Fluorescent Dyes and Their Supramolecular Host/Guest Complexes with Macrocycles in Aqueous Solution Roy N. Dsouza, Uwe Pischel,* and Werner M. Nau* School of Engineering and Science, Jacobs University Bremen, Campus Ring 1, D-28759 Bremen, Germany Centro de Investigaci on en Química Sostenible (CIQSO) and Departamento de Ingeniería Química, Química Física y Química Org anica, Universidad de Huelva, Campus de El Carmen s/n, E-21071 Huelva, Spain

Fluorescent dyes and their supramolecular host/guest complexes with macrocycles in aqueous solution.

Environment-related parameters, including viscosity, polarity, temperature, hypoxia, and pH, play pivotal roles in controlling the physical or chemical behaviors of local molecules. In particular, in a biological environment, such factors predominantly determine the biological properties of the local environment or reflect corresponding status alterations. Abnormal changes in these factors would cause cellular malfunction or become a hallmark of the occurrence of severe diseases. Therefore, in recent years, they have increasingly attracted research interest from the fields of chemistry and biological chemistry. With the emergence of fluorescence sensing and imaging technology, several fluorescent chemosensors have been designed to respond to such parameters and to further map their distributions and variations in vitro/in vivo. In this work, we have reviewed a number of various environment-responsive chemosensors related to fluorescent recognition of viscosity, polarity, temperature, hypoxia, and pH that have been reported thus far.

Macro-/micro-environment-sensitive chemosensing and biological imaging

Knowing the underlying photophysics of thermally activated delayed fluorescence (TADF) allows proper design of high efficiency organic light-emitting diodes. We have proposed a model to describe reverse intersystem crossing (rISC) in donor–acceptor charge transfer molecules, where spin–orbit coupling between singlet and triplet states is mediated by one of the local triplet states of the donor (or acceptor). This second order, vibronically coupled mechanism describes the basic photophysics of TADF. Through a series of measurements, whereby the energy ordering of the charge transfer (CT) excited states and the local triplet are tuned in and out of resonance, we show that TADF reaches a maximum at the resonance point, substantiating our model of rISC. Moreover, using photoinduced absorption, we show how the populations of both singlet and triplet CT states and the local triplet state change in and out of resonance. Our vibronic coupling rISC model is used to predict this behaviour and describes how rISC and TADF are affected by external perturbation.

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Revealing the spin-vibronic coupling mechanism of thermally activated delayed fluorescence.

Redox-active fullerenes can be covalently bound to a variety of donors; their photophysical properties have been investigated. Their photochemical processes, including electron transfer and energy transfer, are varied, depending on the donor, linkage between the donor and C 60 , and solvent. Regardless of the solvent and linkage, the charge-separated state is produced efficiently in zinc porphyrin-C 60 systems, showing that C 60 is a good electron acceptor. The most intriguing characteristic of C 60 in electron transfer is that C 60 accelerates photoinduced charge separation and retards charge recombination in the dark. The long-lived charge-transfer state of the C 60 -porphyrin dyad was successfully converted to photocurrent using a self-assembled monolayer technique. These findings will provide a new strategy for the design and synthesis of artificial photosynthethic systems and photoactive materials using C 60 as a building block.

Donor‐Linked Fullerenes: Photoinduced electron transfer and its potential application

The attainment of a better understanding of the dependence of photoinduced electron transfer reaction rates on the molecular structures of the donor and acceptor entities results in improving the capture and storage of solar energy. Here, the intermolecular and supramolecular electron transfer processes from electron donors (porphyrins (P), chlorophylls (Chl), phthalocyanines (Pc) and naphthalocyanines (Nc)) and their metal derivatives to electron acceptors (fullerenes such as C 60 and C 70 ) studied by nanosecond and picosecond laser flash photolysis techniques in polar and nonpolar solvents are reviewed. For intermolecular systems in polar solvents, photoinduced electron transfer takes place via the excited triplet states of C 60 /C 70 or via the excited triplet states of P/Pc/Nc, yielding solvated radical ions in polar solvents; thus, the back electron transfer rates are generally slow. In the case of the supramolecular dyads and triads formed by axial coordination, hydrogen bonding, crown ether complexation, or rotaxane formation, the photoinduced charge separation takes place mainly from the excited singlet state of the donor; however, the back electron transfer rates are generally quite fast. The relations between structures and photochemical reactivities of these novel supramolecular systems are discussed in relation to the efficiency of charge separation and charge recombination.

Intermolecular and supramolecular photoinduced electron transfer processes of fullerene–porphyrin/phthalocyanine systems

Photophysical properties of the fullerenes, C60 and C70

Photoexcitation of complexes between fullerence C[sub 60] and organic amines in benzene solutions is known to result in charge separation (CS) and subsequent charge recombination (CR) reactions, which lead to varying yields of fullerence triplet formation. Picosecond flash photolysis studies are carried out on C[sub 60]-diphenylamine (DPA), C[sub 60]-triethylamine (TEA), C[sub 60]-diazabicyclooctane (DABCO), and C[sub 60]-triphenylamine (TPA) systems to find out mechanistic details of the triplet formation on CR by inducing heavy atom and polarity effects by using suitable solvents. It is found that in the case of C[sub 60]-DPA, C[sub 60]-TEA, and C[sub 60]-DABCO systems proton transfer from the amine cation to the C[sub 60] anion in the ion pair state dominates, leading to poor triplet yields, which improve in heavy atom containing solvents. In TPA, proton transfer is not possible and hence fullerene triplet yields are high. Increase of solvent polarity for this system results in decreased C[sub 60] triplet yields with a consequent increase in the ion dissociation yield. A suitable reaction scheme is proposed to explain the results obtained. 34 refs., 5 figs., 2 tabs.

Charge recombination reactions in photoexcited fullerene C60-amine complexes studied by picosecond pump probe spectroscopy

Effect of solvent polarity on the aggregation of C60

Effect of solvent polarity on the aggregation of fullerenes: a comparison between C60 and C70

Abstract— The effect of solvent polarity on the electronic absorption and fluorescence properties of neutral red (NR), a phenazine‐based dye of biological importance has been investigated in several neat and mixed solvents. An unusual dual solvatochromic behavior has been observed that reveals the existence of two closely spaced electronic excited states in NR. In low‐polarity solvents the fluorescence of the NR is mainly emitted from the localized excited state, whereas in high‐polarity solvents the emission from the charge transfer state dominates. The dipole moments of the localized and charge transfer states of NR have been estimated from the solvatochromic shifts. The dipole moment of the localized excited state (4.8 D) is only slightly higher than that of the ground state (2.0 D), while that of the charge transfer state is drastically higher (17.5 D). Fluorescence quantum yields and the life‐times of NR have been determined in different solvents and correlated with the solvatochromic shifts.

Avinash V. Sapre

Papers

Photophysical properties of the fullerenes, C60 and C70

Charge recombination reactions in photoexcited fullerene C60-amine complexes studied by picosecond pump probe spectroscopy

Effect of solvent polarity on the aggregation of C60

Effect of solvent polarity on the aggregation of fullerenes: a comparison between C60 and C70

Dual Solvatochromism of Neutral Red