Photo-oxidation of 2-(1H-inden-1-ylidene)-1-methyl-1,2-dihydropyridine (IMDP)

1
Photo-oxidation of 2-(1H-inden-1-ylidene)-1-meth
yl-1,2-dihydropyridine (IMDP) S. Cogan and Y.
Haas The Farkas Center for Light Induced
Processes, Physical Chemistry Department, The
Hebrew University in Jerusalem
Motivation
Experimental Results
However, other possibilities of must also be
taken into account, as the reaction in CCl4 is
also quite rapid. A much larger solvent effect
was detected in presence of Piperylene. In
non-polar solvent (MCH) the Piperylene addition
caused discoloration acceleration while in
methylene chloride the reaction was slower.
IMDP (Figure 1) was synthesized by Berson1 in
late 60s. We are interested in this particular
systems since E-Z photoisomerisation can proceed
through conical intersection which is defined by
two alternative thermal routes biradical and
zwitterionic.
The FTIR spectrum (Figure 4) shows good
correspondence with the calculated spectra.
Comparison of the calculated and experimental
spectra demonstrates that both E and Z isomers
are present.
Very intriguing effect was detected when IMDP was
irradiated at different spectral regions (S3 ? S0
, S1 ? S0) as shown on Fig. 7. Excitation to S3
leads to much faster (at least 10 times)
photo-oxidation then excitation to S1. This is a
violation of Kashas rule2.
Figure 1 IMDP E-Z isomerization.
Figure 4 IMDP experimental FTIR spectrum (top)
E- and Z-Isomer IR calculated (B3LYP/cc-pVDZ)
spectra (bottom)
The reason for this unusual behavior is the fact
that the electronic wavefunction (EWF) of this
compound in the ground state is a superposition
of two VB structures (Fig. 2).
UV-VIS IMDP spectra were recorded in different
solvents (Fig. 5). The hypsochromic shift in the
spectra indicates that the GS equilibrium
structure more polar than S1. This was in good
agreement with QM calculations according to which
the dipole moments of both the E and Z-isomers
are quite large (5 D)
Figure 7 Kashas rule violation
To rationalize these results TD-DFT calculations
were performed at Equilibrium and Zwitterionic TS
geometries. The results of the calculations are
summarized on Figure 8.
Figure 2 Two VB structures contributing to EWF
of IMDP at its minima.
Recently we suggested that in such situations the
photochemical reaction can be controlled by
solvent polarity. The validation of this
hypothesis was the main motivation of our
research.
Experimental
Figure 5 IMDP UV-Vis spectra at different
solvents
Figure 8 State Correlation along Isomerization
RC (zwitterionic).
IMDP is stable at RT and under irradiation in
inert atmosphere, however it undergoes
discoloration under irradiation in the presence
of oxygen (Fig. 6)
We propose the following mechanism of IMDP
photo-oxidation (Scheme 1)
1IMDP(S0) h?? 1 IMDP (S1)
(1) 1IMDP (S1) ? 3IMDP (T1)
(2) 3IMDP (T1) O2(3S-g) ?
1IMDP (S1) O2(1?g) (3) 1IMDP (S0) O2(1?g) ?
Products (4)
Scheme 1 Proposed Mechanism
Conclusions
The solvent effect predicted by the proposed
model was observed, however it was not large. The
Kashas rule violation experimentally observed in
the IMDP system is consistent with the existence
of an S1/S0 Conical intersection.
The pyperilene
effect on photochemical behavior of this system
suggests that Triplet states can be strongly
involved in photochemistry even when Conical
Intersection does exist.
Figure 3 Experimental Set Up
The IMDP was synthesized by procedure which was
described by Berson et al. and was characterized
by FTIR, MS, and UV-Vis spectroscopy. The
photo-oxidation was followed by the discoloration
of the sample upon irradiation in the presence of
oxygen. Our Experimental Set Up is represented
schematically on Fig. 3.
Figure 6 IMDP photo-oxidation in different
solvents - Methylcyclohexane (10 min intervals,
top) CH2Cl2 (at 15 min intervals, bottom)
Computational Details
The model predicts that in the non-polar MCH
internal conversion (IC) to S0 is very rapid due
to a conical intersection. In polar solvents
(CH2Cl2 and CH3CN) the conical intersection does
not exist, so that oxidation can compete with IC.
Acknowledgments
In order to rationalize experimental details DFT
and TD DFT computations with B3LYP functional and
cc-pVDZ basis set were performed on this
molecular system.
This research was supported by The Israel Science
Foundation founded by The Israel Academy of
Sciences and Humanities . The Farkas Center for
Light Induced Processes is supported by the
Minerva Gesellschaft mbH.

We deeply thanks Prof. Shmuel Zilberg
for enlightening discussions.
1 Berson, J. A.,. Evleth, E. M., and Hamlet, Z.,
J. Am. Chem. Soc. 1965, 87, 2887 2 Turro, N. J.
Ramamurthy, V. Cherry, W. Farneth, W. Chem.
Rev.1978, 78, 125