Shpolskii Matrices Research Articles

The influence of deuteration of complex organic molecules on their fluorescence quantum yield (a review)

Data on the influence of deuteration of organic molecules and/or solvent on their fluorescence quantum yield are summarized. It is demonstrated that the effect of deuteration is normal, i.e., deuteration increases the fluorescence quantum yield due to decrease in the internal conversion probability, for organic molecules exhibiting fluorescence in the red region of spectrum, in which the internal conversion competes with the emission of fluorescence. In the case in which the probability of internal conversion is low, i.e., energy of level S 1 ≥ 16000–20000 cm–1, the intersystem crossing probability from the S 1 state to the set of triplet levels depends on the relative position of S 1 and the nearest triplet level. In so doing, the effect of deuteration can be either normal or anomalous, depending on whether the resonance between the S 1 level and the level nearest to it, the T n level, improves or deteriorates when these levels shift as a result of deuteration. In dilute vapors, cooled supersonic jets, and crystalline matrices, including Shpolskii matrices, the effect of deuteration at helium temperatures depends on exact resonance between the interacting levels. In the case of dilute vapors and cooled jets, the effect also depends on the vibronic level being excited. Deuteration of OH and NH groups, as a rule, slows proton transfer in the S 1 state of the molecule that occurs with their involvement, leading to normal effect of deuteration.

Spectral manifestations of distorted forms of Pd-porphin at helium temperatures: The effect of the matrix

The occurrence of two noninteracting forms of Pd-porphin in the ground electronic state in a wide number of Shpolskii matrices maintained at liquid helium temperature was observed with the help of the finestructure phosphorescence spectra of these systems. The ratio between these forms can be varied in wide limits by choosing a particular matrix, which indicates that the matrix plays a significant role in the conformation formation of the tetrapyrrole macrocycle. Based on comparison both with the resonance Raman spectra and with the results of investigations of metal complexes of porphin by the selective spectroscopy methods, the shortwavelength spectral form has been shown to correspond to the planar molecular structure, whereas the longwavelength form has been attributed to a nonplanar molecular structure with the distortion of the porphin macrocycle of the saddle type. The results obtained indicate that the methods of fine structure spectroscopy make it possible to efficiently study distorted molecular structures stabilized in solid matrices at low temperatures.

Photophysical properties of single terrylene molecules in Shpolskii matrices

We studied distributions of line widths of terrylene in n-alkanes. In dodecane the line broadening is caused by fast dephasing. The additional broadening in hexadecane is attributed to spectral diffusion.

ChemInform Abstract: Triplet‐Triplet Fluorescence and Spin Polarization of 1‐ and 2‐Naphthylphenylcarbenes

As with diphenylcarbene, 1- and 2-NPCs in Shpolskii matrices at low temperatures present broad and red-shifted sharp spectra. The concentration dependence of the relative intensities of these spectra indicates that energy transfer is very efficient in these systems. In the case of 2-NPC in n-heptane at 77 K, a ligth-induced interconversion between two fluorescence spectra is observed and tentatively attributed to conformational isomerization

Triplet-triplet fluorescence and spin polarization of 1- and 2-naphthylphenylcarbenes

As with diphenylcarbene, 1- and 2-NPCs in Shpolskii matrices at low temperatures present broad and red-shifted sharp spectra. The concentration dependence of the relative intensities of these spectra indicates that energy transfer is very efficient in these systems. In the case of 2-NPC in n-heptane at 77 K, a ligth-induced interconversion between two fluorescence spectra is observed and tentatively attributed to conformational isomerization

Solid state photochemistry at 4.2 K: The photocyclization of [5] helicene to dihydrobenzoperylene in hpolskii matrices

The reversible photocyclization reaction of [5]helicene to dihydrobenzoperylene (DHBP) was studied in the Shpolskii matrices n-pentane and n-hexane at 4.2 K. The well-resolved transitions from the photochemical product are attributed to DHBP, occupying a distinct site in n-hexane. The photoinduced formation of DHBP at 4.2 K was monitored during illumination by sequentially recording fluorescence spectra. The multiplet structure of [5]helicene and DHBP in fluorescence and in phosphorescence in n-pentane were determined by high-resolution total luminescence spectroscopy (TLS). The subsequent dehydrogenation of DHBP to benzoperylene (BP), which occurs rapidly at higher temperatures in fluid media and in the presence of hydrogen acceptors, was found to be inhibited in solid matrices.

