Molecular Photochemistry and Photonics Research

My research background is in photochemistry and molecular spectroscopy and since joining the faculty at Imperial College in 1989 I have created some new projects that are a combination of pure, applied and industrial research. The work still has a firm foundation in laser photochemistry, but is focussed more on the development of new optical materials. I have established strong links with other research groups whose interests range from synthesis chemistry through to device physics. My long term objective is a molecular structure-property relationship that will provide the rules for conceiving new optical materials on a molecular level.

My current research programmes fall in to four categories:

Brief descriptions of these projects are given below. The work is funded through grants with the UK funding agencies: The Engineering and Physical Sciences Research Council (EPSRC) and The Royal Society. Additional financial support is also made through Kodak Research UK ltd, Unilever Research Port Sunlight and Photek UK ltd.

Nature of the light emitting species in conjugated molecules.

Picosecond time-resolved photoluminescence spectroscopy and photoluminescence quantum yield measurements are used to determine the nature of the light emitting species in conjugated polymers and model compounds. We are currently focusing our efforts on understanding the migration, trapping and transfer of excitation energy in poly (p-phenylene vinylene) derivatives, with an emphasis in understanding the role of interchain excitations. As the emitting species for photoluminescence and electroluminescence is often the same, by working closely with groups on the synthesis, we can provide information on how to optimise the molecular structures for materials for light-emitting devices,

This work is carried out in collaboration with Dr. Ifor Samuel (University of Durham), Dr. Andrew Holmes (University of Cambridge), Dr. Paul Burn (University of Oxford) and Dr. Keith Davidson (University of Lancaster).

Applications of anti-Stokes luminescence

Anti-Stokes fluorescence is an unusual effect with emission occurring to higher energy of the excitation light. We first observed this phenomenon in a polystyrene waveguide structure doped with the dye, rhodamine B observing a yellow luminescence when guiding the red, 632.8 nm light from a helium-neon laser. The same effect could also be observed in dilute fluid solutions and could be extinguished by cooling the sample to 180 K. The luminescence is attributed to the electronic excitation of vibrationally hot molecules that subsequently relax radiatively to a lower vibrational state, emitting a photon of higher energy. We have been developing ways of utilising this novel effect to control and measure the temperature of materials optically. Two projects that are successfully utilising this effect are detailed below:

Laser induced optical refrigeration: We recently demonstrated that anti-Stokes luminescence could be used to optically cool a condensed phase sample. By focusing 350 mW of laser light into an ethanolic solution of the dye rhodamine 101 for four hours we observed a temperature drop of 4 degrees. We are currently optimising the range and efficiency of the cooling process and in collaboration with the aerospace industry we are developing new applications to which the effect can be put.
All-optical temperature sensing: A material for all-optical temperature sensing has been developed that exhibits both Stokes and anti-Stokes fluorescence. This material has been incorporated into a device that has enormous potential for operating in hostile electrical and magnetic environments, where conventional, metallic sensors cannot work. This idea now has patent protection and a commercial product is being developed with an industrial partner.

