Introduction to the themed issue dedicated to Jan Verhoeven

This issue of Photochemical & Photobiological Sciences is published in honour of the contributions Jan Verhoeven has made to science, and in particular to the field of molecular photophysics. The contributed papers are by his colleagues who have had, and continue to benefit from, the privilege to work alongside him.

Jan's scientific contributions have been monumental and have inspired generations of researchers to study molecular photophysics. His work has had major impact on many areas of chemistry and chemical physics and, by way of his scientific leadership, has established numerous techniques and protocols now taken for granted. He has published more than 250 research articles, together with book chapters, comments and review articles, and accumulated in excess of 9000 citations. His scientific rigour and determination to find genuine solutions are well known and have helped lay the foundation upon which the subject stands. There have been extended collaborations with John Warman and Michael Paddon-Row and with many other colleagues in The Netherlands and abroad.

Jan Verhoeven received his Doctorandus (Drs, cum laude) in 1965 from the University of Amsterdam with specialisation in Physical Organic Chemistry. Subsequently, he was employed as a scientific staff member at the University of Amsterdam from 1966 until 1979, receiving his doctorate in 1969 (University of Amsterdam) for a dissertation covering “Intramolecular Electron Donor–Acceptor Interactions in N-Alkyl-pyridinium Ions”. This research was conducted under the supervision of Prof. Dr Th. J. de Boer. After a visiting scientist position at the E.T.H. Zürich (Labor. für Molekularbiologie und Biophysik) with Prof. Dr R. Schwyzer in 1971, Jan returned to the University of Amsterdam. In 1979, he was appointed Assistant Professor (Lector) at the University of Amsterdam before being promoted in 1980 to Full Professor in Organic Chemistry. In 2001, he moved to his current position as Professor Emeritus and guest researcher at the University of Amsterdam. He was Secretary of the Faculty of Chemistry from 1979 to 1981, Chairman of the Organic Chemistry Department from 1994 to 1996, and between 1994 and 2001 he was Chairman of the Holland Research School of Molecular Chemistry (HRSMC), a graduate-school covering the University of Leiden, The Free University at Amsterdam, and the University of Amsterdam.

Among many prizes, Jan received the Unilever Chemistry Award in 1966, the Gold medal of the Royal Netherlands Chemical Society in 1976, the Theodor-Förster Memorial Award in 1993, the Snellius medal in 1994, the Bender Lectureship at Northwestern University in 1996 and a JSPS (Japan) Fellowship in 1997. In 2001, he received the Royal Decoration of Knight in the Order of the Netherlands Lion. Jan served as Vice-chairman of the European Photochemistry Association (1990–1992) before being Chairman from 1992 to 1996. He was Chairman of the Chemistry Section of the Royal Netherlands Academy of Arts and Sciences (1996–2000). In 1998, Jan was elected Vice-chairman of the Gordon Research Conference on Electron Donor–Acceptor Interactions and served as Chairman in 2000.

Through-bond charge transfer

In the early 1970s, our attention was drawn to the need to identify renewable sources of energy and the term “artificial photosynthesis” began to appear ever more frequently. Research into light-induced electron-transfer processes began in earnest, often centred around photogalvanic cells. Few researchers considered mechanisms other than bimolecular diffusional contact, involving orbital overlap between the reagents. A series of publications by Verhoeven and coworkers took the alternative viewpoint that long-range, intramolecular charge transfer could occur by way of through-bond interactions, without direct orbital contact.¹ It was demonstrated that efficacious charge transfer could take place across three² or even five³ sigma bonds and this opened up new opportunities to design well-defined donor–acceptor molecules capable of transient charge separation. Acceptance of these ideas was slow in many quarters—I well remember discussions where a very famous scientist simply refused to acknowledge the concept of charge transfer via sigma bonds—but the spectroscopic evidence was too clear to ignore for long and especially the collaborative work of Jan with Michael Paddon-Row and John Warman was instrumental in its general acceptance. Within a decade, the distance dependence⁴ for light-induced electron transfer across non-conjugated bridges had been formulated and recognised by most research groups. This was the real beginning of bio-inspired mimics, where advanced synthetic chemistry took up the challenge of producing multi-component molecular arrays intended to duplicate some of the redox chemistry inherent to the bacterial reaction centre complex. The availability of X-ray data for the natural systems added further fuel to the fire and the Verhoeven group remained at the forefront of the field for many years. The combination of sophisticated synthesis and state-of-the-art spectroscopy used in this work remains a supreme example of chemistry at its very best.

