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Inter-American Photochemical Society Newsletter
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| Volume 19 Number 1 |
May 1996 |
Table of Contents
1994-1996 I-APS Officers
Call For Nominations
Nominations should include a statement describing the merit of
the contribution to be recognized and should clearly state whether
the award is to recognize contributions in science or in service.
There is no requirement for seconding letters. The nominee must
be a current member of the Society to be eligible, unless the
nominator argues convincingly that there are exceptional circumstances.
The deadline for nominations is August 1, 1996.
Send nominations for either the I-APS Award or for Fellowship
to:
Professor David R. McMillin
Minutes of the Meetings of Commission III-23 (Photochemistry)
Communicated by
Present : J. Coyle (Chairman), R. Bonneau (Secretary), J. Wirz,
F.D. Lewis J. Bolton, F. Saito, S.C. Shim, Y. Cao
The Commission met on the 5, 6 and 7 August to work on current
projects, definition of new projects, changes in the membership
of the commission, exchanges with other commissions and discussions
on the desirable changes in the way IUPAC Divisions and Commissions
operate. The Open Meeting of the Organic Chemistry Division was
held on the Monday 7th.
The first session was opened by our Chairman who welcomed the
members of the Commission and then reported on the meeting of
the Committee of Division III with Chairmen and Secretaries of
the Commissions of that Division, held on Friday 4th. During this
first session, also attended by the P.Müller, liaison agent
between our Commission and Division III, D.Eaton, a former member
of Commission III-3 and now a member of the IUPAC Executive Committee,
gave further comments on the need of changes in IUPAC and the
modifications under consideration for the operation of Divisions
and Commissions and the Commission was honoured by the visit of
Pr Ôki, the Division President.
Current projects :
The final version of the "Glossary of Terms used in Photochemistry
(2nd edition) including Terms used in Photoinduced Electron Transfer",
taking into account the comments received from IDCNS and various
international sources (after a long delay), should be written
by the coordinator of this project (Pr. I. Verhoeven, no longer
a member of the commission). It is expected to be ready for publication
before the end of 1995.
After revisions and additions suggested by the Commission during
the discussions at Guilford, the documents on the "Methods
for the Evaluation of Transient Absorption Kinetic Data"
and on the "Figures of Merit for the Technical Development
and Application of Advanced Oxidation Processes" (first part
of the project on "Water Treatment and Photocatalysis")
should be very close to their finalized version but should be
examined by external experts before being submitted for publication.
A group of ten experts has been identified for critical examination
of the first of these documents and a finalized version taking
into account their comments should be prepared at the end of 1995.
Expertise of the second document will be obtained by publication
in a specialized journal, the "Journal of Advanced Oxidation
Technologies", with a call for comments from the readers.
A finalized version of this document is expected to be available
within the next year.
Strong modifications and revision of the document on "Quantum
Yields and Turnover Numbers in Heterogeneous Photocatalysis"
has been requested by the Commission. One of the new Associate
Members, Prof. Pelizzetti, and the new Chairman, Prof. Bolton,
will assist the assist the author (N. Serpone, not a member of
the Commission) in this task. This second part of the project
on "Water Treatment and Photocatalysis" has been disjoined
from the first one to avoid undesirable delay in the publication
of the "Figures of Merit...".
Study of new projects :
Our Chairman maintained contacts with Dr.H.H.Tønnesen from
the Institute of Pharmacy of the University of Oslo, but the proposal
of our Commission for expertise in the definition of the procedures
for testing the "Photostability of Drugs" found no (or
very little) echo in the pharmaceutic community. (In fact, one
week after the end of the Guilford meeting, Dr.H.H.Tønnesen
informed our Chairman that guidelines acceptable by the european,
japanese and american pharmaceutic industries are expected to
be signed off in a near future and that, in her opinion, "there
is at present no need for an IUPAC project in the area of photostability
testing of drugs".)
Dr E. Vrachnou-Dorier presented last year, in Prague, a proposal
for a project on "Photochemical Solar Energy Conversion and
Storage" and was asked to give a more precise evaluation
of the content. She proposed, this year, the creation of a Data
Bank accessible via Internet and including the rate constants
and mechanisms of the reactions (potentially) useful for the photochemical
conversion and storage of solar energy as well as the properties,
physical constants, functions and stability of the various chemicals
involved in these reactions. This project faces a major problem,
the financial cost, but its feasibility will be studied and tested
by Dr E. Vrachnou-Dorier, who becomes an Associate Member of the
Commission, and its real cost will be evaluated by J. Bolton and
F.D. Lewis.
