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Excited-state relaxation processes of DPA-DSB: Investigation of the reason for high fluorescence quantum yield of symmetric D-π-D molecule
Chemical Physics Letters 501 (2011) 296–299
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Excited-state relaxation processes of DPA-DSB: Investigation of the reason for high fluorescence quantum yield of symmetric D-p-D molecule Xing He a, Yuqiang Liu a, Xin Du a, Yanqiang Yang a,⇑, Bin Xu b, Wenjing Tian b, Yuguang Ma b a b
Center for Condensed Matter Science and Technology, Department of Physics, Harbin Institute of Technology, Harbin 150001, China State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 23 October 2010 In final form 11 November 2010 Available online 25 November 2010
a b s t r a c t The temporal evolution of excited states of a symmetric D-p-D structure two-photon absorption material, 1,4-di(40 -N,N-diphenylaminostyryl) benzene (DPA-DSB), was investigated by femtosecond transient absorption spectrum and solvent polarity dependent fluorescence properties. The results suggested that the structure of DPA-DSB did not change much during the intramolecular charge-transfer (ICT) process, which could be the reason for good radiative ability of the ICT state. The major non-radiative deactive channel may be a large structure-changed process, which was formed more slowly than the radiative ICT state. Symmetric charge-transfer in this D-p-D structure molecule could make the non-radiative structure-changed process slow and ineffective, which should be the reason for the high fluorescence quantum yield. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Two-photon excited fluorescence (TPEF) materials have attracted much attention due to their wide applications in up-conversion lasing [1], two-photon fluorescence microscopy [2]. For such materials, both large two-photon absorption (TPA) cross-section and high fluorescence quantum yield are desired. Much effort has been made to improve the TPA cross section. Molecules that contain electron donor (D) or electronic acceptor (A) have been reported to have large TPA cross section [3]. However, most of them exhibit low fluorescence quantum yield, such as several D–A structure dipole molecules [4] and planar octupolar molecules [5], while the symmetric D-p-D structure quadrupole molecules were reported to have relatively high fluorescence quantum yield in addition to good TPA behaviors [3]. In order to understand the structure factors that affect the TPA and fluorescence, the excited-states relaxation processes should be investigated. As reported, the extent of intramolecular charge-transfer (ICT) upon photoexcitation determines the dipole moment of molecule in the excited state, which could be the major factor that affects the TPA cross-section [3], while the structure variety of molecule in the ICT state affects the fluorescent emitting ability of molecule [4–7]. The D-p-A structure dipole molecules usually have a twist ICT (TICT) state in which the donor plane or the acceptor plane twists relative to each other [6,7]. The TICT state is either weak radiative [6] or non-radiative [4]. This could be the reason for the low fluorescence quantum yield of such molecules. However, ⇑ Corresponding author. E-mail address: [email protected] (Y. Yang). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.11.037
details of excited-state relaxation processes of symmetric D-p-D structure quadrupole molecules have not been well understood. 1,4-di(40 -N,N-diphenylaminostyryl) benzene (DPA-DSB) is a typical symmetric D-p-D structure compound which has large TPA cross-section (970GM) and high fluorescence quantum yield (0.787) in toluene [8,9]. By using solvent polarity dependent fluorescence properties and femtosecond time-resolved transient absorption spectra, we have investigated the nature of excitedstates upon photoexcitation of DPA-DSB in solution. As reported [8], the two-photon absorption spectrum of DPA-DSB covers a broad band from 700 to 820 nm. When converted to the equivalent one-photon absorption spectrum, it covers the wavelength range from 350 to 410 nm, which includes the 400 and 375 nm used as excitation laser pulse in the experiments of this work. Furthermore, the fluorescence spectra of both one- and two-photon excitations are consistent, indicating that both one- and two-photon excitations generate the same fluorescence state. So we suggest that the FC state generated by one-photon excitation (400 and 375 nm) is the same FC state generated by two-photon excitation. Thus, the investigation of the one-photon excitation spectroscopy in this Letter can give useful information for the understanding of the two-photon excitation optical properties. 2. Experimental The synthesis of the material was described in detail elsewhere [8,9]. The samples for absorption and fluorescence measurement were dissolved with the concentration of 0.1 mmol/L in toluene, tetrahydrofuran (THF) and dimethyl sulfoxide (DMSO), which have different polarity. The absorption spectra were measured using the
