International Journal of Photoenergy

Volume 2012 (2012), Article ID 458126, 10 pages

http://dx.doi.org/10.1155/2012/458126

## Photophysical Parameters, Excitation Energy Transfer, and Photoreactivity of 1,4-Bis(5-phenyl-2-oxazolyl)benzene (POPOP) Laser Dye

^{1}Department of Chemistry, Faculty of Science, Tanta University, Tanta 31527, Egypt^{2}Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia^{3}Department of Physics, Faculty of Science, Tanta University, Tanta 31527, Egypt^{4}Faculty of Engineering, MUST, 6th of October City, Egypt^{5}Center of Excellence for Advanced Materials Research, King Abdul Aziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia

Received 20 February 2012; Accepted 6 April 2012

Academic Editor: Vincenzo Augugliaro

Copyright © 2012 Samy A. El-Daly et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The effect of solvents on the absorption and emission spectra of 1,4-bis(5-phenyl-2-oxazolyl)benzene (POPOP) laser dye has been studied in various solvents at 298 K. A bathochromic shift was observed in absorption and fluorescence spectra upon increase of solvent polarity, which indicates that this transition is *π*-. The ground and excited state dipole moments were calculated as 2.23 and 6.34 Debye, respectively. The dye solution in MeOH, *n*-heptane, and methyl isobutyl ketone gives laser emission in the blue region upon excitation by a 337.1 nm nitrogen pulse; the gain coefficient and emission cross section as well as normalized photostability have been determined. Excitation energy transfer from POPOP to rhodamine B and fluorescine was studied to improve the laser emission from these dyes. Such an energy transfer dye laser system (ETDL) obeys a long range columbic energy transfer mechanism with a critical transfer distance, *R*_{0}, of 25 and 33 Å and *k _{q}* equal to and for the POPOP/RB and POPOP/fluorescine pair, respectively. The POPOP dye is highly photostable in polar protic and polar aprotic solvents, while it displays photodecomposition in chloromethane solvent via formation of a contact ion pair. The photochemical quantum yield and rate of photodecomposition depend on the electron affinity of solvent.

#### 1. Introduction

Fluorescent organic compounds possessing high Stokes shift values [1, 2] are prospective candidates for practical application in various fields of science and technology, where high concentrations or long optical paths are required [3, 4], for example, in scintillation techniques [5, 6], sunlight collection, and conversion of solar energy into electricity [7, 8], electroluminescent light sources (OLEDs) [9–11], and various biological applications, and so forth. Several physicochemical mechanisms can be applied to increase the fluorescence Stokes shift of organic compounds [12]; however, not all lead to emissions with high quantum yields [13]. The most popular and most studied is the excited state proton transfer reaction [14, 15]; however, it is usually connected with high radiationless excitation energy losses [16]. To our understanding, excited state conformational transformations resulting in the formation of more planar molecular structures [17–19] have several advantages in producing high Stokes shifted fluorescence emissions [20] over the alternative twisting mechanisms [21], which also induce radiationless excited state deactivation in most of the known cases [22].

Derivatives of 1,2-bis-(5-phenyloxazol-2-yl)benzene [23–25] are ortho analogs of the well-known scintillation luminophore POPOP [1,4-bis-(5-phenyloxazol-2-yl)benzene]. They belong to the class of efficient fluorescent organic compounds with abnormally high Stokes shifts [26]. In contrast to their planar paraisomers [27], molecules of the title series are characterized by essential nonplanarity caused by the steric repulsion of two bulky heterocyclic moieties introduced into the orthopositions of the central benzene ring [28]. The resulting disruption of intramolecular conjugation shifts the electronic absorption spectra of ortho-POPOPs towards the shorter-wavelength region with respect to the absorption of their planar paraisomers.

Further increase of the Stokes shift values is possible by combination of several photophysical mechanisms in one molecule: for example, excited state planarization and solvatochromic effects. Asymmetrization of the electronic density distribution by introduction of highly electron-donating and/or electron-withdrawing substituents into the ortho-POPOP molecule leads to a significant rise in its excited state dipole moment, increased sensitivity to solvent polarity, and thus enlarges the fluorescence Stokes shift in polar media [29]. In the present we report photophysical parameters, excitation energy transfer, and photoreactivity of POPOP laser dye (Scheme 1).