ChemInform Abstract: Fluorescence Excitation and Emission Spectra of 1,8‐Dihydroxyanthraquinone‐d0 and ‐d2 in n‐Octane at 10 K.

The fluorescence excitation and emission spectra in n‐octane of 1,8‐dihydroxyanthraquinone‐d0 and ‐d2 at 10 K have been measured. Dual excitation and emission were observed as a consequence of excited state intramolecular proton transfer. A model, based on the Lippincott–Schroeder potential function, is proposed to predict the observed energy gaps and relative intensities of the transition. The isotopic effects are also explained. The Shpolskii matrices in n‐octane show only one main site allowing a detailed vibrational analysis of the ground and the excited states. This furnished further evidence for the existence of excited state tautomers. The occurrence of an extra fluorescence was explained in terms of the ν(OH) stretching mode of the high frequency transition enhanced via vibronic coupling between the two ground states.

The S 2 → S 0 fluorescence of pseudoazulenes in Shpolskii matrices

The S 2 → S 0 fluorescence spectra of seven pseudoazulenes (PAs) have been studied in Shpolskii matrices. Highly resolved and structured spectra were obtained by selective excitation of the S 2,0 ← S 0,0 transition. With increasing excitation excess energy the fluorescence quasiline spectra are broadened and red-shifted. These phenomena are discussed in terms of a fast intramolecular vibrational redistribution (IVR) process. A vibrational analysis is presented.

Site-Selection Phosphorimetry via Singlet-State Excitation

This paper describes the technique of excitation energy site-selection (SS) phosphorimetry for chemical analysis of polycyclic aromatic compounds in Shpolskii matrices at 4.2 K. The principle of the SS phosphorescence technique, which uses excitation via the singlet state and produces narrow-band phosphorescence, is discussed. The analysis of a three-component mixture is used to illustrate the spectral selectivity of the SS phosphorimetric technique.

Shpolskii spectra and photophysical properties of dinaphtho[1,2-a;1′,2′-h]anthracene - a strongly non-planar, overcrowded aromatic hydrocarbon: Comparison with benzo[c]phenanthrene

Overcrowded, non-planar molecules of dinaphtho[1,2-a;1′,2′-h]anthracene (DNphA), while structurally related to those of benzo[c]phenanthrene (BPh) exhibit, in contrast to the latter, quasilinear multisite luminescence and absorption spectra in Shpolskii matrices at 77 K. Vibrational analysis of these spectra was performed. Electronic transitions of DNphA and BPh were calculated within the Warshel-Karplus method modified by assuming the linear dependence of the β 0 parameter on the number of π electrons in a molecule. The proximity of S 1 and S 2 DNphA states (energy gap about 800 cm -1), inferred from the structure of the quasilinear absorption spectrum, was corroborated by calculation. Photophysical parameters of the DNphA molecule were determined and compared with those of BPh. An increase in the S 1⇝S 0 internal conversion rate seems to occur in large molecules of heavily overcrowded cata-condensed hydrocarbons.

Fluorescence excitation and emission spectra of 1,8-dihydroxyanthraquinone-d and -d2 in n-octane at 10 K

The fluorescence excitation and emission spectra in n-octane of 1,8-dihydroxyanthraquinone-d0 and -d2 at 10 K have been measured. Dual excitation and emission were observed as a consequence of excited state intramolecular proton transfer. A model, based on the Lippincott–Schroeder potential function, is proposed to predict the observed energy gaps and relative intensities of the transition. The isotopic effects are also explained. The Shpolskii matrices in n-octane show only one main site allowing a detailed vibrational analysis of the ground and the excited states. This furnished further evidence for the existence of excited state tautomers. The occurrence of an extra fluorescence was explained in terms of the ν(OH) stretching mode of the high frequency transition enhanced via vibronic coupling between the two ground states.