An other new Associate Member, Dr. Acuña Fernandez, is
asked to define the content and limits of a future project on
the problems arising from light polarisation in studies of films
of biological or polymeric macromolecular systems During the joint
meeting held on August 6th, Commission I-6 (Colloid and Surface
Chemistry including Catalysis), said to be interested by such
a project and should be informed of any development of this idea.
Concerning a possible project in the field of "Photopolymerisation",
the Commission should contact industrials and specialists to know
what are the problems. Pr J.E. Guillet, Secretary of the Macromolecular
Division, seems to be the first person to contact.
Prof. J. Bolton suggested the possibility of publishing a combined
volume of all the Commission's documents finalised to date should
be pursued : certainly, for many photochemists, this would greatly
facilitate the access to these documents and would thus increase
the impact of the Commission's work on the photochemists community.
In a first step, a list of the published Commission's documents
will be published in the EPA, JPS and IAPS Newsletters
as an Addendum to these Minutes.
Changes in the Commission's membership : (effective in
January 1996)
The Titular Member position of F. Scandola, who resigned his membership,
was attributed to Prof. J. Bolton who was elected as new Chairman.
Four new Associated Members joined the Commission : U.Acuña
Fernandez, H. Bouas-Laurent, E. Pelizzetti and E. Vrachnou-Dorier.
New National Representatives proposed by their National Organisations,
were accepted by the Commission : Dr. V. Toscano (Brazil), Dr.
B. Pandey (India) and Prof. S. Içli (Turkey).
The Commission gratefully acknowledged the action of Dr. J. Coyle
as Chairman
Changes in IUPAC :
The Commission was informed of the efforts made at various levels
of the IUPAC organization to improve its efficiency and its image
in the eyes of the public (specially those of the sponsoring industries).
The Commission strongly supports these attempts and already puts
into practice some of the new suggestions such as those relating
to the appointment of new members : new Members will be allocated
to Commissions, for a limited number of years and from a pool
of Member positions managed by the Divisions, only in direct connexion
with an approved project. Our new TM (and Chairman) is in charge
of an approved project and three of the new AM have been chosen
to get progress in one or the other of the current or future projects
(the fourth one, H. Bouas-Laurent, will think about a possible
project on "Photochromic Materials").
Next Meetings of the Commission :
The next meetings of the Commission will be in Helsinki, during
the next IUPAC Symposium on Photochemistry, 21-26 July 1996, and
then in Geneva, during the 39th IUPAC General Assembly, 23-30
August 1997.
Luminescent Sensors: Sensor Support
B. A. DeGraff,
and
J. N. Demas,
INTRODUCTION
There has been a lively interest in the design and application
of luminescent materials as probes and sensors. Examples of these
molecular reporters include fiber-optic based luminescent sensors
for measurement of O2, pH, pCO2, temperature, and immunoassay.
Typically the desired information is encoded in changes in luminescence
intensity, lifetime (t), or spectral distribution. Most practical
sensors utilize luminophores in or on a polymer support. This
support can function as a passive anchor or play an active role
in detection. However, almost always, the support alters the
behavior of the sensor, sometimes in completely unexpected ways.
Understanding the interactions between the sensor and the support
matrix is a continuing challenge for producing useful devices [1-4].
Among the most promising luminescent reporters are compounds based
on transition metal complexes. Complexes of the platinum metals
group, particularly Ru(II), Os(II) and Re(I) have generated considerable
interest due to their strong visible absorption, good photochemical
stability, efficient luminescence, and relatively long lived metal
to ligand charge transfer (MLCT) excited states [5]. Sufficient
data is now available that the rational molecular engineering
of desirable properties is frequently possible for these materials.
The emitting-state energies and excited state redox properties
of the complexes can be very sensitive to variations in the metal,
coordinating ligand, and solvent or support matrix. Many of these
luminescent complexes exhibit a wide variety of energetically
accessible charge transfer (CT), ligand field (LF), and intra
ligand (IL) excited states. Understanding the structural factors
that affect these various states permits the systematic alteration
of spectroscopic and chemical properties. This flexibility allows
systems to be designed that respond to specific environmental
variables, permits either ionic or covalent attachment to a support
and allows the absorption and emission properties to be tailored
to available excitation sources and detectors. Thus, good guidelines
exist to tailor sensor response to applications [6,7]. However
because of the perturbations caused by the binding or support
of the sensor on the matrix, an intimate understanding of the
detailed interactions responsible for achieving selective binding
to specific sites and for controlling or predicting behavior in
different microenvironments is essential.