#### 2. Experimental

1,4-Bis (5-phenyl-2-oxazolyl)benzene (POPOP), fluorescine, and rhodamine B (Aldrich) were used without further purification. All solvents used in this work were of spectroscopic grade and were preliminary checked for the absence of absorbing or fluorescent impurities within the scanned spectral ranges. UV-Visible electronic absorption spectra were recorded on a Shimadzu UV-Vis 1650-PC spectrophotometer and steady state fluorescence spectra were measured using a rectangular quartz cell of 0.2 cm path length to minimize the reabsorption of emitted photons; the emission was monitored at 90°. The fluorescence quantum yield was measured using the optically diluted solution to avoid reabsorption effects (absorption at excitation wavelength ≤0.1) relative method with solution of 9,10-diphenylanthracene as reference standard [30–32]. Using the same excitation wavelength, the unknown quantum yield is calculated using the following [33–35]:
where , are the fluorescence quantum yield of the unknown and standard, respectively, is the integrated emission intensity, is the absorbance at excitation wavelength, and is the refractive index of the solvent. The oscillator strength of electronic transition and the transition dipole moment from ground to excited state was calculated in different solvents using (2) and (3) [36]:
where is the numerical value for the molar extinction coefficient measured in dm^{3} mol^{−1} cm^{−1} and is the wavenumber value in cm^{−1} and is the energy maximum of absorption band in cm^{−1}. The radiative decay rate constant () of a fluorophore (or fluorescence rate constant) presents a fundamental photophysical property, which determines, together with the rates for radiation less processes, the fluorescence life time; the rate of fluorescence resonance energy transfer (FRET) is also directly proportional to the radiative decay rates of the donor. The inverse of the fluorescence rate constant is the radiative (natural) life time () of the excited state and corresponds to the lifetime expected in the absence of radiation less decay processes. The dependence of the radiative decay rate on the environment is significant; namely, it increases with its polarizability. It can be theoretically predicted from the well-known Strickler-Berg equation, which has its foundations in Einstein’s spontaneous emission rate and Planck’s black body radiation law [37]:
where is the fluorescence intensity, is the wave number, and is the molar extinction coefficient at a particular wave number . For the ideal case of a negligible Stokes shift and a perfect mirror-image relationship of absorption and fluorescence spectrum, (4) simplifies to become the following [38]:
where is the average wave number corresponding to the 0-0 transition. Thus, according to (2) we can write
The fluorescence life time can be calculated by using the equation:
The intersystem crossing rate constant () is related to the quantum fluorescence yield for by the approximate relationship:
The photochemical quantum yield () was calculated using the method that was previously described in detail [39] and light intensity was measured by using ferrioxalate actinometry [40].

#### 3. Results and Discussion

##### 3.1. Optical Absorption and Emission Spectra in Different Solvents

The steady state absorption and emission parameters of 1 × 10^{−5} mol dm^{−3} POPOP were recorded in various nonpolar, polar aprotic, and polar protic solvents and are summarized in Table 1. The absorption and emission spectrum of POPOP are shown in Figures 1 and 2, respectively. As shown in Figure 1, the solvent polarity shows a slight effect on the position of electronic absorption spectral maxima, indicating the polar character of POPOP in the ground state.

Fluorescence spectra, on the other hand, are more sensitive to solvent polarity. As solvent polarity increases, the emission spectra become red-shifted, Figure 2. This indicates that the singlet excited state of the POPOP molecule is more polar than the ground state. There is also a good mirror image relationship between absorption and fluorescence spectra. These facts, together with high molar absorptivities (*ε* = 32500–56500 mol dm^{−3} cm^{−1}) and high oscillator strength ( = 0.57–1.02), are consistent with a strong *π*-*π** transition with a small geometry change between electronic ground and excited states [41]. The fluorescence quantum yields are slightly affected by solvent properties; the values of in methyl isobutyl ketone and acetone are low due to the carbonyl group quenching the singlet excited state via enhancement of intersystem crossing , while the high value of fluorescence quantum yield in ethylene glycol can be explained in term of a cage effect and the role of medium viscosity which decrease the stretching and twisting molecular motion in the excited state, thereby decreasing the nonradiative process [42]. As seen in Table 1, the calculated fluorescence lifetimes obtained using the Strickler-Berg equation are very similar to the measured values. This is expected because the electronic absorption and fluorescence spectrum are nearly mirror image.

