THE PHOTOTRANSFORMATIONS OF ORGANIC COMPOUNDS UNDER POWERFUL LASER EXCITATION IN CONDITION OF LASING AND SPONTANEOUS RADIATION.

Rimma T. Kuznetsova

Siberian Physical Technical Institute at Tomsk State University. 1 Novosobornaja sq. Tomsk, 634050, Russia.

e-mail: kopylova@phys.tsu.ru

ABSTRACT

The present paper reports the experimental results about photostability of laser active media under XeCl laser excitation up to 200 MW/cm². The quantum yield of phototransformation and yield of the photoproduct, absorbing on lase wavelength by stimulated and spontaneous radiation are determined.  It is defined mechanism of formation of this photoproduct, is established, that phototransformations of coumarin 2, 2-4pyridylphenyloxazole and p-terphenyl derivates are slowed by stimulated radiation compare with spontaneous. The modify of photostability under variation of spacing form and size of exciting volume is connected with competition of spontaneous and stimulated processes on radiation.

1.      INTRODUCTION.

The molecular photonics is sufficient good developed for spontaneous radiation processes of organic molecules, however especialities of experimental results, which are correspoding to the conditions of stimulated radiation under powerful excitations (phototransformations, spectra, transparency), unsufficiently or little are discussed. The results about lasing TICT-conformations and their phototrasformations in literature are absent. Also there are not dates about photostability of organic molecules by sufficient rising of transparency under powerful laser excitation up to 200 MW/cm². The necessary of such researches appear from search of optimal conditions of excitations for stable work of power dye lasers and registration of efficient photochemistry of organic molecules for biology and medicine, which are produced under powerful laser irradiation besides of tissue's ablations.

The aim of this paper, in which is presented the review of the researches on photostability, made in department of photonics in Siberian Physical Technical Institute is the investigation of the peculiarities of phototransformations of organic compounds with different radiative properties under XeCl laser excitation depending on parameters of excitng pulse (intensity and duration) on condition of optimal lasing, superfluorescence and spontaneous radiation.

2.      EXPERIMENT.

2.1.  Methods.

The objects of the investigation are coumarine, oxazole and p-terphenyl derivates. These compounds have different structure and capability for intermolecular interaction and there are active media for dye lasers, therefore problem of their photostability is very important for laser's "lifetime".

The excitation of researched compounds was produced by excimer XeCl lasers with different parameters: l_las=308 nm, E_las=30 - 500 mJ/p, t_FWHM=10 - 80 ns. Focusing of radiation permits rising density of power up to 300 MW/cm².

The absolute quantum yields of phototransformations - j and relative yields of the photoproducts- Ф_i, which are produced under irradiation, are characteristics of  molecular photostability. They were determined from changing of absorption before and after the irradiation by spectroscopic method detail  descripted in ^1,2.

j=N_ph/N_abs= (1-D/D₀)C₀N_Ahn V/E_sum

F_i =C_i V/N_abs=K_i/e_i´V hn/Е_sum,

where D and D₀ - the optical density after and before irradiation, C₀ - initial molecular concentration, C_i - concentration,  K_i - absorption coefficient  and e - extinction coefficient of photoproduct,  E_sum - absorbed energy,  N_A - Avogadro number, V- volume of irradiated solution, hn - energy of  irradiating photon.

The "lifetime" of laser media or lasing photostability is determined by summary energy, which absorbed in unity of volume and results of the efficiency decreasing to 0.8 or 0.5 from initial significance - R₈₀ and R₅₀, correspondingly.

The reactionable of organic molecules was studied from protonoaccepting properties of these compounds in the ground and excited states by spectral dates. Also the spectra of radiation before and after irradiation were determined for establishment the structure and mechanism of forming of photoproducts.

The depending of transparency of samples on the intensity of excitation were measured for estimation of the population of excited states. The species of radiation - spontaneous or stimulated or lasing - was established with help spectra of radiation, which were determined from one pulse with spectrometer "Angstrem" (Novosibirsk). The properties of transient shortliving products were studied with laser flash photolis. Shape of pumping  and radiation pulse was researched with "Technical vision" (monochromator MDR-23, photoreceiver PEC-22, oscillograph S8-14, TVcamera, PC).

