Chemical physics of excitation dynamics via amplified spontaneous emission (ASE) laser spike spectroscopy in substituted phenyloxazoles

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CHEMICAL PHYSICS LETTERS Chemical Physics Letters 263 (1996) 154--160

Chemical physics of excitation dynamics via amplified spontaneous emission (ASE) laser spike spectroscopy in substituted phenyloxazoles 1 Juan Carlos del Valle a,2 Michael Kasha 9

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Javier Catalgm b

a Institute of Molecular Biophysics, Florida State University, Tallahassee, FL 32306-3015, USA b Departamento de Quimica Fisica Aplicada, UniversidadAutonoma de Madrid, Cantoblanco 28049, Madrid, Spain

Received 17 June 1996; revised 26 September 1996

Abstract A spectroscopic study of the fluorescence, amplified spontaneous emission (ASE), and gain spectra is made, testing for UV lasing from several phenyloxazoles. ASE laser spikes occur at 333 and 349 nm for 2-phenylbenzoxazole (PBO), 356 and 374 nm for 2,5-diphenyloxazole (PPO) and 365 and 385 nm for 2-(1-naphthyl)-5-phenyl-l,3,4-oxadiazole (ct-NPD), all in hydrocarbon solution at 298 K. For DPOPOP, 1,4-bis(4-methyl-5-phenyloxazol-2-yl)benzene, the ASE laser spike occurs at 420 nm. The gain coefficients range from 5 to 10 c m - 1. Unorthodox features in the ASE spectroscopy serve as a clue to the electronic excitation dynamics of complex composite molecular systems.

I. Introduction The various phenyloxazole molecular structure variations available (Table 1) having strong first absorption bands in the UV, with a long wavelength onset in the range 3 0 0 - 3 7 0 nm, suggested that UV laser action could be observed from the fluorescence of these molecules. This has proved to be the case, with a m p l i f i e d s p o n t a n e o u s e m i s s i o n (ASE) laser spikes measured at 333 and 349 nm for 2-phenylbenzoxazole (PBO) in cyclohexane, 356 and 374 nm for

s Work sponsored under contract with the U.S. Department of Energy, Washington, DC. 2 Spanish Fulbright Scholar, on leave from Universidad Autonoma de Madrid, Department de Quimica Fisica Aplicada, Cantoblanco 28049, Madrid, Spain.

2,5-diphenyloxazole (PPO) in methylcyclohexane, 365 and 385 nm for 2-(1-naphthyl)-5-phenyl-l,3,4oxadiazole (ot-NPD) in methylcyclohexane, and the 'double molecule' 1,4-bis(4-methyl-5-phenyloxazol2-yi)benzene (DPOPOP) with an ASE spike at 420 nm, all of these exhibiting spike gain coefficients ct above 5 c m - t, and in several cases reaching the high value of 10 c m - l (units of reciprocal cell length). In the course of a systematic study of the amplified spontaneous emission spectra and corresponding gain factors, three different types of ASE laser spike observations were made. The occurrence of unorthodox features in the spectra of composite (two unit) molecules consisting of inequivalent moieties opened the door to the use of ASE laser spectroscopy as a tool for accessing the concealed chemical physics of the excitation dynamics in these systems. This led to the conclusion that inner or concealed Franck-Con-

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J. Carlos del Valle et al. / Chemical Physics Letters 264 (1996) 154-160

don effects were revealed, as well as evidence for unusual intramolecular and intermolecular energy transfer, leading to anomalous shifting of ASE peak intensity to subsidiary laser spikes which would have been suppressed ordinarily. The anomalous effects arise from the complexity of intramolecular electronic and vibronic coupling in the composite molecules under study (Table 1). We shall classify the ASE laser spike behavior into three classes, and offer interpretations based on variations in vibrational and electronic interaction coupled with intraand intermolecular energy transfer effects. 1.1. Class 1 This is the normal case characterized by those molecules for which the ASE laser spike correlates with the strongest vibronic band in the fluorescence spectrum. The general development of an ASE laser spike [1-3] arising from the exponentiality of successive induced emission steps is to suppress subsidiary vibronic peak development and to enhance the ASE laser spike associated with the principal fluorescence peak. In the case of molecular excitation in which simple or direct S O-+ S I excitation is involved with no conformational or configurational changes in the molecule, the Franck-Condon envelope corresponds to electronic wavefunctions common to the whole molecular skeleton, with vibronic band contours indicating molecular coordinate displacements for the whole-molecule normal modes. A prototype molecule for this case is 2-phenylbenzoxazole (PBO). This molecule is known from previous spectroscopic studies [4-6] to exist as a single conformer, the 2-phenyl ring being conjugated and coplanar with the benzoxazole moiety in the ground and first excited singlet state. As such we anticipate simple a'r -~ "rr * lowest electronic S o ~ S~ transitions, and whole-skeletal normal modes. In generating ASE laser spikes, lower laser energy excitation leads to only partial suppression of those lower intensity spikes corresponding to subsidiary vibrational peaks in the fluorescence spectrum, as is commonly observed with low gain-coefficient (e.g., c~ = 3 - 5 cm -1) ASE spectroscopy [7]. However, these subsidiary laser spikes usually are suppressed completely at higher laser energy. The 2-phenylbenzoxazole molecule, which has a domi-