##### 3.2. Determination of Dipole Moments

Analysis of the solvatochromic effect allows the estimation of difference in the dipole moment between the excited singlet and the ground state. This was achieved by applying the simplified Lippert-Mataga equation [43–45]
where is the Planck’s constant, is the velocity of light in vacuum, and are the dipole moments in the ground and excited states, and is the Onsager cavity radius of POPOP (taken to be between 4.2–4.7 Å [46]), , are the dielectric and refractive index of the solvent, and is the Stokes shift in cm^{−1}, which increases with increase in solvent polarity, pointing to stabilization of excited state in polar solvents. Figure 3 shows a plot of Stokes shift *versus* the orientation polarizability . The change in dipole moment upon excitation was calculated from the slope of the plot (slope = 902 cm^{−1}) and the cavity radius is = 2.45 Debye, indicating the polar nature of the excited singlet state.

Bakhshiev’s and Kawski-Chamma-Viallet equations [47–51] have been used for the treatment of observed spectral shifts to determine the ground and excited state dipole moments of POPOP.

*Bakhshiev’s formula* is given in
where and are the absorption and fluorescence maxima in wavenumber (cm^{−1}), respectively, (solvent polarity function) and are defined as follows:

*Kawski-Chamma-Vaillet’s formula* is given in
and are defined as
where the symbols have their usual meaning as given in (9). The parameters and can be calculated from (12) and (15); they are the slope of straight lines Figure 4, and the values of and can be obtained from (8) and (10)

The values of and were found to be 553 and 1152 cm^{−1}, respectively. Thus the calculated values of and were found to be 2.23 and 6.34, respectively.

##### 3.3. Lasing Action and Fluorescence Quenching of POPOP

POPOP is characterized by a high fluorescence quantum yield, high molar absorptivity, large Stokes shift, and high photostability in most organic solvents. A solution 2 × 10^{−3} mol dm^{−3} of POPOP in methanol, *n*-heptane, and methl isobutyl ketone(MIK) gives laser emission when using a nitrogen pulsed laser ( nm) of 800 ps duration and 1.48 mJ pulse energy. The solution was taken in oscillator and amplifier cuvettes of 10 mm path length. The output energy of the laser dye was measured as a function of wavelength to determine the lasing range in different solvents Figure 5(a). The maximum gain coefficient was calculated at the maximum laser emission by measuring the intensity of laser emission from the entire cell length and the intensity from the cell half length . One can calculate the laser gain emission from the following [52]:
It is well known that gain occurs when the stimulated emission of photons exceeds the reabsorption or loss due to scattering. Therefore, gain is the increase in the number of emitted photons and is dependent on both wavelength and incident intensity. The cross section for the stimulated laser emission of the dye was calculated at the laser emission maximum according to [53]
Here is the emission wavelength, is the refractive index of the solvent, is the velocity of light, and is the normalized fluorescence line-shape function [54]. The normalized photostability is defined as the accumulated pump energy absorbed by the system per mole of dye molecules before the output energy falls to one half of its initial value using the following relation [55–57]:
where is the pulse energy in Joules, is the number of pulses to get the half of initial emission intensity Figure 5(b), is the radius of the laser beam on surface of sample in cm and is the sample thickness in cm ( cm) and is the dye concentration mol·dm^{−3}. The laser parameters and normalized photostability of POPOP are listed in Table 2.

The fluorescence quenching of POPOP has also been studied using rhodamine B (RB) and fluorescine as a quencher in methanol. Figure 6 shows the Stern-Volmer plot of POPOP using RB and fluorescine as a quencher by applying the Stern-Volmer relation in the form [58, 59]:
where is the bimolecular quenching rate parameter, is the fluorescence lifetime of the donor in the absence of the acceptor taken as 0.93 ns in MeOH, and are the fluorescence intensities in the absence and in the presence of quencher of concentration in mol dm^{−3}. A plot of *versus* acceptor concentration is given in Figure 6. From the slope of Figure 6, has been calculated as 26.2 × 10^{12} dm^{3} mol^{−1} s^{−1}, 10.4 × 10^{12} dm^{3} mol^{−1} s^{−1} for the POPOP/RB and POPOP/fluorescine systems, respectively. These values are much higher than the diffusion controlled rate constant in methanol. We determine the value of as 24.4 × 10^{3} dm^{3} mol^{−1}, 9.66 × 10^{3} dm^{3} mol^{−1} for the POPOP/RB and POPOP/fluorescine system, and we can calculate the quenching sphere by using the quenching sphere of action model [60, 61]:
where the volume of the transient quenching sphere and the Avogadro’s number, the radius of quenching sphere calculated to be 17.9 Å for RB and 13.99 Å for fluorescine.

