Angelo Albini is currently Professor of Organic Chemistry at the University of Pavia, Italy. A native of Milan, he completed his studies in Chemistry at Pavia in 1972. After postdoctoral work at the Max-Plank Institute for Radiation Chemistry in Muelheim, Germany (1973-74), he joined the Faculty at Pavia in 1975 as an assistant and then associate (since 1981) professor. He accepted a Chair of Organic Chemistry at the University of Torino in 1990 and then moved again to Pavia in 1993. He has been Visiting Professor at the Universities of Western Ontario (Canada, 1977-78) and Odense (Denmark, 1983). He is active in the field of organic photochemistry, organic synthesis via radical and ions, photoinitiated reactions, mild synthetic procedure in the frame of the increasing interest for substainable/green chemistry, applied photochemistry (photostability of dyes, drugs, photoinduced degradation of pollutants. He has been responsible of several research projects sponsored by national and international institutions and devoted to the above topics and coordinates the æGreen ChemistryÆ group of the Italian Chemical Society. He is coauthor/editor of three books (Heterocyclic N-Oxides, CRC, Orlando, 1990, Drugs: Photochemistry and Photostability, RSC, Cambridge, 1998, and Handbook of Preparative Photochemistry, Wiley-VCH, 2009), the senior reporter of the Specialist Periodic Reports on Photochemistry (RSC) since 2008, as well as coauthor ca. 280 research articles. He has been the recipient of the Federchimica Prize for creativity in chemistry in 1990.

Photochemistry Volume 37

A Review of the Literature Published Between July 2004 and June 2007

By Paolo Coppo, Rui Fausto, Elena Galoppini, Andrea Maldotti, Miguel A. Miranda, Kazuhiko Mizuno, J. Sérgio Seixas de Melo, Nick Serpone, Takashi Tsuno, Angelo Albini

The Royal Society of Chemistry

Copyright © 2009 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-455-9

Contents

Introduction and review of the period July 2004–June 2007 Angelo Albini, 11,
Photophysical processes in polymers and oligomers Telma Costa, João Pina and J. Sérgio Seixas de Melo, 44,
Light induced reactions in cryogenic matrices Rui Fausto and Andrea Gómez-Zavaglia, 72,
Alkenes, alkynes, dienes, polyenes Takashi Tsuno, 110,
Oxygen-containing functions M. Consuelo Jiménez and Miguel A. Miranda, 149,
Photochemistry of aromatic compounds Kazuhiko Mizuno, 175,
Functions containing a heteroatom different from oxygen Angelo Albini and Elisa Fasani, 213,
Photochemistry and photophysics of transition-metal complexes Andrea Maldotti, 240,
Photocatalysis and solar energy conversion (chemical aspects) Nick Serpone, Alexei V. Emeline and Satoshi Horikoshi, 300,
Multi-component arrays for interfacial electronic processes on the surface of nanostructured metal oxide semiconductors Andrew Kopecky and Elena Galoppini, 362,
The day lighting became organic Paolo Coppo, 393,

CHAPTER 1

Photophysical processes in polymers and oligomers

Telma Costa, João Pina and J. Sérgio Seixas de Melo

DOI: 10.1039/b908480k

Processes occurring upon electronic excitation: general considerations

The initial state of molecules prior to excitation is usually the ground electronic state. Upon electronic excitation the molecule returns to the ground electronic state decaying through several deactivation, radiative (fluorescence and phosphorescence) and radiationless (internal conversion and intersystem crossing), channels, see Scheme 1.

Upon electronic excitation to any state above the first excited singlet state (S1), de-excitation occurs (through vibrational relaxation) commonly to the S1 state. From this state, possible deactivation processes include fluorescence emission, continued de-excitation to the ground-state, with no emission, crossing to the triplet state or any combination of these (Scheme 1). If the triplet becomes populated, phosphorescence and/or de-excitation to the ground-state, with no emission, can occur. It is the overall balance between radiative and non-radiative deactivation pathways that gives rise to the characteristic properties of molecules.

