Accepted Manuscript Title: Role of tert-butyl in the linear and nonlinear optical property of push-pull chromophores Authors: Xiao-Chun Chi, Ran Lu, Yu-Gao, Ying-Hui Wang, Shang-Hang Zhou, Ning Sui, Wen-Yan Wang, Mou-Cui Ni, Yan-Qiang Yang, Han-Zhuang Zhang PII: DOI: Reference: S1010-6030(17)31009-2 https://doi.org/10.1016/j.jphotochem.2017.10.036 JPC 10961 To appear in: Journal of Photochemistry and Photobiology A: Chemistry Received date: Revised date: Accepted date: 12-7-2017 9-10-2017 21-10-2017 Please cite this article as: Xiao-Chun Chi, Ran Lu, Yu-Gao, Ying-Hui Wang, Shang-Hang Zhou, Ning Sui, Wen-Yan Wang, Mou-Cui Ni, Yan-Qiang Yang, HanZhuang Zhang, Role of tert-butyl in the linear and nonlinear optical property of push-pull chromophores, Journal of Photochemistry and Photobiology A: Chemistry https://doi.org/10.1016/j.jphotochem.2017.10.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Role of tert-butyl in the linear and nonlinear optical property of push-pull Chromophores Xiao-Chun Chi1a, Ran Lu2a, Yu-Gao2, 4, Ying-Hui Wang1*, Shang-Hang Zhou1, Ning Sui1, Wen-Yan Wang1, Mou-Cui Ni1, Yan-Qiang Yang3 and Han-Zhuang Zhang1* 1 Femtosecond laser Laboratory, Key Laboratory of Physics and Technology for Advanced Batteries, College of Physics, Jilin University, Changchun, 130012, P. R. China 2 College 3 of Chemistry, Jilin University, Changchun 130012, P. R. China. National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, Sichuan, China. 4 State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun, 130012, P. R. China. a Xiao-Chun Chi and Ran Lu contributed equally to this work. *Corresponding authors at: Femtosecond laser Laboratory, Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, P. R. China. Email addresses: firstname.lastname@example.org email@example.com (Han-zhuang Zhang). Tel: +86-431-85167378; Fax: +86-431-85166112 Graphical abstract (Ying-Hui Wang) and Highlights We discussed the role of tert-butyl in optical property of pull-push chromophores. The tert-butyl unit accelerate the flurorescence relaxation rate of chromophores. The tert-butyl unit accelerate the dynamic rate of ICT process. The tert-butyl unit affect the two-photon optical property of chromophores. Abstract Through comparing the linear and nonlinear optical property of two pull-push chromophores, which are composed of difluoroboron β-diketonate functionalized with carbazole and tert-butyl carbazole, the role of tert-butyl in chromosphere has been discussed in detail. The tert-butyl unit would lead to the absorption and fluorescence peak red shift and accelerate the fluorescence relaxation rate of chromophores. Moreover, the tert-butyl unit would affect obviously the two-photon optical property of chromophore according to the Z-scan test. The transient absorption (TA) data shows that these two pull-push chromopphores both own intramolecular charge transfer (ICT) characteristics and the tert-butyl unit accelerate the dynamic rate of ICT generation and relaxation process, which may be responsible for the variance of fluorescence property. Keywords: push–pull chromophores, ICT state, transient absorption, two-photon Introduction Organic semiconducting oligomers have exhibited a huge potential in the field of optoelectronic field, involving light emitting diodes , field-effect transistors , thin film transistors , polarity probes , nonlinear optics  and solar cells , and been considered as the power alternatives to the inorganic counterparts. In addition, people have confirmed that the oligomers composed of electron-donating (D) and electron-accepting (A) units through π-conjugated linker (with dipole character) own excellent optical property [7-13]. Other units like tert-butyl unit and n, n'-diethyl (Net2) group sometimes are introduced on the chromophores so as to increase the molecular solubility or adjust the space structure of oligomer after photexcitation [14-15]. In addition, the number of substitution influence the inter-systerm crossing probability