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J. Phys. Chem. 1996, 100, 4526-4532
Structures, Spectra, and Lasing Properties of New (Aminostyryl)pyridinium Laser Dyes Chan F. Zhao, Raz Gvishi, Upvan Narang, Gary Ruland, and Paras N. Prasad* Photonics Research Laboratory, Department of Chemistry, State UniVersity of New York at Buffalo, Buffalo, New York 14260-3000 ReceiVed: NoVember 15, 1995X
Recent work from our Photonics Research Laboratory has shown that a new class of hydroxy amino styryl pyridinium derivatives exhibit both one- and two-photon pumped lasing. Because of the exciting prospect of up-conversion lasing by direct two-photon excitation, we have conducted a thorough study of the spectroscopic and lasing properties of a group of related compounds with the objective of understanding the structurespectroscopic properties relationship. These dyes were found to have two mesomeric forms, one predominant in the ground-state and the other in the excited-state, leading to a large Stokes shift. A low fluorescence quantum yield was observed for all the studied dyes and could be possibly attributed to two factors: (1) the presence of a counterion, iodide, which increases the singlet-to-triplet intersystem crossing transition, and (2) a twisted intramolecular charge-transfer (TICT) geometry derived from the rotation of the amino moiety. However, significant lasing efficiencies were observed under pulse pump conditions possibly because the dyes are being stimulated to emit at a faster rate than the nonradiative processes. From the one-photon pumped laser loss calculations, we found that the losses are only due to the cavity effect. Solvent effect studies for the hydroxy amino styryl pyridinium derivatives showed that the chromophore is very sensitive to hydrogen bonding donor (HBD) solvents. In addition, the dye-doped sol-gel:PMMA composite glass solid matrix exhibits a behavior close to the dye dissolved in water, suggesting a microenvironment of pure silica. These results indicate that the dye is attached to the silica skeleton of the composite glass through hydrogen bonding.
I. Introduction Dye lasers are an important class of lasers that are attractive due to their unique feature of tunability over a wide range of wavelengths.1 Moreover, there is a new interest in laser dyes due to progress in the solid-state dye laser field.2-8 This requires the need for new laser dyes which will meet several criteria simultaneously, such as, high laser efficiency, high photostability, tunability in pump wavelength, and capability to be entrapped within a solid matrix with maintained physical and chemical properties. It is well-known that lasing properties of organic dyes are dependent on their molecular structures.9 The past work on solid state dye lasers has focused on onephoton pumping. Recently our Photonics Research Laboratory has reported, to our knowledge for the first time, efficient direct two-photon pumped up-conversion lasing in a solid matrix. Twophoton pumped lasing offers some very distinct merits; some of these are (i) use of a long interaction length, (ii) minimization of local thermal damage because of weaker absorption, and (iii) more suitability for fiber and channel wave guide lasing. The two-photon pumped up-conversion also offers the prospect of a tunable blue laser. Efficient two-photon lasing has been achieved in a new amino styryl pyridinium laser dye synthesized in our laboratory.17 These dyes belong to the general family of hemicyanine dyes and exhibit a two-photon absorption crosssection more that 2 orders of magnitude higher than those of previously reported compounds. The exciting prospect of up-conversion lasing provided the motivation for the present work in which we have conducted a comprehensive study of the spectroscopic and lasing properties of a group of closely related dyes. Our objective is to create an understanding of the structure-spectroscopic properties * Author to whom all correspondence should be sent. X Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4526$12.00/0