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UV–visible spectrophotometer. The fluorescence spectra were collected by the spectroscopy meter (Chromex 500IS/SM, BRUKER) and CCD (DU440, Andor) after excited at 400 nm by a femtosecond laser. The fluorescence lifetime measurement was carried out using the time-correlated single photon counting (TCSPC) system, which was composed of a picosecond diode laser (BDL-375-SMC, Becker & Hickl GmbH), a monochromator (SSM101, Zolix), a single photon detection module (id100-50, id Quantique) and a TCSPC measurement card (SPC-130, Becker & Hickl GmbH). The wavelength of the laser was 375 nm. The instrument response function (IRF) of this measurement system was about 60 ps. The sample for the ultrafast measurement was dissolved in toluene with the concentration of 0.1 mmol/L. The transient absorption spectrum experiment setup was described in our early paper [10]. Briefly, the output laser has a repetition rate of 1 kHz, pulse duration of 130 fs at 800 nm, and the power was about 600 mw. The output light was split into two beams by a beam splitter (90% and 10%). Ninety percent of them passed through the BBO crystal to double frequency to 400 nm used as the pump beam. The other one passed through a sapphire plate to generate the white light continuum used as the probe beam. The pump pulse went through the optical delay line and superposed with the probe pulse in the sample temporally and spatially. After passing through the sample, the pump beam was blocked and the probe beam was transmitted to the spectroscopy meter (Chromex 500IS/SM, BRUKER) and measured by a thermoelectrically cooled charge coupled device (DU440, Andor). The polarization of the pump pulse was turned 54.7° respected to the probe pulse. The measurement and control of optical delay line were performed by a personal computer. 3. Results and discussion The molecular structure is shown in onset of Figure 1. The absorption spectra and fluorescence spectra in all the solvents, toluene, THF and DMSO, are shown in Figure 1. The spectroscopic data are shown in Table 1. The polarity of solvent is expressed using Lippert-Mataga solvent polarity parameter Mf [11]:
Mf ¼
e 1 n2 1 2e þ 1 2n2 þ 1
ð1Þ
whereas e is dielectric constant and n is refractive index. The absorption spectra remain almost the same for all the three solvents
Figure 1. The absorption and fluorescence spectra of DPA-DSB in solvents with different polarity. The absorption spectra in all solvents remain almost the same while the fluorescence spectra redshift with increasing of polarity of solvent. The onset is the molecular structure of DPA-DSB.
Table 1 Spectroscopy data of DPA-DSB in three solvents with different polarity.
Toluene THF DMSO
Mf
kA (nm)
kf (nm)
Stokes shift (cm1)
Lifetime (ns)
0.013 0.207 0.263
409 406 414
457 486 519
2530 4071 4906
0.99 1.25 1.53
while the fluorescence spectra show obvious red-shift with the increasing polarity of the solvent. This indicates that the primarily excited Franck–Condon (FC) state is not the fluorescence-emitting state. The absorption spectra remain almost the same in all solvents, indicating that the dipole moment of the molecule keeps unchanged during transition from ground state to FC state. The obvious red-shift of the fluorescence spectra with the increasing polarity of solvent suggests that the fluorescence state has larger dipole moment than the ground state, which indicates that the fluorescence state is an intramolecular charge-transfer (ICT) state. From the primarily excited FC state to the radiative ICT state, there is a big change of the dipole moment of the molecule, which should be the result of intramolecular charge transfer. The results of the fluorescence lifetime measurement are shown in Figure 2. According to the results summarized in Table 1, both the Stokes-shift and fluorescence lifetime increase with the increasing of polarity of solvent, which suggests that the radiative ICT state has large dipole moment that is stabilized by the polar solvent. Different results were reported [4,7,11,12] that the fluorescence lifetime decreased with the increasing polarity of solvents because the increasing polarity of solvents accelerate the ICT ? TICT and the TICT state is a more polar non-radiative state. For DPA-DSB, the more polar non-radiative TICT state could not exist. If the TICT state existed, the increasing of polarity of solvent would accelerate the ICT ? TICT transition, which would decrease the lifetime of the fluorescence state but it is not the fact for DPA-DSB. The femtosecond transient absorption spectra, which were obtained in the toluene solution, are shown in Figure 3. According to Figure 3a, the transient absorption spectra are composed of ground state bleaching (BL), stimulated emission (SE) and excited state absorption (ESA). The BL part appears on the wavelengths shorter than 450 nm. The SE part appears on the wavelengths region of 430–600nm which shows the same spectra shape and peak position as the fluorescence spectra, suggesting that the SE part comes from the radiative ICT state, which is the same as the fluorescence state. The ESA part appears on the wavelengths longer than 600 nm. The dynamics of wavelengths at 410, 430, 457 and
Figure 2. The fluorescence decay curve of DPA-DSB in three solvents with different polarity measured in the fluorescence peak wavelength. The solid line is the fitted curve. Lifetime of DPA-DSB increases with the increasing of polarity of solvent.
