Article pubs.acs.org/JPCA
Understanding the On−Off Switching Mechanism in Cationic Tetravalent Group-V-Based Fluoride Molecular Sensors Using Orbital Analysis Kosuke Usui,† Mikinori Ando,† Daisuke Yokogawa,*,†,‡ and Stephan Irle*,†,‡ †
Department of Chemistry, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Nagoya 464-8602, Japan
‡
S Supporting Information *
ABSTRACT: The precise control of on−off switching is essential to the design of ideal molecular sensors. To understand the switching mechanism theoretically, we selected as representative example a 9-anthryltriphenylstibonium cation, which was reported as a fluoride ion sensor. In this molecule, the first excited singlet state exhibits two minimum geometries, where one of them is emissive and the other one dark. The excited state at the geometry with bright emission is of π−π* character, whereas it is of π−σ* character at the “dark” geometry. Geometry changes in the excited state were identified by geometry optimization and partial potential energy surface (PES) mapping. We also studied Group V homologues of this molecule. A barrierless relaxation pathway after vertical excitation to the “dark” geometry was found for the Sb-containing compound on the excited-states PES, whereas barriers appear in the case of P and As. Molecular orbital analysis suggests that the σ* orbital of the antimony compound is stabilized along such relaxation and that the excited state changes its nature correspondingly. Our results indicate that the size of the central atom is crucial for the design of fluoride sensors with this ligand framework. Scheme 1. Quantum Yields, Φ, Were Measured Values in CHCl3
1. INTRODUCTION A wide variety of functional molecules have been synthesized in recent years, and promising molecular frameworks have been proposed on the basis of experiments.1,2 In particular, on−off switching molecules wherein emission colors can be controlled by surrounding media have attracted scientists not only in chemistry but also in biochemistry.3−5 To understand the on−off switching mechanism, measurements of its photophysical properties are frequently performed in experiments; however, experimental analyses of excited molecules are particularly limited because of the difficulty in observing short-lived species. That is why chemical intuition and empirical data are still employed to discuss the mechanism. Recently, theoretical approaches to treat the excited state have been developed, providing a significant contribution in the prediction of electronic and geometric structures. A comprehensive understanding of excited states by theoretical approaches is currently in the forefront of molecular sensor design. In 2012, Gabbaı̈ and coworkers reported a fluoride ion sensor, the 9-anthryltriphenylstibonium cation ([1]+ in Scheme 1), which has an anthryl group as a chromophore.6 Emission is switched on by the attachment of a fluoride anion (F−) in solution. The [1] + does not exhibit fluorescence, but fluorescence assigned to the anthracene π−π* transition is observed with the formation of the 1-F in the presence of F−. By utilizing this optical turn-on response and high affinity against F−, the researchers succeeded to detect the F− in water at the parts per million level. The remarkable point of this molecule to work © 2015 American Chemical Society
as a fluoride ion sensor is that [1]+ does not show emission despite it having an anthracene chromophore similar to 1-F. There are some insights into why emission from [1]+ is weak;6−8 however, these insights are very limited, and a thorough theoretical analysis has not been performed yet. Addressing this issue is important for the development of new, more efficient F− sensors. Our theoretical approach will be useful for investigating other systems with on−off switching mechanisms.
2. THEORETICAL METHODS In the present study, quantum-chemical approaches9 were employed for revealing the on−off switching mechanism in Received: October 5, 2015 Revised: November 19, 2015 Published: December 9, 2015 12693
DOI: 10.1021/acs.jpca.5b09709 J. Phys. Chem. A 2015, 119, 12693−12698
Article
The Journal of Physical Chemistry A
Figure 1. Optimized geometries in ground (GS) and first excited (ES) states. Angles are in degrees (deg) and distances are in angstroms (Å).30
[1]+. In particular, we attempted to reveal the geometry change in the first excited singlet state and to obtain knowledge for the molecular design based on theoretical considerations. Density functional theory (DFT)10,11 and time-dependent DFT (TDDFT)12−14 methods were employed in ground- and excited-state calculations, respectively. The CAM-B3LYP15 was chosen as a functional. Basis sets16−22 for geometry optimization were LANL2DZ(dp) for Sb and As, 6-31G for H, and 6-31G(d) for others, while single-point energy calculations were performed with CRENBL for Sb and As, 6-311G for H, and 6-311+G(d) for others. All calculations were performed in vacuo. Because spectral measurements in experiments were performed in CHCl3, we took into account the solvent effect of [1]+ and 1-F by applying the polarizable continuum model23−26 (Tables S2− S4). Even if the solvent effect is considered in this way, the oscillator strength of structure C is still much smaller than that of B and can be assigned to the dark state. In addition, simulated absorption and emission energies in CHCl3 shift by
