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Time-resolved emission spectra of stilbene derivatives in various solvents
Chemical Physics Letters 483 (2009) 268–272
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Time-resolved emission spectra of stilbene derivatives in various solvents Aleksander A. Kubicki * ´ sk, Wita Stwosza 57, 80-952 Gdan ´ sk, Poland Institute of Experimental Physics, University of Gdan
a r t i c l e
i n f o
Article history: Received 17 July 2009 In final form 27 October 2009 Available online 30 October 2009
a b s t r a c t The spectroscopic properties of 4-dimethylamino-40 -bromo-stilbene, 4-dimethylamino-40 -chloro-stilbene were studied by steady-state absorption, emission, polarization spectra and time-resolved emission spectra (TRES) in various solvents at 23 °C. A decrease of fluorescence lifetimes was found for all molecules in polar solvents at the blue edge of emission spectra. The lifetimes shorten with decreasing viscosity of solvents. The shift of maximum wavelength of emission spectra of 15–20 nm is observed within the first few hundred picoseconds after excitation. The emission anisotropy r values increase in the vicinity of the 0–0 transition wavelength. A change in the shape of spectra is observed also in some cases. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Stilbene molecules and its derivatives consist of two phenyl rings, connected by a double ethylenic bond. They are characterized by high quantum efficiency, very short lifetimes of excited states (100 ps), huge changes of dipole moments during transition between ground and excited states [1–6]. Dipole moments of the excited state can be even as much as three times higher than in the ground state. Depending on the polarity of the solvent, a strong shift of absorption and emission spectra is observed. Due to their structure, the stilbene-like molecules are characterized by some interesting internal processes occurring after optical excitation. Light induced photoisomerization, intramolecular charge transfer, fast radiationless relaxation of the exciting energy, the possibility to observe the un-relaxed excited states, are processes, which let the stilbene-like molecules be considered as a model group of molecules for a wide variety of research conducted in physics, chemistry, biophysics and biomedical sciences. General anticancer activities of some stilbene-like molecules, which can block a progress of some kinds of cancer has been described [7–9]. In food and health sciences many authors compete in reports on the existence of resveratrol (3,5,40 -trimethoxy-stilbene) in natural food products like grapes, wines and herbs which are used in the traditional medicine [10,11]. More basic research is needed. By synthesizing stilbene derivatives with different substituted groups one can some kind of programming their properties in such a way, that some internal processes can be blocked, while others can be released. Additionally, by taking into account external electrostatic interactions, one can stimulate or block some of the photophysical processes by changes of the physical environment * Fax: +48 58 523 2063. E-mail address: [email protected] 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.10.090
(viscosity, polarity, and temperature). The photophysics of the stilbene-like molecules is based on complex phenomenas like intramolecular charge transfer existing often in conjunction with twisting of the planar electron-donating and electron-accepting groups of substituents, light induced photoisomerization related to ‘twisting’ of the molecule via rotation over the ethylenic bond, the fast vibrational relaxations, the existence of radiative, un-relaxed excited states. Among the excited states few are of a very specific importance: locally excited, un-relaxed Franck–Condon states which allow to observe fast fluorescence, relaxed charge transfer state in which the spatial stabilization of the charge is achieved after the initial shock of excitation and the phantom state in which ethylenic bond twisting occurs following by the non-fluorescent cis form creation [12–14]. Due to a huge increase of the dipole moment during excitation [1,2], a great Stokes shift is observed in the emission spectra of the stilbene derivatives. This is an evidence of intramolecular charge transfer, which persists until the relaxed, fluorescent state S1 is achieved. At the same time, light induced photoisomerization occurs. The time constants of these simultaneous processes strongly depend on solvent viscosity, solvent polarity and temperature, as well. There are some models existing in the literature which describe all these effects, especially for stilbene derivatives [15–21]. The photoisomerization mechanism proposed by Saltiel [22– 24] involves both, higher singlet excited states and the triplet states, as well. According to the energy diagram proposed by Saltiel et al. [22–26] there is a high potential barrier blocking the twisting of the molecule around the ethylenic bond. Transition of the trans form into the cis form is avoided in that way. The excitation to the first excited state causes a transition to the state, which has a minimum energy for the twisted configuration of the double bond. Transition into the ground state can occur with similar probability from both forms, trans and cis, as well. The existence of the intermediate triplet state for the twisted configuration of the ethylenic
A.A. Kubicki / Chemical Physics Letters 483 (2009) 268–272
bond enables the transitions between both forms, because singlet– triplet intersystem crossings are the processes which make the photoisomerization much easier. The height of the potential energy hindrance, which stabilizes both forms in the ground state, depends strongly on the existence of the intramolecular bonds between hydrogen atoms lying nearby the ethylenic bond and the hydrogen atoms of the phenyl rings of the stilbene-like molecules. The solvent molecules interacting with the solute molecules, can affect the existing intramolecular hydrogen bonds by strongly lowering the potential hindrance for the trans–cis transition, making consistently photoisomerization much easier to occur. Bromide and chloride as a substitute in stilbene-like molecules cause a decrease of the fluorescence lifetimes, what allows to observe deactivation processes by complementary steady-state techniques, like measurements of the emission anisotropy. An increase of the emission anisotropy is expected even for solutions of moderate viscosity. The application of the time-resolved emission spectra measurements, simultaneously with the steady-state emission anisotropy experiments allows the observation of deactivation processes.
