O. Morawski

Fluorescence and phosphorescence emission and nonradiative relaxation of acridine in a crystalline matrix of 2,3-dimethylnaphthalene

Abstract The results of experimental studies of fluorescence and phosphorescence of acridine, selectively excited within its absorption band, in a crystalline host lattice of 2,3-dimethylnaphthalene, are reported. The energies of the first excited singlet and triplet states of acridine in this crystalline matrix were determined. The spectra of both emissions display a broadening caused by the orientational disorder of the host lattice. The intensity and the decay time of the fluorescence and phosphorescence show moderate temperature dependence (in the 4.2–300 K temperature range). It is concluded that over the whole temperature range the most important mode of the nonradiative relaxation of the excited acridine molecule is that of the (nearly) temperature-independent intersystem crossing from the S 1 (π, π ∗ ) singlet to the T 3 (π, π ∗ ) and/or T 2 ( n , π ∗ ) triplet states.

Fluorescence excitation and fluorescence spectra of jet-cooled acridine molecules: acridine dimer formation and structure

Abstract The fluorescence excitation and fluorescence spectra from a supersonic helium jet seeded with acridine molecules have been obtained. The presence of low-frequency vibrational transitions in the fluorescence excitation spectrum, which can be assigned as intermolecular vibrational modes, clearly indicates that acridine dimers are formed under jet-cooling conditions. The acridine dimer ground-state geometry determined by AM1 calculations is a head-to-tail-like structure with an almost collinear arrangement of the short molecular axes of both molecular components. The results of calculations of spectral characteristics predict a large enhancement of the oscillator strength for the transition from the ground to first excited singlet state of the dimer (as compared to acridine molecule). Such enhancement is entirely connected with structure of the dimer (which presumably is also stabilized by the formation of a hydrogen bond).

Aggregation-controlled fluorescence of acridine in a crystalline matrix of anthracene

Abstract The results of experimental studies of the fluorescence of chemically mixed crystals of anthracene with large acridine concentrations (⩾ 1 mol%) at 4.2 K are reported. Excitation of fluorescence, carried out with excitation energies lower than the energy of the bottom of the singlet excitonic band, has revealed the formation of acridine dimers (resonance pairs) in the case of crystals with 1 mol% concentration of acridine. An aggregation-controlled spacing between nearly degenerate (in an isolated molecule) singlet 1 (π, π*) and 1 (n, π*) states of acridine is proposed as an explanation of the fluorescence properties of acridine dimers

Singlet states of chemically mixed anthracene−acridine crystals

Abstract An experimental study of the behaviour of fluorescence emission in chemically mixed crystals of acridine in anthracene is presented. A distinct difference between the fluorescence spectra, their temperature dependence and decay times has been found for different concentrations of acridine in the host lattice. These differences are described either in terms of an amalgamated singlet excitonic band (for very low concentrations of acridine) or in terms of the formation of aggregates of the guest molecules (for high concentrations of acridine).

Laser-induced emission spectroscopy of acridine cooled in a supersonic jet: the role of dimers and hydrogen-bonding in photophysics of acridine

Abstract The formation of acridine (ACD) dimers under supersonic jet-cooling conditions has been established by the observations of laser-induced fluorescence and fluorescence excitation spectra. The structure of ACD dimers predicted by using the semi-empirical AM1 parametric method suggests the possibility of the formation of hydrogen bonds between ACD moieties in the dimers. This possibility is verified with use the of the PM3 parametric method. The configuration interaction calculations performed for optimized dimer structures gives clear insights into the physical mechanism of the enhancement of the radiative processes in dimers (which, however, seems to be limited to specific conditions of supersonic expansions). The well-known role of hydrogen-bonding in the photophysics of ACD in protic liquid solutions is also supported by the results of preliminary observations under supersonic jet-cooling.

Environment‐Sensitive Behavior of DCNP in Solvents with Different Viscosity, Polarity and Proticity

A pyrazoline derivative, 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole (DCNP), is studied by using optical spectroscopy methods in several solvents at room and at low temperatures. The DCNP molecule reveals a complex photophysics behavior, which is sensitive to solvent polarity, proticity, temperature and viscosity and arises from the presence of two rotational degrees of freedom of the dicyanovinyl group--the torsion around the double C=C bond and the s-trans-s-cis isomerization around the single C-C bond--that differently behave in various environmental conditions. The fluorescence yield of a few percent and sub-nanosecond decay times observed at room temperature make the compound useful for optical studies of liquid environments. The proticity of polar solvents can be detected with two-exponential fluorescence decays. At low temperatures, DCNP can be used as solvent viscosity or temperature fluorescent sensor.

The Coumarin-Dimer Spring - The Struggle Between Charge Transfer and Steric Interactions

The synthesis of a weakly coupled, strongly polarized coumarin dimer has been achieved for the first time. The three-step strategy comprises the Skattebøl formylation followed by the Knoevenagel reaction and the formation of a tertiary amide by using a peptide-type procedure. The molecule consists of two different coumarin moieties: One is a classical donor-acceptor system and the second one possesses a weaker amide donor at the 7-position. The polarized coumarin dimer can form an electronically conjugated structure possessing an electric dipole larger than that of 7-(dimethylamino)coumarin-3-carboxylic acid. The limited flexibility of the inter-coumarin connection results in stable conformers of different electric dipole moments and complex photophysics. In the solid state, this compound has a strongly bent conformation with the two coumarin units forming an angle of around 74°. In solution, two conformers are in equilibrium. The existence of the molecule as two conformers in the ground state has been confirmed by optical studies, and further corroborated by molecular calculations. The fluorescence spectra possess a unique feature: A charge-transfer band (ca. 550 nm) is visible only in nonpolar or weakly polar solvents. Optical spectroscopy studies coupled with molecular calculations allowed us to rationalize this phenomenon: The large amplitude of intramolecular motions is responsible for the conformational isomerization as well as producing a conical intersection between the potential energy surfaces of the excited singlet state and the ground state, which opens an internal conversion channel that effectively competes with the fluorescence of the conformers.

Fluorescence of 4-(Diisopropylamino)benzonitrile (DIABN) Single Crystals from 300 K down to 5 K. Intramolecular Charge Transfer Disappears below 60 K

Single crystals of 4-(diisopropylamino)benzonitrile (DIABN) undergo an intramolecular charge transfer (ICT) reaction in the excited singlet state. At 300 K, the fluorescence consists of emissions from the locally excited (LE) and from the ICT state. Upon cooling to 5 K, the ICT fluorescence intensity gradually decreases relative to that of the LE emission and is absent below 60 K. With crystalline 4-(dimethylamino)benzonitrile (DMABN), in contrast, only LE emission is found over the entire range from 300 to 5 K. The phosphorescence spectra of the DIABN and the DMABN crystals do not present any evidence for an additional ICT emission, showing that ICT does not occur in the triplet state. An activation energy Ea of ∼4 kJ/mol is determined for the LE → ICT reaction of DIABN crystals, from the temperature dependence of the fluorescence decay times τ2 and τ1. Ea is attributed to changes in the molecular conformation of DIABN other than a full rotation of the large diisopropylamino group with respect to the benzonitrile moiety. In a comparison with crystal and solution data, literature results from transient vibrational and absorption spectra are discussed and it is concluded that they cannot be employed to favor the TICT (perpendicular twist) over the PICT (planar) model for DIABN and DMABN.