B. Kozankiewicz

Single molecule lines and spectral hole burning of terrylene in different matrices

We observe fluorescence excitation lines of single terrylene molecules in three new polymer matrices (polyvinylbutyral, polymethylmethacrylate, and polystyrene) and in two crystals, n‐hexadecane (polycrystalline Shpol’skii matrix) and anthracene single crystal. We also burn persistent spectral holes in bulk samples of these solutions for comparison to single molecule lines. In all matrices where hole burning is efficient enough, we find good agreement between the average width determined from the distribution of single molecules’ linewidths and the homogeneous width deduced from spectral holes, which demonstrates the consistency and complementarity of the two techniques.

12-hydroxy-1-azaperylene-limiting case of the ESIPT system: enol-keto tautomerization in S0 and S1 states.

Absorption, fluorescence, and fluorescence excitation spectra of 12-hydroxy-1-azaperylene (HAP) and 1-azaperylene were studied in n-alkane matrices at 5 K. Two stable tautomers of HAP, each of them in n-nonane embedded in two sites, were identified and attributed to the enol and keto forms. Theoretical calculations of the energy and vibrational structure of the spectra suggest that tautomer A, with the (0, 0) transition energy at 18,980 ± 10 cm(-1) (and 19,060 ± 10 cm(-1) in the high energy site), should be identified as the keto form, whereas tautomer B, with the (0, 0) energy at 19,200 ± 20 cm(-1) (19,290 ± 20 cm(-1)), as the enol form. Observation of absorption and fluorescence of both tautomeric forms and lack of large Stokes shift of fluorescence of the keto form classify HAP as the limiting case of the excited-state intramolecular proton transfer system.

Intermolecular intersystem crossing in single-molecule spectroscopy: Terrylene in anthracene crystal

We present a spectroscopic study of terrylene in anthracene crystals at the ensemble and single-molecule levels. In this matrix, single-molecule fluorescence is reduced by three orders of magnitude. Correlation measurements allow us to identify a new relaxation channel, matrix-enhanced intersystem crossing. This process starts with a singlet-to-triplet energy transfer from guest to host, after which the triplet exciton is transferred back to the guest. The intermolecular intersystem crossing is expected whenever the lowest triplet state of the host is located between the lowest singlet S(1) and lowest triplet T(1) excited states of the guest. It must be considered when searching for new host-guest systems for single-molecule spectroscopy.

Photoluminescence and quantum chemical studies of electronic and optical properties of m-nitroaniline and m-nitrophenol crystals

Fluorescence, phosphorescence and their excitation spectra of single crystals of m-nitroaniline (m-NA) and of m-nitrophenol (m-NPh) were measured at 5 K. m-NA and m-NPh were also studied in an n-hexane Shpolskii matrix. Phosphorescence and phosphorescence decays were also collected at higher temperatures. On the basis of band position analysis and quantum chemical calculations the phosphorescence is assigned to the molecular emission whereas the fluorescence is supposed to originate from crystal defects—radical ions. Radical ions are photogenerated in both materials and in m-NA by crystal freezing below the glassy phase transition. Computed values of the first order hyperpolarizability, βvec, are larger for radical ions than for neutral molecules which suggests that radical ions are intermediates in the molecular mechanism of optical nonlinearity generation.

Intersystem Crossing Mechanisms and Single Molecule Fluorescence: Terrylene in Anthracene Crystals*

Single molecule spectroscopy requires molecules with low triplet yields and/or short triplet lifetimes. The intersystem crossing (ISC) rate may be dramatically enhanced by the host matrix. Comparing the fluorescence intensity of single terrylene molecules in para-terphenyl, naphthalene, and anthracene crystals, we found a reduction of the saturation intensity by three orders of magnitude in the latter case. The fluorescence autocorrelation function indicates that the bottleneck state is the terrylene triplet. We propose a ping-pong mechanism between host and guest. This intermolecular ISC mechanism, which can open whenever the host triplet lies lower than the guest singlet, was overlooked in previous single molecule investigations.

