Energy Transfer Parameters of Aromatic Compounds

382 Pages·1973·9.33 MB·English

by  Isadore Berlman (Auth.)

ENERGY TRANSFER
PARAMETERS OF
AROMATIC COMPOUNDS
Isadore B. Berlman
MICROWAVE DIVISION
RACAH INSTITUTE OF PHYSICS
HEBREW UNIVERSITY
JERUSALEM, ISRAEL
ACADEMIC PRESS New York and London 1973
A Subsidiary of Harcourt Brace Jovanovich, Publishers
COPYRIGHT © 1973, BY ACADEMIC PRESS, INC.
ALL RIGHTS RESERVED.
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PRINTED IN THE UNITED STATES OF AMERICA
Dedicated to my father and mother to whom I owe so much
PREFACE
Following the appearance of the fluorescence and absorption data con-
tained in both editions of my previous book, ("Handbook of Fluorescence
Spectra of Aromatic Molecules," Second Edition, Academic Press, 1971),
I received requests to develop the data now contained in this work. The
amount of data is so massive it could not be incorporated in the second
edition of above handbook. Thus this separate publication was planned.
The volume has been written in an extensive rather than an intensive
fashion and is intended to supplement other existing work already available
to workers in the field. A short historical sketch is provided to give the
reader a proper perspective of some of the concepts. References are co-
piously given to provide sources for more detailed information. Because of
the logjam of publications, some relevant references may have been in-
advertently omitted. For this we apologize in advance.
I wish to thank Professors W. Low, and S. G. Cohen, Racah Institute
of Physics, Hebrew University and Dr. R. E. Rowland, Director, Radio-
logical and Environmental Research Division, Argonne National Laboratory
for their support of this project. I also wish to thank Dr. F. Hirayama
for suggesting these calculations, Dr. M. Inokuti for stimulating discussions,
Mrs. C. A. Yack for typing assistance, and my wife Saralee for assistance
in compiling the author index.
IX
1
INTRODUCTION
Many reactions in radiation and photochemistry, physics, and biology
depend on the transfer of electronic energy. That radiationless energy
transfer can occur in the gas phase, in liquid and solid solutions, and in
crystals further attests to the generality of the phenomenon. Thus, any
advance in the understanding of the energy-transfer process will find
immediate application in many areas of research.
The number of publications on electronic energy transfer has been
increasing dramatically. Some assistance in coping with the ever-increasing
supply has been afforded by a concomitant proliferation of review papers;
to list a few, Livingston (1957), Ganguly and Chaudhury (1959), Ermolaev
(1963), Wilkinson (1964), Jortner et al (1965b), Windsor (1965), Wilkin-
son (1966), Bennett and Kellogg (1967), Förster (1967), Lamola (1969a),
Cundall (1969), and Wagner (1971). A yearly bibliography on energy
transfer and related subjects has been assembled by Lipsett (e.g., 1967,
1968). The well known books of Parker (1968) and Birks (1970b) are also
recommended, f
The tabulated data in this monograph are restricted to a specific type of
nonradiative mechanism, a dipole-dipole interaction. This process may be
t However, this author strongly disagrees with claims made by Birks concerning
quantum yield and decay time (Birks, 1970b, p. 104, 109).
1
to O 3
39000
38000 0.
1
37000 ber 2
m
36000 is nu
35000 pound
34000 s com
33000 e. Thi
n
(R) 3200 31000 32000 -1R (CM) of indenoinde
WAVELENGTH 3400 29000 30000 WAVE NUMBE acteristics
r
a
28000 ce ch
n
27000 oresce
26000 nd flu
25000 ption a
24000 absor
23000 1. The
22000 FIG.
4800 21000
5000 1.00 b 20000
INTRODUCTION 3
represented by the equation
D* + A -> D + A*,
where D and A refer to donor and acceptor molecules, respectively, and an
asterisk represents a molecule excited to its first excited singlet state. The
donor and acceptor molecules can be like or unlike chemical species. In the
initial condition, the donor is electronically excited to its first excited
singlet state; in the final condition, the excitation energy has been trans-
ferred to the acceptor. For purposes of completeness, other processes and
mechanisms are discussed in the text, but no data by the author are pre-
sented in their support.
Förster (1949) has suggested that certain standard reference parameters
called Ä and Co are useful in evaluating the effectiveness of resonance
0
energy transfer between different compounds in a rigid or viscous medium.
The quantity Ä is defined as a mean distance, in angstroms, between a
0
donor molecule and an acceptor molecule where the probability for reso-
nance energy transfer is equal to the probability for emission. A related
parameter Co is the concentration of acceptor molecules in moles per liter
wherein there is, on the average, one acceptor molecule inside a sphere of
radius R . The tabulation of these quantities (plus an overlap integral to
0
be defined later) in a meaningful manner is the prime concern of this
publication.
The first 209 compounds are listed in the order found in the "Handbook
of Fluorescence Spectra of Aromatic Molecules, Second Edition" (Berlman,
1971). Compound 210, indenoindene, was not available at the time of the
above publication and is included here as Fig. 1.
Cyclohexane is the standard solvent. In those cases where solubility is
a problem, other solvents such as alcohol or benzene are used. The type of
solvent employed is indicated by a letter after each graph number: an A for
alcohol, a B for benzene, and a C for cyclohexane.
All of the spectroscopic data (Berlman, 1971) were taken on solutes that
were soluble in cyclohexane. Solutes that were soluble in other solvents
were studied only partially, i.e., their absorption and fluorescence spectra
were measured but not their quantum yield. Therefore in the calculations
of Ro, only the former compounds were employed as both donors and
acceptors; the latter compounds were used only as acceptors—their fluores-
cence quantum yields were not available.
The short-wavelength portion of the fluorescence spectrum is sometimes
distorted by a process called self-absorption. This distortion is most
prominent when the spectra overlap appreciably; it is minimized by using
very dilute solutions. The fluorescence spectra of compounds in trace
