Volume 85, Issue 7

The XXIVth IUPAC Symposium on Photochemistry was held in the old university city of Coimbra, Portugal from 15 to 20 July 2012, and welcomed 640 participants from 53 countries presenting their research on this important area of chemistry. This series of meetings started in Strasbourg in July 1964 as the International Symposium on Organic Photochemistry, organized by George Hammond and J. Levisalles. Subsequent symposia have seen the meeting expand to embrace all areas of photochemistry. The program topics of the Coimbra symposium ranged from materials aspects of photochemistry through nanostructures and nanomaterials to mechanistic and synthetic aspects of organic photochemistry, photobiology, photomedicine and skin photochemistry, applied photochemistry, and photochemistry and cultural heritage. The symposium had 8 plenary lectures, 22 invited lectures, 105 oral communications, and more than 400 posters, confirming the vitality of this area of chemistry. It is difficult to pinpoint specific highlights, as these depend very much on one's personal interests, but one of the most important presentations was undoubtedly Tom Meyer's Porter Medal Lecture on metal-to-ligand charge-transfer states in polypyridylruthenium(II) complexes and related systems. An IUPAC Photochemistry Symposium was previously held in Portugal, in Lisbon, in 1986, and it is interesting to note that Prof. Meyer also gave a plenary lecture there addressing some of the fundamental photophysics of these systems [ Pure Appl. Chem. 58 , 1193 (1986)]. It is refreshing to see how these have developed from pure science to practical applications. George Porter gave a plenary lecture at the Lisbon symposium in 1986 on the first nanoseconds of photosynthesis. Developments in instrumentation in the intervening 26 years now make interrogation of excited-state behavior on the femtosecond timescale relatively straightforward, and as various presentations in this volume and in the symposium demonstrate, are helping unravel the importance of early events in many photochemical and photobiological processes. In addition to the lectures and poster presentations, the program also included a number of awards for young photochemists and posters, and a variety of social activities, including canoeing on the local River Mondego. We believe that the scientific program has maintained the excellent tradition of the IUPAC Photochemistry Symposia in showing that this continues to be a vibrant, exciting interdisciplinary area of research. This issue of Pure and Applied Chemistry contains a number of the plenary and invited lectures from the symposium, which we feel mirror the current state of the art of photochemistry as a dynamic and important field of chemistry. Hugh D. Burrows J. Sérgio Seixas de Melo Artur J. M. Valente Conference Editors

In 1974, the metal-to-ligand charge transfer (MLCT) excited state, [Ru(bpy) 3 ] 2+ *, was shown to undergo electron transfer quenching by methylviologen dication (MV 2+ ), inspiring a new approach to artificial photosynthesis based on molecules, molecular-level phenomena, and a “modular approach”. In the intervening years, application of synthesis, excited-state measurements, and theory to [Ru(bpy) 3 ] 2+ * and its relatives has had an outsized impact on photochemistry and photophysics. They have provided a basis for exploring the energy gap law for nonradiative decay and the role of molecular vibrations and solvent and medium effects on excited-state properties. Much has been learned about light absorption, excited-state electronic and molecular structure, and excited-state dynamics on timescales from femtoseconds to milliseconds. Excited-state properties and reactivity have been exploited in the investigation of electron and energy transfer in solution, in molecular assemblies, and in derivatized polymers and oligoprolines. An integrated, hybrid approach to solar fuels, based on dye-sensitized photoelectrosynthesis cells (DSPECs), has emerged and is being actively investigated.

We demonstrate the ability of two-dimensional electronic spectroscopy (2DES) to map ultrafast energy transfer and dynamics in two systems: the pigment–protein complex photosystem I (PSI) and aggregates of the conjugated polymer poly(3-hexylthiophene) (P3HT). A detailed description of our experimental set-up and data processing procedure is also given.

A number of soluble platinum(II) and gold(I) complexes have been designed and synthesized. Their luminescence and photophysical behavior have been studied. Their assembly properties, which were governed by the noncovalent metal–metal interactions, have also been investigated with the introduction of various external stimuli. The UV–vis and emission spectral changes of these complexes, which arose from the modulation of their assembly and disassembly behavior, have demonstrated the establishment of a convenient and sensitive detection strategy for various types of analytes. These studies suggested the potential applications of these metal complexes as detection probes for a diverse range of target substrates.