2. Experimental details All chemicals were used as spectroscopically pure. The structures of 4-dimethylamino-40 -bromo-stilbene (DMABrS), 4-dimethylamino-40 -chloro-stilbene (DMAClS) and 4-dimethylamino-40 cyano-stilbene (DMACS), the abbreviations and the dipole moments in the ground and the excited states are shown in Fig. 1. Absorption spectra were recorded using a SHIMADZU UV-1650 PC spectrophotometer with UV Probe software. Emission and excitation spectra were taken by a Varian CARY Eclipse fluorescence spectrophotometer. Emission anisotropy spectra measurements were done by the photon counting setup described in [27]. The streak camera system (C4334-01 Hamamatsu) with the 2501S spectrograph (Bruker Optics) and the solid state Nd:YAG laser (PL 2143A/SS EKSPLA) with the optical parametric generator (PG 401/SH EKSPLA) as the excitation light pulses source were used
Fig. 1. Structures and abbreviations of the investigated molecules. lg, le denote the dipole moments in the ground and the excited states, respectively.
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for the time-resolved emission spectra (TRES) experiments,. The system has been described in [28]. All experiments were carried out with sample concentrations smaller than 1 104 M and at a temperature of 23 °C. 3. Results and discussion All results for bromide (DMABrS) and chloride (DMAClS) in position 40 and with the dimethylamino group in position 4 of stilbene derivatives in solvents of different polarity and viscosity should be discussed in comparison to results for the cyano (DMACS) and the methoxy (DMAMOS) substitutes in position 40 of the stilbene molecule, which were obtained recently [29,30]. Both, DMACS and DMAMOS, are characterized by different values of dipole moments in the ground (lg) and in the excited (le) states as follows: lg is equal to 6.95 D and 4.1 D, respectively, le is equal to 20.2 D and 12.4 D, respectively. For DMABrS lg = 5.6 D, le = 17.6 D, for DMAClS lg = 5.5 D, le = 17.5 D. As one can see, the dipole moments of Br and Cl substitutes are closer to the DMAMOS lg and le values, respectively, so the values obtained from photophysics should be similar to those of DMAMOS rather than to DMACS. Photophysical characteristics of DMACS and DMAMOS are different, because of differences in the internal electronic interaction of the methoxy and the cyano groups in position 40 with the opposite dimethylamino group in position 4 [31]. Fig. 2 presents the TRES measurements results of DMABrS in ethylene glycol (EG), prepared as previously described in detail in [30]. The originally recorded image in ITEX format was colored in RGB standard, but for printing purposes, it was converted into the negative grayscale TIFF image. The saturation of black is proportional to the intensity of the captured light. Both plotted curves, pasted into originally converted grayscale image represent the total spectrum (along the horizontal axis – wavelength) and the total decay (along the vertical axis – time). Total decay means a sum of all decays over all wavelengths, and total spectrum means the sum of all spectra over all times. Point-to-point resolutions are: 2.3 ps and 0.47 nm for the time and the wavelength scales, respectively. Both, Br and Cl, 40 substitutes in non-polar solvents, like n-hexane and cyclohexane, show time-independent, well structured spectra, similar to those presented in previous papers [26,27]. Both molecules, having smaller dipole moments in the ground and the excited states, and weaker solute–solvent interaction, exhibit
Fig. 2. Time-resolved emission spectrum (TRES) of DMABrS in ethylene glycol. The negative of the screen copy is converted into grayscale image, with the decay and the spectrum curves along the vertical and the horizontal axis, respectively. The plotted curves represent the total spectrum and the total decay, taken over the whole measured ranges.
