Jacek Nowacki

Photoinduced intramolecular electron transfer in 4-dimethylaminopyridinesDedicated to Professor Dr Z. R. Grabowski and Professor Dr J. Wirz on the occasions of their 75th and 60th birthdays.

Contrary to 4-(dimethylamino)pyridine (DMAP), which revealed solvent-dependent dual fluorescence, its sterically hindered ortho-methylated derivatives: 3-methyl-4-(dimethylamino)pyridine and 3,5-dimethyl-4-(dimethylamino)pyridine show single charge transfer fluorescence band. The electronic structures of the lowest excited states and molecular geometries were investigated by stationary and kinetic absorption and fluorescence spectroscopy as well as time-dependent density functional response theory (TD-DFT) and semiempirical (INDO/S) calculations. The results can be interpreted in terms of the TICT state model.

Phosphorescent intramolecular charge transfer triplet states

Abstract A comparative study of the electronic structure of the lowest excited triplet state T 1 is presented for a series of N-bonded donor-acceptor derivatives of 3,6-di-tert-butylcarbazole containing benzonitrile, nicotinonitrile or various dicyanobenzenes as electron acceptor. Solvent, temperature and concentration effects on phosphorescence, measurements of luminescence anisotropy and lifetimes, and ESR investigations of selected compounds show the dependence of the electronic structure of their T 1 states on the electron affinity of the acceptor moiety and point to the 3 CT character of the emitting triplet states in 3,6-di-tert-butylcarbazol-9-yl dicyanobenzenes.

ReI(CO)3+ complexes with N∩O− bidentate ligands

Simple reactions between Re(CO)5Cl and 2-benzoxazol-2-ylphenol, 2-benzothiazol-2-ylphenol or 2-(1-methyl-1H- benzoimidazol-2-yl)phenol (performed in boiling toluene) lead to dimeric complexes of general formula Re2(CO)6(N∩O−)2. The obtained dimeric species are quite stable in non-coordinating solvents, but in coordinating media (e.g., pyridine – pyr) they undergo disscociative solvolysis to form monomeric derivatives of general formula Re(CO)3(N∩O−)pyr. Molecular structures of both the dimeric and monomeric forms have been confirmed by means of X-ray measurements, electron impact mass spectrometry and IR spectroscopic investigations. UV-VIS absorption and emission properties of the newly synthesized Re(CO)3+ complexes have been investigated and briefly compared with those previously reported for their 8-oxyquinolinato analogues.

Radiative and nonradiative electron transfer in donor–acceptor phenoxazine and phenothiazine derivatives

Abstract Radiative and nonradiative electron transfer (exemplified by the CT fluorescence and nonradiative charge recombination process in the singlet manifold, respectively) in a series of donor–acceptor phenoxazine and phenothiazine derivatives is reported. An analysis of the CT fluorescence leads to the quantities relevant for the radiative electron transfer in the Marcus inverted region. Using a nonadiabatic theory of electron transfer and the latter parameters, the rate constants for nonradiative electron transfer can be predicted. Electronic coupling elements V 0 between the 1 CT state and the ground state obtained from the radiative rates are in agreement with those calculated from the simple LCAO MO model which assumes that V 0 are mainly determined by the interactions between the atoms forming the A–D bond.

Radiative electron transfer in planar donor–acceptor quinoxaline derivatives

Abstract Synthetic and spectral studies have been performed for a family of electron donor–acceptor (D-A) quinoxaline derivatives possessing an aromatic amine as an electron donor. A photophysical behaviour of the compounds with an internal degree of freedom for internal D-A rotation and their rigid analogues with a fixed planar conformation appears to be very similar. Electronic transition dipole moments related to the charge-transfer (CT) absorption and fluorescence are determined by both the direct interactions between the 1 CT and ground states and by the contributions from the locally excited configurations. The radiative properties of the D-A systems under study can be explained in terms of the simple model which assumes that the electronic coupling elements are mainly determined by the interactions between the atoms forming the A-D bond.

Highly efficient electrochemical generation of fluorescent intramolecular charge-transfer states

Abstract The electrochemically generated chemiluminescence of 4-(3,6-di-tert-butylcarbazol-9-yl)-benzonitrile (CBP) and 3,6-di-tert-butylcarbazol-9-yl- terephthalonitrile (CTO) was studied using the triple-potential-step method. In the electrogenerated emission spectra the charge-transfer (CT) bands (the same as in photoluminescence) were observed. The Feldberg plot analysis indicates that the 1,3 CT states are formed directly by the electron transfer between the radical anion and cation. High emission efficiencies (0.027 for CBP and 0.011 for CTO) were found with population yields of the fluorescent state markedly different for both compounds (0.066 for CBP and unusually large, 0.64 for CTO). Evidence is also presented that at low temperatures the efficiency of the fluorescent state formation (for CTO) is still higher, approaching unity. This finding is interpreted in terms of the different electronic structure of the lowest excited triplet states of the two compounds.