For sensor design, a polymeric support cannot be treated as a
continuous medium because there frequently exists a number of
microdomains. These domains are usually essentially static during
the excited-state lifetime of the sensor and thus, the sensor
in different domains experiences different average environments
during it decays. This heterogeneity of environments is referred
to as microheterogeneity. If the domain properties remain constant
during the excited lifetime of the sensor, the term persistent
heterogeneity is appropriate. The presence of these interactions
in most polymer supported luminescent sensors makes it very difficult
to extrapolate sensor behavior based solely on characterization
in homogeneous media. At the moment, a comprehensive understanding
of the factors that control sensor/support interactions and how
these modify luminescent response is still in the most formative
stages.
Our recent studies have been an attempt to elucidate the types
of interactions that occur when luminescent metal complexes of
Ru(II), Os(II), and Re(I) are supported by a family of polymers
based on a polydimethylsiloxane (PDMS) backbone. These support
materials were chosen for several reasons. First, we were interested
in developing an O2 sensor and thus good gas permeability was
a major concern. Second, we envisioned that this probe might
have medical applications and thus biocompatibility would be important.
Finally, there are a large number of modifications that can be
made by using various terminators and cross linkers for this family
of polymers. A typical PDMS polymer and a few of the many available
crosslinkers we have explored are shown below.
While a variety of sensors have been tested, much of our work
has employed the Ru(II) complex, [Ru(Ph2phen)3]Cl2 (Ph2phen =
4,7-diphenyl-1,10-phenanthroline), as the molecular reporter.
In homogeneous media such as methanol, this complex shows a 20
fold change in emission intensity on going from a nitrogen to
air atmosphere. Thus, the complex has potential as an oxygen
sensor [8,9]. Its long lifetime ( >5 µs) allows the use
of inexpensive monitoring equipment. In homogeneous media the
complex typically displays simple Stern-Volmer behavior as shown
by
to/t = 1 + Ksv [Q] (1a)
Io/I = 1 + Ksv [Q] (1b)
KSV = k2to (1c)
where t's and I's are the luminescent lifetimes and intensities,
respectively. Ksv is the Stern-Volmer quenching constant, and
k2 is the bimolecular quenching constant. Plots of to/t or
Io/I versus [Q] should be linear with identical slopes equal to
Ksv. In systems where static quenching can occur, the intensity
expression, (1b), must be modified to
Io/I = 1 + (Ksv + Keq)[Q] + KeqKsv[Q]2 (1d)
in which Keq is the association constant for binding of the quencher
to the complex. If static quenching is present, then the lifetime
and intensity quenching curves do not track the same and the intensity
curve will lie above the lifetime curve.
SENSOR/SUPPORT INTERACTIONS
In many microheterogeneous systems simple homogeneous models fail.
The intensity Stern-Volmer plots are generally curved downwards,
which is the opposite of mixed dynamic-static quenching plots
[10]. Further, the decays are invariably multiexponential. It
is dangerous to predict supported sensor behavior as a simple
extrapolation of homogeneous trends. Several factors can dramatically
change the observed quenching when the media is altered. These
are related to quencher availability and transport and the efficiency
of the quencher/sensor encounter. Frequently the presence of microdomains
within the support which rise to multiple decays by virtue of
different Ksv's. Thus, changes in quencher diffusion rates and
solubility in the support matrix, as well as changes in the efficiency
of the quencher/sensor encounter from domain to domain can alter
behavior. From our work with metal complexes, we have found that
a two site model fits virtually all our quenching data [9,11].
However, for some organic systems, a Gaussian distribution of
sites seems more appropriate (e.g. organic sensors bound to silica).
Two-Site Model
The downward curvature of the Stern-Volmer intensity plots necessitates
a model more complex than a single species quenched bimolecularly.