Charge-transfer luminescence

The fluorescence properties of hetero-exciplexes, and especially the inherent solvent effects, as pioneered by, for example, Albert Weller and Noboru Mataga, have played a prominent role in the development of molecular photophysics as a rigorous subject. Jan Verhoeven has made many important contributions to this field, including the notable discovery of intramolecular charge-transfer (CT) emission.⁵ The use of donor–acceptor cyclophanes⁶ confirmed the idea that the probability of CT fluorescence, as well as CT absorption, depends critically on the mutual orientations of the reactants. This ground-breaking work was followed by detailed comparisons of inter- and intramolecular CT emission and into formulating descriptions of the configuration interactions between CT and locally-excited states. A key example of the latter relates to collaborative work carried out with Joshua Jortner⁷ where the importance of coupling with upper-lying excited states was firmly demonstrated. This work, in turn, allowed precise examination of conformational effects and of how changes in excited state energy levels could appear in CT emission spectra and yields. These results,⁸ aided by many other studies along similar lines, facilitated establishment of the conditions that govern the appearance of optical CT transitions in cases where through-bond electronic interactions prevail.⁹ The power of this kind of systematic approach is that robust theoretical understanding accompanies experimental observation.

Solvation effects

Examination of the photophysical properties of a molecule in condensed media invariably requires knowledge of how the excited state responds to changes in the polarity of the surrounding environment. In certain cases, changes in solvent polarity can be used to monitor the dipole moment of the excited state and how the dipole is dissipated upon relaxation. Applying the technique of time-resolved microwave conductivity, in collaboration with John Warman,¹⁰ to study the discharge of giant dipolar excited states allowed the Verhoeven group to produce unique information about the key process of charge recombination.^11,12 Solvent effects on exciplex emission and for controlling the thermodynamic driving forces for light-induced charge separation have also been examined and quantified. Much of this work relied upon the synthesis of simple but well-defined donor–acceptor dyads where the geometry could be varied systematically and where orbitals on the bridge remained isolated from those of the reactants. Methoxylated aromatics found prominent use as the donor and 1,1-dicyanoethylene became a favourite acceptor for much of this work; more recently, fullerenes have been employed as electron acceptors and this work included the first such example. The precise function of the polar solvent came under close scrutiny, especially under extreme conditions of polarity.¹³ This research, much of which was completed in the 1980s, remains contemporary and has helped innumerable PhD students pad out a chapter or two. The disappearance of exciplex emission in polar solvents, as explained in detail by this work, remains one of the standards of molecular photophysics and there are few examples to the contrary. A novel aspect of the work has centred around polarity-induced switching between dipolar states.¹⁴

Conformational effects

Can we expect to derive useful information from flexibly-linked donor–acceptor dyads when studied in fluid solution? Most of us would say “no” and try to work with rigid molecular systems. Such pessimism has not stopped the Verhoeven group from conducting careful studies of the photophysical properties of dyads subjected to varying degrees of internal mobility.¹⁵ Among many factors submitted to critical evaluation has been the folding mechanism whereby flexible linkages bring active terminals into close proximity. The role of electrostatic interactions in such harpoon steps has been explored,¹⁶ together with temperature and viscosity effects. A major outcome of these studies has been critical comparison of through-space and through-bond electron-transfer events, most notably how subtle changes in molecular flexibility affect the dynamics of charge recombination.¹⁷ Conformational changes of intramolecular CT species, studied by means of time-resolved fluorescence spectroscopy, were used to describe a compartmental model by which to explain emission spectral-shape parameters.¹⁸ Ways to circumvent the harpooning mechanism have also been considered in terms of both environmental effects and substitution patterns. The overall outcome is an object lesson in how to avoid following the flow but stick to basic scientific principles and exact protocols. Current research in the photophysics field once again favours flexibly-linked systems!