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a
b
c
d
Figure 3. Transient absorption (TA) spectra and dynamics of several wavelengths of DPA-DSB measured by femtosecond pump–probe technique. Part a shows the TA spectra compared as the steady absorption spectra and fluorescence spectra. Part b shows kinetics of several wavelengths at first several picoseconds. Part c shows TA spectra at different delay time. Part d shows kinetics of several wavelengths in long time region.
489 nm in the first several picoseconds are shown in Figure 3b, in order to investigate the excited-states dynamics in the first several picoseconds. The kinetic of 410 nm shows a rapid rising process of amplitude within few hundreds of femtoseconds and then decays in several picoseconds while the dynamics of 430 nm also show a similar rapid rising process followed by a slower decay process than that of 410 nm. For 457 and 489 nm which implies the SE process, they show a rapid rising process, the same to the previous two wavelengths, but followed by another relative slowly rising process. The relative slowly rising process of 457 nm shows the same time character as the decay process of 410 nm, suggesting that they come from the same state. Two processes contribute to the kinetic of 410 nm. While the rising process of 410 nm is contributed mainly from the ground state bleaching, the decay process of 410 nm is contributed mainly from the ESA of the ICT state. The rising process of both 457 and 489 nm are assigned to the SE of the ICT state. Two processes could be involved in the formation of ICT state according to the two rising process of SE part. After excited by the femtosecond laser pulse, the high vibrational excited state of FC state is reached. The fast rising process comes from higher vibrational excited state of FC state while the second one comes from the lower vibrational state of FC state. Both processes occur within as fast as less than 5 ps, which indicates that the structure of molecule does not have much change accompanying the ICT process. Time-resolved transient absorption spectra at different delay times are shown in Figure 3c. With the increasing of delay time, the amplitude of SE and BL parts decreases while ESA part shows a blue-shift from wavelength region longer than 600 nm to almost all wavelengths that detected. The kinetics of several wavelengths in long time region are shown in Figure 3d.
The kinetics of three wavelengths clearly show that in the beginning the dominant process is SE which shows a negative absorption change and then an ESA process arises slowly. The slowly rising of this ESA process implies that there is a large structural change accompany with the formation of the state from which the ESA comes. We consider that molecule with this large structural change is in an X state, so that the ESA process comes from the X state. The fitted curves of kinetics are also shown in Figure 4d.
Sn 430nm
560nm
410nm
457nm
FC *
X
ICT
400nm
489nm
430nm
457nm 560nm
S0 ξ
ξ
Figure 4. Diagram for the excited states relaxation processes of DPA-DSB after excited by femtosecond laser pulse. n refers to reactive coefficient.