We have evaluated several possible models. The one that satisfactorily
duplicates our data involves the complex existing in two distinctly
different environments with both being quenched but with different
rate constants. This model yields an intensity Stern-Volmer equation
Io/I = {fo1/(1+Ksv1[Q] + fo2/1+Ksv2[Q]}-1 (2)
where foi are the fractions of the total emission from each component
under unquenched conditions and the Ksv's are the associated Stern-Volmer
quenching constants for each component. If either of the f's
is zero than the other term dominates and the normal Stern-Volmer
equation results.
A cartoon showing one possible concept of this model is shown
above. For our studies to date, the two regions differ in their
hydrophobic nature or polarity. The pure PDMS backbone is very
hydrophobic and quite permeable to oxygen, with a Tg well below
room temperature. We find that even adding a small amount of
certain crosslinkers or subtle modifications of the terminator
groups can dramatically change the sensor response. Indeed, several
of our attempts to improve the physical properties of the support
matrix have resulted in disastrous effects on the actual probe
response. This can occur for several reasons. First, the metal
complex may have preferential solubility in one region or another
and the Ksv for the two regions may be quite different. Second,
a little crosslinking goes a long way in restricting segmental
motion of the backbone and can produce significant variations
in the ease of oxygen access to sensor in a particular type of
site. At present, the exact basis of the profound effects found
for very small amounts of copolymer is still being unraveled.
However, the changes that can occur, even for 1% additions, represents
an elephant trap for the unwary and "simple" modifications
should be approached with caution. A typical set of results for
the addition of several crosslinkers to a primarily PDMS backbone
are shown below. Here we see both an increase and dramatic decrease
in sensitivity as compared to the unmodified polymer by addition
of very small amounts of various crosslinkers. This type of response
is typical for a number of complexes and a variety of polymer
supports [12].
A final word of caution about the two site model which has served
us well to this point. Though this model has been shown to fit
a number of systems well, it is tempting to over-interpret the
parameters derived from the model [13]. This is first and foremost
an empirical approach to extracting fits from difficult systems.
One still needs to be cautious when ascribing chemical characteristics
to the two sites and we suggest that under interpretation is the
lesser evil.
Temperature Effects
A problem not unique to but frequently present in the use of a
supported sensor, is the loss of intensity and lifetime shortening
that usually accompanies increased sensor temperature. This problem
has received extensive attention for homogeneous media [14] and,
alas, the problem is evident in polymer supported systems as well.
The most studied are the Ru(II) complexes and here the general
model is illustrated by the figure shown below.
Here the first excited level is the MLCT, and kr and kq represent
emissive and non-emissive depopulation of that state respectively.
For this model the expressions
1/t = (kr + kq) + knr(T) (3a)
knr(T) = k'exp(-ÆE/kT) (3b)
are appropriate where t is the observed lifetime, knr(T) the rate
constant for thermal population of the d state from the MLCT state.
k' is the pre-exponential factor for the thermally activated
deactivation of the emitting state via the nonemissive d excited
state located ÆE above the MLCT state. This model assumes
that once the d state is reached, loss of emission is certain.
However, the alternative model of an excited state equilibrium
between the emitting MLCT state and the d state yields slightly
different expressions which are indistinguishable experimentally
from eqns (3a) and (3b) [13].
It appears that the upper d levels are accessible thermally from
the MLCT level for many Ru(II) based sensors. Population of this
upper d level leads to two possible outcomes, both undesirable.
First, the sensor can simply return to the ground state via a
non-emissive transition. This results in loss of sensor intensity.
Second, and more damaging, while in the upper d state photochemistry
can occur. While quantum yields are frequently small for these
processes, over time the sensor performance can degrade significantly.
The problem of photostability can be ameliorated significantly
by use of the Os(II) complex, but like all medicines, there are
side effects including lower quantum yields and decreased lifetimes.
We are currently exploring the use of Os(II) complexes for sensor
applications [15]. Finding a way to anchor the sensor to the
polymer that will reduce the level of photochemical loss is also
a major challenge presently being explored.
THE FUTURE
The two examples discussed are to give the reader a flavor of
the interesting problems associated moving from the cuvette to
a practical device. The understanding and ultimately the rational
manipulation of sensor/support interactions is in its very early
stages. Given the number of possible sensors and an even wider
variety of supports, some systematic framework will be needed
if fabrication of practical devices is not to be a hit-or-miss
affair. Just like the tennis or golf player who gets a spark
from that one sweet shot, we conclude this part with an illustration
of emission intensity variation as a function time for a coworker
breathing over a PDMS/Ru(II) complex sensor. The change in breath
composition is clearly visible. While there are still many problems
to be solved, such results do make the quest worthwhile.