Time-resolved resonance Raman spectrum of chrysene in the S1 and T1 states

The time-resolved Sn–S1 and Tn–T1 resonance Raman spectra of highly purified chrysene in tetrahydrofuran were observed at room temperature using the third harmonic of a Q-switched Nd:YAG laser as an electronic excitation pulse and a dye laser pumped by the second harmonic of another Q-switched Nd:YAG laser as a Raman probe pulse. We observed the Raman lines attributed to the S1 state at 1549, 1369, 1344, 1110, and 990 cm−1, and to the T1 state at 1528, 1494, 1353, 1199, 1110, 990, 678, 554, 472, and 286 cm−1. The decay time of 1369 cm−1 (S1) line, the formation time of 1528 cm−1 (T1) line, that of 1199 cm−1 (T1) line, and the fluorescence lifetime were obtained to be 35+6−5, 51+19−12, 41+10−7, and 41+− 1 ns, respectively. The observed Sn–S1 and Tn–T1 resonance Raman spectra were compared with the previously reported fluorescence and phosphorescence spectra in Shpolskii matrices, Sn–S1 and Tn–T1 resonance CARS spectra, and Tn–T1 spontaneous resonance Raman spectra. Discussion was made on the geometrical changes in the excited states on the assumption that the changes were not very large for the rigid molecular structure. We came to the following conclusions: (1) The resonance-coupled excited singlet state is different from the ground state in geometry mainly along the normal coordinates of the vibrational modes which have the Sn–S1 resonance Raman peaks at 1549 and 1344 cm−1. (2) The geometry of the resonance-coupled excited triplet state differs from that of the ground state mainly along five coordinates. Four of them correspond to the vibrational bands observed in the Tn–T1 resonance Raman spectrum at 1528, 1353, 1110, and 990 cm−1. The other corresponds to the vibrational bands observed in the Shpolskii spectrum of phosphorescence at 1385 cm−1 in literature. (3) The S1 and T1 states differ along the normal coordinates of the modes which have the Tn–T1 resonance Raman bands at 678, 554, 472, and 286 cm−1.

Reinvestigation of the absorption spectrum of triphenylene: Vibronic structure of the S 1 ← S 0 transition and location of higher excited singlet states

Vibrational analysis of the absorption spectrum of triphenylene- h 12 in Shpolskii matrices, supplemented by calculation of excited-state vibrations performed within the Warshel-Karplus formalism, has resulted in an improved set of vibrations active in the S 1← S 0 transition. Experimental evidence and calculation of electronic transitions point to the location of the S 2 state at ≈ 32000 cm −1.

Site-selection spectroscopy of o- and m-xylylenes and their methylated derivatives in Shpolskii matrices

The vibronic structures of the site-selected fluorescence and excitation spectra of methylated o- and m-xylylenes in Shpolskii matrices at 5–10 K are analyzed. These biradicaloid species are photochemically generated from methylbenzenes.

Luminescence of 1,4-naphthoquinone and the vitamin K system in Shpolskii matrices at 4 K

This work investigates the high-resolution phosphorescence spectra of 1,4-naphthoquinone and the vitamin K system in Shpolskii solvents at 4 K. The quasi-linear vibronic bands are discussed with regard to spectral assignments and polarization data. The effect of non-totally symmetric vibrations is also discussed.

