1. Photo-oxidation of 2-(1H-inden-1-ylidene)-1-methyl-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 Kasha’s rule2.
Figure 1: IMDP E-Z isomerization.
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).
Figure 4: IMDP experimental FTIR spectrum (top) E- and Z-Isomer IR calculated (B3LYP/cc-pVDZ) spectra (bottom)
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: Kasha’s 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) + hν 1 IMDP (S1) (1)
1IMDP (S1)  3IMDP (T1) (2)
3IMDP (T1) + O2(3Σ-g)  1IMDP (S1) +O2(1Δg) (3)
1IMDP (S0) + O2(1Δg)  Products (4)
Scheme 1: Proposed Mechanism
Conclusions
The solvent effect predicted by the proposed model was observed, however it was not large.
The Kasha’s 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.
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.
Acknowledgments
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