Table Of ContentContributors
Waldemar Adam Keith A. Horn
Robert Charles Allen Karl R. Kopecky
Denis J. Bogan Frank H. Quina
Giuseppe Cilento Gary B. Schuster
Nelson Duran Osamu Shimomura
Larry R. Faulkner Klaus Zinner
Robert S. Glass
Chemical and
Biological
Generation of
Excited States
Edited by
Waldemar Adam
Organic Chemistry Institute
University of Würzburg
Würzburg, West Germany
and
Department of Chemistry
University of Puerto Rico
Rio Piedras, Puerto Rico
Giuseppe Cilento
Department of Biochemistry
University of Säo Paulo
Säo Paulo, Brazil
1982
ACADEMIC PRESS
A Subsidiary of Harcourt Brace Jovanovich, Publishers
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COPYRIGHT © 1982, BY ACADEMIC PRESS, INC.
ALL RIGHTS RESERVED.
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United Kingdom Edition published by
ACADEMIC PRESS, INC. (LONDON) LTD.
24/28 Oval Road, London NW1 7DX
Library of Congress Cataloqinn in Publication Data
Main entry under title:
Chemical and bioloqical generation of excited states.
Includes bibliographies and index.
Contents: Photophysical concepts in condensed
media / Frank H. Ouina — Gas-phase dioxetane chemi-
luminescence / Denis J. Bogan -- Synthesis of 1,2-di-
oxetanes / Karl R. Kopecky— [etc.]
1. Bioluminescence. 2. Chemiluminescence. I. Adam,
Waldemar. II. Cuento, G.
QH641.C47 574.19«125 82-6808
ISBN 0-12-044080-6 AACR2
PRINTED IN THE UNITED STATES OF AMERICA
82 83 84 85 9 8 7 6 5 4 3 2 1
List of Contributors
Numbers in parentheses indicate the pages on which the authors' contributions begin.
Waldemar Adam (115, 153), Organic Chemistry Institute, University of
Würzburg, D-8700 Würzburg, West Germany, and Department of Chemistry,
University of Puerto Rico, Rio Piedras, Puerto Rico 00931
Robert Charles Allen (309), U.S. Army Institute of Surgical Research and the
Department of Clinical Investigation, Brooke Army Medical Center, Fort
Sam Houston, Texas 78234
Denis J. Bogan (37), Combustion and Fuels Branch, Naval Research Laboratory,
Washington, D.C. 20375
Giuseppe Cilento (277), Department of Biochemistry, Instituto de Química,
Universidade de Säo Paulo, Säo Paulo, Brazil
Nelson Duran (345), Instituto de Química, Universidade Estadual de Campinas,
Campinas, Säo Paulo, Brazil
Larry R. Faulkner (191), Department of Chemistry, University of Illinois,
Urbana, Illinois 61801
Robert S. Glass1 (191), Department of Chemistry, University of Illinois, Urbana,
Illinois 61801
Keith A. Horn (229), Department of Chemistry, University of Illinois, Urbana,
Illinois 61801
Karl R. Kopecky (85), Department of Chemistry, University of Alberta, Edmon-
ton, Alberta, Canada T6G 2G2
Frank H. Quina (1), Group for Interfacial Studies (GIST), Instituto de Química,
Universidade de Sao Paulo, Säo Paulo, Brazil
Gary B. Schuster (229), Department of Chemistry, University of Illinois, Urbana,
Illinois 61801
Osamu Shimomura2 (249), Department of Biology, Princeton University, Prince-
ton, New Jersey 08544
Klaus Zinner (153), Department of Biochemistry, Instituto de Química,
Universidade de Säo Paulo, Säo Paulo, Brazil
1 Present address: Sandia National Laboratories, Albuquerque, New Mexico 87116.
2 Present address: Marine Biological Laboratory, Woods Hole, Massachusetts 02543.
x
Preface
For decades the chemistry underlying the phenomenon of chemi- and
bioluminescence has been a subject of considerable interest, but only in
the last 15 years has great progress been made toward an understanding of
the chemical generation of the electronically excited states that are in-
volved in these processes. This is a field which has important implications
for chemistry, physics, biology, and medicine. In view of its interdiscipli-
nary diversity, research in this field is intellectually very demanding but
also more rewarding.
In August 1978 a Brazil-USA Joint Seminar and an International Con-
ference were held on this topic in Brazil. The success of these activities
encouraged us to prepare a book featuring the major aspects of chemical
and biological generation of electronic excitation. Such a volume must by
necessity be a multiauthored book. Therefore, it has been one of our
fundamental motivations to secure contributions from experts who would
provide the necessary broad coverage.
Excellent books are available on chemiluminescence, bioluminescence,
singlet oxygen, and related subjects, but to the best of our knowledge
there is no book that widely covers the field of chemi- and bioenergized
processes. The book intends to attract young scientists as well as estab-
lished research workers to broaden the horizons of this rapidly growing
and potentially very important field.
The editors remain very grateful to the individual contributors. They
wish also to thank Academic Press for their patience and cooperation. We
are pleased to acknowledge the able secretarial work of Sra. Lydia de
Freitas Butturini and Frau Anita Melber.