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absorption and the emission spectra lying closer to the UV region than those of DMACS, but lying further than those of DMAMOS. The shape and the maximum intensity of TRES for DMABrS remain the same over the whole time range (1.0 ns). The maximum of the fluorescence intensity stays at 415 nm. The fluorescence decay time is constant over the spectral range and is equal to 0.18 ns. For DMAClS the fluorescence decay time is 0.215 ns. In polar solvents like ethanol (ET), propanol (PR) and ethylene glycol (EG) both, DMABrS and DMAClS molecules, have structureless spectral shapes, with the maximum at: 455 nm (ET), 465 nm (EG) and 445 nm (PR), 462 nm (EG), respectively. From these data, one can see, that for polar solvents there is a larger Stokes shift of the emission spectra, due to stronger solute–solvent interactions for both molecules. For ethylene glycol, the Stokes shift is a little bit greater for DMABrS than for DMAClS molecule, due to a little bit greater moment. From the time analysis of TRES of DMABrS in ethanol (by slicing TRES data along the time axis with the slice window width in the wavelength scale of 5 points = 2.35 nm) we have obtained the increasing values of decay time from 0.10 to 0.14 ns in the blue edge of emission spectrum (between 395 and 430 nm). For wavelengths longer than 430 nm, the decay time remains the same, at the level around 0.16 ns. The effect is shown in Fig. 3. For DMABrS in ethylene glycol, the time analysis shows, that at the blue edge of TRES, decays consist of two components: the shorter (0.04 ns) and the longer one (0.31 ns). The ratio of their intensities changes from 1000:1 to almost 1:1 upon moving from 397 nm toward 420 nm. For wavelengths longer than 425 nm, decays remain monoexponential with a decay time of 0.33 ns (data not shown). Similar results in the time analysis were obtained for DMAClS in propanol and ethylene glycol. For propanol, the short time component of s = 0.07 ns was found for k between 385 nm and 420 nm, and the only longer component with s = 0.17 ns was found for k 430 nm. For ethylene glycol, a biexponential decay was found (s1 = 0.04 ns, s2 = 0.53 ns) for k between 400 nm and 430 nm, and for k 440 nm decays remain monoexponential, with s = 0.57 ns (data not shown). These effects clearly also appear in the wavelength scale. For DMABrS and DMAClS in polar solvents one can observe the irregularity in the shape of TRES, visible as a narrower line at the left top side of the main shape of the time-resolved emission spectra. By
slicing along the wavelength axis and moving the slice window toward higher time values, one can obtain the proof for time shift of the maximum intensity of the emission spectra. The shifts observed for DMABrS, DMAClS (Fig. 4a and b) and the comparison with DMACS (Fig. 4c) are shown. The shifts are 15–16 nm for DMAClS and 18–20 nm for DMABrS, and they occur within the first 200 ps after excitation. For comparison, the shifts are shown for DMACS in solvents of a wide range of polarity and viscosity values. As one can see, for more polar molecules like DMACS, the values of the maximum intensity shift changes from 10 to even 66 nm, and it depends rather more on the viscosity, than on the dielectric constant values of the solvent.
a
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c
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Fig. 3. TRES of the DMABrS in ethanol – changes of the decay time in the emission range.
Fig. 4. TRES of DMACS (a), DMABrS (b) and DMAClS (c). Shifts of the intensity maximum in the time. In the parts (b) and (c) the internal windows show spectra for the initial moments and for the moments longer than 160 ps. The results are taken by slicing the TRES results along the wavelength scale, with the time width of the slice equal to 2.5 ps.
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.
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. .
. . .
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Fig. 5. The steady-state absorption and the emission spectra (both are the solid lines, normalized to the same height of maximum) as well as the steady-state excitation and the emission polarization spectra (points) of DMABrS in cyclohexane, ethanol and ethylene glycol.