2.2.  Results.

The more reactioncapable centers of the researched molecules are established by analysis of spectral-luminescent and photochemical properties. There is shown that efficiency of photocathion forming increases by excitation for compounds, which consist of oxygen atoms on carbonyl groups, nitrogen atoms on pyridyl or oxazole cycles, and decreases for compounds, which consist of aminogroups^3,4,5. The study of spectral characteristics of photoproducts , which absorbs  on lasing range, is shown, that they coincides with spectra of photocathions (Fig.1).

This is means, that photoproduct, which absorbs  on lasing, and his yield F_las correlated with lifetime, is responsible for lasing photostability. It is forming as the same as photocathion by interaction of protonacceptor center of excited molecules with protondonor or electronacceptor reagents^1,6-11.This result is confirmed with investigation the effects of inhibition and sensibilisation on photoreactions^5,8-10 (Tabl.1), phase state of media (Tabl.2) and concentration depending of phototransformations (Tabl.1, Fig.2).

The complex of experimental and theoretical studies are shown that dimethylaminosubstituted of pyridylphenyloxazole (4PyPON(CH₃)₂), having capability of rotation of donor and acceptor fragments is lasing under XeCl laser excitation as in nonpolar hexan, in which there is high quantum yield of fluorescence, and in polar ethanol and acetonitryl, in which there are TICT- conformations forming^2,12,13 (Tabl.3). The efficiency of fluorescence for TICT- conformation is less on three than in hexan, however lasing efficiency is higher (Tabl.3).  This is explained more slightly forming of inversion population, because TICT - conformation in the graund state is absent. The peculiarities in photostability of this compound also is connected with forming TICT - conformation (anomal large Stoces shift, and increasing protonoacceptor properties of nitrogen by excitation^2,11.

It is established, that many organic compounds operating in 400 nm range decrease efficiency and photostability by increase duration of pulse from 10 to 40 ns (Tabl.4). The study of transient absorbance is shown that the time of their formation is differed: 40 ns for 4PyPO and PDPDP, and @100 ns for PEDP. It means, that influence of transient absorption on inversion population as amplification and also photostability is there for 4PyPO and PDPDP by changing pulse duration from 10 to 40 ns and for PEDP up to 80 ns. Transient absorption for coumarines is absent, therefore photoprocesses in coumarines are less depending on pulse duration. DABCO addition decreases intensity of  transient absorption (Fig.3), increases lifetime and increases pulse duration of lasing and efficiency (Tabl.4).

It is means, that efficiency and lifetime of laser media decreases with increasing  exciting pulse duration from t₁ up to t₂, when the time of transient absorption formation t is limited of changing of pulse excitation duration, i.e. t₁<t <t_2.

The research of photostability of laser active media on the base of dicarboxipropylsubstituted p-terphenyl (LOS1) is shown (Fig.4) that it depends on exciting intensity, kind of radiation (spontaneous or stimulated) and shape of excited volume.

Efficiency and photostability are maximal by excitation in modification 1, when excited volume has preferencial direction (transversial pump with cylinder lens). Efficiency is 45 % , quantum yield of phototransformation @ 2 10^-4, lifetime

@ 175 J/cm³  in this modification. In modification 2 excited volume  is the cylinder with diameter and high 0.1 cm, and density of pump is  150-200 MW/cm². Efficiency decreases up to 4.5 % by 100 MW/cm² yield of phototransformation increases to @ 10^-3 and lifetime drops to 11 J/cm³ (Fig.4). Spectral characteristics of photoproducts are the same however photostability on lasing is higher than on spontaneous by 25 MW/cm². Increasing of photostability on lasing compare with spontaneous was got for coumarine 2 and 4PyPO solutions in amplifier of laser system MZhL-03, by irradiation as with injection of radiation from master oscillator  and without.

