Marzena Banasiewicz

Benzo[a]imidazo[5,1,2-cd]indolizines – a new class of molecules displaying excited state intramolecular proton transfer

The new class of ESIPT-capable molecules, benzo[a]imidazo[5,1,2-cd]indolizines, bearing the 2-hydroxyphenyl substituent were prepared in a straightforward manner from imidazo[1,2-a]pyridines via a tandem [8+2]cycloaddition–[2+6+2]dehydrogenation reaction. The relationship between the structure and photophysical properties was thoroughly elucidated by comparison with simple analogues i.e. 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridines. Compared with parent 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridines the new chromophores strongly absorb blue light and emit in the yellow part of the spectrum. In contrast to 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridines, emission of π-expanded dyes in protic solvents is usually weaker than in aprotic ones, where they exhibit large Stokes shifts (5500–7000 cm−1). Unlike in solution, in the solid state luminescence for benzo[a]imidazo[5,1,2-cd]indolizines is shifted on average by 100 nm relative to the fluorescence of the corresponding imidazo[1,2-a]pyridines, which results in a relatively strong (Φfl = 0.18 to 0.27) emission of the red light. Benzo[a]imidazo[5,1,2-cd]indolizines possessing a methoxy substituent have much higher fluorescence quantum yields than analogues bearing fluorine and methyl substituents.

Dipolar Dyes with a Pyrrolo[2,3‐b]quinoxaline Skeleton Containing a Cyano Group and a Bridged Tertiary Amino Group: Synthesis, Solvatofluorochromism, and Bioimaging

Two strongly polarized dipolar chromophores possessing a cyclic tertiary amino group at one terminus of the molecule and a CN group at the opposite terminus were designed and synthesized. Their rigid skeleton contains the rarely studied pyrrolo[2,3-b]quinoxaline ring system. The photophysical properties of these regioisomeric dyes were different owing to differing π conjugation between the CN group and the electron-donor moiety. These dipolar molecules showed very intense emission, strong solvatofluorochromism, and sufficient two-photon brightness for bioimaging. One of these regioisomeric dyes, namely, 11-carbonitrile-2,3,4,5,6,7-hexahydro-1H-3a,8,13,13b-tetraazabenzo[b]cyclohepta[1,2,3-jk]fluorene, was successfully utilized in two-photon imaging of mouse organ tissues and showed distinct tissue morphology with high resolution.

Vertically π‐Expanded Imidazo[1,2‐a]pyridine: The Missing Link of the Puzzle

The dehydrogenative coupling of imidazo[1,2-a]pyridine derivative has been achieved for the first time. In cases in which the most-electron-rich position of the electron-excessive heterocycle was blocked by a naphthalen-1-yl substituent, neither oxidative aromatic coupling nor reaction under Scholl conditions enabled the fusion of the rings. The only method that converted the substrate into the corresponding imidazo[5,1,2-de]naphtho[1,8-ab]quinolizine was coupling in the presence of potassium in anhydrous toluene. Moreover, we discovered new, excellent conditions for this anion-radical coupling reaction, which employed dry O2 from the start in the reaction mixture. This method afforded vertically fused imidazo[1,2-a]pyridine in 63% yield. Interestingly, whereas the fluorescence quantum yield (Φ(fl)) of compound 3, despite the freedom of rotation, was close to 50%, the Φ(fl) value of flat naphthalene-imidazo[1,2-a]pyridine was only 5%. Detailed analysis of this compound by using DFT calculations and a low-temperature Shpolskii matrix revealed phosphorescence emission, thus indicating that efficient intersystem-crossing from the lowest-excited S1 level to the triplet manifold was the competing process with fluorescence.

Intersystem crossing of single pentacene molecules in Shpol'skii matrices

We have measured the fluorescence decays of pentacene (Pc) dispersed in Shpolskii matrices of n-heptane, n-tetradecane and n-hexadecane in the temperature range 1.7–200 K. In all of these matrices the decay times were about 25 ns at 1.7 K and shortened to less than 15 ns at high temperatures. This result suggests a low value of the intersystem crossing (ISC), S1→T1, yield at 1.7 K. To check this hypothesis we studied the fluorescence intensity auto-correlation decays of single Pc molecules in n-tetradecane at 1.7 K. The analysis led to a value of the triplet population yield of about 1.5%.

Symmetry Breaking in Pyrrolo[3,2-b]pyrroles: Synthesis, Solvatofluorochromism and Two-photon Absorption