X. He et al. / Chemical Physics Letters 501 (2011) 296–299 Table 2 Fitted parameters of TA kinetics. The value of s1 is fixed at 990 ps which was obtained by the TCSPC measurement. Nanometers
A1
s1 (ps)
A2
s2 (ps)
A3
s3 (ps)
430 457 560
0.15 0.14 0.06
990 990 990
0.056 0.02 0.103
4.73 6.07 3.69
0.19 0.10 0.21
254 229 240
The fitted parameters are given in Table 2. The curves were fitted to three processes with a time constant si for each process. The parameter s1 which indicates the lifetime of ICT state was fixed to 990 ps according to the fitting result of TCSPC measurement. The time constant s2 indicates the vibrative relaxation of the ICT state which is shorter than 10 ps while the time constant s3 indicates the rising time of X state which is about 240 ps. The rising time of X state is shorter than the lifetime of the ICT state, which suggests that X state does not come from the ICT state but from the primarily excited FC state. The relative long population time (240 ps) of X state implies that the large structural change is more slowly than the charge transfer process which does not involve any large structural change process. After giving our experimental results, we will discuss excitedstates relaxation in detail. Upon photoexcitation, an ICT state occurs. In the ICT state, charge-transfer takes place from electron donor to electron acceptor so that the dipole moment is much larger than that of ground state and FC state. In stilbene derivation whose connecting bridge between electron donor and acceptor is styryl, two additional processes could be include in the relaxation of excited state besides the ICT process: the rotation of double bonds of styryl to form a non-radiative P* [13], and the rotation of electron donor around single bond of styryl to form highly polar electron decouple TICT state [4]. The TICT may be weak-radiative or non-radiative. Both processes would decrease the fluorescence quantum yield of molecules. For DPA-DSB, the fast formation of ICT state (<5 ps) indicates that there is no big structural change from FC to ICT state, because big change of structure could not complete in such a short time. The similar structure between the ICT state and ground state could be the reason for the good radiative ability of ICT state. The absence of the non-radiative TICT state increases the fluorescence quantum yield. The large structural change state X, which may be the C@C double-bond twist state P*, is not efficient because of its relatively long rising time (240 ps), which could not largely decrease the fluorescence quantum yield. The details of excited-states relaxation in DPA-DSB excited by femtosecond laser pulse are shown in Figure 4. After excitation, FC state is reached via Franck–Condon transition, which is followed by two processes. One is the ICT process which occurs in several picoseconds. The ICT state is produced and relaxes radiatively with time-constant of 990 ps. The other one is a large structural change to produce the X state which relaxes thermally to the ground state. The rising time of X state is about 240 ps while the decay time of
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this state is too long to be determined by our measurement system. The little structural change from FC to ICT state, which leads to the good radiative ability of ICT state, could result from the symmetric structure of molecule, as in non-symmetric D–A structure molecule there usually be a large structural change during ICT process such as the twist of donor or acceptor compound [6]. In the symmetric D-p-D structure molecule, the structural change may be somewhat limited because of the interaction between two donor compounds [14], which could be the reason for the little structural change accompanying the ICT process and the absence of the TICT state. This should be the reason for the high fluorescence quantum yield of DPA-DSB. Then, we suggest that the symmetric D-p-D structure quadrupole molecule which usually has high fluorescence quantum yield could be a good choice for the two-photon fluorescence materials. 4. Conclusion Excited-states relaxation in DPA-DSB upon excitation was investigated using solvent-polarity dependent fluorescence properties and femtosecond time-resolved transient absorption spectra. The results suggest that the structure of molecule in ICT state do not change much compared to the ground state which could be the reason for high emission ability of the ICT state. The non-radiative TICT state could not be involved in the excited states, resulting in the high fluorescence quantum yield. The major non-radiative process may be a large structural change process which is formed more slowly than the radiative ICT state. The symmetric D-p-D structure could prevent structural change within charge transfer process which should be the reason for high fluorescence quantum yield. Acknowledgement We thank the National Natural Science Foundation of China (Grant Nos. 60478015 and 20973050) for its financial support. References [1] S. Oliveira, D. Correa, L. Misoguti, C. Constantino, R. Aroca, S. Zilio, C. Mendonca, Adv. Mater. 17 (15) (2005) 1890. [2] W. Denk, J. Strickler, W. Webb, Science 248 (4951) (1990) 73. [3] M. Albota et al., Science 281 (5383) (1998) 1653. [4] M. Shaikh et al., J. Phys. Chem. A 114 (13) (2010) 4507. [5] B. Cho et al., J. Am. Chem. Soc 123 (41) (2001) 10039. [6] Z. Grabowski, K. Rotkiewicz, W. Rettig, Chem. Rev. 103 (10) (2003) 3899. [7] G. Zhou, D. Wang, Y. Tian, Z. Shao, M. Jiang, Appl. Phys. B: Lasers Opt. 78 (3) (2004) 397. [8] F. He et al., Adv. Funct. Mater. 