Waldemar Adam
Giuseppe Cuento
xi
1
Photophysical Concepts in Condensed Media
FRANK H. QUINA
I. Introduction I
IL Unimôlecular Decay Processes 2
A. Kinetic Considerations 2
B. Radiative Transitions 4
C. Nonradiative Transitions II
D. Heavy-Atom Effect 14
III. Bimolecular Interactions Involving an
Electronically Excited State 17
A. Excimers and Exciplexes 18
B. Energy Transfer 20
References 32
I. INTRODUCTION
The promotion of a molecule to an electronically excited state can be
induced by both electromagnetic and ionizing radiation or can occur during
certain types of highly exothermic chemical reactions (chemiexcitation). At
the instant of excitation, each of these excitation routes is, in principle,
distinct. Thus, both the relative proportions of excitation into the singlet and
triplet manifolds and the precise nature (geometry, electronic and vibrational
energy content) of the initially created excited species may be quite sensitive
functions of the excitation mode. In condensed media, much of the potential
complexity that might arise from such differences is eliminated as a result
of rapid thermal equilibration of the initially created excited species. Thus,
within about a picosecond this initially excited species undergoes radiation-
less relaxation (internal conversion and vibrational relaxation) to the lowest
thermally equilibrated excited state of the same multiplicity (S or TJ (e.g.,
1
Turro et ai, 1978b; Wiesenfeld and Ippen, 1979; Morgante and Struve, 1979).
Chemical and Biological Generation Copyright © 1982 by Academic Press, Inc.
of Excited States All rights of reproduction in any form reserved.
ISBN 0-12-044080-6
2 Frank H. Quina
As long as other potential decay routes do not compete with this fast
relaxation, the observed photochemical and photophysical decay will occur
exclusively from Si and Ύ (for a comprehensive review of exceptions to
ί
this generalization, see Turro et al, 1978b). In this chapter, we touch on
some of the fundamental aspects of the description of excited state behavior
in condensed media, restricting our attention principally to thermally equil-
ibrated excited states. Although we are concerned almost exclusively with
excited states created by the absorption of light, about which much more
is known, in condensed media the nature and reactivity of each of the relaxed
states S and Ύ are insensitive to the details of their creation. Thus, even
x ί
though other excitation routes can result in quite different initial proportions
of excitation into the singlet and triplet manifolds, this residual "memory"
of the excitation process may be readily taken into account in any kinetic
analysis of the system.
II. UNIMOLECULAR DECAY PROCESSES
A. Kinetic Considerations
The unimolecular decay processes that can deactivate the lowest excited
singlet (M*1) and triplet (M*3) states of a thermally equilibrated molecule
are outlined in the following kinetic scheme:
k
Fluorescence: M*1 —L-► M + hV (1)
0
Intersystem crossing: —iIS-+ M*3 (2)
k-
Internal conversion : —^-> M (3)
0
k
Singlet photoreaction: —rj-+ product (4)
Phosphorescence: M*3 —*-► M + hv" (5)
0
k
Intersystem crossing: —Ë-+ M (6)
0
k
Triplet photoreaction: —rJ-+ product (7)
Equations (1) and (5) represent radiative transitions in which the photon
fulfills the criterion of conservation of energy. Equations (2)-(4) and Eqs. (6)
and (7) correspond to nonradiative transitions in which the initial state
1 Photophysical Concepts in Condensed Media 3
converts isoenergetically to the final state, followed by rapid liberation of
heat to the medium (vibrational relaxation within the final state).
Assuming initial optical excitation to the singlet manifold, i.e.,
Excitation : M + hv - ^- M* l (8)
0
where I(t) represents the portion of the total incident intensity that is actually
absorbed by the ground state M , the rates of change of the concentrations
0
of the relaxed states M*1 and M*3 are given by
L J _ fejfM*1] (9)
= î{t) {k{ + fcisc + K +
dt
¿[M*3]
= kiscüM*1] - (fcp + k + fc )[M*3] (10)
dt d rt
For instantaneous pulsed excitation at t = 0 \_I(t) = δ pulse], the probability
of finding the system excited in the state M*1 at some later time t is given
by the decay function
p¿(í) = [M^MM*1^ = exp[-(/c + fc + k + k„)t] = exp(-i/T0) (11)
f isc ic f
Thus, the decay of M*1 is exponential, with a singlet state (or fluorescence)
lifetime defined as
Tf° = (fcf +kisc+ fcic + U-1 (12)
Similarly, the probability of finding the system excited in the state M*3 is
given by
PÍ(t) = [Mn/lM^lo = % ^4 [exp(-iAÎ) - expí-ί/τ?)] (13)
τ - T
Τ f
Under the usual circumstances (τ? » τ?), M*3 decays exponentially (for
t » τ^) with a lifetime equal to the triplet state (or phosphorescence) lifetime
τ?, defined as
T? = (fcp + /c + /c )"1 (14)
d rt
The quantum yield (Φ0) for a given decay route may be defined as the
number of excited molecules which decay via that route divided by the
total number of excited molecules created by the excitation process. For
decay from M*1 (fc¿ = fe, fe , fc , or fc ), all of the following expressions
f isc ic rs
are equivalent:
Φ?=k-^p^=*. /; ut)dt=^=w (is)
P
4 Frank H. Quina
For decay from M*3 (k = fe , k , or /c ), equivalent expressions are
} p d rt
ο MMpll . J» 3 θ Λ_ 0 ,
φ = = k p (f)dt = φ ο = O cfc τ? (16)
The expressions ^[M*1]^// and kj[_M*3~\JI refer to conditions of continu-
s
ous irradiation [I(t) = I = constant], where the steady-state approximation
may be applied to all excited species (d[M*l~\ /dt = d[M*3] /di = 0); the
ss ss
integrated decay functions refer to conditions of ¿-pulse excitation. The quan-
tum efficiency q° may be defined as the fraction of molecules created in a
given state that decay via one of the primary processes deactivating that
state. Thus, for M*1:^? = fc^fc, = fc^?; for M*3:^ = kj/^jkj = fyr?.