Such results are an evidence of strong influence of viscosity on the sensitivity of light induced trans–cis photoisomerization. Viscosity blocks the photoisomerization by difficulties in molecular reorientation. The solvent polarity acts only as an energy level moderator, shifting emission spectra with increasing polarity towards longer wavelength. It facilitates the energy level separation of the relaxed intramolecular charge transfer states (fluorescent states) and other un-relaxed levels which are involved in the other non-fluorescent deactivation pathways like trans–cis photoisomerization. Results of steady-state measurements of absorption, emission, emission anisotropy excitation and emission spectra for DMABrS in cyclohexane, ethanol and ethylene glycol are shown in Fig. 5. They confirm conclusion of TRES results for polar solvents. The emission spectra exhibit a strong Stokes shift for polar solvents in comparison to non-polar ones like cyclohexane. The absorption spectra show the polarity effect on spectra broadening. In both, emission anisotropy spectra measured across the absorption region and the emission region, as well, the viscosity effect is easily visible. For all, polar and non-polar solvents, especially for ethanol, there is evidence for the existence of different fluorescent centers, one of which appears at the blue edge of the emission spectrum, right in the 0–0 transition region. Increasing values of r in that region are an evidence for the existence of a fluorescent center, the lifetime of which is shorter than in the other emission region. Shortening the lifetime causes the increase of emission anisotropy, according to an earlier explanation in [31]. The emission anisotropy measured at kobs = 400 nm remains the same, higher than for kobs = 450 nm over the whole range. This conclusion is consistent with earlier described results of TRES for both molecules, DMABrS and DMAClS [31].
dy-state measurements, too. Bromide and chloride at position 40 have an influence on spectra shifts similar to the methoxy group. For DMABrS in ethanol maximum intensity of TRES is located initially at 437 nm and during the next 160 ps the maximum of TRES moves 18 nm to the red edge of the emission spectra. Similarly for DMABrS in ethylene glycol the maximum of TRES moves 20 nm toward the red edge of emission spectra within 130 ps time frame. All processes of deactivation like vibrational, solute–solvent and internal charge relaxations and non-radiative ones can evidently occur within this time frame. Steady-state polarization spectra confirm these conclusions with regard to deexcitation pathways of stilbene derivatives. Smaller spectral shifts for DMABrS and DMAClS, in comparison to the more polar molecule DMACS, is due to weaker internal interactions. The modified interaction between electric fields of the excited molecule and the surrounding molecules of the solvent continuously changes the distribution of excited states of the investigated molecule. The energy barrier for radiationless processes changes also, so the radiation deactivation pathway becomes more and more probable. Smooth changes of TRES indicate the evolution of the same single state, which corresponds to a continuously changing atomic configuration of the molecule after the moment of excitation. The existence of the TICT state and its effect on the deactivation of DMACS and other stilbene derivatives still remains open [20,21,32,33] and investigations are to be continued. Acknowledgements The author would like to express his gratitude to Prof. Dieter Gloyna for synthetizing and donation of the stilbene derivatives. The work has been sponsored by the University of Gdan´sk within the framework of the grant: BW 5200-5-0052-8.
4. Conclusions References TRES of all investigated stilbene derivatives show the short time component maximally at the blue edge of emission spectra in polar solvents. In non-polar solvents the spectra remained structured and unchanged and lifetimes are shorter as well. By comparison with recently measured TRES for molecules with cyano or methoxy groups in position 40 and dimethylamino group in position 4 one can find a strong derivative group effect which is shown in stea-
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A.A. Kubicki / Chemical Physics Letters 483 (2009) 268–272 E.N. Frankel, Lancet 341 (1993) 1103. P. Kopp, Eur. J. Endocrine 138 (1998) 619. C. Counet, D. Callemion, S. Collin, Food Chem. 98 (2006) 649. E. Celotti, R. Ferrarini, R. Zironi, L.S. Conte, J. Chromatogr. A 730 (1996) 47. K.A. Zachariasse et al., J. Photochem. Photobiol. A: Chem. 102 (1996) 59. Y.V. Il’ichev, W. Kühnle, K.A. Zachariasse, Chem. Phys. 211 (1996) 441. E. Abraham, J. Oberlé, G. Jonusauskas, R. Lapouyade, C. Rullière, J. Photochem. Photobiol. A: Chem. 105 (1997) 101. N. Lin, X. Zhao, A. Rizzo, Y. Luo, J. Chem. Phys. 126 (2007) 244509. Y. Amatatsu, Theor. Chem. Acc. 103 (2000) 445. Y. Amatatsu, Chem. Phys. 274 (2001) 87. Y. Amatatsu, Chem. Phys. Lett. 369 (2003) 673. Y. Amatatsu, Y. Ohara, Chem. Phys. Lett. 373 (2003) 245. M. Dekhtyar, W. Rettig, Phys. Chem. Chem. Phys. 3 (2001) 1602. S.A. Kovalenko, R. Schanz, T.A. Senyushkina, N.P. Ernsting, Phys. Chem. Chem. Phys. 4 (2002) 703. J. Saltiel, J. D’Agostino, E.D. Megarity, L. Metts, K.R. Neuberger, M. Wrighton, O.C. Zafiriou, in: O.L. Chapman (Ed.), The cis–trans Photoisomerization Olefins in Organic Chemistry, vol. 1, Marcel Dekker, New York, 1973.