From spectral data it is seen that there is a significant overlap between electronic absorption of acceptor (RB, fluorescine) and the emission spectrum of donor (POPOP) Figures 7(a) and 7(b). Applying Forester’s resonance energy transfer mechanism, the critical transfer distance has been calculated for POPOP/RB and POPOP/fluorescine pair using the following relation [62]: where is the corrected normalized fluorescence intensity of donor in the absence of the acceptor at wavelength , and is the molar absorption coefficient of the acceptor at , Avogadro’s number, the orientation factor was taken to be a reference state. Accordingly, the critical transfer distance was found as 25 and 33 Å for POPOP/RB and POPOP/fluorescine pair, respectively. This value is higher than that for collision energy transfer in which values are in the range of 4–6 Å [63]. The high value of the critical transfer distance as well as the quenching rate constant indicates that the underlying mechanism of energy transfer is that of resonance energy transfer due to long-range dipole-dipole interaction between excited donor and ground state acceptor.

##### 3.4. Photoreactivity of POPOP in Chloromethane Solvents

The photoreactivity of POPOP was studied in CCl_{4}, CHCl_{3} and CH_{2}CH_{2} solvents. Upon irradiation of a 2 × 10^{−5} mol dm^{−3} solution of POPOP at 366 nm ( Einstein min^{−1}), the absorbance of dye decreases upon increasing the irradiation time and a new absorption peak appears at 260 nm Figure 8. This indicates the formation of photoproduct in a solvent cage. The net photochemical quantum yields of the underlying reaction were calculated as 0.61, 0.103, and 0.067 for CCl_{4}, CHCl_{3}, and CH_{2}Cl_{2}, respectively (Figure 9). The formation of photoproduct is a one photon process as represented by the well-known mechanism [64–67]:

It was proposed that the electron transfer from the excited singlet POPOP to within the transient excited charge transfer complex (exciplex) is the main primary photochemical process. This leads to the POPOP radical cation, a chloride ion and a chloromethyl radical in the solvent cage Step .

The formation of constant ion pair usually occurs by electron transfer from excited donor molecule (POPOP)* to the acceptor (). The rate constant of photoreactivity of POPOP was calculated by applying the simple first order rate equation:
where , , and are the initial absorbance, absorbance at time , and infinity, respectively. The rate constant was found to be 0.024, 0.011, and 0.005 min^{−1} in CCl_{4}, CHCl_{3}, and CH_{2}Cl_{2}, respectively. It was found that the net photochemical quantum yield and the observed rate constant increases with increase in the electron affinity of chloromethane solvents ( equal 2.12, 1.75 and 1.36 eV for CCl_{4}, CHCl_{3} and CH_{2}Cl_{2}, resp.), indicating that the electron affinity of the solvent plays a role in the photoreactivity and controls the photochemical reaction. The POPOP dye is highly photostable in polar protic and aprotic solvents, since no change in absorbance of dye was observed upon prolonged irradiation by using either 336 or 254 nm light for about 24 hours.

#### 4. Conclusion

The ground and excited state dipole moments of POPOP were determined as 2.3 and 6.34 Debye, respectively. The dye solution in methanol, *n*-heptane, and methyl-isobutyl ketone gives laser emission in blue region with emission maximum in the range 417–450 nm upon pumping by nitrogen laser pulse. POPOP act as a good energy donor for rhodamine B and fluorescine laser dyes, the energy transfer rate constant and critical transfer distance are determined. POPOP dye is highly photostable in polar solvents but displays photodecomposition in chloromethane solvents via electron transfer from excited dye to solvent molecules.