A class of molecular electron transfer cascades—those based on Pt II complexes of 2-(1-pyrazole)-pyridine (pzpy) ligands—are reported. The synthesis of a new electron-acceptor imide-modified pzpy ligands is reported, and their application to transition-metal chemistry demonstrated by the synthesis of the Pt II chloride and acetylide complexes. These donor–acceptor assemblies are promising models for investigation of photoinduced charge separation. Accordingly, picosecond time-resolved infrared (TRIR) and femtosecond transient absorption (TA) studies have been undertaken to elucidate the nature and dynamics of the lowest excited states in Pt(NAP-pyr-pyrazole)(–CC–Ph–C 7 H 15 ) 2 . It has been established that the initial population of an MLL'CT excited state in the chromophoric [Pt(pyridine-pyrazole)(acetylide)] core is followed by an electron transfer to the naphthalimide (NAP) acceptor, forming a charge-separated state. This state is characterized by a large shift in ν(CO) vibrations of the NAP acceptor, as well as by a very intense and broad [×10 times in comparison to ν(CO)] asymmetric acetylide stretch which incorporates –CC–Pt–CC– framework and occurs at approximately 300 cm –1 lower in energy than its ground-state counterpart. In CH 2 Cl 2 at room temperature, the charge-separated state with the lifetime of 150 ps collapses into an almost isoenergetic NAP-localized triplet state; the rate of this transformation changes upon decreasing the temperature to 263 K. This final excited state, 3 NAP-(pyr-pyrazole)Pt(–CC–Ph–C 7 H 15 ) 2 , has an unusually long, for Pt II complexes, excited-state lifetime of tens of microseconds. The work demonstrates the possibility of tuning excited-state properties in this new class of Pt II chromophores designed for electron-transfer cascades.

A pair of covalently linked molecular dyads is described in which two disparate boron dipyrromethene dyes are separated by a tolane-like spacer. Efficient electronic energy transfer (EET) occurs across the dyad; the mechanism involves important contributions from both Förster-type coulombic interactions and Dexter-type electron exchange processes. The energy acceptor is equipped with long paraffinic chains that favor aggregation at high concentration or at low temperature. The aggregate displays red-shifted absorption and emission spectral profiles, relative to the monomer, such that EET is less efficient because of a weaker overlap integral. The donor unit is insensitive to applied pressure but this is not so for the acceptor, which has extended π-conjugation associated with appended styryl groups. Here, pressure reduces the effective π-conjugation length, leading to a new absorption band at higher energy. With increasing pressure, the overall EET probability falls but this effect is nonlinear and at modest pressure there is only a small recovery of donor fluorescence. This situation likely arises from compensatory phenomena such as restricted rotation and decreased dipole screening by the solvent. However, the probability of EET falls dramatically over the regime where the π-conjugation length is reduced owing to the presumed conformational exchange. It appears that the pressure-induced conformer is a poor energy acceptor.

In the past few decades, charge transfer in DNA has attracted considerable attention from researchers in a wide variety of fields ranging from bioscience and physical chemistry to nanotechnology. Charge transfer in DNA has been investigated using various techniques. Among them, time-resolved spectroscopic methods have provided information on charge-transfer dynamics in DNA, an important basis for therapy applications, nanomaterials, and so on. In charge transfer in DNA, holes and excess electrons act as positive and negative charge carriers, respectively. Hole-transfer (HT) dynamics have been investigated in detail, while the dynamics of excess electron transfer (EET) have only become clear rather recently. In the present paper, we summarize studies on the dynamics of HT and EET by several groups including ourselves.

Experimental investigations of photoinduced charge transport in synthetic DNA capped hairpins possessing electron acceptor and donor stilbene chromophores at either end have established the mechanism, dynamics, and efficiency of charge transport in DNA. The mechanism for charge transport in repeating A-T base pairs (A-tracts) was found to change from single-step superexchange at short distances to multistep incoherent hole hopping at longer distances. The rate constants for base-to-base hole hopping in longer A- and G-tract sequences are 1.2 × 10 9 s –1 and 4.3 × 10 9 s –1 , respectively, considerably slower than the rate constants associated with molecular wires. Even slower rate constants are observed for alternating or random base sequences such as those encountered in natural DNA. The efficiency of charge separation in capped hairpins with A-tract sequences is also low as a consequence of the competition of hole hopping with charge recombination. Significantly higher efficiencies for charge separation are possible using diblock purine base sequences consisting of two or three adenines followed by a larger number of guanines. The short A-block serves as a molecular rectifier, slowing down charge recombination. More efficient charge separation can also be achieved using non-natural bases or by using the triplet acceptor anthraquinone for hole injection.