through changing energy gap between the first excited singlet state (S 1) and the lowest-lying triplet excited states (T1) . Similarity, new difluoroboron β-diketonate complexes functionalized with carbazole and tert-butyl carbazole have been synthesized by Lu and coworkers  and named CBM and TCBM, respectively, as shown in Fig. 1(A). Through linking the tert-butyl unit, the optical properties of pull-push compound would change apparently. However, the corresponding mechanism still remains unclear. In this paper, we compare the optical properties of the two chromophores as mentioned above through multi-spectroscopy techniques. We clarify the single-photon and two-photon optical property of compounds and the corresponding ultrafast photo-physical relaxation process, so as to discuss the role of tert-butyl unit in the optical property of chromophore. 2. Experimental The compounds of CBM and TCBM were obtained from Lu’s group in college of chemistry in Jilin University. The two compounds were dissolved in CH2Cl2 solution with concentrations of 510-4 M for optical measurements. The samples were placed in 1 mm and 2 mm thick quartz cuvette for linear and nonlinear optical measurements, respectively. Ground state absorption measurements were carried out in UV-Vis absorption spectrometer (Purkinje, TU-1810PC). Fluorescence measurements were made with a fiber optic spectrometer (Ocean Optics, USB4000) with an excitation wavelength of 400 nm. The femtosecond transient absorption (TA) technique is reported in previous work . The excitation spot was about 2.0 mm in diameter. The excitation wavelength was 400 nm. The TA spectrum were carried out by a spectrometer (AvaSpec-2048×16). The fluorescence dynamics are detected by time-correlated single-photon counting (TCSPC) technique , . All quantum chemistry calculations were done with Gaussian 09 Software . The ground-state geometries were optimized with density functional theory (DFT)  at B3LYP/6-311G(d, p) level . All the measurements were performed at room temperature. 3. Results and discussion F F B O F F B O N N TCBM 1.0 (B) 1.0 0.8 CBMabs 0.8 0.6 TCBMabs 0.6 0.4 CBMPL 0.4 TCBMPL 0.2 0.2 0.0 300 400 500 600 700 800 900 0.0 1000 Normalized PL/a.u. Normalized abs/a.u. CBM Normalized Intensity/a.u. (A) O O /nm 1.0 CBM TCBM 0.8 (C) 0.6 0.4 0.2 0.0 -5 0 1 10 Time/ns Fig. 1(A) Molecular structure of CBM and TCBM; (B) Normalized ground state absorption and steady-state fluorescence spectra of CBM (black) and TCBM (red). (C) Normalized fluorescence decay curve of CBM (black) and TCBM (red). Fig. 1(B) exhibits that both CBM and TCBM show two absorption bands and one fluorescence band in visible-near UV region. After the tert-butyl unit is linked on the chromophore, it is found that the absorption peak in the high-frequency region shift from 320 (CBM) to 323 nm (TCBM) and the main absorption peak shifts from 412 (CBM) to 432 nm (TCBM). Meanwhile, the fluorescence peak shifts from 597 to 643 nm. Apparently, the main absorption peak and the fluorescence peak are both much sensitive to the tert-butyl unit. And then, the fluorescence decay curve of CBM and TCBM are given in Fig. 1(C), which are fitted with mono-exponential function. The fitted results show that the fluorescence lifetime of CBM is 1.73 ns and that of TCBM is 0.30 ns, indicating that the tert-butyl unit would apparently accelerate the fluorescence relaxation rate of compounds. 