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Fig. 1. Fluorescence spectrum (Areax = 332 nm) of 2-phenylbenzoxazole (PBO, Table 1) at t.0X 10 -5 M, with mean lifetimes (circles) as a function of wavelength. The ASE laser spikes are observed at 333 and 349 nm (0.002 M). Both fluorescence and ASE were carried out in cyclohexane at 298 K.

nant ASE laser spike at 333 nm in cyclohexane (Fig. 1), shows a secondary ASE spike persisting even when the dominant spike has a measured gain-coefficient value of a = 9 cm- I. The subsidiary ASE laser spike appears at 349 nm, corresponding to the third vibronically resolved fluorescence band. The mean lifetimes (Fig. 1) range from 1.23 to 1.30 ns (average 1.27 ns), which we consider to be invariant within the 0.05 ns error of the phase-modulation lifetime measurement. A single exponential was found to be the best fit of the decay data. This result confirms the presence of a single conformer for this Class 1 molecule. In the simple excitation case studied under Class 1, a molecular three-level laser mechanism must be involved, the laser electronic transition ending on non-equilibrium vibronic levels of the ground electronic state depopulated by rapid intramolecular vibrational relaxation (IVR), preserving the population inversion required for lasing action. This mechanism supplants the simple two-level system, for which population inversion cannot be achieved in spite of prevalent induced emission. Analogously, the 0,0 fluorescence band cannot appear as an ASE laser spike, even if in the Franck-Condon envelope for the fluorescence emission the 0, 0 vibronic transition is the dominant band.

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Table 1 Classification of substituted phenyloxazole structures Class 1

Main ASE spike coincides with main fluorescence peak, secondary spike suppressed but observed. Normal gain spectrum relation. Single electronic

structure. PBO

2-phenylbenzoxazole

Class 2

Main ASE coincides with main fluorescence peak, no other ASE spike appears. Normal gain spectrum relation. Symmetrical double molecule. H3CvN ` ~ N...,.,.~C H3

DPOPOP

1,4-bis(4-methyl-5-phenyloxazol-2-yl)benzene

Class 3

Main ASE spike correlates with main fluorescence peak, subsidiary laser spike anomalously enhanced. Gain spectra show reversal of intensity ratio. Unsymmetrical composite molecule.

ct-NPD

2-( 1-naphthyl)-5-penyl- 1,3,4-oxadiazole

PPO

2,5-diphenyloxazole

unorthodox ASE laser spike phenomena, the most common feature being that a secondary fluorescence vibronic peak may develop unusual prominence as an ASE laser spike, and even dominate the ASE laser spike development. The corresponding gain spectra frequently show dual peaks with anomalous relative intensities compared with the ASE spike intensities. The first example of this class is represented by the a-NPD molecule (Table 1). This molecule as an unsymmetrical oxazole may be expected to show unusual excited state properties. A hidden complexity of the fluorescence spectrum (Fig. 2a) shows up immediately in the development of two strong ASE laser spikes, roughly proportional in relative intensity to the two strongest fluorescence vibronic bands, instead of a dominant and a suppressed ASE spike. The gain spectrum (Fig. 2b) shows the striking feature that the strongest peak (ct = 6.5 cm -~) corresponds to the weaker fluorescence band and corresponding ASE laser spike at 385 nm, whereas the dominant ASE spike correlates with a secondary gain spectrum peak ( a = 4.75 cm - l ) at 365 nm. The fluorescence spectrum would appear to arise from a single molecular conformer, based on comparisons with the corresponding absorption spectrum. A study of the excitation spectroscopy of the absorption/fluorescence relation showed no variation in the apparent fluorescence Franck-Condon envelope with variation of excitation wavelength. With ordinary conformers and solvent-solute complexes (hydrocarbon solvents) excluded from consideration, we

