Al³⁺ regulated competition between TICT and ESIPT of a chemosensor

Highlights

• Competition between TICT and ESIPT is observed which coordinates the photophysics of the sensor.

• Interaction between the sensor and Al³⁺ is comprehensively studied.

• Al³⁺ is capable to control the competition between TICT and ESIPT.

Abstract

Twisted intramolecular charge transfer (TICT) and excited state intramolecular proton transfer (ESIPT) are of fundamental importance during the excited state dynamics of dyes and fluorescent sensors. Co-existence of the two processes usually complicates the whole photophysical process of dyes and leave the fluorescence detection mechanism of sensors uncovered. In this contribution, competition between the two processes are observed during the excited state relaxation of a fluorescence sensor 1-[(2-Amino-phenylimino)-methyl]-naphthalen-2-ol (short for L). Al³⁺ is proved to be capable of regulating this competition during the excited state deactivation process. With the addition of Al³⁺, the original barrier for the ESIPT process is removed and ESIPT fluorescence peak of the sensor is significantly enhanced. TICT process, which is originally barrier-less, is prohibited by the strong coordination between Al³⁺ and the sensor. The regulation of TICT and ESIPT process to achieve Al³⁺ detection is interesting and intrinsically different from the previously proposed sensing mechanism. The newly proposed mechanism is quite rare in the field of metal ion detection and may inspire scientists to design sensors from a different perspective.

Introduction

Development of fluorescence sensor has drawn increasing attention in the field of metal ion sensing because of its good selectivity, low cost and excellent portability [[1], [2], [3]]. By exhibiting detectable fluorescence quenching, enhancing or color changing signals, precise detection of the target ion can be achieved. A delicately designed sensor can not only show significant changing of fluorescent signal toward the target ion but also rule out the interfering ions. Thus, it is usually a pretty tricky task to design the ideal sensor for a specific metal ion. A common strategy is to incorporate a fluorophore and some coordination sites into the sensor. The coordination sites are usually Lewis bases which can form strong interactions between the metal ion and the sensor [[4], [5], [6]]. Fluorescence signal, thereafter, can be significantly affected and ion detection is achieved.

The photophysical process of the fluorescence sensor is usually complicated. During the excitation and excited state relaxation processes, multiple minimums on the excited state potential energy surface (PES) namely local excited (LE) state, intramolecular charge transfer (ICT) state, twisted intramolecular charge transfer (TICT) state and proton transfer state can be reached [[7], [8], [9], [10], [11]]. Competitions between these states may exist which adds to the complexity of its fluorescence mechanism. Moreover, after interacting with the target metal ion, the presences of the above states are likely to be affected and the competition between them should be regulated. Most experimental results focus on reporting the performance of the sensor without substantial investigations on the sensing mechanism. The fluorescence mechanism and underlying detecting mechanism of the sensor usually remains unsubstantial or uncovered, which hinders the development of metal ion sensors. Thus, fundamental understandings on the photophysics of the sensor are required.

As a typical bio-functional element, aluminum usually participates in many bio-chemical processes such as enzyme-catalyzed reactions and biotechnological transformation [12,13]. Aluminum is the third abundant element in the earth and can be released into the water cycle via various human activities. Although aluminum can be utilized as food additives and water purifying agents [14,15], an excess intake can lead to severe diseases such as Alzheimer's disease and Parkinson's disease [[16], [17], [18]]. Herein, precise detection of aluminum is of medical and physiological importance.

In 2016, a sensitive Al³⁺ sensor 1-[(2-Amino-phenylimino)-methyl]-naphthalen-2-ol (short for L) based on a turn-on fluorescence signal is reported by Zhu and coworkers [19]. The as-synthesised sensor shows very weak fluorescence which peaks at 499 nm. After adding Al³⁺ ion, a 63-fold enhancement of the 499 nm signal is observed together with the presence of a second strong emission peak centered at 473 nm. This sensor has been successfully utilized to detect Al³⁺ in living cells and has a detection limit of 1.08 × 10⁻⁷ mol/L, which makes it a good candidate in the fabrication of Al³⁺ sensor. Photo-induced electron transfer (PET) of the sensor may be present and the detecting mechanism is simply attributed to the inhibition of PET by Al³⁺ (Scheme 1). As is stated above, the photophysics of the sensor can be very complicated. Besides, the interaction between the sensor and Al³⁺ may form organometallic compound, which further complicates the detection mechanism. Thus, the original detection mechanism can be problematic.

Density functional theory (DFT) and time-dependent density functional theory (TDDFT) are very useful theoretical approaches on the studies of photophysical properties from small organic dyes to complicated organometallics [[20], [21], [22], [23], [24], [25]]. It is a powerful approach to investigate proton transfer in both ground state as well as excited state [[41], [42], [43],45,46]. In this contribution, the photophysics of sensor L is studied first. An Excited state intra-molecular proton transfer (ESIPT) process is observed during the relaxation of L on the PES. A TICT state is also observed which competes with the ESIPT state. The coexistence and competition between TICT and ESIPT is quite rare and interesting for small organic sensors. Consequently, the turn-on mechanism of Al³⁺ is fully discussed and it is proved that the presence of Al³⁺ can tune the energy barrier of the proton transfer process and the twisting process, which plays a key role during the competition between the two processes. The regulation of the competition between TICT and ESIPT by metal ion may shed light on the design of metal ion sensor from a new perspective.