Tuneable white fluorescence from intramolecular exciplexes

Crystal violet lactone (CVL) in solution displays unusually broad (FWHM > 9100 cm(-1)) dual fluorescence with the characteristics of white light. The emission combines a blue CT band from a local chromophore with an orange CT band from an intramolecular exciplex formed adiabatically at appropriate medium polarity. The fluorescence spectrum can be controlled by solvent polarity to yield tuneable emission colours in a broad range of coordinates in the CIE chromaticity diagram including the white region. We show that such dual emission is a general property of CVL-like D-A structures built on sp(3) carbon atoms. The dependence of excited state energetics on molecular structure allows the prediction of width, shape and other parameters of the dual fluorescence spectrum, and so enables the engineering and customised design of white fluorophores. The photophysics-structure relationship found for CVL and its analogues can be generalized into a novel concept of white light generation by small molecules. These D-A systems are studied as a template basis for design and development of white fluorophores.

Cyclometallated iridium(III) complexes with 2-phenylbenzimidazole derivatives - spectroscopic, electrochemical and electrochemiluminescence studies

Summary Simple synthetic procedure (reactions between iridium trichloride hydrate with a series of fluorine substituted 2-phenyl-N-methyl-benzimidazole derivatives L) leads to the cyclometallated Ir(III) complexes IrL2(acac) and IrL2(pic) where acac = acetylacetonate and pic = α-picolinate anions, respectively. All the investigated complexes exhibit strong absorption in the UV region, due to the spin allowed intra-ligand (IL) transitions, and moderately intense bands in the visible region, due to the metal-to-ligand-charge-transfer (MLCT) transitions. The complexes exhibit also strong luminescence in the deoxygenated 1:1 acetonitrile/dioxane solutions at 298 K as well as in the butyronitrile glasses at 77 K. In both experimental conditions the structured emissions (with the 0–0 transition within the 480–510 nm range of UV-Vis radiation) have been observed. Cyclic voltammetry investigations point out to reversible character of the electrochemical reactions leading to stable IrL2(acac)+ or IrL2(pic)+ cations and IrL2(pic)− anions. The corresponding IrL2(acac)− species have been found to be unstable under the applied experimental conditions (1:1 acetonitrile/dioxane solutions containing 0.1 M (C4H9)4NPF6 as the supporting electrolyte) most probably due to the irreversible electrochemical reduction of the acac− ligand. Appropriate combination of the electrochemical and luminescence properties found for the investigated complexes allows for the electron transfer generation of the excited 3∗IrL2(pic) and 3∗IrL2(acac) species. Triple-potential-step technique was used to create electrochemiluminescence (ECL) emission by annihilation of the electrochemically generated IrL2(acac)+ or IrL2(pic)+ cations and 4,4’-dicyano-biphenyl radical anion. Very high ECL emission efficiencies (close to the excited 3∗IrL2(acac) and 3∗IrL2(pic) luminescence yields) have been found.

EXCITED CHARGE TRANSFER STATES IN DONOR-ACCEPTOR INDOLE DERIVATIVES

Abstract Solvent-dependent electronic structure of the selected donor (D)–acceptor (A) derivatives of 5-methylindole and 5-methoxyindole containing benzonitrile as an electron acceptor in the fluorescent charge transfer (CT) states has been investigated. The mechanism of the radiative charge recombination 1 CT → S 0 is discussed in terms of the Mulliken–Murrell model of the CT complexes and the Marcus theory of photoinduced electron transfer (ET). Solvatochromic effects on the spectral position and profile of the stationary fluorescence spectra point to a considerable CT character of the emitting singlet states of all the compounds studied. The determination of the electronic transition dipole moments corresponding to the 1 CT → S 0 fluorescence and the band-shape analysis of the corresponding spectra leads to the quantities relevant for the charge recombination in the Marcus inverted region. It is shown that some of the photophysical properties of the compounds can be predicted in terms of a simple model from the properties of individual chromophores.

Intramolecular excited charge-transfer states in donor–acceptor derivatives of naphthalene and azanaphthalenes

Electronic structure and molecular conformation of selected donor–acceptor isomeric derivatives of naphthalene, quinoline and quinazoline as well as a family of isomeric triazanaphthalenes containing N,N-dimethylaniline as an electron donor in the excited charge transfer singlet (1CT) states have been investigated. The mechanism of the 1CT←S0 absorption and 1CT → S0 emission is discussed in terms of the Mulliken–Murrell model of the CT complexes and the Marcus formalism of radiative charge transfer. The determination of the electronic transition dipole moments corresponding to the CT absorption and fluorescence and the band-shape analysis of the corresponding spectra lead to the quantities relevant for the 1CT → S0 charge recombination in the Marcus inverted region. It is shown that some of the photophysical properties of the compounds can be predicted in terms of a simple LCAO MO description from the properties of individual chromophores.