17 (9) (2007) 1551. [9] B. Xu et al., New J. Chem. 33 (12) (2009) 2457. [10] X. He, Y. Wang, Z. Yang, Y. Ma, Y. Yang, Appl. Phys. B: Lasers Opt. 100 (4) (2010) 773. [11] A. Barik, M. Kumbhakar, S. Nath, H. Pal, Chem. Phys. 315 (3) (2005) 277. [12] B. Li, R. Tong, R. Zhu, F. Meng, H. Tian, S. Qian, J. Phys. Chem. B 109 (21) (2005) 10705. [13] R. Lapouyade, K. Czeschka, W. Majenz, W. Rettig, E. Gilabert, C. Rulliere, J. Phys. Chem. 96 (24) (1992) 9643. [14] Y. Huang et al., J. Phys. Chem. B 106 (39) (2002) 10031.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Excited-state relaxation processes of DPA-DSB: Investigation of the reason for high fluorescence quantum yield of symmetric D-p-D molecule Xing He a, Yuqiang Liu a, Xin Du a, Yanqiang Yang a,⇑, Bin Xu b, Wenjing Tian b, Yuguang Ma b a b
Center for Condensed Matter Science and Technology, Department of Physics, Harbin Institute of Technology, Harbin 150001, China State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 23 October 2010 In final form 11 November 2010 Available online 25 November 2010
a b s t r a c t The temporal evolution of excited states of a symmetric D-p-D structure two-photon absorption material, 1,4-di(40 -N,N-diphenylaminostyryl) benzene (DPA-DSB), was investigated by femtosecond transient absorption spectrum and solvent polarity dependent fluorescence properties. The results suggested that the structure of DPA-DSB did not change much during the intramolecular charge-transfer (ICT) process, which could be the reason for good radiative ability of the ICT state. The major non-radiative deactive channel may be a large structure-changed process, which was formed more slowly than the radiative ICT state. Symmetric charge-transfer in this D-p-D structure molecule could make the non-radiative structure-changed process slow and ineffective, which should be the reason for the high fluorescence quantum yield. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Two-photon excited fluorescence (TPEF) materials have attracted much attention due to their wide applications in up-conversion lasing [1], two-photon fluorescence microscopy [2]. For such materials, both large two-photon absorption (TPA) cross-section and high fluorescence quantum yield are desired. Much effort has been made to improve the TPA cross section. Molecules that contain electron donor (D) or electronic acceptor (A) have been reported to have large TPA cross section [3]. However, most of them exhibit low fluorescence quantum yield, such as several D–A structure dipole molecules [4] and planar octupolar molecules [5], while the symmetric D-p-D structure quadrupole molecules were reported to have relatively high fluorescence quantum yield in addition to good TPA behaviors [3]. In order to understand the structure factors that affect the TPA and fluorescence, the excited-states relaxation processes should be investigated. As reported, the extent of intramolecular charge-transfer (ICT) upon photoexcitation determines the dipole moment of molecule in the excited state, which could be the major factor that affects the TPA cross-section [3], while the structure variety of molecule in the ICT state affects the fluorescent emitting ability of molecule [4–7]. The D-p-A structure dipole molecules usually have a twist ICT (TICT) state in which the donor plane or the acceptor plane twists relative to each other [6,7]. The TICT state is either weak radiative [6] or non-radiative [4]. This could be the reason for the low fluorescence quantum yield of such molecules. However, ⇑ Corresponding author. E-mail address: [email protected] (Y. Yang). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.11.037
details of excited-state relaxation processes of symmetric D-p-D structure quadrupole molecules have not been well understood. 1,4-di(40 -N,N-diphenylaminostyryl) benzene (DPA-DSB) is a typical symmetric D-p-D structure compound which has large TPA cross-section (970GM) and high fluorescence quantum yield (0.787) in toluene [8,9]. By using solvent polarity dependent fluorescence properties and femtosecond time-resolved transient absorption spectra, we have investigated the nature of excitedstates upon photoexcitation of DPA-DSB in solution. As reported [8], the two-photon absorption spectrum of DPA-DSB covers a broad band from 700 to 820 nm. When converted to the equivalent one-photon absorption spectrum, it covers the wavelength range from 350 to 410 nm, which includes the 400 and 375 nm used as excitation laser pulse in the experiments of this work. Furthermore, the fluorescence spectra of both one- and two-photon excitations are consistent, indicating that both one- and two-photon excitations generate the same fluorescence state. So we suggest that the FC state generated by one-photon excitation (400 and 375 nm) is the same FC state generated by two-photon excitation. Thus, the investigation of the one-photon excitation spectroscopy in this Letter can give useful information for the understanding of the two-photon excitation optical properties. 2. Experimental The synthesis of the material was described in detail elsewhere [8,9]. The samples for absorption and fluorescence measurement were dissolved with the concentration of 0.1 mmol/L in toluene, tetrahydrofuran (THF) and dimethyl sulfoxide (DMSO), which have different polarity. The absorption spectra were measured using the