1.2. Class 2

This case involves moieties or molecular segments made up of two equivalent molecular units. DPOPOP (Table 1) would seem to fit this description, and exhibits a corresponding simple ASE laser ~pike at 420 nm and a single corresponding gain spectrum peak. We shall not discuss this case further here as it falls outside of the UV laser cases, deferring a discussion to our final complete report. 1.3. Class 3

This group consists of composite molecules characterized by unlike moieties, and exhibit a range of

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Fig. 2. (a) Fluorescence spectrum (AreaX= 365 nm) of a-NPD (Table l) (5.0× l0 -6 M) and ASE laser spikes (365 and 385 nm) (0.005 M) both in methylcyclohexane at 298 K. The anomalous intensity ratio of the ASE spikes is attributed to a molecular mixture of A and B localized normal mode electromers (Scheme l). (b) Gain spectrum of ct-NPD (0.005 M in methylcyclohexane at 298 K) with peaks at 365 and 385 rim.

J. Carlos del Valle et a l . / Chemical Physics Letters 264 (1996) 154-160 A

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Scheme 1. Model of partitioning of composite molecules into molecular segments or vibratory electromers.

must seek another explanation for the ASE laser spike anomalous intensities. The et-NPD molecule presents the dilemma that by all ordinary spectroscopic criteria it behaves like a normal single molecule, yet by the ASE laser spike performance it behaves like a molecular mixture [8]. We consider that these results can indicate a special case of molecular conformational isomerism, as localized normal mode electromers, which may arise because of the heterocomposite nature of the substituted et-NPD molecule. We define the localized normal mode electromers as having an electronic structure extending over the entire molecular skeleton, whereas the normal modes of vibration are localized on two molecular segments (Scheme 1). For example, for et-NPD we may have coplanar segments A, with the naphthyl group twisted somewhat out of plane, or a coplanar segment B, with the phenyi ring somewhat twisted out of plane. In this visualization, the electronic wavefunction covers the entire molecular skeleton, but the normal mode vibration systems are isolated, i.e. consist of the coplanar skeletal segment and having out-of-plane phenyl or naphthyl rings with an independently partitioned normal mode system. In a literal sense, these localized normal mode electromers are spectroscopically differentiated molecular conformers. The S 1 initially excited state could be represented by a population of two species of localized normal mode electromers, each having its own Franck-Condon envelope for fluorescence (Fig. 3). We may then picture two types of energy transfer: intramolecular energy transfer arising from a soliton [9,10] transfer of the large excess of vibrational excitation distortion from one localized normal mode system to the other, with the general transfer to the normal mode system represented by the lowest average zero-point energy. Intermolecular energy transfer from one set

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Fig. 3. Schematicpotential diagram (harmonic approximation)for vibronic excitation soliton coupling between the molecular segments A and B of a heterocompositemolecule, with corresponding Franck-Condon electronic transition contours. In the S t state, the zero-pointvibronic levels are shown for the lowest frequency normal mode and a high frequency normal mode of A and B segments (Scheme 1). of localized normal mode electromer systems to the lower energy set could occur, this transfer being promoted by the high density of excited molecules in an extreme population inversion, and by the high molecular concentration used in the ASE laser peak study. We consider that the intramolecular transfer of excess vibrational excitation is a unidirectional solitonic transfer of skeleton distortion from one normal mode system to the other, and not an oscillatory vibrational exciton effect, the driving force being energetic (to the lowest average zero-point energy system) and entropic (to the system with the largest complexity and number of normal modes). The phenyloxazole PPO (Table 1) offers a second example of a completely unorthodox ASE laser spike development. We may consider this molecule to consist of, e.g., a molecular segment in which a coplanar 2-phenyloxazole is vibrationally weakly coupled to the 5-phenyl ring, on account of the torsional mode at the 5-bond position, or alternatively a coplanar 5-phenyloxazole coupled to the

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J. Carlos del Valle et a l . / Chemical Physics Letters 263 (1996) 154-160

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Fig. 4. (a) Fluorescence spectrum of PPO (Table 1) (Amx = 356 nm, 6.9X l0 -6 M) and ASE laser spikes (356 and 374 nm) (0.0053 M), both in methyicyclohexane at 298 K. The anomalous intensity ratio of the ASE spikes is atlrihuted to a molecular mixture of A and B localizod normal mode elcctromers (Scheme l). (b) Gain spectrum of PPO (0.0053 M) in methylcyclohexane at 298 K with peaks at 356 and 374 nm.