concentrations, when a * ailable, were therefore used in the calculation of R.
0
2
SPECTROSCOPIC CONCEPTS
2.1 Energy Levels
Aromatic compounds possess electrons, called τ electrons, that occupy
nonlocalized molecular orbitale. It is the transitions of these electrons from
one orbital to another that produce the phenomena of absorption and
fluorescence. Although π electrons contribute to the chemical binding, they
are not primarily concerned with holding the molecular framework to-
gether.
Since the ground state of these compounds, So in Fig. 2, is a singlet state,
this means that their lowest molecular orbitale have paired electrons of
antiparallel spin. The small arrows in the boxes at the extreme left in the
figure represent such electrons. In the absorption of ultraviolet radiation
one of the π electrons in a filled orbital is raised to a higher unfilled orbital,
as shown by a solid arrow pointing upward in the figure. When the spin
direction is retained, the molecule ends up in an excited singlet state, and
when reversed, in a triplet state. For each excited electronic singlet state
there is a corresponding triplet state of lower energy. When excitation
is by uv the probability of a singlet-singlet transition is many orders of
magnitude higher than that of a singlet-triplet transition. When excitation
is by means of an ionizing particle, the probability of populating a triplet
state relative to a singlet state is a function of the energy of the particle.
The upper states are termed π, π* states and the transitions, π-π* transi-
tions. With each electronic state there are associated vibrational and
4
2.1 ENERGY LEVELS
£ΕΞΜ?
Π
FIG. 2. An energy-level diagram that illustrates the relative spacing of electronic and
vibrational levels. The large arrows depict the directions of the various transitions.
rotational states. In Fig. 2, several electronic and vibronic levels but not
rotational levels are depicted. It should be noted that the energy interval
between the ground level and first excited electronic level is much larger
than the interval between successively higher levels.
At room temperature most of the molecules are found in their zero
vibrational state, so that in the absorption process an electron is raised
from the zero vibrational state of the ground state to one of the higher
electronic and vibrational states, depending on the energy of the absorbed
photons. The pathways of deexcitation can be by internal and external
processes, as explained below. In a condensed medium, excess vibrational
energy can be transferred as vibrational quanta to the medium by collisions
(external process) and the compound is cooled to its lowest vibrational
level (zero) in about 10~n sec. In other words, the vibrationally hot
molecule is brought into thermal equilibrium with its environment by
collisions. This vibrational relaxation is shown as a wavy line in Fig. 2.
If the molecule is in the zeroth vibrational state of an upper electronic state,
it can be transformed into a molecule with a lower excited electronic state,
but with high vibrational energy by a process called internal conversion
(internal process). Here, too, the vibrationally rich molecule is cooled by
interacting with the medium. Because internal conversion is adiabatic, i.e.,
involves no net energy change, it is depicted by a horizontal dashed arrow
in the figure.
6 2. SPECTROSCOPIC CONCEPTS
When a molecule ends up in the zero vibratiohal state of its first excited
state, it can remain in this state for relatively long times, i.e., nanoseconds,
because the energy gap between this state and the ground state is relatively
large so that the (competing) rate constant for internal conversion is
relatively small. The molecule can lose its remaining excitation energy by
radiation, called fluorescence, by energy transfer to another molecule, or
by converting adiabatically to the triplet manifold, called intersystem
crossing. In the latter case, the triplet-state molecule donates its surplus
vibrational energy (if any) to the environment and ends up in the zero
vibrational state, from which it can either produce radiation, called phos-
phorescence, or transfer its energy to another molecule, or, under the
proper conditions, cross back to the singlet manifold by intersystem
crossing, as shown in the figure. Kotsubanov et al. (1968) have shown that
the triplet manifold forms a relatively isolated system of states and that
upon excitation to a high triplet state relaxation proceeds to the lowest
triplet state. There is little or no crossing from a higher triplet state to the
singlet manifold.
In summary, internal conversion takes place between states of like
multiplicity and intersystem crossing between states of different multi-
plicity. The first step in each is adiabatic, followed by vibrational relaxation.
Whereas absorption takes place from the zero vibrational level of the
ground state to one of the vibrational levels of an upper state, fluorescence
and phosphorescence both take place from the zero vibrational level of the
excited state to one of the vibrational levels of the ground state. In general,
fluorescence is defined as a transition between states of like multiplicity,
i.e., singlet-singlet or triplet-triplet transitions, and phosphorescence is
between states of different multiplicity, i.e., triplet-singlet. In our study,
fluorescence always (with the exception of azulene) involves a transition
from the first excited singlet state to the ground state.
The natural fluorescence lifetime for organic molecules is usually in the
range from 10~7 to 10~9 sec. As discussed later, the fluorescence quantum
yield is unity when the competing processes for the excitation energy are
weak, and the measured decay time is then equal to the theoretical natural
lifetime. When there are effective competing processes, the measured decay
time is less than the theoretical value, and the quantum yield is less than
one.
Phosphorescence is a much slower process. Because of the spin-forbidden
character of the transition, the natural phosphorescence lifetime is many
orders of magnitude longer than that of fluorescence, being sometimes
many seconds. Here, too, competing processes reduce the measured decay
time. It is of interest to note that intersystem crossing from an excited