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UV–visible spectrophotometer. The fluorescence spectra were collected by the spectroscopy meter (Chromex 500IS/SM, BRUKER) and CCD (DU440, Andor) after excited at 400 nm by a femtosecond laser. The fluorescence lifetime measurement was carried out using the time-correlated single photon counting (TCSPC) system, which was composed of a picosecond diode laser (BDL-375-SMC, Becker & Hickl GmbH), a monochromator (SSM101, Zolix), a single photon detection module (id100-50, id Quantique) and a TCSPC measurement card (SPC-130, Becker & Hickl GmbH). The wavelength of the laser was 375 nm. The instrument response function (IRF) of this measurement system was about 60 ps. The sample for the ultrafast measurement was dissolved in toluene with the concentration of 0.1 mmol/L. The transient absorption spectrum experiment setup was described in our early paper [10]. Briefly, the output laser has a repetition rate of 1 kHz, pulse duration of 130 fs at 800 nm, and the power was about 600 mw. The output light was split into two beams by a beam splitter (90% and 10%). Ninety percent of them passed through the BBO crystal to double frequency to 400 nm used as the pump beam. The other one passed through a sapphire plate to generate the white light continuum used as the probe beam. The pump pulse went through the optical delay line and superposed with the probe pulse in the sample temporally and spatially. After passing through the sample, the pump beam was blocked and the probe beam was transmitted to the spectroscopy meter (Chromex 500IS/SM, BRUKER) and measured by a thermoelectrically cooled charge coupled device (DU440, Andor). The polarization of the pump pulse was turned 54.7° respected to the probe pulse. The measurement and control of optical delay line were performed by a personal computer. 3. Results and discussion The molecular structure is shown in onset of Figure 1. The absorption spectra and fluorescence spectra in all the solvents, toluene, THF and DMSO, are shown in Figure 1. The spectroscopic data are shown in Table 1. The polarity of solvent is expressed using Lippert-Mataga solvent polarity parameter Mf [11]:
Mf ¼
e 1 n2 1 2e þ 1 2n2 þ 1
ð1Þ
whereas e is dielectric constant and n is refractive index. The absorption spectra remain almost the same for all the three solvents
Figure 1. The absorption and fluorescence spectra of DPA-DSB in solvents with different polarity. The absorption spectra in all solvents remain almost the same while the fluorescence spectra redshift with increasing of polarity of solvent. The onset is the molecular structure of DPA-DSB.
Table 1 Spectroscopy data of DPA-DSB in three solvents with different polarity.
Toluene THF DMSO
Mf
kA (nm)
kf (nm)
Stokes shift (cm1)
Lifetime (ns)
0.013 0.207 0.263
409 406 414
457 486 519
2530 4071 4906
0.99 1.25 1.53
while the fluorescence spectra show obvious red-shift with the increasing polarity of the solvent. This indicates that the primarily excited Franck–Condon (FC) state is not the fluorescence-emitting state. The absorption spectra remain almost the same in all solvents, indicating that the dipole moment of the molecule keeps unchanged during transition from ground state to FC state. The obvious red-shift of the fluorescence spectra with the increasing polarity of solvent suggests that the fluorescence state has larger dipole moment than the ground state, which indicates that the fluorescence state is an intramolecular charge-transfer (ICT) state. From the primarily excited FC state to the radiative ICT state, there is a big change of the dipole moment of the molecule, which should be the result of intramolecular charge transfer. The results of the fluorescence lifetime measurement are shown in Figure 2. According to the results summarized in Table 1, both the Stokes-shift and fluorescence lifetime increase with the increasing of polarity of solvent, which suggests that the radiative ICT state has large dipole moment that is stabilized by the polar solvent. Different results were reported [4,7,11,12] that the fluorescence lifetime decreased with the increasing polarity of solvents because the increasing polarity of solvents accelerate the ICT ? TICT and the TICT state is a more polar non-radiative state. For DPA-DSB, the more polar non-radiative TICT state could not exist. If the TICT state existed, the increasing of polarity of solvent would accelerate the ICT ? TICT transition, which would decrease the lifetime of the fluorescence state but it is not the fact for DPA-DSB. The femtosecond transient absorption spectra, which were obtained in the toluene solution, are shown in Figure 3. According to Figure 3a, the transient absorption spectra are composed of ground state bleaching (BL), stimulated emission (SE) and excited state absorption (ESA). The BL part appears on the wavelengths shorter than 450 nm. The SE part appears on the wavelengths region of 430–600nm which shows the same spectra shape and peak position as the fluorescence spectra, suggesting that the SE part comes from the radiative ICT state, which is the same as the fluorescence state. The ESA part appears on the wavelengths longer than 600 nm. The dynamics of wavelengths at 410, 430, 457 and
Figure 2. The fluorescence decay curve of DPA-DSB in three solvents with different polarity measured in the fluorescence peak wavelength. The solid line is the fitted curve. Lifetime of DPA-DSB increases with the increasing of polarity of solvent.