Since all excited state must eventually decay, ^qf = Σ/4? = 1. Numerous
authors have treated the experimental aspects of the measurement of emis-
sion quantum yields (Demas and Crosby, 1971; Wrighton et al, 1974),
emission lifetimes (Ware, 1971; O'Connor et al, 1979), intersystem crossing
quantum yields (Wilkinson, 1975; Carrol and Quina, 1976; Kittel et al,
1979), and photoreaction quantum yields (Johns, 1971; Shetlar, 1973).
B. Radiative Transitions
1. A bsorption and Emission
When the frequency of the light satisfies the resonance condition hv =
\Εγ — E \, the probability of an electric dipole transition between two elec-
F
tronic states Ψ and Ψ , with energies E and E , is proportional to the
Ι Ρ Y F
square of the transition moment integral M given by
IF
M = JVîer^dr = <Ψ |^|Ψ,>, (17)
IF Ρ
where r is the electron position operator and dx is over all configuration
space (see Henderson, 1979, for a concise discussion of the dynamic aspects
of the electronic transition). The transition moment integral is subject to a
series of selection rules (McGlynn et al., 1972), which determine whether the
transition is allowed (M Φ 0) or forbidden (M = 0). The most important
IF IF
for our purposes are the spin selection and the symmetry selection rules.
The former prohibits (by a factor of about 108) transitions between states
of different spin multiplicity, i.e., M = 0 if Δ5 φ 0. This selection rule is
IF
relaxed by spin-orbit coupling (vide infra), which mixes states of different
spin multiplicity. The symmetry restrictions arise from the fact that, like
any other molecular property, M must be invariant under the symmetry
IF
operations of the molecular point group. Thus, if the symmetry direct prod-
1 Photophysical Concepts in Condensed Media 5
uct Γ(Ψρ) x Γ(?) Χ Γ(Ψ!) does not contain the totally symmetric representa-
tion, M = 0 and the transition is symmetry forbidden. A special case of
IF
the symmetry selection rule is the parity selection rule for systems with a
center of symmetry, in which the electronic wavefunctions can be classified
as symmetric (g = gerade or even parity) or antisymmetric (u = ungerade
or odd parity) with respect to reflection through the center. In accord with
this rule, electric dipole transitions between states of different parity (u <-+ g)
are allowed and those between states of like parity (gegoru^u) are
forbidden. Since symmetry restrictions refer only to the equilibrium nuclear
geometry, which defines the molecular point group, symmetry-forbidden
transitions can usually steal intensity from allowed transitions via the inter-
mediacy of nontotally symmetric vibrations of the appropriate symmetry
(Herzberg-Teller coupling). As a result, symmetry forbiddenness normally
introduces a prohibition factor of only ca. 102-104.
The intensity of an electronic transition is conveniently expressed in
terms of the oscillator strength,
/ = 4.39x 10"9Je(v)dv (18)
osc
which is related to the experimental optical density through the Beer-
Lambert law,
A = log(/0//) = e(v)fc[C] Π9Ϊ
where ε(ν) is the molar extinction coefficient (M-1 cm"1), b is the optical
pathlength (in centimeters), and [C] is the molar concentration. For a
typical organic molecule in solution, with an absorption bandwidth of
ca. 2500 cm-1, / ~ 10~5e ; thus, e ~ 105 corresponds to a totally
osc max max
allowed (f — 1) transition in such a molecule.
osc
Within the framework of the Born-Oppenheimer approximation (separa-
bility of electronic and nuclear motion), the transition moment for absorp-
tion may be related to the modified absorption spectrum via the expression
(Strickler et α/., 1974; Hirayama and Phillips, 1980)
(20)
S^-xX^WU^'W
where n is the refractive index of the medium in the region of the transition,
c is the velocity of light in vacuo, N is Avogadro's number, |M | is the electronic
a
part of the transition moment, and the <χ |χ ) are vibrational overlap
3 0
integrals for a transition to the wth vibrational level of the upper state.
Similarly, the transition moment for emission may be related to the modified