#### References

- J. R. Lakowicz,
*Principles of Fluorescence Spectroscopy*, Springer Science, Singapore, 3d edition, 2006. - B. Valeur,
*Molecular Fluorescence*, Wiley-VCH GmbH, Germany, 2002. - M. Eichhorn, “Fluorescence reabsorption and its effects on the local effective excitation lifetime,”
*Applied Physics B*, vol. 96, no. 2-3, pp. 369–377, 2009. View at Publisher · View at Google Scholar - G. Bordeau, R. Lartia, and M. P. Teulade-Fichou, “Meta-Substituted triphenylamines as new dyes displaying exceptionally large Stokes shifts,”
*Tetrahedron Letters*, vol. 51, no. 33, pp. 4429–4432, 2010. View at Publisher · View at Google Scholar · View at Scopus - J. B. Birks,
*The Theory and Practice of Scintillation Counting*, Pergamon Press, Oxford, UK, 1967. - C. Zorn, M. Bowen, S. Majewski et al., “Pilot study of new radiation-resistant plastic scintillators doped with 3-hydroxyflavone,”
*Nuclear Instruments and Methods in Physics Research. Section A*, vol. 273, no. 1, pp. 108–116, 1988. View at Google Scholar - N. S. Lewis and D. G. Nocera, “Powering the planet: chemical challenges in solar energy utilization,”
*Proceedings of the National Academy of Sciences of the United States of America*, vol. 103, no. 43, pp. 15729–15735, 2006. View at Publisher · View at Google Scholar - W. R. McNamara, R. C. Snoeberger, G. Li et al., “Acetylacetonate anchors for robust functionalization of TiO2 nanoparticles with Mn(II)-terpyridine complexes,”
*Journal of the American Chemical Society*, vol. 130, no. 43, pp. 14329–14338, 2008. View at Publisher · View at Google Scholar · View at Scopus - C. W. Tang and S. A. Vanslyke, “Organic electroluminescent diodes,”
*Applied Physics Letters*, vol. 51, no. 12, pp. 913–915, 1987. View at Publisher · View at Google Scholar - N. Tamoto, C. Adachi, and K. Nagai, “Electroluminescence of 1,3,4-oxadiazole and triphenylamine-containing molecules as an emitter in organic multilayer light emitting diodes,”
*Chemistry of Materials*, vol. 9, no. 5, pp. 1077–1085, 1997. View at Publisher · View at Google Scholar - A. P. De Silva, D. B. Fox, A. J. M. Huxley, and T. S. Moody, “Combining luminescence, coordination and electron transfer for signalling purposes,”
*Coordination Chemistry Reviews*, vol. 205, no. 1, pp. 41–57, 2000. View at Google Scholar - F. Vollmer, W. Rettig, and E. Birckner, “Photochemical mechanisms producing large fluorescence stokes shifts,”
*Journal of Fluorescence*, vol. 4, no. 1, pp. 65–69, 1994. View at Publisher · View at Google Scholar · View at Scopus - V. V. Volchkov and B. M. Uzhinov, “Structural relaxation of excited molecules of heteroaromatic compounds,”
*High Energy Chemistry*, vol. 42, no. 3, pp. 153–169, 2008. View at Publisher · View at Google Scholar · View at Scopus - S. M. Ormson and R. G. Brown, “Excited state intramolecular proton transfer part 1: ESIPT to nitrogen,”
*Progress in Reaction Kinetics*, vol. 19, pp. 45–91, 1994. View at Google Scholar - D. Le Gourrierec, S. M. Ormson, and R. G. Brown, “Excited state intramolecular proton transfer part 2: Esipt to oxygen,”
*Progress in Reaction Kinetics*, vol. 19, no. 3, pp. 211–275, 1994. View at Google Scholar · View at Scopus - F. Vollmer and W. Rettig, “Fluorescence loss mechanism due to large-amplitude motions in derivatives of 2,2′-bipyridyl exhibiting excited-state intramolecular proton transfer and perspectives of luminescence solar concentrators,”
*Journal of Photochemistry and Photobiology A*, vol. 95, no. 2, pp. 143–155, 1996. View at Publisher · View at Google Scholar - R. Siebert, A. Winter, U. S. Schubert, B. Dietzek, and J. Popp, “Excited-state planarization as free barrierless motion in a