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a
b
c
d
Figure 3. Transient absorption (TA) spectra and dynamics of several wavelengths of DPA-DSB measured by femtosecond pump–probe technique. Part a shows the TA spectra compared as the steady absorption spectra and fluorescence spectra. Part b shows kinetics of several wavelengths at first several picoseconds. Part c shows TA spectra at different delay time. Part d shows kinetics of several wavelengths in long time region.
489 nm in the first several picoseconds are shown in Figure 3b, in order to investigate the excited-states dynamics in the first several picoseconds. The kinetic of 410 nm shows a rapid rising process of amplitude within few hundreds of femtoseconds and then decays in several picoseconds while the dynamics of 430 nm also show a similar rapid rising process followed by a slower decay process than that of 410 nm. For 457 and 489 nm which implies the SE process, they show a rapid rising process, the same to the previous two wavelengths, but followed by another relative slowly rising process. The relative slowly rising process of 457 nm shows the same time character as the decay process of 410 nm, suggesting that they come from the same state. Two processes contribute to the kinetic of 410 nm. While the rising process of 410 nm is contributed mainly from the ground state bleaching, the decay process of 410 nm is contributed mainly from the ESA of the ICT state. The rising process of both 457 and 489 nm are assigned to the SE of the ICT state. Two processes could be involved in the formation of ICT state according to the two rising process of SE part. After excited by the femtosecond laser pulse, the high vibrational excited state of FC state is reached. The fast rising process comes from higher vibrational excited state of FC state while the second one comes from the lower vibrational state of FC state. Both processes occur within as fast as less than 5 ps, which indicates that the structure of molecule does not have much change accompanying the ICT process. Time-resolved transient absorption spectra at different delay times are shown in Figure 3c. With the increasing of delay time, the amplitude of SE and BL parts decreases while ESA part shows a blue-shift from wavelength region longer than 600 nm to almost all wavelengths that detected. The kinetics of several wavelengths in long time region are shown in Figure 3d.
The kinetics of three wavelengths clearly show that in the beginning the dominant process is SE which shows a negative absorption change and then an ESA process arises slowly. The slowly rising of this ESA process implies that there is a large structural change accompany with the formation of the state from which the ESA comes. We consider that molecule with this large structural change is in an X state, so that the ESA process comes from the X state. The fitted curves of kinetics are also shown in Figure 4d.
Sn 430nm
560nm
410nm
457nm
FC *
X
ICT
400nm
489nm
430nm
457nm 560nm
S0 ξ
ξ
Figure 4. Diagram for the excited states relaxation processes of DPA-DSB after excited by femtosecond laser pulse. n refers to reactive coefficient.
X. He et al. / Chemical Physics Letters 501 (2011) 296–299 Table 2 Fitted parameters of TA kinetics. The value of s1 is fixed at 990 ps which was obtained by the TCSPC measurement. Nanometers
A1
s1 (ps)
A2
s2 (ps)
A3
s3 (ps)
430 457 560
0.15 0.14 0.06
990 990 990
0.056 0.02 0.103
4.73 6.07 3.69
0.19 0.10 0.21
254 229 240
The fitted parameters are given in Table 2. The curves were fitted to three processes with a time constant si for each process. The parameter s1 which indicates the lifetime of ICT state was fixed to 990 ps according to the fitting result of TCSPC measurement. The time constant s2 indicates the vibrative relaxation of the ICT state which is shorter than 10 ps while the time constant s3 indicates the rising time of X state which is about 240 ps. The rising time of X state is shorter than the lifetime of the ICT state, which suggests that X state does not come from the ICT state but from the primarily excited FC state. The relative long population time (240 ps) of X state implies that the large structural change is more slowly than the charge transfer process which does not involve any large structural change process. After giving our experimental results, we will discuss excitedstates relaxation in detail. Upon photoexcitation, an ICT state occurs. In the ICT state, charge-transfer takes place from electron donor to electron acceptor so that the dipole moment is much larger than that of ground state and FC state. In stilbene derivation whose