2-phenyl ring (see Scheme 1). The electronically excited state $1 can be ascribed to whole-molecule electronic distribution, whereas we can consider the localized normal mode electromers to soliton couple the two normal mode vibrationally excited systems. The same energy transfer possibilities would then exist as noted for ~t-NPD. The fluorescence spectrum of PPO (Fig. 4a) has relative vihronic peak intensities in the band sequence 2 > 1 > 3, which in physical optics terms should yield a dominant ASE laser spike corresponding to band 2, with a possible weaker subsidiary spike corresponding to vibronic band 3, like the case of 2-phenylbenzoxazole (Fig. I). Instead, we see a remarkable reversal of the ASE laser spike intensity relative to the intensities of the originating fluorescence vibronic bands, indicating that some unusual physical phenomenon occurring here is altering the gain spectrum intensities. Fig. 4b shows a gain spectrum peak of a = 9.9 cm -~ corresponding to the weaker fluorescence vibronic peak 3, whereas the strongest fluorescence vibronic peak has a value of only a = 5.5 cm -l. We believe that the ASE laser spike intensities and especially the reversed gain spectrum intensities offer strong evidence for vibronic energy transfer between the two populations of A - B system molecules which we have described as localized normal mode electromers, centered on each of the molecular segments of the hetero-composite molecule.

We shall now consider the molecular spectroscopic basis for our description of the localized normal mode electromers, and their consequent interconversion. Fig. 3 depicts two harmonic potential sets Ill] (pictured in one plane) for two excited state normal vibrational modes for each system of molecular segments A and B (Scheme 1). In the S~ state for each system, all of the normal vibrational modes correspond to the electronic state (S~) energy term D e, corresponding to respective molecular segments A and B, but have different zero-point energy D O spectroscopic term values. That molecular segment which has the lowest frequency normal modes will have of course the lowest zero-point energy D O values (e.g., B). We discuss the bases for our presentation in our complete paper. Therefore, that molecular vibronic system could act as an energy sink for a competitive A < > B soliton transfer for a thermal equilibration of the initial high vibrational excitation of the electronic excited state. If a large excited state population inversion exists at high molecular concentration and high driving laser energy, an intermolecular transfer with a mixed population of A and B system molecules could further enhance the dominance of a B system molecular segment population even if the A system molecules were favored by Franck-Condon absorption peaks in the primary laser excitation. A critical feature of the present research is the study of the effect of the driving laser energy on the behavior of the multiple ASE laser spikes. In the case of Class 1 species, the principal ASE spike is progressively reinforced, and the weaker ASE spike progressively suppressed, as the laser energy is increased. For the Class 3 cases, the ASE laser spike corresponding to subsidiary fluorescence vibronic bands is progressively increased and the ASE spike corresponding to the main fluorescence peak is suppressed. These results vividly discriminate between the behavior of Class 1 and Class 3 molecules. A parasitic transient absorption excited in the laser pumping step cannot offer a selective suppression of one of the two ASE laser spikes because their spacing is small 0 4 0 0 cm -~) compared with the band width of 7r ~ 7r * transition in these large skeleton aromatics (6000-10000 c m - l ) . This would apply to the S 1 ~ S~ transients, as well as the T I

J. Carlos del Valle et a l . / Chemical Physics Letters 264 (1996) 154-160

T, transients, as indicated for the latter by the work of Pavlopoulos and Hammond [12]. Moreover, the intrusion of a transient absorption quenches the lasing ability of a fluorescent molecule, as illustrated by the case of 3-hydroxychromone compared with 3-hydroxyflavone [13].

159

as supplied. Purity was evaluated by NMR spectroscopy for ot-NPD and PPO. The cyclohexane and methylcyclohexane were spectrograde quality and were dried by freshly pressed sodium introduced into the solvent.

