Paweł Borowicz

Unusual, Solvent Viscosity-Controlled Tautomerism and Photophysics: Meso-Alkylated Porphycenes

Stationary and time-resolved studies of 9,10,19,20-tetramethylporphycene and 9,10,19,20-tetra-n-propylporphycene in condensed phases reveal the coexistence of trans and cis tautomeric forms. Two cis configurations, cis-1 and cis-2, play a crucial role in understanding the excited-state deactivation and tautomer conversion dynamics. The trans-trans tautomerization, involving intramolecular transfer of two hydrogen atoms, is extremely rapid (k ≥ 10(13) s(-1)), both in the ground and lowest electronically excited states. The cis-1-trans conversion rate, even though the process is thermodynamically more favorable, is much slower and solvent-dependent. This is explained by the coupling of alkyl group rotation with the hydrogen motion. Excited-state deactivation is controlled by solvent viscosity: the S(1) depopulation rate decreases by more than 2 orders of magnitude when the chromophore is transferred from a low-viscosity solution to a polymer film. Such behavior confirms a model for excited state deactivation in porphycene, which postulates that a conical intersection exists along the single hydrogen transfer path leading from the trans to a high energy cis-2 tautomeric form. For this process, the tautomerization coordinate includes not only hydrogen translocation but also large-amplitude twisting of the two protonated pyrrole moieties attached to the opposite sides of the ethylene bridge.

Tautomerization in fluorescent states of bipyridyl-diols: A direct confirmation of the intramolecular double proton transfer by electro-optical emission measurements

Abstract Integral electro-optical emission measurements (IEOEM) and INDO/S calculations were carried out for 2,2′-bipyridyl- 3,3′-diol (BP(OH) 2 ), and its 5-methyl- (MeBP(OH) 2 ) and 5,5′-dimethyl- (Me 2 BP(OH) 2 ) derivatives. All three molecules were reported earlier as effective lasing dyes [5,6] undergoing photoautomerization in the S 1 state. From the IEOEM experiment an upper limit of about 1.0 D (debye) is derived for the fluorescent state dipole moment, μ e , in each case, while the INDO/S calculations predict μ e of the excited singlet (π, π ∗ ) state of the diketo and monoketo forms in the range: 0.0–0.5 D and 4.0–4.9 D, respectively. Comparison of both results leads to an unequivocal discrimination between the two possible photoreactions, single or double proton transfer (SPT or DPT), and shows that the fluorescent species are diketo tautomers produced by DPT.

Solvent dependence of (sub)picosecond proton transfer in photo-excited [2,2′-bipyridyl]-3,3′-diol

Abstract We report on (sub)picosecond fluorescence studies of the mono- to diketo tautomer reaction in photo-excited [2,2′-bipyridyl]-3,3′-diol in aprotic and protic solvents. In aprotic solvents no influence of the nature of the solvent, temperature and deuteration of the hydroxyl groups on the proton transfer dynamics was found. In protic solvents, however, the proton transfer time is found to change proportionally with the viscosity coefficient of the solvent. We briefly discuss (i) the formation of the mono- and diketo forms from the dienol form and (ii) the nature of the vibrational modes assisting in the proton transfer process.

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.

Triplet states of molecules undergoing internal double-proton transfer in the S1 state: 2,2′-bipyridyl-diol and its 5,5′-dimethylated derivative

Abstract 2,2′-bipyridyl-3,3′-diol and its dimethyl derivative undergo an ultrafast double-proton transfer upon excitation to S 1 state, and no direct population of T 1 state is observed. Thus, the primary (dienol) triplet (T 1 ) was populated, both by sensitization from the toluene triplet and by enhancement of the forbidden T 1 ←S 0 absorption. The low-energy phosphorescence of the phototautomeric (diketo) triplet was detected by irradiation in bromobenzene and was quenched by O 2 ( 3 Σ − g ) to yield O 2 ( 1 Δ g ). The triplet state is located at 21300 ± 300 and 12700 ± 100 cm −1 for the primary and phototautomeric form, respectively, in good agreement with the values predicted by the INDO/S method.

Strongly modified [2,2′-bipyridyl]-3,3′-diol (BP(OH)2): a system undergoing excited state intramolecular proton transfer as a photostabilizer of polymers and as a solar energy collector

Abstract Six new compounds, derivatives of [2,2′-bipyridyl]-3,3′-diol (BP(OH) 2 ) are recommended as useful modifications of the parent structure. One group, three molecules with conjugate aromatic substituents (6-methyl-6′-styryl-BP(OH) 2 , 6-(4-methoxystyryl)-6′-methyl-BP(OH) 2 , 6-methyl-6′-(4-phenylstyryl)-BP(OH) 2 ), are potential solar energy concentrators — they have absorption spectra red shifted with respect to BP(OH) 2 . To the second group belong the derivatives with the aliphatic side chains, better “accepted” by polyethylene than the parent molecule. These compounds are designed to improve their function as photostabilizers of polymers. All six new compounds were synthesized and their photophysical characteristics were studied. Absorption spectra, phototautomeric fluorescences and their excitation spectra, as well as quantum yields and lifetimes are reported.

Wavelength- and temperature-dependent measurement of refractive indices

A new method for determination of the refractive index of any transparent solvent or glass was developed and tested for use at temperatures of 133–293 K in the ultraviolet and visible spectral ranges. The wavelength and temperature dependencies of the refractive indices of several solvents and of fused silica are reported. The data obtained for tetrachloromethane and fused silica are compared with those available in the literature. The advantages and limitations of the method are discussed, based on the accuracy of the data presented. Knowing the values of refractive indices of organic solvents should be useful in luminescence spectroscopy.

New proton transfer lasing systems—derivatives of 2,2′-bipyridyl. Synthesis and photophysical characteristics

Abstract Two new bipyridyl derivatives undergoing the excited state intramolecular proton transfer were synthesized: 5-methyl-[2,2′-bipyridine]-3,3′-diol and 5,5′-dimethyl-[2,2′-bipyridine]-3,3′-diol. Both reveal a strong phototautomeric, largely Stokes-shifted fluorescence and can be used as proton transfer laser dyes. Tuning ranges and lasing efficiences are also reported.

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.

Deep-ultraviolet Raman investigation of silicon oxide: thin film on silicon substrate versus bulk material

Raman spectroscopy is a powerful experimental technique for structural investigation of silicon based electronic devices such as metal‐oxide‐semiconductor-type structures. It is widely used for characterization of mechanical stress distribution in silicon substrate. However, in the case of Raman measurements of oxide layer on silicon substrate visible excitation makes this technique almost useless. The reason for this difficulty is two-phonon scattering from silicon substrate which masks the signal from oxide layer. Application of deep-ultraviolet (deep-UV) excitation reduces the penetration depth of the radiation into silicon substrate about 30 times. As a result, the simultaneous measurement of one-phonon scattering from silicon substrate and the Raman spectrum of the oxide layer become possible. This work presents the study of thin silicon oxide film on silicon substrate with application of deep-UV Raman scattering. The spectra measured for thin film are compared with reference spectra obtained for bulk material.