Theories of radiationless conversions and of chemical processes were employed to design better photosensitizers for photodynamic therapy (PDT). In addition to photostability and intense absorption in the near infrared, these photosensitizers were required to generate high yields of long-lived triplet states that could efficiently transfer their energy, or an electron, to molecular oxygen. The guidance provided by the theories was combined with the ability to synthesize large quantities of pure photosensitizers and with the biological screening of graded hydrophilicities/lipophilicities. The theoretical prediction that halogenated sulfonamide tetraphenylbacteriochlorins could satisfy all the criteria for ideal PDT photosensitizers was verified experimentally.

An intermolecular light-driven hydrogen evolution system with free glutamic-acid-modified zinc tetra( p -phenyl) porphyrin ( Glu-ZnP ) as a photosensitizer and [Fe 2 (CO) 6 (μ-adt)C 6 H 5 ] [μ-adt = N(CH 2 S) 2 ] ( Badt ) as a catalyst has been constructed. Using phenylmercaptan ( BSH ) as electron donor and acetic acid (HOAc) as proton source, hydrogen was obtained after irradiation with visible light for 2 h; the efficiency is comparable to that of the similar intramolecular dyad. Steady-state and time-resolved spectroscopy and cyclic voltammetry show that both the first and the second electron transfer from singlet 1* Glu-ZnP to Badt and reduced Badt are thermodynamically feasible. However, the competition of electron transfer from singlet 1* Glu-ZnP to Badt with intersystem crossing from singlet 1* Glu-ZnP to triplet 3* Glu-ZnP , inefficient electron transfer from triplet 3* Glu-ZnP to Badt , and the lower energy of triplet 3* Glu-ZnP and possible 3* Badt to that of yielded charge-separated state of Glu-ZnP + · -Badt −·  were believed to be the obstacles for efficient hydrogen evolution.

As part of a study directed toward design of good catalytic systems based upon a hydrophobic vitamin B 12 , heptamethyl cobyrinate perchlorate, we describe the preparation of various nanomaterials using the vitamin B 12 derivative and photosensitizers. Examples include vitamin B 12 -hyperbranched polymers (HBPs), human serum albumin (HSA) containing vitamin B 12 derivatives, a vitamin B 12 -titanium dioxide hybrid catalyst, a vitamin B 12 -Ru complex combined system, and a vitamin B 12 -rose bengal combined system. These bioinspired materials have the potential as catalytic systems for the degradation of organic halide pollutants and for molecular transformations via radical intermediates during irradiation by UV or visible light, and offer a variety of applications that are of great interest in terms of green chemistry.

Photoinduced electron-transfer processes are enhanced or quenched by application of electric fields, depending on the donor–acceptor pairs. Electric-field-induced quenching of photoluminescence, which results from the field-induced dissociation of the exciton state that depends on the photoexcitation wavelength, is observed in π-conjugated polymers. These electric-field effects on photoinduced dynamics have been confirmed by the measurements both of electroabsorption and electrophotoluminescence spectra and of time-resolved electrophotoluminescence decays. Time-resolved measurements of photocurrent, with which novel material function in electrical conductivity of organic materials induced by photo-irradiation and application of electric fields is confirmed, are also reviewed.

Sunlight is a clean, cheap, and abundant reagent. Many light-initiated reactions can be carried out in water, making photochemistry an ideal tool for pollution abatement and/or elimination. We studied the photoreactivity of different families of common organic micropollutants: light-initiated processes in the absence or presence of co-oxidants, and photocatalyzed reactions using different photocatalysts and composites. Based on the experimental evidences found, detailed transformation mechanisms have been proposed that help understand the reactivity of organic micropollutants and predict their environmental fate. Our approach includes the study of the photophysics for each family of compounds, its reactivity upon direct photolysis, adsorption onto photocatalysts, photocatalytic reactivity, thermodynamics, and kinetics of the processes involved (p K a , E º, rate constants, etc.), product analysis, and ecotoxicological assessment. Different commonly overlooked problems, related to the kinetics of the process, are reported, and a model is proposed that includes the possibility of adsorption on different types of active sites, leading to different reactivities.

Unlike most other electron donor–acceptor (EDA) molecules, aminochalcones exhibit unusual solvent polarity-dependent fluorescence behavior. The photophysical behavior of two aminochalcones, namely, 4-aminochalcone (AC) and 4-dimethylaminochalcone (DMAC), has been studied in a viscous room-temperature ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF 6 ], by steady-state and time-resolved fluorescence techniques. The observation of a single emission band in viscous IL, which is similar to the one observed in less viscous polar conventional solvents, suggests no twisting is necessary for the formation of the charge-transfer state from which the emission of aminochalcones originates. The fluorescence decay profiles, solvation dynamics, and excitation-wavelength-dependent emission behavior of AC are found to be quite different from those of DMAC in the IL. The observed difference is attributed to specific H-bonding interaction between AC and [bmim][PF 6 ].