COLLABORATION
We could not conclude this little piece without noting that many
of the results here and from other aspects of our work are the
result of a long collaboration. This interaction has proved productive
both from a scientific and personal standpoint. One unique feature
is the role that undergraduates have played. James Madison has
no graduate chemistry program and the JND group is very pro-active
about including undergraduates in the research program. The collaboration
between the two groups has proved to be a win-win situation.
The JMU faculty and students can be part of a larger undertaking
than would normally be possible at an undergraduate institution.
The U.Va. group gains considerable extra effort and expertise
in areas not represented in that group. Further, funding agencies
like the N.S.F. have been very supportive of this type of interaction
and with the increased emphasis on the educational aspects of
academic research at all levels, this is a non-trivial feature.
At a time when resources are scarce and students fewer, we would
urge more faculty at both undergraduate and graduate institutions
to explore this type of collaboration as both the students and
the faculty involved will profit.
REFERENCES
1. Lieberman, R.A.; Wlodarczyk, M.T. (eds.), Chemical, Biochemical,
and Environmental Sensors, Vol. 1172, International Society
for Optical Engineering, Bellingham, WA, 1988.
The Trail of This Photochemist's Beginnings
Jack Saltiel
I write this personal account of my early steps toward and in
photochemistry in response to several requests by Kirk Schanze
and by V. Ramamurthy before him. Perchance it may make the lives
of future Heinz Roths a little easier.
I started school immediately after World War II when my family’s
hiding days in Athens ended and we returned to Thessaloniki, our
hometown. Learning was a great virtue to my parents and I was
always encouraged to be a good student. The holocaust had robbed
us of most of my uncles and aunts and cousins and friends, and
my father's business never recovered. "Become an
engineer," my mother would say, "they can take all of
your material possessions, but they can never take what is in
your head." In 1951 we immigrated to Houston, Texas and
three years later in Mr. Finfrock's class in San Jacinto
High School I first encountered chemistry. Mr. Finfrock was a
dedicated teacher who knew and taught his subject well. He challenged
the three or four best students in his class to volunteer for
a second year of chemistry in which experimental work and fun
in the laboratory were heavily emphasized. I was disappointed
to receive a rejection letter from the Rice Institute where a
high quality tuition-free education could be had. Since I was
graduating close to the top of my high school class, this apparent
injustice to a poor immigrant boy did not sit well with my bosses
at Gordon's Jewelry Co., and with the help of rabbinical
intervention was not allowed to stand. I heard rumors of quotas,
but have no first hand knowledge of what brought about Rice's
change of heart. Soon after returning the rejection letter as
requested, I received a standard acceptance letter from Rice with
no explanation.
My initial notion of becoming a mechanical engineer was soon dispelled
as I struggled through an engineering drawing class as a freshman.
Chemistry, on the other hand, that was so difficult to so many,
was, thanks to Mr. Finfrock, a breeze to me. With all the Texas
oil around, it was also big in Houston. After briefly considering
Chemical Engineering (Do you really want to be a glorified plumber?
one of my Chemistry Professors queried) I settled on Chemistry
as a major. Undergraduate research in Professor E. S. Lewis'
laboratory on the kinetics of reactions of diazonium salts introduced
me to Physical Organic Chemistry. Working with Ted was a thoroughly
enjoyable experience and as one of the few holders of a key to
the Cary 14 room, where I spent many hours, was wonderful for
my ego.
I entered my senior year with big plans. I would graduate and
find a job as a chemist in one of the several oil companies in
the area. Shell, where an acquaintance of my family worked, seemed
especially attractive. Pursuing a higher degree was furthest
from my mind when Professor R. B. Turner stopped me one day in
the hallway and asked me where I was going to graduate school.