relation which can lead to the design of new dyes with improved one- and two-photon lasing properties. Hemicyanine dyes containing long alkyl chains have drawn considerable attention in recent years because of the possible optoelectronic and molecular electronic applications by incorporating them into Langmuir-Blodgett (LB) films.10-16 Hemicyanine dyes containing short alkyl chains such as, amino styryl pyridinium derivatives (ASPD) are more suitable for lasing applications. The dyes: trans-4-[p-[N-ethyl-N-(hydroxyethyl)amino]styryl]-N-methylpyridinium salts with a tetraphenylborate or an iodide as the counterion (hereafter ASPT and ASPI, respectively) have been demonstrated to be efficient laser dyes in the orange-red region under one- and/or two-photon excitation pumping. The medium successfully used has been liquid solution, solid-state matrices such as sol-gel:PMMA or a Vycore: PMMA composite glass, a poly 2-hydroxyethyl methacrylate (poly-HEMA) rod, and a polymer epoxy film.17-20 The main features observed in our studies on ASPT and ASPI are a large Stokes shift (118 nm in ethanol) and a low fluorescence quantum yield (7 × 10-3 in ethanol).18 On the other hand, they exhibit a large two-photon absorption cross-section (about 2-3 orders of magnitude greater than the corresponding values of rhodamine dyes)17,19 and significant lasing efficiency under either onephoton (13.5%)18 or two-photon (3.5%)17,19 excitation pumping. II. Experimental Section A. Sample Preparation. Dyes. The investigated compounds are trans-4-[p-[N-ethyl-N-(hydroxyethyl)amino]styryl]N-methylpyridinium tetraphenylborate (ASPT), trans-4-[p-(Nethyl-N-(hydroxyethyl)amino]styryl]-N-hydroxyethyl(or methyl)pyridinium iodide (ASPI), phenylene-1,4-diethylene-1(N-methylpyridinium iodide)-4-[4-(dimethylamino)benzene] (AB[Ph]PI), 2,5-dimethoxyphenylene-1,4-diethylenebis[(dimethylamino)benzene] ([Ph](AB)2), and 2,5-dimethoxyphenylene-1,4© 1996 American Chemical Society
New (Aminostyryl)pyridinium Laser Dyes
Figure 1. Molecular structure of ASPI, ASPT, AB[Ph]PI, [Ph](AB)2, and [Ph](PI)2.
diethylenebis(N-methylpyridinium iodide) ([Ph](PI)2). Molecular structures of these dyes are shown in Figure 1. They were synthesized in our laboratory, and different counterions (ASPT and ASPI) were used to increase the solubility in nonpolar and polar solvents, respectively. Solutions. Dilute dye solutions (1-10 µM; to avoid aggregation and self-absorption effects) were prepared in a series of solvents, for absorption and emission measurements, in order to study the effects of solvent and its structure on the fluorescence quantum yield. Solid State. The procedure used to prepare the solid sample of dye-doped composite glasses has been described previously by Gvishi et al.21,22 and is based on the composite glass preparation method of Pope et al.23 Briefly, highly porous silicagel bulk glasses were prepared by the sol-gel process. The porous monolith was impregnated with methyl methacrylate (MMA) by diffusion into the pores. The MMA in the pores was subsequently polymerized using 2,2′-azobis(isobutyronitrile) (AIBN) as the initiator. The final glass was cleaned and polished. This procedure produced a high optical quality nanostructured composite glass. This method has the advantage that two (or more) different optically responsive materials can be doped in different phases of the matrix (the silica phase, the PMMA phase, and the interfacial phase), to make multifunctional bulk materials for photonic applications.18,24,25 For lasing studies, ASPT and ASPI were doped at the interfacial phase18,24,25 by placing the porous glass in a ASPTcyclopentanone solution (2.1 × 10-3 M) or ASPI-ethanol solution (4.4 × 10-3 M), respectively. After the solution completely impregnated the glass, the glass monolith was removed from the solution and placed on a hot plate at ∼45 °C for several hours until the solvent evaporated out of the glass, leaving the dye on the surface of the pores. This procedure resulted in a concentration of ∼1.5 × 10-3 M ASPT and ∼3.1 × 10-3 M ASPI. This concentration is based on the volume of the pores of the glass being ∼70% of the total volume, and it assumes that none of the chromophore evaporated/decomposed upon the removal of the solvent.