The lifetime of an excited state of a molecule is one of its four main characteristics; the others are the energy, quantum yield (φ) and polarization (P or anisotropy, r). Following pulsed excitation, fluorescence decays exponentially with time, according to I = I0e-t/τ, where I0 is the initial intensity settled at an arbitrary time zero, I is the intensity at some latter time, t, and τ is a constant. When the time t is equal to τ, the intensity has fallen to 1/e of its initial value. The value of τ is therefore defined as the mean decay time for the emission process or the mean life of the excited state. When fluorescence is the only deactivation process, the value of τ is commonly designated as τ0 with the meaning of intrinsic or natural lifetime.

The observed or measured lifetime τ will only be equal to τ0 in the absence of any deactivating process, either internally or externally induced. The observed lifetime, τF, is determined by all the deactivation processes and is expressed as:

τF = 1/kF + kIC + kISC + kq[Q],

where kF(φF/τF), kIC (φIC/τF), kISC (φISC/τF) and kq are the rate constants for, respectively the fluorescence, internal conversion, intersystem crossing and quenching processes, and Q is a quencher.

Additionally, several other factors can influence and add new pathways for dissipation of energy. Among these, is the formation of new species (for example excimer formation or acid–base equilibria) and/or quenching. Oxygen, present in all solvents in equilibrium with air, acts as a very efficient quencher. This frequently is due to energy transfer to the ground-state of oxygen (a triplet) to generate singlet molecular oxygen (1270 nm, [congruent to]1 eV). Obviously, the efficiency of oxygen quenching depends on the lifetime of the probe being quenched and, particularly, on the state undergoing quenching. In the case of triplet states, due to their longer lifetimes, rigid matrices (frozen solutions or glasses for example) can be used to prevent diffusional collision between molecular oxygen and the molecular probe, thus avoiding quenching.

A comprehensive study of the excited states of a molecule involves the determination of the above mentioned characteristics (summarized in Scheme 1) for the singlet and triplet excited states.

1. Hydrophobically modified polymers

Fluorescence is an important and useful tool to investigate physicochemical, biochemical and biological systems. Several techniques, based on the detection of fluorescence, have been developed in terms of spectrometry, microscopy, flow cytometry, and of photophysical methods. Included in the photophysical methods are steady-state and time-resolved fluorescence, anisotropy, fluorescence resonance energy transfer (FRET), two-photon absorption. Fluorescence techniques offer a huge variety of advantages due to its high sensitivity and selectivity to a high number of analyte targets. Intrinsic or natural probes are frequently found in proteins, where the aromatic amino acids, such as tryptophan (trp), tyrosine (tyr), and phenylalanine (phe), exhibit fluorescence emission. However, the majority of the systems under investigation (polymers, micelles, lipids, DNA, …) are non-fluorescent and the use of an extrinsic or intrinsic fluorescence probe must be therefore considered in order to investigate these systems at a molecular level. The choice of the fluorescence probe should be made taking into consideration its sensitivity to the property of the system to be measured. There is a wide range of fluorophores, or fluorescence probes, that can be used: coumarins, fluoresceins, eosin, rhodamines, cyanines, etc. However, the design of new fluorescent molecular sensors, that are selective to a given analyte is, nowadays, an area of growing research.

1.1 The use of fluorescence probes in the investigation of polymer dynamics and self-assembly phenomena

1.1.1 Fluorescent probes in polymers and models of analysis. The grafting of a fluorescent probe onto the polymer backbone provides information at a molecular level. By employing photophysically active groups, either randomly distributed or at both ends of the polymer, a direct molecular-level study of the association and, consequently, adopted molecular conformation can be made. From photophysical studies it is possible to follow events occurring on a very short time scale and, in ideal situations, of “zero intramolecular concentration”. Probes with long lifetimes are required in order to probe large scale motions of the polymer chains. Steady-state and time-resolved fluorescence measurements, based on excimer formation/decay kinetics, are also often used as tools to follow the self-association of these polymers in aqueous solution. Excimer (excited dimers) imply the diffusive encounter between a molecule in its ground state with one in its excited state. Due to its long lifetime (100 to 600 ns depending on the solvent and substitution) pyrene is the most popular probe known to present excimer formation. There are however situations where an “excimer-like” emission can be observed, resulting from pre-associated (or preformed) dimers in the ground state, GSD. This is commonly seen when the chromophores are linked by a polymer chain, when they are bound or absorbed on silicas, aluminas, clays or zeolites or when they form complexes with cyclodextrins, calixarenes or metal ions. The presence of ground-state dimers has a strong influence on the excimer-to-monomer steady-state emission ratio (IE/IM) and in the obtained excimer association and dissociation rate constants. Ground-state dimers are known to have a lower fluorescence quantum yield (φFGSD) compared to the excimers formed through a dynamic mechanism. Duhamel et al. reported the first quantitative information about the fluorescence quantum yield of pyrene dimers (φFGSD), for the interaction between Hydrophobically modified Alkali Swellable Emulsion (HASE) copolymer labelled with pyrene (Fig. 1a) and surfactant sodium dodecyl sulfate (SDS), where a decrease of the ground state association was observed by increasing the SDS concentration. It was found that the φFGSD is 4.5 times smaller than that of dynamic excimers.