Five centrosymmetric and one dipolar pyrrolo[3,2-b]pyrroles, possessing either two or one strongly electron-withdrawing nitro group have been synthesized in a straightforward manner from simple building blocks. For the symmetric compounds, the nitroaryl groups induced spontaneous breaking of inversion symmetry in the excited state, thereby leading to large solvatofluorochromism. To study the origin of this effect, the series employed peripheral structural motifs that control the degree of conjugation via altering of dihedral angle between the 4-nitrophenyl moiety and the electron-rich core. We observed that for compounds with a larger dihedral angle, the fluorescence quantum yield decreased quickly when exposed to even moderately polar solvents. Reducing the dihedral angle (i.e., placing the nitrobenzene moiety in the same plane as the rest of the molecule) moderated the dependence on solvent polarity so that the dye exhibited significant emission, even in THF. To investigate at what stage the symmetry breaking occurs, we measured two-photon absorption (2PA) spectra and 2PA cross-sections (σ2PA ) for all six compounds. The 2PA transition profile of the dipolar pyrrolo[3,2-b]pyrrole, followed the corresponding one-photon absorption (1PA) spectrum, which provided an estimate of the change of the permanent electric dipole upon transition, ≈18 D. The nominally symmetric compounds displayed an allowed 2PA transition in the wavelength range of 700-900 nm. The expansion via a triple bond resulted in the largest peak value, σ2PA =770 GM, whereas altering the dihedral angle had no effect other than reducing the peak value two- or even three-fold. In the S0 →S1 transition region, the symmetric structures also showed a partial overlap between 2PA and 1PA transitions in the long-wavelength wing of the band, from which a tentative, relatively small dipole moment change, 2-7 D, was deduced, thus suggesting that some small symmetry breaking may be possible in the ground state, even before major symmetry breaking occurs in the excited state.

Modulation of the fluorescence properties of diketopyrrolopyrroles via various electron-rich substituents

Four diketopyrrolopyrroles have been synthesized starting from heterocyclic aromatic nitriles. It was found that the negative influence of electron-donating groups on the reactivity of nitriles can be overcome by the presence of an electron-deficient pyridine ring. The absorption and emission properties of the diketopyrrolopyrroles and their N-substituted derivatives were evaluated in a range of solvents revealing that the exact position of the electron-donating substituents significantly modulated their fluorescence response. The presence of a dialkylamino moiety at position 3 of the aryl substituents led to the occurrence of very fast nonradiative deactivation processes. Formation of both the anion (located on the core) and cation (located on the pyridine ring) changes the relative energy of the excited states leading to strong red fluorescence. On the other hand, the presence of a pyrrole moiety at position 4 of the aryl substituents resulted in a record high fluorescence quantum yield (0.88). The combination of the two dialkylamino-pyridine moieties and the oligoethylene glycol substituent made it possible to obtain a compound possessing reasonable water-solubility, which was applied in fluorescence microscopy for the selective staining of mitochondria in living cells.

Excited singlet state relaxation yields of pentacene in Shpol'skii matrices

Abstract We studied absorption and fluorescence spectra and decays of pentacene (Pc) in Shpolskii matrices of n -heptane (C 7 ), n -nonane (C 9 ), n -decane (C 10 ), n -dodecane (C 12 ), n -tetradecane (C 14 ) and n -hexadecane (C 16 ) in the temperature range 1.7–200 K. The fluorescence quantum yields of Pc in C 12 , C 14 and C 16 at 1.7 K were determined to be 20±6%, 26±6% and 22±6%, respectively. The kinetic analysis provided values of the radiative, internal conversion and intersystem crossing rate constants for the relaxation channels of the S 1 state of Pc in different matrices. Calculated fully saturated emission rates of single molecule were in very good relation with previous observation of single Pc molecules in Shpolskii matrices.

Building Molecular Complexity from Quinizarin: Conjoined Coumarins and Coronene Analogs

The double Knoevenagel condensation of 1,4-dibenzoyloxyanthraquinone with methyl esters of arylacetic acids affords a series of compounds based upon a previously unknown 1,8-dioxa-benzo[e]pyrene-2,7-dione heterocyclic core. The aryl groups incorporated in the 3- and 6-positions can be oxidatively coupled to the π-expanded backbone to produce a further new heterocyclic core: 1,10-dioxa-dibenzo[dj]coronene-2,9-dione. The intriguing optical properties of these π-expanded coumarin derivatives are discussed and rationalized through quantum chemical calculations. The broad absorption bands of 1,8-dioxa-benzo[e]pyrene-2,7-dione-based dyes are attributed to both HOMO-1→LUMO and HOMO→LUMO transitions, which have a similar energy. Weakly coupled electron-donating aryl substituents result in a moderate bathochromic shift of both the absorption and emission by 30-60 nm in toluene. The emissive properties of these compounds are in part determined by the oscillator strength of the main transition, lifetimes of the excited state, and by the energy match of the excited state with a triplet state of a similar energy. The 1,10-dioxa-dibenzo[dj]coronene-2,9-dione displays a much smaller Stokes shift, yet a markedly increased fluorescence quantum yield of 90 % owing to the increased rigidity compared with the 1,8-dioxa-benzo[e]pyrene-2,7-dione core.

New quinacridone derivatives with π-extended conjugation in central core

A novel synthetic strategy for the preparation of quinacridone derivatives with extended conjugation in central aromatic core is proposed allowing for fine tuning of their electronic properties. Two new quinacridone derivatives with carbazole (1) and naphthalene (2) core are described, which are excellent candidates for optoelectronic applications as evidenced by their electrochemically determined redox properties combined with strong photoluminescence (the fluorescence quantum yield of 0.83 and the radiative rate constant of 90.2 × 107 s−1 in the case of 2).