[23] J. Saltiel, J.L. Charlton, Cis – Trans Isomerization of Olefins, in: P. de Mayo (Ed.), Rearrangements in Ground and Excited States, vol. 3, Academic Press, New York, 1980, p. 25. [24] J. Saltiel, Y.P. Sun, Cis–trans isomerization of C@C double bonds, in: H. Dürr, H. Bonas-Laurent (Eds.), Photochromism: Molecules and Systems, Elsevier, Amsterdam, 1990, p. 64. [25] D.H. Waldeck, Chem. Rev. 91 (1990) 415. [26] D.H. Waldeck, J. Mol. Liq. 57 (1993) 127. [27] A. Kubicki, Exp. Tech. Phys. 37 (1989) 329. [28] A.A. Kubicki, P. Bojarski, M. Grinberg, M. Sadownik, B. Kuklin´ski, Opt. Commun. 263 (2006) 275. [29] A.A. Kubicki, Chem. Phys. Lett. 439 (2007) 243. [30] A.A. Kubicki, Chem. Phys. Lett. 457 (2008) 246. [31] A.A. Kubicki, Chem. Phys. Lett. 373 (2003) 471. [32] S.I. Druzhinin, V.A. Galievsky, T. Yoshihara, K.A. Zachariasse, J. Phys. Chem. A 110 (2006) 12760. [33] S. Arzhantsev, K.A. Zachariasse, M. Maroncelli, J. Phys. Chem. A 110 (2006) 3454.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Time-resolved emission spectra of stilbene derivatives in various solvents Aleksander A. Kubicki * ´ sk, Wita Stwosza 57, 80-952 Gdan ´ sk, Poland Institute of Experimental Physics, University of Gdan
a r t i c l e
i n f o
Article history: Received 17 July 2009 In final form 27 October 2009 Available online 30 October 2009
a b s t r a c t The spectroscopic properties of 4-dimethylamino-40 -bromo-stilbene, 4-dimethylamino-40 -chloro-stilbene were studied by steady-state absorption, emission, polarization spectra and time-resolved emission spectra (TRES) in various solvents at 23 °C. A decrease of fluorescence lifetimes was found for all molecules in polar solvents at the blue edge of emission spectra. The lifetimes shorten with decreasing viscosity of solvents. The shift of maximum wavelength of emission spectra of 15–20 nm is observed within the first few hundred picoseconds after excitation. The emission anisotropy r values increase in the vicinity of the 0–0 transition wavelength. A change in the shape of spectra is observed also in some cases. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Stilbene molecules and its derivatives consist of two phenyl rings, connected by a double ethylenic bond. They are characterized by high quantum efficiency, very short lifetimes of excited states (100 ps), huge changes of dipole moments during transition between ground and excited states [1–6]. Dipole moments of the excited state can be even as much as three times higher than in the ground state. Depending on the polarity of the solvent, a strong shift of absorption and emission spectra is observed. Due to their structure, the stilbene-like molecules are characterized by some interesting internal processes occurring after optical excitation. Light induced photoisomerization, intramolecular charge transfer, fast radiationless relaxation of the exciting energy, the possibility to observe the un-relaxed excited states, are processes, which let the stilbene-like molecules be considered as a model group of molecules for a wide variety of research conducted in physics, chemistry, biophysics and biomedical sciences. General anticancer activities of some stilbene-like molecules, which can block a progress of some kinds of cancer has been described [7–9]. In food and health sciences many authors compete in reports on the existence of resveratrol (3,5,40 -trimethoxy-stilbene) in natural food products like grapes, wines and herbs which are used in the traditional medicine [10,11]. More basic research is needed. By synthesizing stilbene derivatives with different substituted groups one can some kind of programming their properties in such a way, that some internal processes can be blocked, while others can be released. Additionally, by taking into account external electrostatic interactions, one can stimulate or block some of the photophysical processes by changes of the physical environment * Fax: +48 58 523 2063. E-mail address: [email protected] 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.10.090