0.04 0.02 (A) PIA2 0.3 ps PIA1 0.3 ps (C) 0.03 1 ps 0.6 ps 0.02 20 ps 14 ps 0.01 0.00 0.00 -0.01 SE2 -0.02 SE1 -0.04 0.02 -0.03 (D) (B) 20 ps 14 ps 1850 ps 1916 ps 0.01 T/T T/T -0.02 0.02 0.01 0.00 0.00 -0.01 -0.01 -0.02 -0.02 400 500 600 /nm 700 400 500 600 700 /nm Fig. 2: Transient absorption spectra of CBM (A, B) and TCBM (C, D) in CH2Cl2. The black arrows represent the evolution of photo-induced absorption peak and stimulated emission peak. In order to further investigate the role of tert-butyl unit on the photo-excitation relaxation mechanism of compounds, the transient absorption is employed to probe the ultrafast photo-excitation process of CBM and TCBM. Fig. 2 exhibits the time-dependent transient absorption (TA) spectra of CBM (A and B) and TCBM (C and D). As shown in Fig. 2(A), two negative peaks around 540 and 610 nm are attributed to the stimulated emission (SE), since they are overlapped with the steady emission spectrum of CBM. The negative bands in short wavelength region are noted as SE1, and that in long wavelength region are noted as SE2. The positive bands located at about 425 nm and 500 nm are assigned to the photo-induced absorption (PIA). The positive bands in short wavelength region and long wavelength region are labeled PIA1 and PIA2, respectively. After photo-excitation, the amplitude of PIA1 gradually enhance with delay time, which reaches the maximum together with the signal of SE2, suggesting that the PIA1 and SE2 should correspond to the same new transient species after photo-excitation. Meanwhile, the intensity of SE1 and the PIA2 both weaken with delay time. The PIA2 is overlapped with SE2, therefore the spectral feature at 500 nm gradually change from the positive region to the negative region after 1 ps time delay. After 20 ps, there are only PIA1 and SE2 in the TA spectra and both of them gradually weaken with delay time. The evolution of TA spectra of TCBM is also given in Fig. 2(C) and (D). The time-dependent spectral feature and the evolution of TCBM is much similar to those of CBM. This can be ascribed to similar molecular structure. In comparison with CBM, the spectral feature around 500 nm of TCBM quickly change from the positive region to the negative region after 0.6 ps. According to Fig.2, we believed that the tert-butyl unit is able to affect the relaxation rate of each channel in the relaxation process. To further clarify the dynamic relaxation mechanism of compound and make sure the role of tert-butyl unit, the kinetics recorded at significant wavelength of CBM and TCBM are given in Fig. 3(A) and (B), and the corresponding relaxation mechanism is summarized in Fig. 3(C). The initial relaxation lifetime (τv) recorded at 495 and 630 nm is much similar to each other in CBM and is estimated to be about 1.0 ps, which should be assigned to the vibration relaxation from the initial high energy excited state to the low energy excited state according to their ultrafast relaxation lifetime. In addition, the dynamic component with lifetime (τICT) of 9.1 ~ 9.4 ps appears in the TA curve at 425 nm and 558 nm is attributed to the intramolecular charge transfer (ICT) process, owing to the molecular structure of pull-push chromophore , . Moreover, the transition from low highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) which is mentioned in Fig. S1 also confirms our speculation. 425 nm 495 nm 0.04 (A) 0.02 T/T 558 nm 0.00 -0.02 -0.04 630 nm T/T 0.02 (B) 440 nm 505 nm 0.00 585 nm 656 nm -0.02 0 5 10 100 1000 Delay time/ps S1 (C) ICT kICT kV kr S0 Normalized T/T 0.02 (D) 0.00 -0.02 SEICT of CBM SEICT of TCBM -0.04 -2 0 2 4 6 8 10 500 1000 1500 2000 Delay time/ps Fig. 3: Kinetics recorded at significant wavelength of CBM (A) and TCBM (B). The relaxation mechanism of compounds (C) and the kinetic curves at SE peak of CBM and TCBM (D). k represents the relaxation rate. The dynamic component with lifetime of 9.1 ps can be also observed at 495 and 630 nm, suggesting that the population of ICT state is originated from the initial excited state (S1). During the ICT process, the population in ICT state gradually increases and the intensity of fluorescence from this intermediate state also enhance at the same time. When the ICT process finished, the intensity of fluorescence from ICT state begins to weaken. The relaxation of TCBM is similar to that of CBM, but the corresponding generation lifetime of ICT state in TCBM is about 6.7 ~ 6.8 ps. Moreover, relaxation lifetime of ICT state in CBM and TCBM is 865 ~ 970 ps and 247 ~ 305 ps, respectively. The tert-butyl unit is responsible for