Energy and electron transfer

Some 20 years ago, the Verhoeven group set out to make a critical examination of the distance dependence for electronic energy transfer between rigidly-linked donor–acceptor pairs.¹⁹ Their conclusion, remarkable at that time, was that the rates greatly exceeded what might reasonably be expected on the basis of Förster theory. It was also discovered that the rate of electronic energy transfer was highly sensitive to the configuration of the bridge, in much the same way as had been found for through-bond electron transfer. Thus, Dexter-type electron-exchange interactions promote through-bond electronic energy transfer, a feature well accepted today but unknown at the time. Over recent years the Verhoeven group have become more occupied with the role of spin correlation in the formation, decay, and detection of long-lived, intramolecular charge-transfer states.²⁰ In particular, attention has been given to using spin to control the lifetime of an intramolecular charge-transfer excited state and the group has pioneered²¹ the technique of generating long-lived, short-distance intramolecular charge separation by way of intermolecular triplet sensitisation. Charge recombination to produce locally excited triplet states has also been examined as a crucial component for shortening the lifetime of charge-separated states.²² Reversible intramolecular electron transfer at the single-molecule level has also been demonstrated.²³

Apart from research, Jan has devoted a large part of his academic career to inspiring new generations of researchers, having received two separate awards for the “Best Chemistry Teacher” at the University of Amsterdam. He has presented “3e Cycle en Chimie” courses in Switzerland, served as member of the QANU Visitation Committee for Chemistry Teaching at the Netherlands Universities, held many guest professorships in physical organic chemistry and helped to organise several international workshops on molecular photophysics. Jan has supervised some 38 PhD candidates. He serves on many committees charged with the responsibility of awarding prizes and related honours. Jan was a member of the IUPAC Commission on Photochemistry from 1981 until 1993.

 