When Turner understood that I had given no thought to that possibility,
he asked about my grades. "You will go to graduate school"
he said. "Every Rice student with an average of B or better
goes to graduate school." He offered to provide guidance
and not long after that all graduating chemistry majors, about
six of us, met in his office. I recall his ranking the top 5-10
programs in the country according to the overall quality of program,
the quality in specific research areas and snob appeal. With
my interest in physical organic chemistry my list included Caltech
(Hammond, Roberts), Illinois (Curtin), MIT (Cope) and UCLA (Cram,
Winstein). I eliminated Yale because I did not feel up to working
for "the best chemist in the world", Doering. Caltech
offered a choice of two excellent mentors and in addition ranked
very high on TurnerÕs snob appeal list. When asked to
narrow the list down to my top choice I said Caltech and Turner
immediately got Jack Roberts on the phone and, in my presence,
made my case. "It is not 100%, but 99% certain that you
are in" he said. Another university on Turner's lists
that did not seem important then, but became of major importance
later was Florida State University. It was at the top of the
lists of schools of the next rank for students with grades somewhat
below B. Two of the six students in my class went to graduate
school at FSU including Joel Gilbert, my best friend from San
Jacinto High School days. Ted Lewis a man of very few words,
had something to say only after he found out that I had accepted
Caltech's offer. "You cannot go wrong by working
for either Roberts or Hammond," he said. And after a long
pause, "Hammond would be a good choice if you wanted to
go very deeply into a subject." He said nothing of his
friendship with George from their days together as graduate students
in Paul Bartlett's laboratory.
At Caltech it did not take me long to realize that Turner had
pulled something of a miracle by seeing to it that I be included
in that select group of twenty new students. Suffice it to say
that Nick Turro and George Whitesides ended up as the top students
of my class. Roberts seemed somewhat intimidating, and probably
because he is so much taller than I, somewhat distant. It also
did not help that (knowing nothing of his hearing problem) he
scared me out of my wits when I went to talk to him about research.
In response to my knock he had opened his office door and asked
me what I wanted. When he did not hear my soft reply and retorted
from his heights with a loud "what?" I deduced that
asking about his research was terribly inappropriate. Hammond
was now the only choice and one of the two available spots in
his laboratory was already claimed by Nick Turro. Nick had started
at Caltech during the Summer and, it seemed to me, was well on
his way to irradiating all the chemicals in Caltech's stockroom.
To make matters worse several other of the new students wanted
the remaining spot in Hammond's laboratory, and Hammond
was moving away from free radicals and other areas as he was becoming
more and more involved in the chemistry of triplets, mysterious
entities to me. As time was of the essence, I rushed to his office
and after confessing that I had no idea what triplets were, I
offered to work on any other project that he wished to see completed.
I landed the open spot and was soon working on the quantitative
determination of bibenzyl yields from the thermolysis of different
concentrations of benzoyl peroxide in toluene. We were searching
for evidence for p-complexes between radicals and aromatics.
Two other graduate students and a postdoctoral fellow had worked
on the project and bibenzyl remained as the last unquantified
product. After an ominous beginning when Bob Neuman found me
distilling 14C-labeled toluene without the benefit of even a hood,
my research proceeded smoothly. Isotopic dilution was employed,
the bibenzyl augmented hydrocarbon product mixture was subjected
to SeO2 oxidation and the benzil separated by column chromatography.
Reaction of the benzil with o-phenylenediamine yielded the quinoxaline
whose activity before and after recrystallization was determined
by scintillation counting.
When upon completion of my part of the free radical project I
sought Hammond's advice on future research, he said "Well
Jack, you have been here almost a year and you know now what triplets
are. Why don't you do some photochemistry?" Not
having a clue as to what I should do, I looked for guidance in
what others in the photochemistry part of the research group were
doing. The project upon which Turro and Leermakers were embarking
provided my inspiration. They were investigating the role of
triplet biradical intermediates in the Diels Alder reaction by
determining whether benzophenone could photosensitize the addition
of the piperylenes to maleic anhydride. My free radical project
had made me especially fond of benzil and of phenyl groups in
general. Not wishing to venture very far into unfamiliar territory,
I thought I would investigate the [4+2] addition of benzil triplets
to trans-stilbene. "Great idea. Do it." was George's
reaction. George was never one to discourage his students! Irradiation
of a benzene solution in a Hanovia reactor yielded, upon concentration,
not the beautiful white crystals I expected but a yellow viscous
oil that defied attempts at crystallization. Well, this was my
first irradiation and something must have gone wrong, so down
the sink it went. On repetition the experiment yielded identical
results and the yellow oil met the identical fate. I had reached
the conclusion that it was time to search for another project
when Karl Kopecky, who was on his second year as a postdoctoral
fellow with George, entered the lab. He whiffed the air with
obvious concern. "Who has been messing with my cis-stilbene?"
he asked. Karl had synthesized cis-stilbene for some thermal
isomerization experiments and was very familiar with its characteristic
fragrance. I knew immediately that I had hit pay dirt. Days
earlier Turro and Leermakers had shown that benzophenone triplets
sensitized the cis-trans photoisomerization of piperylene, a finding
that suddenly loomed large. The benzil-sensitized photoisomerization
of stilbene yielded photostationary states that were nearly pure
cis-stilbene (92-95%), a result not matched by any of the myriad
of sensitizers employed since then. Serendipity had smiled upon
me and I was finally embarking on my dissertation research.