In addition to GSD, conformationally different excimers, monomers that give rise to excimer (MAGRE) with different association rate constants and monomers that do not give rise to excimer, i.e., free monomers, can co-exist in randomly labelled polymers as a result of the different interpolymeric distances between adjacent chromophores.

Efforts and different methods of analysis have been developed over the past years in order to solve the observed complex time dependent luminescence behavior. The more widespread methods involve fittings to multiexponential decay laws; however, these differ in methodology. The compartmental analysis, sum of exponentials, and the blob model are nowadays the main methods of analysis of fluorescence decays. The compartmental analysis has been extensively used by De Schryver, Boens, Amellot et al. for different systems, in which these are divided into ground- and excited-state subsystems composed of distinct species which act kinetically in a unique way. The compartmental analysis was the topic of a recent work by Boens and Amellot where the concepts of this modelling were revised.

The exponential analysis is the classical model of analysis where experimental fluorescence decays are fitted to a sum of exponentials. The obtained decay times and preexponential factors are later on used to determine the value of the excimer association (ka) and dissociation (kd) rate constants. This is probably the most direct and clean method to identify species in a given system. However, in systems where more than two species exist, for example in polymers randomly labelled with fluorescent probes, the analysis of such a system becomes much more complex and sometimes impossible through the derivatization of analytical expressions.

Poly(acrylic acid) (PAA) randomly labelled with naphthalene (Np) and pyrene (Py) (Fig. 1b) have showed to present different excited state behavior depending on external conditions such as the pH, temperature and solvent. In aqueous solution, its behavior is ruled out by the balance between the hydrophobic and electrostatic interactions. In view of these competing processes, the polymer chain can undergo severe conformational changes. The presence of excited-state (as well as ground-state) dimers in addition to monomer emission, due to locally excited probe, gives evidence for hydrophobic association between chromophores. This association was found to become much less important at higher pH due to the electrostatic repulsion between different chain segments. However, it was noted that even at high pH there is a significant self-association. For the naphthalene labelled polymer, from time-resolved fluorescence data, three kinetically coupled species were found and attributed to the MAGRE monomers, free monomers and one excimer (formed either by a dynamic and/or static mechanism). For the pyrene labelled-PAA instead of one excimer, two excimers, with two different conformations (twisted and parallel), were observed. For the Np labelled-PAA the apparent and the intrinsic activation energies (for excimer formation) were determined from Stevens-Ban plots and from time-resolved fluorescence data, respectively. A comparison between the two values revealed that the intrinsic is always smaller than the apparent activation energy, which shows the importance of the contribution of ground-state association to the activation energy for excimer formation. When a good solvent (for the chromophore groups), such as dioxane or methanol, is gradually introduced in the system, a decrease of the chromophore–chromophore interactions was observed, which was shown to be consistent with the presence of two types of monomers and one excimer. Both monomers were found to be able to form excimers in the excited state (two MAGRE monomers): one involving the movement of long distance chromophores and the other involving a local reorientation of nearby chromophores. In pure organic solvents (methanol and dioxane), one MAGRE, one excimer (more stable) and free monomers were observed and with this somehow less complex kinetic scheme, the determination of all the rate constants in these polymers was achieved.

The effect of the size of the polymer backbone was investigated and showed to influence the photophysics of PAA polymers. For low molecular weight polymers (Mn= 2000 g/mol), a number of peculiar effects were observed which were attributed to the adoption of “micelle-like” conformation, at intermediate pH values, where the PAA chain surrounds the hydrophobic core formed by the pyrene groups to shield it from water; these structures promote the occurrence of GSD. At higher pH values, the higher electrostatic repulsion expands the polymer chain, as happens with the long PAA chain polymers.