Anthony Harriman

Newcastle University, Newcastle upon Tyne, United Kingdom

References

1. C. Worrell, J. W. Verhoeven and W. N. Speckamp, Through-bond interaction in 1-aza-adamantane derivatives, Tetrahedron, 1974, 30, 3525–3531.
2. P. Pasman, J. W. Verhoeven and Th. J. De Boer, Intramolecular charge-transfer interaction via three sigma bonds, Tetrahedron, 1976, 32, 2827–2830.
3. P. Pasman, J. W. Verhoeven and Th. J. De Boer, Charge-transfer interaction via five sigma bonds, Tetrahedron Lett., 1977, 18, 207–210.
4. N. S. Hush, M. N. Paddon-Row, E. Cotsaris, H. Oevering, J. W. Verhoeven and M. Heppener, Distance dependence of photoinduced electron transfer through non-conjugated bridges, Chem. Phys. Lett., 1985, 117, 8–11.
5. P. Pasman, J. W. Verhoeven and Th. J. De Boer, Fluorescence of intramolecular electron donor–acceptor systems: The importance of through-bond interactions, Chem. Phys. Lett., 1978, 59, 381–385.
6. J.-H. Borkent, J. W. Verhoeven and Th. J. De Boer, Charge-transfer fluorescence from electron donor–acceptor cyclophanes: Influence of geometry and solvent polarity, Chem. Phys. Lett., 1976, 42, 50–53.
7. M. Bixon, J. Jortner and J. W. Verhoeven, Lifetimes for radiative charge recombination in donor–acceptor molecules, J. Am. Chem. Soc., 1994, 116, 7349–7355.
8. P. Pasman, F. Rob and J. W. Verhoeven, Intramolecular charge-transfer absorption and emission resulting from through-bond interaction in bichromophoric molecules, J. Am. Chem. Soc., 1982, 104, 5127–5133.
9. P. Pasman, G. F. Mes, N. W. Koper and J. W. Verhoeven, Solvent effects on photoinduced electron transfer in rigid bichromophoric systems, J. Am. Chem. Soc., 1985, 107, 5839–5843.
10. J. M. Warman, M. P. De Haas, J. W. Verhoeven and M. N. Paddon-Row, Photoinduced electron transfer within donor-spacer-acceptor molecular assemblies studied by time-resolved microwave conductivity, Adv. Chem. Phys., 1999, 106, 571–601.
11. G. F. Mes, B. De Jong, H. J. van Ramesdonk, J. W. Verhoeven, J. M. Warman, M. P. De Haas and L. E. W. Horsman van den Dool, Excited-state dipole moment and solvatochromism of highly fluorescent rod-shaped bichromophoric molecules, J. Am. Chem. Soc., 1984, 106, 6524–6528.
12. J. M. Warman, M. P. De Haas, H. Oevering, J. W. Verhoeven, M. N. Paddon-Row, A. M. Oliver and N. S. Hush, Donor–acceptor and self-quenching of the giant dipole state of a rigid, σ-bond separated donor–acceptor molecular assembly, Chem. Phys. Lett., 1986, 128, 95–99.
13. J. M. Warman, K. J. Smit, M. P. De Haas, S. A. Jonker, M. N. Paddon-Row, A. M. Oliver, J. Kroon, H. Oevering and J. W. Verhoeven, Long-distance charge recombination within rigid molecular assemblies in non-dipolar solvents, J. Phys. Chem., 1991, 95, 1979–1987.
14. S. I. van Dijk, P. G. Wiering, C. P. Groen, A. M. Brouwer, J. W. Verhoeven, W. Schudderboom and J. M. Warman, Solvent-dependent switching between two dipolar excited states in a rigidly extended trichromophoric system, J. Chem. Soc., Faraday Trans., 1995, 91, 2107–2114.
15. B. Wegewijs, R. M. Hermant, J. W. Verhoeven, M. P. De Haas and J. M. Warman, Exciplex-type emission from folded and extended conformations of a donor–acceptor molecule with limited flexibility, Chem. Phys. Lett., 1990, 168, 185–190.
16. A. M. Brouwer, R. D. Mout, P. H. M. van der Brink, H. J. van Ramesdonk, J. W. Verhoeven, S. A. Jonker and J. M. Warman, Electrostatically driven folding following light-induced intramolecular electron transfer in a trichromophoric electron donor–acceptor molecule, Chem. Phys. Lett., 1991, 186, 481–489.
17. W. Jager, S. Schneider and J. W. Verhoeven, Influence of solvent viscosity and permittivity on the harpooning mechanism in semi-rigidly bridged electron donor–acceptor systems, Chem. Phys. Lett., 1997, 270, 50–58.
18. X. Y. Lauteslager, I. H. M. van Stokkum, H. J. van Ramesdonk, A. M. Brouwer and J. W. Verhoeven, Conformational dynamics of semi-flexibly bridged donor–acceptor systems studied with a streak camera and spectrotemporal parametrization of fluorescence, J. Phys. Chem. A, 1999, 103, 653–659.
19. J. Kroon, A. M. Oliver, M. N. Paddon-Row and J. W. Verhoeven, Observation of a remarkable dependence of the rate of singlet-singlet energy transfer on the configuration of the hydrocarbon bridge in bichromophoric systems, J. Am. Chem. Soc., 1990, 112, 4868–4873.
20. L. Hviid, W. G. Bouwman, M. N. Paddon-Row, H. J. van Ramesdonk, J. W. Verhoeven and A. M. Brouwer, Spin control of the lifetime of an intramolecular charge-transfer excited state, Photochem. Photobiol. Sci., 2003, 2, 995–1001.
21. L. Hviid, A. M. Brouwer, M. N. Paddon-Row, H. J. van Ramesdonk and J. W. Verhoeven, Long-lived short-distance intramolecular charge separation via intermolecular triplet sensitisation, ChemPhysChem, 2001, 2, 232–236.
22. A. C. Benniston, A. Harriman, P. Y. Li, J. P. Rostron, H. J. Van Ramesdonk, M. M. Groeneveld, H. Zhang and J. W. Verhoeven, Charge shift and triplet state formation in the 9-mesityl-10-methylacridinium cation, J. Am. Chem. Soc., 2005, 127, 16054–16064.
23. T. D. M. Bell, A. Stefan, S. Masuo, T. Vosch, M. Lor, M. Cotlet, J. Hofkens, S. Bernhardt, K. Mullen, M. van der Auweraer, J. M. Verhoeven and F. De Schryver, Electron transfer at the single molecule level in a triphenylamine-perylene imide molecule, ChemPhysChem, 2005, 6, 942–948.

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