It was becoming clear in the Hammond group that substrate response
could be controlled by changing triplet sensitizer. This was
at first puzzling, although we were convinced that triplet energy
transfer was involved. While others in the group were trying
to find a sensitizer structure/response relationship, I concentrated
on the sensitizer triplet energy as the controlling factor. Bill
Herkstroeter had an old-fashioned phosphoroscope running in Wilse
Robinson's laboratory. With the classic Lewis and Kasha
paper as a guide, I selected potential sensitizers for purification.
Bill would measure their phosphorescence on photographic plates,
and I would run to one of the biochemistry laboratories where
I had secured access to a densitometer in order to record the
spectra. Such were the beginnings of the Saltiel plots.
This story would not be complete without a word about the exciting
and nourishing atmosphere of the Hammond research group and of
Caltech generally. The sharing of ideas and the friendships were
invaluable. The photochemists in the group are well known to
you as the core of the "Hammond Mafia". Others whose
help in forming my first steps I remember vividly include Guy
Moses, Bob Neuman, Hal Waits and Jim Clovis, and outside our group,
Mike Fisch and George Whitesides. Above all there was George
Hammond. Some impressions of George are expressed in my letter
to David Eaton, sent on the occasion of the Symposium "Photochemistry
Faces the 21st Century: A Tribute to George S. Hammond" held
at Bowling Green State University in March 1990:
Dear David:
Reaching into my memories for my contribution for this booklet
for George Hammond is both a most pleasant and most humbling task.
George, the man, the scientist, was bigger than life for me when
I was privileged to be in his group and the passing of time has
only served to make him still bigger. How fortunate I was to
have had such a teacher. Always approving, encouraging and expansive.
"Great idea. Sure do it!" That was George to me.
And how exciting it was to do research in George's laboratory!
In that heady atmosphere we, the students and postdocs, learned
to be curious about each others projects, and we learned to be
students and teachers for each other. We were truly a research
group, not just a group of researchers. Not only did George bring
out the best in each of us, he also made us feel we were the best.
On one of our weekly group lunches I still remember George's
toast "To the best research group in the world."
And, if George said it, how could it be otherwise.
Nor will I ever forget George coming into the lab to explain to
us for the first time nonvertical triplet energy transfer, or
the mechanism for triplet sensitized 1,3-diene photodimerization.
What a pleasure it was to watch George think.
It was George who gave me a much needed push to try an academic
position and take the job at Florida State University, and it
was also George who was full of encouragement after reading my
first research proposal. In a letter dated November 6, 1964 he
advised me to put some indication in the proposal that the time
scale for nonvertical triplet energy transfer is "enormously
longer than the times involved in absorption from the radiation
field. You and I know this is true, but you must remember that
the proposal is likely to be reviewed by one or more philistines
who haven't read our papers carefully and are inclined to
the notion that Saltiel and Hammond are full of crap." Valuable
advice that, and faithfully followed. George taught me to search
for the loophole in my scientific argument and not to disregard
intuition and emotion when they did not wish to follow the apparent
path of scientific logic. Concerning one of my ideas that turned
out to be quite wrong he wrote "It really makes perfect
sense! My only reservation is the generalized fear that such
things are really too good to be true." And concerning
nonvertical transitions he wrote: "Logically it seems to
have to be right, but emotionally I keep feeling that there may
be a loophole that we haven't seen." But George
is never one to be bluffed out of a good idea, and if that attitude
may cost him money in a poker game, it has served him remarkably
well in his science.
This last paragraph is addressed to you George. The above words
are long overdue. It is fitting though that they are written
in celebration of your post "retirement" careers
and in anticipation of your future valuable contributions. In
closing, I return the toast to the best research director in the
world.
Sincerely,
Jack Saltiel
Upcoming Meetings
Please send new meeting announcements to: Kirk Schanze or
Willie Leigh.
30-apr-96
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