(viscosity, polarity, and temperature). The photophysics of the stilbene-like molecules is based on complex phenomenas like intramolecular charge transfer existing often in conjunction with twisting of the planar electron-donating and electron-accepting groups of substituents, light induced photoisomerization related to ‘twisting’ of the molecule via rotation over the ethylenic bond, the fast vibrational relaxations, the existence of radiative, un-relaxed excited states. Among the excited states few are of a very specific importance: locally excited, un-relaxed Franck–Condon states which allow to observe fast fluorescence, relaxed charge transfer state in which the spatial stabilization of the charge is achieved after the initial shock of excitation and the phantom state in which ethylenic bond twisting occurs following by the non-fluorescent cis form creation [12–14]. Due to a huge increase of the dipole moment during excitation [1,2], a great Stokes shift is observed in the emission spectra of the stilbene derivatives. This is an evidence of intramolecular charge transfer, which persists until the relaxed, fluorescent state S1 is achieved. At the same time, light induced photoisomerization occurs. The time constants of these simultaneous processes strongly depend on solvent viscosity, solvent polarity and temperature, as well. There are some models existing in the literature which describe all these effects, especially for stilbene derivatives [15–21]. The photoisomerization mechanism proposed by Saltiel [22– 24] involves both, higher singlet excited states and the triplet states, as well. According to the energy diagram proposed by Saltiel et al. [22–26] there is a high potential barrier blocking the twisting of the molecule around the ethylenic bond. Transition of the trans form into the cis form is avoided in that way. The excitation to the first excited state causes a transition to the state, which has a minimum energy for the twisted configuration of the double bond. Transition into the ground state can occur with similar probability from both forms, trans and cis, as well. The existence of the intermediate triplet state for the twisted configuration of the ethylenic
A.A. Kubicki / Chemical Physics Letters 483 (2009) 268–272
bond enables the transitions between both forms, because singlet– triplet intersystem crossings are the processes which make the photoisomerization much easier. The height of the potential energy hindrance, which stabilizes both forms in the ground state, depends strongly on the existence of the intramolecular bonds between hydrogen atoms lying nearby the ethylenic bond and the hydrogen atoms of the phenyl rings of the stilbene-like molecules. The solvent molecules interacting with the solute molecules, can affect the existing intramolecular hydrogen bonds by strongly lowering the potential hindrance for the trans–cis transition, making consistently photoisomerization much easier to occur. Bromide and chloride as a substitute in stilbene-like molecules cause a decrease of the fluorescence lifetimes, what allows to observe deactivation processes by complementary steady-state techniques, like measurements of the emission anisotropy. An increase of the emission anisotropy is expected even for solutions of moderate viscosity. The application of the time-resolved emission spectra measurements, simultaneously with the steady-state emission anisotropy experiments allows the observation of deactivation processes.
2. Experimental details All chemicals were used as spectroscopically pure. The structures of 4-dimethylamino-40 -bromo-stilbene (DMABrS), 4-dimethylamino-40 -chloro-stilbene (DMAClS) and 4-dimethylamino-40 cyano-stilbene (DMACS), the abbreviations and the dipole moments in the ground and the excited states are shown in Fig. 1. Absorption spectra were recorded using a SHIMADZU UV-1650 PC spectrophotometer with UV Probe software. Emission and excitation spectra were taken by a Varian CARY Eclipse fluorescence spectrophotometer. Emission anisotropy spectra measurements were done by the photon counting setup described in [27]. The streak camera system (C4334-01 Hamamatsu) with the 2501S spectrograph (Bruker Optics) and the solid state Nd:YAG laser (PL 2143A/SS EKSPLA) with the optical parametric generator (PG 401/SH EKSPLA) as the excitation light pulses source were used
Fig. 1. Structures and abbreviations of the investigated molecules. lg, le denote the dipole moments in the ground and the excited states, respectively.
