Photochemical Sciences Ph.D. Dissertations

Title

Structure and Exciton Coupling in Jet-Cooled Bichromophores

Date of Award

2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

John R. Cable

Second Advisor

Douglas C. Neckers (Committee Member)

Third Advisor

Sheila J. Roberts (Committee Member)

Fourth Advisor

Deanne L. Snavely (Committee Member)

Abstract

The spectroscopy and exciton coupling in a series of conformationally flexible bichromophoric molecules have been investigated in a supersonic jet. The investigated molecules are: 1,2-diphenylethane (DPE), 1,2-bis(4-methylphenyl)ethane (M2DPE), 5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene (THDC), cis and trans isomers of 1,2-diphenylcyclopropane (DPCP), 2-phenylindane(2PI), and 2-(4-fluorophenyl)indane (2FPI). Resonant enhanced two-photon ionization (R2PI) spectra of these compounds and many of their deuterated isotopomers have been recorded for the first time. The observed spectral features have been assigned. The experimental results are compared with the predictions of the dipole-dipole based Forster theory, and the supramolecular model of bichromophoric molecules with identical chromophores.

Analysis of the experimental data is facilitated by the spectral analysis of the single chromophore analogues of the investigated bichromophoric molecules. These include ethylbenzene-d0, α-ethylbenzene-d1, cis and trans isomers of 1-methyl-2-phenylcyclopropane, and 2-methylindane.

The molecular structures and other properties of the investigated molecules in their ground and exited singlet states have been computed at various levels of theory. The calculations predict energetic preference of localized electronic excitation of most of the investigated molecules, suggesting weak inter-chromophore interactions in the excited states. Transition density surfaces indicate that the lowest excited singlet states of most of the investigated molecules have Lb ππ* character.

Two conformers of DPE, M2DPE, THDC, and 2-methylindan have been observed. The rest of the investigated molecules are found to exist in a single conformation. The S2←S0 transitions of both anti and gauche conformers of DPE, the anti conformer of M2DPE, the chair conformer of THDC, and the trans isomer of DPCP are forbidden. However, the S2←S0 transitions of the asymmetrically deuterated isotopomers are observed, which allows for the estimation of the exciton splittings in these bichromophores. The exciton splittings are ca. three times higher in the more compact C2 symmetric gauche conformers of DPE and M2DPE than in the C2h symmetric anti conformers. The experimental exciton splittings of DPE conformers are higher than those of the corresponding M2DPE conformers. Dipole-dipole based Forster theory predicts higher splittings in M2DPE conformers, and incorrect ordering of the excited states of the anti conformers of DPE and M2DPE The exciton splitting in the C2 symmetric twist-boat conformer of THDC is seven times higher than that of the C2h symmetric chair conformer. The exciton splittings in THDC conformers are substantially larger than those of the structurally similar conformers of DPE and M2DPE.

Experimental evidence is provided in support of the conjugated nature of cyclopropyl ring. The trans isomer of DPCP has C2 symmetry. The spectrum of trans-DPCP is devoid of and like transitions. The cis isomer of DPCP is asymmetric. The S1←S0 and S2←S0 transitions of cis-DPCP are localized on the bisected and perpendicular phenyl rings, respectively. The S2←S0 minimum is calculated to lie just 60 cm-1 below the transition state along the phenyl torsional coordinates; hence, S2←S0 vibronic transitions are not observed in the spectrum of cis-DPCP. The S1←S0 and S2←S0 transitions of 2PI and 2FPI are found to be completely localized on the ortho-xylyl and phenyl (4-fluorophenyl) chromophores, respectiv...

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