*π*-Conjugated terpyridine,”*Journal of Physical Chemistry C*, vol. 114, no. 14, pp. 6841–6848, 2010. View at Publisher · View at Google Scholar - C.-C. Yang, C.-J. Hsu, P.-T. Chou, H. C. Cheng, Y. O. Su, and M.-K. Leung, “Excited state luminescence of multi-(5-phenyl-1, 3, 4-oxadiazo-2-yl) benzenes in an electron-donating matrix: exciplex or electroplex?”
*Journal of Physical Chemistry B*, vol. 114, no. 2, pp. 756–768, 2010. View at Publisher · View at Google Scholar - R. Y. Iliashenko, N. Y. Gorobets, and A. O. Doroshenko, “New and efficient high Stokes shift fluorescent compounds: unsymmetrically substituted 1,2-bis-(5-phenyloxazol-2-yl)benzenes via microwave-assisted nucleophilic substitution of fluorine,”
*Tetrahedron Letters*, vol. 52, no. 39, pp. 5086–5089, 2011. View at Publisher · View at Google Scholar - A. O. Doroshenko, “Physicochemical principles of the creation of highly efficient organic luminophores with anomalously high stokes' shifts,”
*Theoretical and Experimental Chemistry*, vol. 38, no. 3, pp. 135–155, 2002. View at Google Scholar - W. Rettig, “Charge separation in excited states of decoupled systems—TICT compounds and implications regarding the development of new laser dyes and the primary processes of vision and photosynthesis,”
*Angewandte Chemie*, vol. 25, pp. 971–988, 1986. View at Google Scholar - Z. R. Grabowski, K. Rotkiewicz, and W. Rettig, “Structural changes accompanying intramolecular electron transfer: focus on twisted intramolecular charge-transfer states and structures,”
*Chemical Reviews*, vol. 103, no. 10, pp. 3899–4031, 2003. View at Publisher · View at Google Scholar · View at Scopus - A. O. Doroshenko, L. D. Patsenker, V. N. Baumer et al., “Structure of sterically hindered aryl derivatives of five-membered nitrogen containing heterocyclic ortho-analogs of POPOP,”
*Molecular Engineering*, vol. 3, no. 4, pp. 353–363, 1994. View at Publisher · View at Google Scholar - A. O. Doroshenko, L. D. Patsenker, V. N. Baumer et al., “Structure of sterically hindered aryl derivatives of five-membered nitrogen containing heterocyclic ortho-analogs of POPOP,”
*Molecular Engineering*, vol. 3, no. 4, pp. 353–363, 1994. View at Publisher · View at Google Scholar · View at Scopus - A. O. Doroshenko, V. N. Baumer, A. A. Verezubova, and L. M. Ptyagina, “Molecular structure of unsubstituted oxadiazolic analog of ortho-POPOP and peculiarities of conformational structure of this class of sterically hindered organic compounds,”
*Journal of Molecular Structure*, vol. 609, no. 1–3, pp. 29–37, 2002. View at Publisher · View at Google Scholar - A. O. Doroshenko, A. V. Kirichenko, V. G. Mitina, and O. A. Ponomaryov, “Spectral properties and dynamics of the excited state structural relaxation of the ortho analogues of POPOP—effective abnormally large Stokes shift luminophores,”
*Journal of Photochemistry and Photobiology A: Chemistry*, vol. 94, no. 1, pp. 15–26, 1996. View at Publisher · View at Google Scholar · View at Scopus - R. Ambats and E. Marsh, “The crystal structures of 2,2'-
*p*-phenylenehis(5-phenyloxazole)—`POPOP',”*Acta Crystallographica*, vol. 19, pp. 942–948, 1965. View at Google Scholar - A. O. Doroshenko, V. N. Baumer, A. V. Kirichenko, V. M. Shershukov, and A. V. Tolmachev, “Molecular structural features of unsymmetrical ortho analogs of POPOP,”
*Chemistry of Heterocyclic Compounds*, vol. 33, no. 11, pp. 1341–1349, 1997. View at Google Scholar · View at Scopus - A. O. Doroshenko, “The effect of solvent on the spectral properties and Dynamics of structural relaxation of two excites-state ortho-analogs of popop with essentially different polarities,”
*Chemical Physics Reports*, vol. 18, no. 5, pp. 873–879, 1999. View at Google Scholar - W. H. Melhuish, “Quantum efficiencies of fluorescence of organic substan ces: effect of solvent and concentration of the fluorescent solute,”
*Journal of Physical Chemistry*, vol. 65, no. 2, pp. 229–235, 1961. View at Google Scholar - J. B. Birks and D. J. Dyson, “The relations between the fluorescence and absorption properties of organic molecules,”