J. Phys. Chem., Vol. 100, No. 11, 1996 4527 The ASPT-doped Vycore glass:PMMA composite were prepared using a Corning Vycore glass (with a pore diameter of approximately 40 Å) as the porous bulk with the same impregnation procedure, as above, of preparing the dye-doped sol-gel-PMMA composite.26 B. Measurements. Spectra. The absorption spectra were obtained using a Shimadzu UV-Vis 260 spectrophotometer with a resolution of (1 nm. The spectra of the solution were obtained using quartz cuvettes (1 cm path length). The emission spectra were collected on a SLM 48000 MHF spectrofluorimeter (90° geometry) using a xenon arc lamp as the excitation source. For solution measurements, a fluorimetric quartz cuvette was used. For the composite glass, the fluorescence was obtained from the surface of the glass (90° geometry) due to significant primary absorption. The resolution of the spectrofluorimeter is (0.5 nm. Fluorescence spectra were background-subtracted and corrected for detector and monochromator transmission nonlinearities. Fluorescence quantum yields were obtained using a comparative method, which is discussed in more detail elsewhere.18,27 Lasing. In this paper we report lasing behavior under onephoton excitation. The excitation source used in our lasing performance studies (for both the solution and the composite glass) was a frequency-doubled Quanta-Ray DCR Nd:YAG Q-switched laser with a repetition rate up to 30 Hz producing 8 ns pulses at 532 nm. During all of the lasing experiments, the outcoming pump beam was reflected using a 2nd harmonic selector and passed through an IR filter. The beam was then passed through an aperture, followed by a cylindrical lens which focused the beam to a linear shape on the sample. We used a transverse pump cavity configuration. Lasing was obtained in a cavity with either a combination of a ∼100% reflecting flat mirror (at 0 °C) and a ∼70% reflecting flat outcoupler (at 0 °C through the range of 550 to 630 nm) or a combination of a grating (1200 groove/mm) as the back reflector with the same output coupler. The lasing output vs the wavelength of the composite glass was measured by passing the lasing output through a monochromator (SPEX Triplemate Model 1460) and detecting it with an optical multichannel analyzer vidicon system (OMA-III, EG&G Princeton Applied Research). The spectrum is a collection of 11 pulses at a 1 Hz repetition rate. The laser output for the lasing slope efficiency measurement was measured with a United Detector Technology 350 linear/log optometer; each collected data point was an average of 10 pulses. The laser output for tunability study was measured with a Jobin-Yvon UV monochromator and a Si PIN photodiode. The photodiode output signal was measured with an oscilloscope (Tektronix 350 MHz, Model 2467). The wavelength was scanned manually, and several measurements were made at each point. III. Results and Discussion A. Spectroscopic Characteristics. Structure Effects. Figure 2 presents the UV-visible absorption and emission spectra of ASPI in ethanol solution (∼2.6 × 10-6 M). The absorption spectrum is centered at 483 nm with a molar absorptivity of 6.5 × 104 M-1 cm-1 at the peak.18 The emission spectrum exhibits a mirror image to the absorption spectrum with a large Stokes shift of 117 nm. In addition, we recently reported the fluorescence quantum yield of ASPI (in ethanol solution) to be ∼7 × 10-3.18 Both the Stokes shift and the fluorescence quantum yield are crucial parameters for lasing performance. Generally, a large Stokes shift is attributed to a different charge distribution (or geometry) in the excited-state compared to the ground-state.9 For example, in coumarine dyes a nonpolar
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Figure 2. Absorption (solid curve) and fluorescence emission (dashed curve) spectra of ASPI in ethanol solution (∼2.6 × 10-6 M).
Figure 3. Two mesomeric forms proposed for ASPI and ASPT: form A, positively charged at the pyridinium moiety; form B, positively charged at the amino moiety.