Duhamel and co-workers have developed an alternative model to interpret excimer formation kinetics with different kinds of pyrene-labelled polymers, based on the fact that excimer formation can be described by a distribution of rate constants. This distribution is due to the different distances between the interacting chromophores, attached to the polymer backbone, and since cyclization rate constants depend on the chain length (between the chromophores), different rate constants could be obtained. The model divides the polymer coil into several blobs and is known as the “fluorescent blob model” (FBM). The FBM was applied to four different randomly labelled polystyrene polymers with different pyrene derivatives (see for example Fig. 1c), and it was concluded that the more efficient excimer formation was achieved with longer and more flexible linkers. Additionally, it was also found that different grafting procedures lead to different distributions of pyrene groups along the polymer chain.

The FBM was also applied to two different poly(2-vinylpyridine) polymers, i.e., mid- and end-tagged anthracene-labelled poly(2-vinylpyridine), which was previously investigated by Clements et al. and whose behaviour showed to be dependent on the degree of ionization (α). By increasing α, the intrapolymer fluorescence quenching becomes efficient due to the electron transfer from the excited anthracene to neighbouring pyridinium units. Similar results were obtained with the FBM, in which the quenchers randomly distribute themselves within the blobs.

This method has been successfully applied to the study of pyrene-labelled polystyrene, and poly(dimethylacrylamide) polymers. A study on a series of double pyrene labelled end-capped polystyrene [PS(X)Py2, where X is the polymer molecular weight] and pyrene randomly labelled polystyrene using steady-state and time-resolved techniques, where the nature of the labelling and of the linker connecting the pyrene group to the polystyrene chain has been taken into account, was reported. It was observed that the random labelling of the polymer backbone generates excimer formation with higher efficiency and with a higher value for the association rate constant, than in the case of end-capped polymers, due to the formation of pyrene rich-domains in the former case. On the other hand, the flexibility and stiffness of the linker also showed to be of relevance; short and stiff linkers are associated with slower rate constants than longer and flexible ones. However, in both cases, from the normalization of the obtained IE/IM ratio and of the cyclization rate constants, taking into account the different pyrene content in the polymer, it was found that, independently of the labelling degree, nature of the linker and of the labelling, they merge into a single value and have the same tendencies of variation with the solvent viscosity. These observations lead to an important conclusion: the long range polymer chain dynamics depend only on the polymer backbone. The use of randomly labelled polymers seems therefore an advantage for the study of long-range interactions, since they are easier to prepare (than the monodisperse end-capped polymers) and the encounters between the chromophores occur more frequently.

The FBM was also used to the determination of the radius of the polymer chain ratio through the number of ground-state pyrene groups per chain, the mean number of ground-state pyrene groups in each blob and the excimer association rate constant within a blob. The obtained values were further compared with the hydrodynamic radius obtained through dynamic light scattering experiments, where a fairly good agreement between the values was found. Moreover, the exponential and the blob models were used to investigate the effect of the addition of SDS on the intramolecular interactions within a HASE copolymer labelled with pyrene. Both the methodologies of analysis yielded identical results, leading to the conclusion that the obtained rate constants seem independent of the model of choice.

1.1.2 Hydrophobically modified polymers with fluorescent probes: trends and developments. Hydrophobically modified polymers (HMP) have important applications for surface modification, structuring and, in particular, rheology control. The rheological behavior of hydrophobically modified water soluble polymers is usually followed for polymers in which alkyl chains are incorporated into the water soluble backbone. In order to validate and correlate the information obtained at the molecular level with the macroscopic viscoelastic properties found for these kind of polymers, pyrene groups were incorporated onto the polymer chain instead of the alkyl chains. It was observed that pyrene confers the necessary hydro- phobic properties for associative thickeners, i.e., a large shear thinning effect was observed. Complementary information about HMP in solution in different concentration regimes can be found using fluorescence and rheological techniques.

(Continues…)Excerpted from Photochemistry Volume 37 by Paolo Coppo, Rui Fausto, Elena Galoppini, Andrea Maldotti, Miguel A. Miranda, Kazuhiko Mizuno, J. Sérgio Seixas de Melo, Nick Serpone, Takashi Tsuno, Angelo Albini. Copyright © 2009 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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