connecting bridge between electron donor and acceptor is styryl, two additional processes could be include in the relaxation of excited state besides the ICT process: the rotation of double bonds of styryl to form a non-radiative P* [13], and the rotation of electron donor around single bond of styryl to form highly polar electron decouple TICT state [4]. The TICT may be weak-radiative or non-radiative. Both processes would decrease the fluorescence quantum yield of molecules. For DPA-DSB, the fast formation of ICT state (<5 ps) indicates that there is no big structural change from FC to ICT state, because big change of structure could not complete in such a short time. The similar structure between the ICT state and ground state could be the reason for the good radiative ability of ICT state. The absence of the non-radiative TICT state increases the fluorescence quantum yield. The large structural change state X, which may be the C@C double-bond twist state P*, is not efficient because of its relatively long rising time (240 ps), which could not largely decrease the fluorescence quantum yield. The details of excited-states relaxation in DPA-DSB excited by femtosecond laser pulse are shown in Figure 4. After excitation, FC state is reached via Franck–Condon transition, which is followed by two processes. One is the ICT process which occurs in several picoseconds. The ICT state is produced and relaxes radiatively with time-constant of 990 ps. The other one is a large structural change to produce the X state which relaxes thermally to the ground state. The rising time of X state is about 240 ps while the decay time of
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this state is too long to be determined by our measurement system. The little structural change from FC to ICT state, which leads to the good radiative ability of ICT state, could result from the symmetric structure of molecule, as in non-symmetric D–A structure molecule there usually be a large structural change during ICT process such as the twist of donor or acceptor compound [6]. In the symmetric D-p-D structure molecule, the structural change may be somewhat limited because of the interaction between two donor compounds [14], which could be the reason for the little structural change accompanying the ICT process and the absence of the TICT state. This should be the reason for the high fluorescence quantum yield of DPA-DSB. Then, we suggest that the symmetric D-p-D structure quadrupole molecule which usually has high fluorescence quantum yield could be a good choice for the two-photon fluorescence materials. 4. Conclusion Excited-states relaxation in DPA-DSB upon excitation was investigated using solvent-polarity dependent fluorescence properties and femtosecond time-resolved transient absorption spectra. The results suggest that the structure of molecule in ICT state do not change much compared to the ground state which could be the reason for high emission ability of the ICT state. The non-radiative TICT state could not be involved in the excited states, resulting in the high fluorescence quantum yield. The major non-radiative process may be a large structural change process which is formed more slowly than the radiative ICT state. The symmetric D-p-D structure could prevent structural change within charge transfer process which should be the reason for high fluorescence quantum yield. Acknowledgement We thank the National Natural Science Foundation of China (Grant Nos. 60478015 and 20973050) for its financial support. References [1] S. Oliveira, D. Correa, L. Misoguti, C. Constantino, R. Aroca, S. Zilio, C. Mendonca, Adv. Mater. 17 (15) (2005) 1890. [2] W. Denk, J. Strickler, W. Webb, Science 248 (4951) (1990) 73. [3] M. Albota et al., Science 281 (5383) (1998) 1653. [4] M. Shaikh et al., J. Phys. Chem. A 114 (13) (2010) 4507. [5] B. Cho et al., J. Am. Chem. Soc 123 (41) (2001) 10039. [6] Z. Grabowski, K. Rotkiewicz, W. Rettig, Chem. Rev. 103 (10) (2003) 3899. [7] G. Zhou, D. Wang, Y. Tian, Z. Shao, M. Jiang, Appl. Phys. B: Lasers Opt. 78 (3) (2004) 397. [8] F. He et al., Adv. Funct. Mater. 17 (9) (2007) 1551. [9] B. Xu et al., New J. Chem. 33 (12) (2009) 2457. [10] X. He, Y. Wang, Z. Yang, Y. Ma, Y. Yang, Appl. Phys. B: Lasers Opt. 100 (4) (2010) 773. [11] A. Barik, M. Kumbhakar, S. Nath, H. Pal, Chem. Phys. 315 (3) (2005) 277. [12] B. Li, R. Tong, R. Zhu, F. Meng, H. Tian, S. Qian, J. Phys. Chem. B 109 (21) (2005) 10705. [13] R. Lapouyade, K. Czeschka, W. Majenz, W. Rettig, E. Gilabert, C. Rulliere, J. Phys. Chem. 96 (24) (1992) 9643. [14] Y. Huang et al., J. Phys. Chem. B 106 (39) (2002) 10031.