mesomeric form is predominant in the ground-state while a polar mesomeric form is predominant in the excited-state.9 On the basis of quantum chemical calculations, Fromherz28 recently reported for zwitterionic hemicyanine dyes that the positive charge of the chromophore is displaced upon excitation from the pyridinium moiety toward the amino moiety. In addition, Hall et al.29 reports that hemicyanine dyes have a larger dipole moment in the ground-state than in the excited-state. On the basis of the previous reports9,28,29 and on our observation of a large Stokes shift, we propose that ASPI has a predominant mesomeric form with a positive charge at the pyridinium moiety (Figure 3, structure A) in the ground-state, but another predominant mesomeric form with a positive charge at the amino moiety (Figure 3, structure B) in the excited-state. The low fluorescence quantum yield observed was previously attributed to intersystem crossing occurring from the S1 state to the T1 state.18 In order to understand more about this chromophore, we investigated the influence of the structure (influence of electron withdrawing-donating groups) and the solvent effect on the spectroscopic properties of this chromophore. An intramolecular charge-transfer process between a strong electron-donating and a withdrawing group(s) greatly reduces the electronic-excitation (absorption) probability.9 In our case, the studied chromophore ASPI contains an electron-donating amino group (one end) and an electron-withdrawing pyridinium group (the other end). Therefore, the effect of the electronwithdrawing-donating groups was studied by comparing the spectroscopic properties of the following modified structures: AB[Ph]PI, [Ph](AB)2, and [Ph](PI)2. (AB[Ph]PI) is based on ASPI modified by an additional phenylene group, ([Ph](AB)2) is a modified chromophore containing two electron-donating groups, and ([Ph](PI)2) is a modified chromophore containing two electron-withdrawing groups. Figure 4A presents the absorption spectra of these three modified chromophores in
Figure 4. Absorption (panel A) and fluorescence emission (panel B) of AB[Ph]PI (curve 1), [Ph](AB)2 (curve 2), and [Ph](PI)2 (curve 3) in ethanol solution (∼2.6 × 10-6 M).
solution (in ethanol; ∼2.6 × 10-6 M). All the three modified chromophores exhibit a significant decrease in the molar absorptivity () compared to ASPI (AB[Ph]PI by a factor of ∼2 and the other two by a factor of ∼3.5). The AB[Ph]PI spectrum exhibits two systems of peaks: a main peak centered at 493 nm, which is a 10 nm red shift compared to the peak of ASPI, and an additional peak centered at 347 nm. The red shift in the main peak can be explained by the increase in conjugation.1 In contrast, the absorption of the first excited singlet state in both [Ph](PI)2 and [Ph](AB)2 (464 and 411 nm, respectively) occurs at shorter wavelengths than that for ASPI (483 nm). The observed blue shift in the absorption spectra may be due to a displacement of charge from the end groups (pyridinium and amino, respectively) to the center of the chromophore (phenylene moiety), resulting in a less polar mesomeric form (charges with δ(+) and δ(-), respectively). These results support our assumption that the mesomeric form (Figure 3B) is predominant in the excited-state of ASPI/ASPT. Figure 4B presents the fluorescence emission spectra of the three modified chromophores AB[Ph]PI, Ph(AB)2, and Ph(PI)2. The emission spectra for all three chromophores exhibit a single emission peak with Stokes shifts of 113, 116, and 79 nm for AB[Ph]PI, [Ph](PI)2, and [Ph](AB)2, respectively. In addition, compared to ASPI, the relative fluorescence quantum yield increases considerably by factors of ∼2.3, ∼22.5 and ∼7.8 for AB[Ph]PI, [Ph](PI)2 and [Ph](AB)2, respectively. Moreover, the relative fluorescence quantum yield of ASPT (in ethanol) was also observed to increase by a factor of ∼5.30 All the observed absorption and fluorescence parameters for the five studied dyes are summarized in Table 1. The Stokes shifts for all the studied dyes, except [Ph](AB)2, are about the same (∼115 nm). [Ph](AB)2, the only dye that is not a cation and exhibits
New (Aminostyryl)pyridinium Laser Dyes
J. Phys. Chem., Vol. 100, No. 11, 1996 4529
TABLE 1: Spectroscopic Parameters of (Aminostyryl)pyridinium Derivative Dyes in Ethanol Solution dye
absmax (nm)
molar abs (M-1 cm-1)
emmax (nm)
∆λa (nm)
Φ
ASPI ASPT AB[Ph]PI [Ph](PI)2 [Ph](AB)2
483 486 493 464 411
65000 + 2%b ∼43500 ∼30000 ∼18000 ∼18000
600 603 606 580 490
117 117 113 116 79
∼0.007b ∼0.035c ∼0.016 ∼0.160 ∼0.054
a
∆λ ) emmax - absmax. b From ref 18. c From ref 30.
a much smaller Stokes shift than the other dyes, indicating a relatively low polarity for both mesomeric forms (ground- and excited-state). Low fluorescence quantum yields (