269
for the time-resolved emission spectra (TRES) experiments,. The system has been described in [28]. All experiments were carried out with sample concentrations smaller than 1 104 M and at a temperature of 23 °C. 3. Results and discussion All results for bromide (DMABrS) and chloride (DMAClS) in position 40 and with the dimethylamino group in position 4 of stilbene derivatives in solvents of different polarity and viscosity should be discussed in comparison to results for the cyano (DMACS) and the methoxy (DMAMOS) substitutes in position 40 of the stilbene molecule, which were obtained recently [29,30]. Both, DMACS and DMAMOS, are characterized by different values of dipole moments in the ground (lg) and in the excited (le) states as follows: lg is equal to 6.95 D and 4.1 D, respectively, le is equal to 20.2 D and 12.4 D, respectively. For DMABrS lg = 5.6 D, le = 17.6 D, for DMAClS lg = 5.5 D, le = 17.5 D. As one can see, the dipole moments of Br and Cl substitutes are closer to the DMAMOS lg and le values, respectively, so the values obtained from photophysics should be similar to those of DMAMOS rather than to DMACS. Photophysical characteristics of DMACS and DMAMOS are different, because of differences in the internal electronic interaction of the methoxy and the cyano groups in position 40 with the opposite dimethylamino group in position 4 [31]. Fig. 2 presents the TRES measurements results of DMABrS in ethylene glycol (EG), prepared as previously described in detail in [30]. The originally recorded image in ITEX format was colored in RGB standard, but for printing purposes, it was converted into the negative grayscale TIFF image. The saturation of black is proportional to the intensity of the captured light. Both plotted curves, pasted into originally converted grayscale image represent the total spectrum (along the horizontal axis – wavelength) and the total decay (along the vertical axis – time). Total decay means a sum of all decays over all wavelengths, and total spectrum means the sum of all spectra over all times. Point-to-point resolutions are: 2.3 ps and 0.47 nm for the time and the wavelength scales, respectively. Both, Br and Cl, 40 substitutes in non-polar solvents, like n-hexane and cyclohexane, show time-independent, well structured spectra, similar to those presented in previous papers [26,27]. Both molecules, having smaller dipole moments in the ground and the excited states, and weaker solute–solvent interaction, exhibit
Fig. 2. Time-resolved emission spectrum (TRES) of DMABrS in ethylene glycol. The negative of the screen copy is converted into grayscale image, with the decay and the spectrum curves along the vertical and the horizontal axis, respectively. The plotted curves represent the total spectrum and the total decay, taken over the whole measured ranges.
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absorption and the emission spectra lying closer to the UV region than those of DMACS, but lying further than those of DMAMOS. The shape and the maximum intensity of TRES for DMABrS remain the same over the whole time range (1.0 ns). The maximum of the fluorescence intensity stays at 415 nm. The fluorescence decay time is constant over the spectral range and is equal to 0.18 ns. For DMAClS the fluorescence decay time is 0.215 ns. In polar solvents like ethanol (ET), propanol (PR) and ethylene glycol (EG) both, DMABrS and DMAClS molecules, have structureless spectral shapes, with the maximum at: 455 nm (ET), 465 nm (EG) and 445 nm (PR), 462 nm (EG), respectively. From these data, one can see, that for polar solvents there is a larger Stokes shift of the emission spectra, due to stronger solute–solvent interactions for both molecules. For ethylene glycol, the Stokes shift is a little bit greater for DMABrS than for DMAClS molecule, due to a little bit greater moment. From the time analysis of TRES of DMABrS in ethanol (by slicing TRES data along the time axis with the slice window width in the wavelength scale of 5 points = 2.35 nm) we have obtained the increasing values of decay time from 0.10 to 0.14 ns in the blue edge of emission spectrum (between 395 and 430 nm). For wavelengths longer than 430 nm, the decay time remains the same, at the level around 0.16 ns. The effect is shown in Fig. 3. For DMABrS in ethylene glycol, the time analysis shows, that at the blue edge of TRES, decays consist of two components: the shorter (0.04 ns) and the longer one (0.31 ns). The ratio of their intensities changes from 1000:1 to almost 1:1 upon moving from 397 nm toward 420 nm. For wavelengths longer than 425 nm, decays remain monoexponential with a decay time of 0.33 ns (data not shown). Similar results in the time analysis were obtained for DMAClS in propanol and ethylene glycol. For propanol, the short time component of s = 0.07 ns was found for k between 385 nm and 420 nm, and the only longer component with s = 0.17 ns was found for k 430 nm. For ethylene glycol, a biexponential decay was found (s1 = 0.04 ns, s2 = 0.53 ns) for k between 400 nm and 430 nm, and for k 440 nm decays remain monoexponential, with s = 0.57 ns (data not shown). These effects clearly also appear in the wavelength scale. For DMABrS and DMAClS in polar solvents one can observe the irregularity in the shape of TRES, visible as a narrower line at the left top side of the main shape of the time-resolved emission spectra. By
slicing along the wavelength axis and moving the slice window toward higher time values, one can obtain the proof for time shift of the maximum intensity of the emission spectra. The shifts observed for DMABrS, DMAClS (Fig. 4a and b) and the comparison with DMACS (Fig. 4c) are shown. The shifts are 15–16 nm for DMAClS and 18–20 nm for DMABrS, and they occur within the first 200 ps after excitation. For comparison, the shifts are shown for DMACS in solvents of a wide range of polarity and viscosity values. As one can see, for more polar molecules like DMACS, the values of the maximum intensity shift changes from 10 to even 66 nm, and it depends rather more on the viscosity, than on the dielectric constant values of the solvent.