Single molecules detect ultra-slow oscillators in a molecular crystal excited by ac voltages

We monitored the spectral shifts of single molecule lines under an applied ac voltage in a molecular crystal at a low temperature. When varying the voltage modulation frequency, we found pronounced resonances in the oscillating shift of the optical lines. The resonance frequencies are surprisingly low and are found anywhere in the explored region, from some tens of kHz to a few MHz. Their width and amplitude depend very steeply on temperature, with quality factors as high as a few hundred at 1.4K. Probing the resonant modes with single molecules at different locations, we find a clear spatial correlation of the modes within microcrystalline domains, extending over 10-100µm. For large amplitudes, the oscillations become anharmonic and display a range of nonlinear effects: decrease of the frequency with amplitude, hysteretic behavior depending on the scan direction and shift of the frequency whenan external strain is applied to the sample. Put together, our observations point to low-frequency acoustic modes localized at or around crystal defects. This work is closely related to earlier observations of similar resonances in Shpolskii matrices deposited on a semiconductor. We speculate on the relation of these acoustic modes to the low-energy modes (quasi-localized modes and Boson-peak excitations) already known in disordered solids.

Low-Temperature Spectra of the Analogues of 10-Hydroxybenzo[h]quinoline as an Indication of Barrierless ESIPT

The absorption and fluorescence spectra of two analogues of 10-hydroxybenzo[h]quinoline (10-HBQ), namely, 1-hydroxy-7-methylbenzo[c]acridine (HMBA) and 4-hydroxybenzo[c]phenanthridine (HBPA), were studied in n-alkane matrices at 5 K. Considerable energy separation between the onsets of the spectra and broadening of the bands was an indication that intramolecular proton transfer (ESIPT) takes place at such a low temperature. DFT and ab initio methods were used to calculate the electronic transition energies and oscillator strengths and the vibronic structure of the electronic spectra. Shortcomings in our knowledge of the shape of the potential energy surface for ESIPT systems are highlighted in the context of the discussion of the shape of the electronic spectra. The π-expansion of the 10-HBQ chromophore achieved by adding a benzene moiety at various positions adjacent to the pyridine ring led to compounds possessing diverse photophysical properties, ranging from the non-ESIPT strongly fluorescent molecule of 10-hydroxy-1-azaperylene to weakly emitting (or nonemitting) molecules, where ESIPT occurs very efficiently.

Photo-stability of single terrylene molecules in 2,3-dimethylnaphthalene crystals

Fluorescence excitation spectra and fluorescence intensity traces of single terrylene molecules in 2,3-dimethylnaphthalene crystals were studied at 5 K and also at temperatures between 200 and 290 K. Terrylene molecules were found to be relatively photo-stable in crystals obtained by cosublimation of both components under an argon atmosphere whereas these molecules were more easily photo-bleached in crystals obtained under a helium atmosphere. It is propose that the diverse behaviour in both groups of crystals is due to different penetration by oxygen and bleaching of terrylene molecules by photo-oxidation. The fully saturated fluorescence detection rates for the (0, 0) and vibronic transitions were studied.

The origin of delayed fluorescence in charge-transfer crystals: pyromellitic dianhydride-phenanthrene crystal

Abstract The temperature dependence of emission spectra and their decay parameters for pyromellitic dianhydride-phenanthrene chargetransfer crystals have been investigated between 1.7 and 300 K. It has been established that the delayed fluorescence originates from triplet-triplet annihilation at temperatures between 30 and 60 K. (activation energy 290 ± 20 cm−1) and from thermal activation of triplet excitons to the singlet excitonic band for temperatures higher than 60 K (activation energy 600 ± 30 cm−1). This mechanism may be considered as typical for charge-transfer crystals characterized by intermediate (50–80%) charge-transfer character of triplet excitons.

Micellar Structure and Water Penetration Studied by NMR and Optical Spectroscopy

The molecular structure of aqueous micellar solutions and microemulsions is studied by spectroscopic methods, employing molecules that respond to polarity. The spectroscopic methods used are: fluorescence (spectra, lifetimes and quenching studies) and NMR. In the fluorescence experiments, the probe molecules are present in extremely low concentration (below 10−6 M), thereby minimizing the possible perturbation of the probe environment. The probes are predominantly solubilized in the micellar phase. Their location can be investigated utilizing NMR spectroscopy (ring current effect).