*Proceedings of the Royal Society A*, vol. 275, pp. 135–148, 1963. View at Publisher · View at Google Scholar - W. R. Dawson and M. W. Windsor, “Fluorescence yields of aromatic compounds,”
*The Journal of Physical Chemistry*, vol. 72, pp. 3251–3260, 1968. View at Publisher · View at Google Scholar - G. A. Crosby and J. N. Demas, “Measurement of photoluminescence quantum yields. Review,”
*The Journal of Physical Chemistry*, vol. 75, no. 8, pp. 991–1024, 1971. View at Google Scholar - J. C. Scaiano, Ed.,
*Handbook of Organic Photochemistry*, Harwood, Chichester, UK, 1991. - A. Credi and L. Prodi, “From observed to corrected luminescence intensity of solution systems: an easy-to-apply correction method for standard spectrofluorimeters,”
*Spectrochimica Acta. Part A*, vol. 54, no. 1, pp. 159–170, 1998. View at Google Scholar - B. J. Coe, J. A. Harris, I. Asselberghs et al., “Quadratic nonlinear optical properties of
*N*-aryl stilbazolium dyes,”*Advanced Functional Materials*, vol. 12, no. 2, pp. 110–116, 2002. View at Publisher · View at Google Scholar - B. Briks,
*Photophysics of Aromatic Molecules*, Wiley, London, UK, 1970. - G. A. Kumar and N. V. Unnikrishnan, “Energy transfer and optical gain studies of FDS: Rh B dye mixture investigated under cw laser excitation,”
*Journal of Photochemistry and Photobiology A: Chemistry*, vol. 144, no. 2-3, pp. 107–117, 2001. View at Google Scholar · View at Scopus - E.-Z. M. Ebeid, R. M. Issa, M. M. Ghoneim, and S. A. El-Daly, “Emission characteristics and micellization of cationic 1,4-bis(
*β*-pyridyl-2-vinyl)benzene laser dye,”*Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases*, vol. 82, no. 3, pp. 909–919, 1986. View at Publisher · View at Google Scholar - S. L. Murov,
*Handbook of Photochemistry*, Marcel Dekker, New York, NY, USA, 1973. - S. A. El-Daly, S. M. Al-Hazmy, E. M. Ebeid et al., “Spectral, acid-base, and laser characteristics of 1,4-Bis[
*β*-(2-quinolyl)vinyl]benzene (BQVB),”*Journal of Physical Chemistry*, vol. 100, no. 23, pp. 9732–9737, 1996. View at Google Scholar - E.-Z. M. Ebeid, M. H. Abdel-Kader, R. M. Issa, and S. A. El-Daly, “Viscosity and medium effects on the fluorescence and photochemical behaviour of some aryl chalcones,”
*Chemical Physics Letters*, vol. 146, no. 3-4, pp. 331–336, 1988. View at Google Scholar - E. Lippert, “Spektroskopische bestimmung des dipolmomentes aromatischer verbindungen im ersten angeregten Zustand,”
*Zeitschrift für Elektrochemie*, vol. 61, pp. 962–975, 1957. View at Google Scholar - P. Suppan, “Invited review solvatochromic shifts: the influence of the medium on the energy of electronic states,”
*Journal of Photochemistry and Photobiology, A: Chemistry*, vol. 50, no. 3, pp. 293–330, 1990. View at Google Scholar - S. A. El-Daly, M. Gaber, S. S. Al-Shihry, and Y. S. E. Sayed, “Photophysical properties, excitation energy transfer and laser activity of 3-(4′-dimethylaminophenyl)-1-(2-pyridinyl) prop-2-en-1-one (DMAPP). A new potential laser dye,”
*Journal of Photochemistry and Photobiology A*, vol. 195, no. 1, pp. 89–98, 2008. View at Publisher · View at Google Scholar - G. H. Malimath, G. C. Chikkur, H. Pal, and T. Mukherjee, “Role of internal mechanisms in energy transfer processes in organic liquid scintillators,”
*Applied Radiation and Isotopes*, vol. 48, no. 3, pp. 359–364, 1997. View at Publisher · View at Google Scholar · View at Scopus - L. Bilot and A. Kawski, “Zur theorie des einflusses von Lösungsmitteln auf die elektronenspektren der moleküle,”
*Zeitschrift Naturforschung*, vol. 17, p. 621, 1962. View at Google Scholar - A. Kawski, “Uber die temperaturbhangigkeit der absorptions—und fluoreszenzspektren von 4-aminophthalimid,”
