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Dynamics of the Photodimerization of 9-Anthroic Acid and Sodium 9-Anthroate.

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They occur at 2055 (vasN3),1240 (v,N,), 470 cm-' (6N3).
Further deformational vibrations of the azido group are
probably masked by the strong bands at 670 (r) and 740
cm-' (yC-H). The "X-sensitive" vibrations q, r, t, and
yC3]of the C,HsAs group are shifted by only about 10 cmtowards longer wavelengths compared to C,H,ASC~,[~'.
However, a very strong and poorly structured band between 96G1020 cm- ', which is absent from the spectrum
of C,H,AsCl,, is particularly noticeable. Although the
phenyl ring vibration will be present at 1000 cm- ', the
main contribution certainly comes from As-N stretching
vibrations. The frequency can only be explained in terms
of extensive As-N K bonding, as in the arsenic nitride
compounds [(C,H,),ASN],['~ and [C,H,AsClN],
(2)
where association gives rise to an alternating sequence of
As-N single and double bonds. The x-N atom of the azo
group can admittedly function as donor atom owing to
its lone pair, as in the organometal azides of Group IIIB
elements[2. however, in (1) the negative inductive effect
of the three chlorine atoms and the phenyl group should
rather promote an interaction of the lone pair with an
empty d orbital of arsenic. In this way a (p+d)n bond is
superimposed on the As-N o bond.
'
Table. Characteristic IR frequencies of compounds (Z) and (2)
Dynamics of the Photodimerization of 9-Anthroic
Acid and Sodium 9-Anthr0ate['l[**~
By Dwaine 0. Cowan and Walter W Schmiegel"]
While many anthracene derivatives are known to photodimerize only the photodimerization of unsubstituted
anthracene has been studied in mechanistic detail['I. To
learn more about the effect of solvent and substituents on
radiative and nonradiative processes we have studied the
photodimerization of 9-anthroic acid in ethanol and
benzonitrile and the photodimerization of sodium 9anthroate in water, deuterium oxide, and benzonitrile.
The material balance (dimer and starting compound) for
the preparative photolysis of degassed solutions of the
purified acid and sodium salt in each of the solvents was
greater than 93% with only traces of nonacidic material.
The non-acidic material was principally anthraq~inone[~l.
Process
Rate
A + h v -A'
A' - A + h v
A' + A
As + A'
A'+A + 2A
A'+A + D
kF[ASI
klc"4A'l
klSC[AS1
k C Q r ~ sl.41
i
1,
kDIM~Asl
LA]
Light absorption
Fluorescence
Internal conversion
Intersystem crossing
Concentration quenching
Dimerization
Scheme I.
2055 vs [b]
1240 m
1070 s
V.P3
v,N 3
4 [CI
vAs-N-As(
r(+GAs-N-As)
2 1
I
VAS-CI
[cl
+v,C6H,)
[c]
970 vs (br)
670 vs
470 s-m
450 vs
400 vs ( h r )
290 vs hr)
1090 vs
945 vs
930 sh
640 vs
468 vs
{
-
{
390 vs
375 sh
330 vs
[a] Assignment based on vibrational spectrum of C,H,AsCl,.
[b] Values given in cm-I.
[c] Designation of "X-sensitive" vibrations (q, r, y, 1) according to
Whiffrn[3].
By analogy with tetrameric diphenylarsenic nitridel'] a
ring structure can be assumed for trimeric phenylarsenic(v)
chloride nitride.
To obtain meaningful assessments of the photochemical
reactivity it is necessary to determine specific rate constants.
Quantum yield measurements represent rate ratios and
are therefore insufficient to indicate specific reactivities.
To obtain the specific rate constants and verify the mechanism suggested in Scheme 1 the following was carried out :
first, the fluorescence lifetimes in dilute solutions (5:) were
measured by the phase shift methodc4];second, the absolute
fluorescence quantum yields in dilute solutions (a:) were
determined by comparison with the fluorescence quantum
yield of quinine sulfate after the spectra had been corrected
for phototube wavelength response ; third, relative quantum yields of fluorescence as a function of concentration
were measured over the concentration range at which the
dimerization was studiedts1;and fourth, the low conversion
quantum yield of dimer formation as a function of concentration was determined using the equipment previously described'".
The mechanism given in Scheme 1 would predict the
following relationships :
Q g = k , '(k, +kac+krc)
I/@, = ( X , +A isc+ A Ic):k,
+ { (k,,,
+ X co/ A \ . [A]
i / @ D i ~ = ( k ~ i ~ + k ~+
~ ){ (iXkF D
i~
+kis+kic),L
ixu
The six-membered ring system can have C3", C,, or C ,
symmetry, depending upon the positions of the ligands C1
and R. It is not possible to distinguish between a planar
and a puckered As3N, ring on the basis of the I
R spectrum
alone.
Received: February 19, 1971 [Z 433 IE]
German version: Angew. Chem. 83,504 (1971)
[l] Part of Dissertation V. Krieg, Universitat Stuttgart 1970
[2] V. Krieg and J . Weidlein, 2. Anorg. Allg. Chem. 368,44 (1969)
[3] D. H. Whiffen,J. Chem. SOC.1956, 1350.
[4] H . Schindlbaur and H. Srenzenberger, Spectrochim. Acta 26 A. 1707
(1970).
[5] W 7: Reichle, Tetrahedron Lett. 1962, 51.
161 J. MuIler and K . Dehnicke, J. Organometal Chem. 12, 37 (1968);
see further literature cited therein.
Angew. Chem. internal. Edit. 1 Vol 10 (1971) 1 No. 7
.
