Study of the electronic structure and spectra of diphenylcarbene conformers in their ground state and lower excited states

Abstract

The triplet-triplet fluorescence of perprotonated and perdeuterated diphenylcarbene (DPC-hlo and -dlo) in n-hexane at 4.2-25 K are both composed of a sharp spectrum and a broad blue-shifted one. They are attributed to two different conformers, the sharp spectrum corresponding to DPC in a quasi-planar geometry. The excitation spectra of both emissions consist of broad bands with a large Stokes shift. A detailed CS-INDO CI investigation (configuration interaction based on an intermediate neglect differential overlap method adapted to conformation and spectroscopy problems) is performed for threedifferent geometriesof DPC having CZ,, C2, and Cl symmetries. The computed triplet-triplet transition energies which indicate an hypsochromic shift going from planar CZ, to nonplanar C1 structures compare favorably with the experimental data. The fluorescence decays of DPC-hlo and -dl0 are nonexponential and attributed to the emission from different sublevels of the first excited triplet state. The measured decays of the broad fluorescence are longer than those of the sharp one. This is in agreement with CS-INDO CI calculations which predict a decrease of the oscillator strength of the TFT1 transition going from CZ, to C1 structures, as due to the inversion between the two excited triplets corresponding to the n r * 1 and nr*2 configurations. The zero-field splitting (ZFS) parameter D(T1) of DPC in the first excited triplet T I has been estimated from the magnetic field effect on the fluorescence decays. D(T1) = 0.007 and 0.20 cm-l for measurements carried out respectively on the sharp and on the broad fluorescence. Tentative ZFS calculations give D(T1) = 0.0155 cm-l for the Cb structure and D(T1) = 0.01 15 cm-l for the CI structure.The measured and calculated D(T1) values are always smaller than the D(T0) = 0.405 cm-1 value of DPC in the ground triplet state, which indicates the delocalization of the r electron density onto the aromatic rings upon excitation. It is therefore reasonable to detect a specific chemistry of this carbene in the presence of an intense laser which creates a high concentration of DPC in the first excited triplet T1.

Conformational dependence of electronic spectra and zero-field splitting parameters in 2-naphthylphenylcarbene

“…The To-TI transition energies of 1.96 and 1.98 eV calculated for the planar 2-NPC .conformers are lower than the corresponding energy of 2.62 eV computed for the planar (C2J DPC (9), in good agreement with the expected energy shift for more extended conjugated systems. This indicates that the CS-INDO parameters can be transferred from a medium-size molecule, like DPC, to larger compounds.…”

“…In previous reports (8,9), it was shown that two distinct fluorescence spectra are emitted from DPC in an n-hexane Shpolskii matrix over the temperature range between 4.2 and 77 K: a sharp fluorescence with an origin band at 502.8 nm and a broad blueshifted fluorescence with A, , , = 482 nm. The two fluorescence spectra were attributed to two different conformations of DPC trapped in this Shpolskii matrix.…”

“…This implies that DPC has nearly the same geometry in n-hexane and in benzophenone, which is consistent with the fact that the broad fluorescence with A, , , = 482 nm observed in nhexane is very similar to the broad structureless phonon band with A, , , = 476 nm observed in benzophenone (1 1). A detailed CS-INDOICI investigation was performed for three different geometries of DPC including both the abovementioned structures (9). This theoretical approach is based on a carefully designed multireference perturbation and uses a semiempirical valence-Hamiltonian specifically developed and parametrizedforconformational and spectroscopic problems.…”

Direct Detection of the Excited Triplet State of Diphenylmethylene by Time-Resolved EPR Spectroscopy

“…The lowest excited triplet state (T 1 ) was detected by triplet−triplet (T−T) fluorescence and transient absorption measurements . The zero field splitting (ZFS) parameter of D in the T 1 state of DPM has been estimated on the basis of the results of the external magnetic field effects on the T−T fluorescence decay . However, there are no reports on the EPR spectrum of the T 1 state of carbenes.…”

Second-order perturbation theory using correlated orbitals. II. A coupled MCSCF perturbation strategy for electronic spectra and its applications to ethylene, formaldehyde and vinylidene

“…It was also applied to the determination of accurate potential energy surfaces (for a recent example, see Ref. [70]), spin-orbit couplings in the ab initio scheme [71] or zero-field splitting parameters in a semi-empirical framework [72]. It was furthermore recently extended to semiempirical electronic Hamiltonians (PPP or CS/ INDO), using zeroth-order spaces designed to deal with the optical spectra of large conjugated systems or related ions [73][74][75][76][77][78].…”