The transparency of samples also temporal and spectral characteristics of its radiation was measured for explanation of yield of phototransformation of decrease by powerful excitation of nonlasing LOS1 solution (0.1 mmol/l, modification 2)¹⁵. It is got, that spectra of radiation are narrowed by increasing of excitation intensity compare with fluorescence spectra of such solutions. The same narrowing of spectra is observed by excitation lower threshold of lase in modification 1 (W=5 – 10 MW/cm²) is connected with amplification of stimulated of radiation (superluminescence). The estimation of excited state’s population versus density of exciting power from transparency by method, described in¹⁴, is shown, that population is decreased by rising of stimulated processes’s  influence¹⁵.  Thus, decreasing of phototransformation’s yield under powerful excitation (curve 4 in fig.4) is connected with decreasing of excited state’s population by formation of superluminescence.

The study of temporal characteristics of radiation pulse is shown, that they sufficiently are the same as the pump pulse when the spectra of radiation is the spectra of fluorescence¹⁵ . The duration of radiation’s pulse decreases compare with pump, when the spectra of  superfluorescence or lasing are observed. However, duration increases with forming of oscillations under powerful excitation (>100 MW/cm²) of nonlasing solution in modification 2. Possible it is connected with changing of population by formation of superfluorescence or collective superradiation. This results are shown, that variation of phototransformation’s characteristics depending on intensity of excitation and configuration of excited volume connected with competition of spontaneous and stimulated processes in radiation.

3. CONCLUSION

Thus, the based principles for creation of effective photostable laser media and turning laser system are:

1.      Selection of compounds with highest quantum yield of fluorescence and lowest yield of phototransformation.

2.      Selection of solvents and additions to it for decreasing of yield of photoproduct, responsible for lifetime of laser media.

3.      Using of regeneration system for removing photoproducts.

4.      Treatment and creation of laser system with optimal construction for highest laser efficiency and minimal spontaneous radiation, owing to selection of concentration, density of pump, spaceal configuration of excited volume.

4. ACKNOWLEDGEMENTS

These researches were supported in part by Russian Foundation of Basic Research (grant 98 –03 – 32082).

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Table 1. Effects of concentration and additions on phototransformation’s characteristics of ethanol dye’s solutions.

Table 2. Lasing and photochemical characteristics of polymeric and ethanol samples of LOS1 under excitation  ХеС1 laser. W=30 МW/сm². * -nonfocusing beam W=3 МW/сm².

Table 3. Lasing and photochemical characteristics of oxazole substituted under excitation  ХеС1 laser (t_puls=10 ns, W=30 МW/сm²).

Table 4. Lasing characteristics of  some active media under excitation  ХеС1 lasers.

Subscripts for figures:

Figure 1. Spectra of fluorescence of neutral – 1 and cathion – 2 form of 4PyPO, quaternary salt 4PyPOCH₂PCl – 3 and photoproducts of 4PyPO – 4,5. l_ex: 1 – 3  -308 nm, 4 – 400 nm, 5 – 430 nm.

Figure 2. Dependence of quantum yields of phototransformation j – 1, lifetime – 2 and yields of photoproducts Ф_las –3, Ф₃ – 4, Ф₂ – 5 on concentration of coumarin 153.

Figure 3.Spectra of 4PyPO transient absorption   –1-3 in ethanol, 4-5 in water. Additions: NaOH (1mmol/l) – 2,5; DABCO (25 mmol/l) – 3.

Figure 4. Dependence of phototransformation’s characteristics and efficiency of LOS1 on density of excitation power. a) quantum yield of phototransformation: modification 1 –1, 2; modification 2 – 3 – 5. Lasing – 1 – 3; nonlasing – 4, 5. C=1mmol/l –1, 3, 5; 0.1 mmol/l – 2, 4. b) efficiency – 1, 2; lifetime –3, 4; yield of photoproduct  F_las – 5, 6. Modification 1 –1, 3, 5; modification 2 – 2, 4, 6. C=1mmol/l.