3. Conclusion 2. Experimental All of the spectroscopic observations were conducted for hydrocarbon solutions of the various phenyloxazoles at 298 K in the concentrations and solvents indicated in the text and the figure legends. The absorption spectra were recorded by a Shimadzu UV-2100 spectrophotometer. Fluorescence spectra and mean lifetimes were obtained from a Fluorologtau-2 spectrofluorimeter (Spex Industries, Edison, N J). The ASE laser spike measurements were made by primary excitation with an Nd:YAG laser, using the fourth harmonic (266 rim) for all phenyloxazoles except DPOPOP for which the third harmonic (355 nm) sufficed. The ASE was detected through a pinhole (0.05 cm diameter) placed at a distance of ca. 50 cm from the sample cell, in order to optimize the excitation averaging by the laser spike. Aside from the exciting source, the optical arrangement was the same as that reported for previous ASE work from this laboratory [3]. The ASE gain coefficient is given [1-3] by a(A) L ~-(2/L)In[IL/IL/2 --1], where L is the ASE cell length (in cm). In a standardized series in which experimental conditions are fixed, the a value can be used to indicate relative laser efficiency. We use the a values as a qualitative indication of performance. It is possible to relate the observed ct values to an absolute figure of merit. An absolute FOM formulation based on independent optical-spectroscopic data has been proposed by Pavlopoulos [14]. In the laser experiments, the solutions were degassed by bubbling Ar gas through the small oblique-windowed ASE laser cell for 30 min prior to excitation. The solutions were stirred during the Nd:YAG laser excitation, with no evidence of photo-instability. The phenyloxazoles PBO, DPOPOP, (x-NPD, and PPO (Table 1) were CP grade from Aldrich and used

The substituted phenyloxazoles have been shown to yield efficient UV lasing action in the range 333-385 nm, with gain coefficient ot reaching the high value of 10 cm - l , in the several examples studied. The observations were made in a small cell with stirring, after deaeration of the liquid solution with argon (hydrocarbon solvents at 298 K), and exhibited high relative photostability. It is demonstrated that the study of amplified spontaneous emission (ASE) reveals concealed spectroscopic complexity in normal appearing fluorescence bands having moderate vibronic resolution in which a discrete fluorescence band structure is observable. Violating the expected exponential gain of intensity, in the ASE laser peak developments it is found that in the case of hetero-composite molecules in which the two moieties or molecular segments are dissimilar, a weaker fluorescence vibronic band can give rise to a dominant ASE laser spike. This suggests that such an unsymmetrical molecule can act like a mixture of two 'localized normal mode electromers', their presence being revealed by the excitation dynamics of the composite molecule system. It is argued that an intramolecular soliton transfer of the excess vibrational energy produced by selective Franck-Condon non-equilibrium primary excitation, and intermolecular electronic energy transfer in the exceptionally high population inversion produced in the laser excitation, account for the anomalous gain spectrum peak intensities, and their corresponding ASE laser spikes anomalies.

Acknowledgements We are pleased to acknowledge the valuable assistance in laser optics by Dr. L. van de Burgt and Dr. David A. Gormin of the IMB Laser Laboratory, and in chemical nomenclature by Professor Martin

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Schwartz. We thank Dr. van de Burgt for penetrating comments on our manuscript preparation. One of us (JCV) acknowledges with thanks the granting of a Fulbright Scholarship by the Fulbright Commission and the Ministry of Education and Science of Spain. References [1] C.V. Shank, Rev. Mod. Phys. 47 (1975) 649. [2] C.V. Shank, A. Dieser and W.T. Silfvast, Appl. Phys. Lett. 17 (1970) 307. [3] P. Chou and D. McMorrow, T.J. Aartsma and M. Kasha, J. Phys. Chem. 88 (1984) 4596. [4] C. Rulliere and J. Joussot-Dubien, Opt. Commun. 24 (1978) 38.

[5] J. Catalan, E. Mena, F. Fabero and F. Amat-Guerri, J. Chem. Phys. 96 (1992) 2005. [6] P. Chou, W.C. Cooper, J.H. Clements, S.L. Studer and C.P. Chang, Chem. Phys. Lett. 216 (1993) 2005. [7] J.R. Heldt and J. Heldt, Acta Phys. Pol. A 55 (1) (1979) 79. [8] P. Chou and T.J. Aartsma, J. Phys. Chem. 90 (1986) 721. [9] R.K. Bullough and P.J. Cauchy, eds., Topics in current physics, Solitons (Springer-Verlag, Berlin, 1980). [I 0] A.C. Newell, Solitons in mathematics and physics (Society for Industrial and Applied Mathematics, Philadelphia, PA, 1985). [11] F. Dushinsky, Acta Physicochim. URSS 7 (1937) 551. [12] T.O. Pavlopoulos and P.R. Hammond, J. Am. Chem. Soc. 96 (1974) 6568. [13] D.A. Parthenopoutos, D. McMorrow and M. Kasha, J. Phys. Chem. 95 (1991) 2668. [14] T.G. Pavlopoulos, Opt. Commun. 38 (1981) 393.