Photochemistry and photophysics of molecular materials

One of the most exciting prospects of using molecular materials for optical applications is the opportunity to exploit the established procedures of chemical synthesis to modify and optimise the molecular structure to suit specific objectives. This requires the understanding of the relationship between the property of the material and the structure of the molecule from which it is prepared. One of the main aims of my research is the development of these structure-property relationships. The case of electroluminescent polymers, discussed above, is one such example.The following list highlights some of these projects, where we use photoluminescence spectroscopy to analyse new compounds that exhibit novel photophysical properties. The work forms the part of key collaborations with both industry and academe.
Porphyrazines: These are an example of a tetra pyrrolic macrocycle that belong to the same class of compounds as porphyrines and phthalocyanines. The electronic properties depend strongly on peripheral substitution, like porphyrines, but the difficult synthesis procedure has been developed, like phthalocyanines. In a recent study the efficient photosensitized oxidation of one of these molecules was demonstrated. We are also exploring the possibility of developing materials based on porphyrazines that exhibit optical limiting properties and can be used to protect sensitive light detectors from laser light. (Professor A.G.M. Barrett, Dr. A. Garido-Montalban)
II-IV nanoparticles: Semi-conductor materials synthesised as solvated quantum dots exhibit photophysical properties that differ markedly from both bulk materials and molecules and offer the opportunity to investigate excited state species confined in a particle of nanometer dimensions. The materials are prepared using a new single-pot synthesis procedure with photoluminescence spectroscopy used to augment conventional characterising procedures. (Professor P.O'Brien)
Organic molecular beam deposition: Using a modified molecular beam epitaxy vacuum system thin films of molecular materials are being grown under highly controlled deposition conditions. The long term objective is the development of molecular materials that can be deployed in specialised electronic and photonic devices. Photoluminescence and Raman spectroscopy form two of the ex-situ diagnostic techniques that are being used to monitor the growth of the materials and the impact of surfaces and interfaces with other materials. (Professor T.S. Jones)
Ionic liquid solvents: Using photophysical probe molecules we study the solvent environment offered by ambient temperature ionic liquids and compare them with those of conventional molecular solvents. This unusual solvent, apart from having no vapour pressure, is proving to be an ideal system in which to examine oxygen sensitive fluorophores. (Dr. T.Welton)
Optical brighteners: In collaboration with Unilever Research, Port Sunlight, we are examining the photophysics of a series of stilbene derivatives that are used as optical brighteners in washing detergents. The sensitivity of time-resolved photoluminescence anisotropy spectroscopy is allowing us to investigate the aggregating properties of these luminescent molecules in solution and when bound to fabric surfaces.
Photographic dyes: The degradation of photographic dyes is being investigated as part of a long term collaboration with Kodak Research UK ltd. The evanescent wave-induced fluorescence technique, that we have developed in our laboratory, is being used to investigate the impact of surfaces and interfaces on the photodegradation mechanism.

Techniques

Time-resolved photoluminescence
Time-correlated single-photon counting is an elegant, simple and sensitive technique for measuring photoluminescence decay profiles. It is based on a sampling technique that times the arrival of an emitted photon following excitation of the sample with a pulse of light.
Using a cavity-dumped, synchronously pumped, mode-locked dye laser pumped by the frequency doubled or tripled output of a mode-locked, cw, Nd:YAG laser we can generate picosecond excitation pulses at repetition rates of up to 5 MHz. We spectrally resolve the emission and then using a fast photon counting microchannel plate photomultiplier tube as a detector we can measure luminescence decays at a fixed wavelength with a time resolution of <. 50 picoseconds. Two laser systems are available, providing us with the flexibility of exciting luminescence over a range of excitation wavelengths from 250 nm to 850 nm, allowing us to study a wide variety of materials of photophysical interest.
Steady-state photoluminescence
Two small frame argon ion lasers and a dye laser are used as excitation sources in a steady-state luminescence spectrometer. We employ two different detection systems that use either a 1 m monochromator with single-photon counting detection or a small spectrograph with a CCD camera detector. We can configure this apparatus to detect both Raman or luminescence signals. A commercial photon counting spectrometer, using a xenon lamp excitation source, is also available for routine spectroscopy, along with conventional uv/vis spectrometers.
Evanescent wave-induced luminescence
Using the evanescent wave associated with the total internal reflection process we are able to investigate both the steady-state and time-resolved fluorescence properties of fluorophores at a dielectric interface. This technique has been developed over a number of years and allows us to investigate the impact of surfaces and interfaces, both of which can be controlled, on the photophysical properties of a variety of different molecules ranging from polymers to dyes and optical brighteners.

Research funding

1998 - 2001 EPSRC Physics GR/L84179
'Condensed phase laser cooling'.
1998 - 2000 EPSRC Materials GR/M34478
'Nature of the excited state in electroluminescent polymers'.
1997 EPSRC Industrial CASE studentship with Kodak Research UK limited.
1997 Royal Society Small Grant RSRG 18663
'IR-emitting molecular materials'.
1995 EPSRC Industrial CASE studentship with Unilever Research Port Sunlight.
1993-1996 SERC Materials GR/J02063
'Time-resolved and steady-state luminescence properties of conjugated polymers and related molecules'.
1992 Royal Society Small Grant RSRG 11836
'Picosecond coherent anti-Stokes Raman scattering from conjugated molecules in solution'.
1992 SERC CASE studentship with Photek UK limited.
1991 - 1994 SERC Physics/Materials GR/H10016
'Coherent Raman spectroscopy of organic non-linear optical materials'.
1991 SERC CASE studentship with Kodak Research UK limited.
1990 RSRG Royal Society Small Grant RSRG 10344
'Coherent anti-Stokes Raman scattering of soluble polydiacetylenes'.
1990 SERC CASE studentship with Applied Photophysics limited.

Updated 5th March 1999