Highly resolved electronic absorption and luminescence spectra of some phenanthrolines in Shpolskii matrices: Part II. Fluorescence and S1←S0 absorption spectra

Highly resolved fluorescence and S 1←S 0 absorption spectra of some phenanthrolines in n-alkane matrices were obtained at 77 K. The vibrational analysis of the spectra was carried out. Mirror symmetry distortions of the fluorescence and absorption spectra were attributed to vibronic coupling between 1(π, π ∗) states. It was shown that vibronic mixing of S 1(π, π ∗) and S 2(π, π ∗) states occurs in phenanthroline molecules with S 1-S 2 energy gap not exceeding 3000 cm -1 (i.e., 4,7-, 1,7- and 1,10-Phen), whereas in other phenanthrolines the coupling of S 1 and S 3(π, π ∗) plays a dominant role. Fluorescence quantum yields of phenanthroline liquid solutions were measured. Changes of luminescence efficiencies due to the change of the polarity of the media point to a weak coupling of the lowest 1(π, π ∗) and 1(n, π ∗) states.

Highly resolved electronic spectra and electronic structure of benzo[f]quinoxaline

Vibrational analysis of highly resolved phosphorescence (P), flourescence (F) and absorption spectra of benzo[f]quinoxaline (BQ) in Shpolskii matrices, at 77 K, was carried out. The spectra were dominated by phenanthrene-type fundamental vibrations. The appearance of several out-of-plane (op) modes in the P spectrum as well as the P lifetime, much shortened relative to that of the parent hydrocarbon, point to the proximity of the lowest n, π ∗ and π, π ∗ states of the BQ molecule. High activity of op modes in the F spectrum and complex structure of the absorption spectrum onset have been explained in terms of the pseudo-Jahn-Teller interaction between close-lying S 1 (π, π ∗)and S 2(n, π ∗) states. According to calculations of the BQ electronic structure, performed using a modified INDO CI method, the T 1(π, π ∗)-T 2(n, π ∗) and S 1(π, π ∗-S 2(n, π ∗) energy gaps are about 1200 and 700 cm -1, respectively.

Spectroscopic investigations of photolysis products of organic compounds in Shpolskii matrices. Application to acenaphthene and monomethylnaphthalenes

It is shown that Shpolskii matrices can be very suitable to acquire quasilinear spectra of trapped unstable species. Photolysis of acenaphthene and monomethylnaphthalenes in Shpolskii matrices yields photoproducts exhibiting highly resolved spectra. It is proposed that in addition to the acenaphthenyl radical the 1-acenaphthenium cation is formed, whereas photolysis of monomethylnaphthalenes produces the methylnaphthyl radicals, the methylnaphthenium cations and the benzocycloheptenium cation.

Low-temperature vibronic spectra of 1, 1′-binaphthyl

Vibronic spectra of 1, 1′-binaphthyl in pentane Shpolskii matrices at 4 K are reported. Taken in conjunction with the IR and Raman spectra, it can be seen that these arise from two non-interconverting ground-state conformers. The fluorescence origins of these conformers are split by 4850 cm−1. A vibronic analysis suggest that the two naphthalene moieties are approximately perpendicular in the conformer with the higher-energy origin, while being approximately planar in the other. The absence of progressions in the torsional mode in the spectra of each conformer suggests that their conformations are unaltered in the excited state. Spectra in other media suggest that the two ground-state conformers also exist in many liquid solutions.

Low-temperature luminescence spectrum of crystalline biquinoline: trap phosphorescence and delayed enlimer fluorescence

The sI~ectroscopy of the so-called double molecules, e.g. biphenyl [1 --5[, has attracted considerable attention as such compounds each represent, roughly speaking. two strongly interacting monomers [2] ; these systems have thus served as intriguing models with respect to exciton theory and other energy transfer phenomena [2]. Recent work in this laboratory has been concerned with the spectroscopy of 2,2'-biquinoline (BQ) [4,5]. While past efforts have been spent examining the low-temperature phosphorescence and optically detected magnetic resonance (ODMR) of BQ dispersed in Shpolskii matrices [4], some emission features of the neat crystalline phase have recently been observed and have proven to be of interest. It is our purpose here to report these results and to enumerate the various energy transport processes manifest in the spectra.