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Fig. 3. TRES of the DMABrS in ethanol – changes of the decay time in the emission range.
Fig. 4. TRES of DMACS (a), DMABrS (b) and DMAClS (c). Shifts of the intensity maximum in the time. In the parts (b) and (c) the internal windows show spectra for the initial moments and for the moments longer than 160 ps. The results are taken by slicing the TRES results along the wavelength scale, with the time width of the slice equal to 2.5 ps.
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Fig. 5. The steady-state absorption and the emission spectra (both are the solid lines, normalized to the same height of maximum) as well as the steady-state excitation and the emission polarization spectra (points) of DMABrS in cyclohexane, ethanol and ethylene glycol.
Such results are an evidence of strong influence of viscosity on the sensitivity of light induced trans–cis photoisomerization. Viscosity blocks the photoisomerization by difficulties in molecular reorientation. The solvent polarity acts only as an energy level moderator, shifting emission spectra with increasing polarity towards longer wavelength. It facilitates the energy level separation of the relaxed intramolecular charge transfer states (fluorescent states) and other un-relaxed levels which are involved in the other non-fluorescent deactivation pathways like trans–cis photoisomerization. Results of steady-state measurements of absorption, emission, emission anisotropy excitation and emission spectra for DMABrS in cyclohexane, ethanol and ethylene glycol are shown in Fig. 5. They confirm conclusion of TRES results for polar solvents. The emission spectra exhibit a strong Stokes shift for polar solvents in comparison to non-polar ones like cyclohexane. The absorption spectra show the polarity effect on spectra broadening. In both, emission anisotropy spectra measured across the absorption region and the emission region, as well, the viscosity effect is easily visible. For all, polar and non-polar solvents, especially for ethanol, there is evidence for the existence of different fluorescent centers, one of which appears at the blue edge of the emission spectrum, right in the 0–0 transition region. Increasing values of r in that region are an evidence for the existence of a fluorescent center, the lifetime of which is shorter than in the other emission region. Shortening the lifetime causes the increase of emission anisotropy, according to an earlier explanation in [31]. The emission anisotropy measured at kobs = 400 nm remains the same, higher than for kobs = 450 nm over the whole range. This conclusion is consistent with earlier described results of TRES for both molecules, DMABrS and DMAClS [31].
dy-state measurements, too. Bromide and chloride at position 40 have an influence on spectra shifts similar to the methoxy group. For DMABrS in ethanol maximum intensity of TRES is located initially at 437 nm and during the next 160 ps the maximum of TRES moves 18 nm to the red edge of the emission spectra. Similarly for DMABrS in ethylene glycol the maximum of TRES moves 20 nm toward the red edge of emission spectra within 130 ps time frame. All processes of deactivation like vibrational, solute–solvent and internal charge relaxations and non-radiative ones can evidently occur within this time frame. Steady-state polarization spectra confirm these conclusions with regard to deexcitation pathways of stilbene derivatives. Smaller spectral shifts for DMABrS and DMAClS, in comparison to the more polar molecule DMACS, is due to weaker internal interactions. The modified interaction between electric fields of the excited molecule and the surrounding molecules of the solvent continuously changes the distribution of excited states of the investigated molecule. The energy barrier for radiationless processes changes also, so the radiation deactivation pathway becomes more and more probable. Smooth changes of TRES indicate the evolution of the same single state, which corresponds to a continuously changing atomic configuration of the molecule after the moment of excitation. The existence of the TICT state and its effect on the deactivation of DMACS and other stilbene derivatives still remains open [20,21,32,33] and investigations are to be continued. Acknowledgements The author would like to express his gratitude to Prof. Dieter Gloyna for synthetizing and donation of the stilbene derivatives. The work has been sponsored by the University of Gdan´sk within the framework of the grant: BW 5200-5-0052-8.
4. Conclusions References TRES of all investigated stilbene derivatives show the short time component maximally at the blue edge of emission spectra in polar solvents. In non-polar solvents the spectra remained structured and unchanged and lifetimes are shorter as well. By comparison with recently measured TRES for molecules with cyano or methoxy groups in position 40 and dimethylamino group in position 4 one can find a strong derivative group effect which is shown in stea-
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