1
[A1
This mechanism was found to be consistent with the observations that a plot of the reciprocal of the relative quantum yield of fluorescence 1/QFus the anthracene concentration [A] yields a straight line and a plot of the reciprocal
of the quantum yield of dimerization I/@
us,,
the
, reciprocal of the anthracene concentration I/[A] gives a straight
[*I Prof. Dr. D. 0. Cowan'"'' and Dr. W. W. Schmiegel
Physikalisch-Chemisches Institut der Universitat
CH-4056 Basel, Klingelbergstr. 80 (Switzerland)
[**I This work was supported in part by the National Science Foundation and constitutes a part of the PhD thesis by W W Schmiegel, Johns
Hopkins University 1969.
[,**I Guggenheim Fellow. On leave from the Johns Hopkins Univers1ty.
517
line. Since the intercept of I/@,,, GS 1/[A] is not unity the
concentration quenching process (kcQ)must be important.
This may suggest an excimer intermediate but our kinetic
data does not demand this intermediate.
A’+ A
[A,. .A]”
[A ...A]’
ii [A.. . A]’
+
Dimer
4
2A
anion (6-9 nm)r71should place the lowest excited singlet
state of the acid below the second triplet state. Since intersystem crossing is normally to the second triplet which is
approximately isoenergetic with the first excited singlet
state this lowering of the singlet state energy can nicely
account for the lower rate constants found for intersystem
crossing of the acid‘*].
excirner
Received: April 5, 1971 [Z 434 IE]
German version: Angew. Chern. 83,545 (1971)
From the slope and intercept of the dimerization graphs
and the values of :T and @: it is possible to calculate the
rate constants given in Table 1. Only qualitative trends in
rate constants will be considered in this communication.
[I] Photochemical Reactions, Part 7.- Part 6: [6].
[2] R . Licingston in F. Daniels: Photochemistry in the liquid and solid
states. Wiley, New York, 1960, p. 76; J . 8. Birks: Photophysics of
Aromatic Molecules. Interscience, New York, 1970.
All of
values for k~ are similar except for the sodium
salt in benzonitrile. For this system the sum of the second
Table 1 Specific rate constants for the photodynamics of 9-anthroic acld and sodium 9-anthroate at 25°C
DIM
Q;
:T
System [a]
x102
(ns)
kF
xlO-’
hisc+kic
x10-
x10-7
x~o-7
k DIM + A
x10-7
9-COOH FtOH. He
9-COOH PhCN
9-COONa PhCN
9-C0OY.t H,O
9-COONa D,O
21.8
64.6
4.88
6.89
5.54
4.55
12.3
2.65
1.50
1.35
4.79
5.24
1.84
4.59
4.17
17.2
2.88
31.9
62.1
69.8
102
35.9
11.3
73.0
76.9
428
298
1440
657
307
530
334
1451
730
384
~ C Q
CQ
k DlFFN
x10-7
613
532
532
739
598
[a] First order rate constants (s-’); second order rate constants (mol-‘ 1 s - ’ )
order rate constants (kc,+ k,,,) exceeds the diffusion
controlled collision rate constant (kDIFFN).This may reflect
an aggregation phenomenon resulting from ion-pairing.
The small rate constant for dimerization and abnormally
large concentration quenching rate constant may then be
a result of an unfavorable dimerization geometry in the
ion pair. The similar values of k,,, of the 9-anthroic acid
ion in water, deuterium oxide and the acid in acidic ethanol
indicate that coulombic repulsion between the anions is
not as large as expected.
The large Stokes shift observed between the absorption
and fluorescence spectra of the acid (40 nm in benzonitrile,
50 nm in ethanol) compared to the small Stokes shift of the
(31 A . W Bradshaw and 0. L. Chapman, J . Amer. Chem. SOC.89, 2372
(1967).
[4] J . 8. Birks and 1. H . Munro I n G. Porrri.: Progress in Reaction
Kinetics, Vol. 4, Pergamon Press, New York, I~b8.We wish to thank
the Central Research Department of E. I. Du Pont de Nemours & Co.
for generously allowing us to use their phase shift instrument.
[5] Details of this technique will be described in another paper. Emission spectra were measured with an Hitachi-Perkin Elmer MPF-2A
fluorescence spectrophotometer.
[ 6 ] D.0.Cowan and A. A . Buum, J. Amer. Chem. SOC.93, 1153 (1971).
[7] T C. Werner and D. M . Hercules. J. Phys. Chem. 73, 2005 (1969);
74, 1030 (1970).
[8) Preliminary experiments indicate that k,, IS small in comparison
with k,, consequently the values of k,,, + k,, in Table 1 can be regarded
as values of k,,,.
CONFERENCE REPORTS
Structural Problems in Boron-Nitrogen and BoronSulfur Chemistry
is 1.58 A. Compounds in which the B-N bond is seriously
weakened by steric or eiectronic effects represent excep-
By Heinz Hess“]
The past few years have seen various structural determinations, employing X-ray and electron diffraction techniques,
on boron-nitrogen compounds. The results provide material for a fairly detailed discussion of the boron-nitrogen
bond from a structural-chemical viewpoint.
Compounds with tetravalent boron (“borazanes”, “cycloborazanes”, “amine-boronium salts”) generally exhibit only
slight variations in the B-N bond lengths; the mean value
[*I
518
Priv.-Doz Dr H. Hess
Institut fur Anorganische Chemie der Universitat
7 Stuttgart, Schellingstrasse 26 (Germany)
MezN-BnB-NMe2
0
B
NMe,
(1)
(Me@).$
SiMe,
I
N
/ \
-B, ,B-N(Sible3),
rSiMeg
13)
Angew. Chem. internal. Edit. 1 Voi. 10 (1971) J No. 7
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