the difference between relaxation rates of CBM and TCBM, even though the dynamic behavior at different wavelength of TCBM are all similar to those of CBM, as seen Table 1. Table 1: Ultrafast dynamic relaxation fitting parameters for the significant wavelength. All the kinetic curves are fitted with three-exponential function. τv is the lifetime of vibration relaxation. τICT is the lifetime of rising or decreasing population which is corresponding to the intramolecular charge transfer progress. τr is the relaxation lifetime of intramolecular charge transfer state. Sample CBM TCBM λ/nm τv /ps τICT/ps τr /ps 425 1.3 9.1 872 495 1.0 9.2 974 558 1.4 9.4 865 630 1.0 9.1 940 440 1.0 6.7 247 505 1.2 6.2 305 585 1.0 6.7 249 656 1.1 6.3 288 In order to further discuss the difference of fluorescence behavior, the SE curve of CBM (at 558 nm) and TCBM (at 585 nm) are given in Fig. 3(D), showing that the relaxation rate of TCBM is rapider than that of CBM, which would be responsible for the difference of fluorescence curves between CBM and TCBM. After comparing the linear optical property of the two compounds, we also test their nonlinear optical property. We found that CBM would emit fluorescence when it is excited by 800 nm femtosecond laser. Therefore, the excitation intensity-dependent fluorescence spectra of CBM are given in Fig. 4(A), meanwhile the integrated fluorescence intensity vs the square of excitation intensity is also given in inset of Fig. 4(A).The integrated fluorescence intensity linearly increases with the square of excitation intensity, suggesting that the fluorescence of CBM excited 800 nm femtosecond laser is assigned to two-photon fluorescence. Therefore, the two-photon absorption cross section of CBM are detected by Z-scan technique. The nonlinear absorption coefficient (β) and the two-photon cross absorption section (σ) can be calculated using the same method as described in our previous work . The obtained β and σ values of CBM is 6.710−2 cm∙GW−1 and 5824 GM, respectively. Unfortunately, there is not signal of Z-Scan, when TCBM is excited by 800 nm femtosecond laser. This variance of nonlinear optical property should be assigned to the tert-butyl unit. 5 3 IPLI/10 3 PL intensity/10 1.6 (A) 1.4 0.470 J 0.904 J 1.2 1.104 J 1.0 1.466 J 1.807 J 0.8 0.6 0.4 0.2 0.0 400 500 2 1 0 0 1 2 3 2 2 (Pump Intensity) /J 600 700 800 900 1000 Normalized Transmittance/a.u. /nm 1.05 (B) 1.00 0.95 0.90 0.85 0.80 CBM 0.75 -5 -4 -3 -2 -1 0 1 2 3 4 5 Z/mm Fig. 4: (A) two-photon fluorscence of CBM; (B) the Z-scan curve of CBM. Insert: two-photon integrated fluorescence intensity as a function of square of excitation intensity. Fig. 5: The optimization structure of CBM in the ground state (A) and the excited state (B). Fig. 5 shows the structure of CBM in the ground state (A) and the excited state (B) which are calculated using Gaussian 09. HOMO and LUMO of CBM and TCBM are provided in Fig. S1. The CBM would twist in excited state. The dihedral angle between carbazole and difluoroboron β-diketonate (C11-N21-C22-C24) in excited state (90.04°) is bigger than that in ground state (49.71°). Therefore, it would be reasonable speculated that the tert-butyl unit linked on carbazole unit would affect the transition dipole through influencing the twist behavior of TCBM originated from the space steric effect. Therefore, the linear and nonlinear optical property of CBM and TCBM would have a huge difference. 4. Conclusions The linear and nonlinear optical properties of CBM and TCBM has been compared in detail, which confirms that the introduction of tert-butyl unit would lead to redshift of spectral feature, accelerate the fluorescence relaxation rate and weaken the two-photon optical property of compound. The TA data confirms that the ICT state exist in the photo-excitation relaxation process and the variance of fluorescence relaxation trace is originated from the change of fluorescence relaxation rate of ICT. 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