occurs from the antioxidant carotenoids to the silanol acceptor sites of MCM-41. This transfer forms carotenoid radical cations that are stable for time long enough to be detected by EPR techniques. If cis ground state carotenoids are used, then electron and proton transfer occurs before adsorption [18] and then the radicals produced are adsorbed on the surface. The radical concentration of all trans carotenoids has been shown to be dependent on the metal present. In Cu-MCM-41, distance (3 Å) dependent reversible electron transfer [14] occurred while in Ti-MCM-41, this phenomena did not occur but rather isomerization of the carotenoid [13]. In Fe MCM-41, Fe aggregates formed [12]. In Ti-MCM-41 [19], the all trans carotenoid radical concentration increased upon light irradiation. This was determined to be due to the presence of surface oxygen [11] despite extensive degassing and the presence of metal V-center formed upon light irradiation that generated proton acceptor sites. Proton loss from the carotenoid radical cations caused an increase in the total radical concentration.

Published studies [11], [12], [13], [14], [18] showed the importance of proton transfer in the presence or absence of light and led to a publication examining the hydrogen bond formation between the carotenoid canthaxanthin and the silanol group on MCM-41 [4]. The calculated H-bonding properties fit well with those obtained by experiments.

In the current study, DFT calculations are performed to examine the H-bonds between carotenoids containing hydroxyl (single bondOH) groups, such as retinol (ROL) (see Scheme 1), and the single bondSiOH groups on the surface of MCM-41. Two types of H-bond can be formed between ROL and the single bondSiOH group: H-bond 1, the oxygen atom of ROL interacts with the proton of the single bondSiOH group on the surface of MCM-41, and H-bond 2, the proton of the single bondOH group of ROL interacts with the oxygen atom of single bondSiOH group on the surface of MCM-41. In order to examine this difference, the surface of MCM-41 was modified by esterification of the single bondSiOH group on the surface of MCM-41. The single bondOH group was replaced by single bondOCH2CH3 so that only H-bond 2 can be formed for ROL imbedded in the surface modified MCM-41 (R-MCM-41). The efficiency of the photo-induced electron transfer of ROL imbedded in R-MCM-41 was compared to that in MCM-41 by measuring the EPR intensities of the radicals of ROL produced in the two hosts at 77 K in the presence and absence of light.

Also another question arises because the formation of H-bond 1 decreases the energy of the LUMO of a carotenoid and stabilizes the neutral species more than the radical cation [4]. Is the efficiency of the photo-induced electron transfer of the carotenoid with the formation of H-bond 1 higher or lower than that of the carotenoid physisorbed on the surface of MCM-41? To answer this question, The efficiency of the photo-induced electron transfer of retinal (RAL) (see Scheme 1) imbedded in R-MCM-41 was compared to that in MCM-41 by measuring the EPR intensities of the radicals of RAL produced in the two hosts at 77 K. RAL can form H-bond 1 with the single bondSiOH group on the surface of MCM-41, and no H-bond can be formed on the surface of R-MCM-41.

Section snippets

Synthesis and characterization of MCM-41

The procedure used for the preparation of the siliceous material (MCM-41) has been previously described [14]. X-ray powder diffraction (XRD) data of MCM-41 were obtained from thin layers of samples, and measurements were carried out with a Philips 1840 diffractometer using Cu K radiation (λ = 1.541 Å) within the scattering angle 2θ range of 0.5–10°.

Surface modification of MCM-41

Esterification of the silanol groups on MCM-41 surface was carried out according to the method described by Kimura et al. [20] Briefly, 0.5 g of MCM-41

Results and discussion

Previously in a published study [4], a model for the single bondSiOH group on a siliceous surface was built and the formation of the H-bond between single bondSiOH and acetone and that between single bondSiOH and CAN were examined by the B3LYP method with a posteriori counterpoise correction (CP-SP) [4]. In order to perform efficient, yet sufficiently accurate calculations, a combination of basis sets was implemented. The mixed basis set includes 6-311++G(d,p) for the oxygen and hydrogen atoms of the silanol group and the

Conclusions

Two types of H-bond can be formed when a carotenoid containing an single bondOH group interacts with a single bondSiOH group on MCM-41 surface: H-bond 1, the oxygen atom of the carotenoid interacts with the proton of the single bondSiOH group on the surface of MCM-41, and H-bond 2, the proton of the single bondOH group of the carotenoid interacts with the oxygen atom of −SiOH group on the surface of MCM-41. The formation of H-bond 1 decreases the LUMO of the carotenoid and stabilizes the neutral species more than the radical cation, and

Acknowledgments

Prof. Michael K. Bowman at the chemistry department of The University of Alabama is thanked for useful discussion. This work was financially supported by the Faculty Research Grants Program at the College of Sciences of Nanjing Agricultural University.

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