*Acta Physica Polonica*, vol. 29, p. 507, 1966. View at Google Scholar - A. Kawski, “On the estimation of excited-state dipole moments from solvatochromic shifts of absorption and fluorescence spectra,”
*Zeitschrift für Naturforschung A*, vol. 57, no. 5-6, pp. 255–262, 2002. View at Google Scholar - R. Ghazy, S. A. Azim, M. Shaheen, and F. El-Mekawey, “Experimental studies on the determination of the dipole moments of some different laser dyes,”
*Spectrochimica Acta. Part A*, vol. 60, no. 1-2, pp. 187–191, 2004. View at Publisher · View at Google Scholar · View at Scopus - L. Liu, Y. Sun, S. Wei, X. Hu, Y. Zhao, and J. Fan, “Solvent effect on the absorption and fluorescence of ergone: Determination of ground and excited state dipole moments,”
*Spectrochimica Acta Part A*, vol. 86, pp. 120–123, 2012. View at Publisher · View at Google Scholar - M. Rinke, H. Güsten, and H. J. Ache, “Photophysical properties and laser performance of photostable UV laser dyes. 1. Substituted p-quaterphenyls,”
*Journal of Physical Chemistry*, vol. 90, no. 12, pp. 2661–2665, 1986. View at Google Scholar - L. G. Nair, “Dye lasers,”
*Progress in Quantum Electronics*, vol. 7, no. 3-4, pp. 153–268, 1982. View at Google Scholar - G. Valverde-Aguilar, “Photostability of laser dyes incorporated in formamide SiO
_{2}ORMOSILs,”*Optical Materials*, vol. 28, no. 10, pp. 1209–1215, 2006. View at Publisher · View at Google Scholar - Y. Yang, M. Wang, G. Qian, Z. Wang, and X. Fan, “Laser properties and photostabilities of laser dyes doped in ORMOSILs,”
*Optical Materials*, vol. 24, no. 4, pp. 621–628, 2004. View at Publisher · View at Google Scholar - M. S. Mackey and W. N. Sisk, “Photostability of pyrromethene 567 laser dye solutions via photoluminescence measurements,”
*Dyes and Pigments*, vol. 51, no. 2-3, pp. 79–85, 2001. View at Publisher · View at Google Scholar - G. R. Penzer,
*An Introduction to Spectroscopy for Biochemistry*, S. B. Brown, Editor, Academic Press, London, UK, 1980. - N. J. Turro,
*Molecular Photochemistry*, Benjamin, New York, NY, USA, 1967. - A. Gilbert and J. Baggott,
*Essential of Molecular Photochemistry*, Black-Well, London, UK, 1991. - N. J. Turro,
*Modern Molecular Photochemistry*, Benjamin, New York, NY, USA, 1968. - I. M. Frank and S. J. Vavilov, “Über die wirkungssphare der auslöschunsvargänge in den flureszierenden flussing-keiten,”
*Zeitschrift für Physiologie*, vol. 69, p. 100, 1931. View at Google Scholar - S. G. Schulman, “Acide-base chemistry of excited singlet state: fundamental and analytical implications,” in
*Modern Fluorescence spectroscopy*, E. L. Wehry, Ed., vol. 2, Plenum, New York, NY, USA, 1976. View at Google Scholar - M. C. Biondic and R. Erra-Balsells, “Photochemical reaction of full-aromatic
*β*-carbolines in halomethanes 2. CHCl_{3}: Electronic spectra and kinetics,”*Journal of Photochemistry and Photobiology, A: Chemistry*, vol. 77, no. 2-3, pp. 149–159, 1994. View at Google Scholar · View at Scopus - M. C. Biondic and R. Erra-Balsells, “Photochemical reaction of
*β*-carbolines in carbon tetrachloride-ethanol mixtures,”*Journal of Photochemistry and Photobiology, A: Chemistry*, vol. 51, no. 3, pp. 341–353, 1990. View at Google Scholar - M. C. S. Mastsuda, R. Kokado, and H. E. Inou, “The Photoconductivity in a CCl
_{4}Solution of N,N-Dimethylaniline,”*Bulletin of the Chemical Society of Japan*, vol. 43, p. 2994, 1970. View at Google Scholar - R. E. Balsells and A. R. Farsca, “Photochemical reactions of aliphatic-amines in dichloromethane solution,”
*Australian Journal of Chemistry*, vol. 41, no. 1, pp. 103–110, 1988. View at Google Scholar - L. Wolinski, Z. Turznski, and K. Witkowski, “Lichtstreubefunde zur kettenspaltung von polystyrol in sauerstoffreien CCl
_{4}- und CHCl_{3}-lösungen bei lichteinwirkung der wellenlängen*λ*≥ 270 nm,”*Die Makromolekulare Chemie*, vol. 199, no. 12, pp. 2895–2907, 1987. View at Google Scholar