Excited-state intramolecular proton-transfer (ESIPT) compounds and intramolecular charge-transfer (ICT) compounds have attracted attention due to their interesting and even unique emission properties. The intense and environment-sensitive emission showed by some members of the two families has been exploited in fluorescent probes for various forms of environmental sensing. In this paper, we summarize our recent study on the utilization of novel ESIPT and ICT compounds with intense emission as fluorescent probes after an introduction to the ESIPT and/or ICT processes and related photophysics mechanism.

Irradiation of the three isomeric (chlorobenzyl)trimethylsilanes in methanol–water generates the corresponding didehydrotoluenes (DHTs). The process involves expulsion of a chloride ion to give the triplet phenyl cation and ensuing elimination of the trimethylsilyl cation. This straightforward generation of DHT intermediates overcomes a shortcoming of previous methodology (cycloaromatization of enyne-allenes), which is limited to the meta -isomer, and opens the path for understanding the chemistry (and possibly the biological action) of these unusual intermediates.

Benzhydryl radicals and cations are reactive intermediates central to the understanding of organic reactivity. They can be generated from benzhydryl halides by UV irradiation. We performed transient absorption (TA) measurements over the range from femtoseconds to microseconds to unravel the complete reaction scheme. The 290–720-nm probe range allows the unambiguous monitoring of all fragments. The appearance of the radical is delayed to the optical excitation, the onset of the cation signal is found even later. Ab initio calculations show that this non-rate behavior in the 100 fs range is due to wavepacket motion from the Franck–Condon region to two distinct conical intersections. The rise of the optical signal with a quasi-exponential time of 300 fs is assigned to the planarization and solvation of the photoproducts. The bond cleavage predominantly generates radical pairs. A subsequent electron transfer (ET) transforms radical pairs into ion pairs. Due to the broad interradical distance distribution and the distance dependence, the ET is strongly non-exponential. Part of the ion pairs recombine geminately. The ET and the recombination are terminated by the depletion of close pairs and diffusional separation. The remaining free radicals and cations undergo further reactions in the nanosecond to microsecond regime.

Three photo-click ligation strategies described in this account provide scientists with efficient and selective tools for derivatization of various molecules, polymers, and surfaces. Fast photochemical reactions that are utilized in these techniques permit spatiotemporal control of the process. The absence of activating reagents and catalysts, as well as compatibility with aqueous media, makes photo-click ligations suitable for biomedical applications. The first of these approaches relies on the photochemical decarbonylation of cyclopropenones to produce cyclooctynes. The latter undergo rapid catalyst-free strain-promoted azide–alkyne cycloaddition (SPAAC) to azide-tagged substrates. The second method is based on a very fast (>10 4 M –1 s –1 ) light-triggered hetero-Diels–Alder reaction and permits efficient derivatization of substrates bearing vinyl ether moiety. An even faster reaction between photochemically generated naphthoquinone methides ( o NQMs) and thiols (~2 × 10 5 M –1 s –1 ) serves as a basis for a third method. This thiol photo-click chemistry allows for the selective derivatization of thiol-functionalized substrates or labeling of free cysteine residues in proteins. The thioether linkage produced by the reaction of o NQMs and a thiol is stable under ambient conditions, but can be cleaved by UV irradiation, regenerating free thiol. This feature permits the removal or replacement of immobilized compounds, as well as traceless substrate release.

This document contains recommendations for terminology in mass spectrometry. Development of standard terms dates back to 1974 when the IUPAC Commission on Analytical Nomenclature issued recommendations on mass spectrometry terms and definitions. In 1978, the IUPAC Commission on Molecular Structure and Spectroscopy updated and extended the recommendations and made further recommendations regarding symbols, acronyms, and abbreviations. The IUPAC Physical Chemistry Division Commission on Molecular Structure and Spectroscopy’s Subcommittee on Mass Spectroscopy revised the recommended terms in 1991 and appended terms relating to vacuum technology. Some additional terms related to tandem mass spectrometry were added in 1993 and accelerator mass spectrometry in 1994. Owing to the rapid expansion of the field in the intervening years, particularly in mass spectrometry of biomolecules, a further revision of the recommendations has become necessary. This document contains a comprehensive revision of mass spectrometry terminology that represents the current consensus of the mass spectrometry community.