Solution and gas-phase photochemistry of 2-cyclopropyl and 3-cyclopropylindene and an investigation of the delocalized excited states of silyl-substituted chlorobenzonorbornenes
Abstract
The cyclopropylcarbinyl radical rearrangement was used to probe the photochemistry and photophysics of S2 and S1 in 2-cyclopropylindene (2CPI) and 3-cyclopropylindene (3CPI) in the solution and gas phases. Population of S2 with 254 nm excitation in the gas phase produced the anticipated ring expansion products 2, 3, 3a, 8-tetrahydro-cyclopenta[a]indene (RE1, Φ = 0.1) and 1, 2, 3a, 8-tetrahydrocyclopenta[a]indene (RE2, Φ = 0.06) from 2CPI and 3CPI, respectively. The S1 singlet lifetimes of 2CPI (.39 ns) and 3CPI (1.0 ns) are short relative to the corresponding methylindenes (2.3 and 13.9 ns) and are attributed to cyclopropane ring opening from S 1 with rates of 2.1 × 109 s–1 and 9.2 × 108 s–1 for 2CPI and 3CPI, respectively. Direct population into S2 (254 nm) state of 2CPI in solution produces REL The efficiency of which is excitation wavelength dependent (Φ254 = 0.022 and Φ280 = 0.006). The solution phase excitation spectrum and the fluorescence of 2CPI are dependent on wavelength. Both phenomena are explained by a ground state conformational equilibrium where the rotamers have different extinction coefficients. The second part of this thesis describes an effort to enhance the photolytic efficiency of C-Cl cleavage of exo-2-chlorobenzonorbornene (BExoCl) by increasing the σ*C-Cl character in S1. Preliminary ab initio computations predict that placement of a silyl group at the bridgehead position might enhance π*σ*C-Cl orbital interactions, presumably increasing the σ*C-Cl character in S1 by mixing the σ*C-Si, σ*C-Cl and π* orbitals. Subsequent calculations show that it is primarily steric interactions between the exo chlorine and the bridgehead group that bring the π* and σ*C-Cl orbitals closer in space and lead to enhanced π*/σ* C-Cl interactions. For exo-2-substituted norbornyl derivatives, the practice of modeling Si(CH3)3 as SiH 3 is not valid. The efficiency of C-Cl photocleavage of exo -3-chloro-1-trimethylsilylbenzonorbornene (1TMSe3Cl) is greater in cyclohexane but lower in methanol, relative to those of BExoCl.
Degree
Ph.D.
Advisors
Morrison, Purdue University.
Subject Area
Organic chemistry
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