Tunable Electronic Structures and Optical Properties of Fluorenone

Jan 15, 2010 - Thiemo Gerbich , Jörg Herterich , Juliane Köhler , and Ingo Fischer. The Journal of Physical Chemistry A 2014 118 (8), 1397-1402...
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Tunable Electronic Structures and Optical Properties of Fluorenone-Based Molecular Materials by Heteroatoms Peng Song and Fengcai Ma* Department of Physics, Liaoning UniVersity, Shenyang 110036, People’s Republic of China ReceiVed: October 7, 2009; ReVised Manuscript ReceiVed: December 22, 2009

The ground-state and excited-state electronic structures as well as the tunable optical properties of a variety of newly designed fluorenone-based molecular materials have been theoretically investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The substitutes on the O atom in the carbonyl group of the fluorenone (FN) molecule with S (FNsCdS), Se (FNsCdSe), and Te (FNsCdTe) atoms can significantly influence their electronic structures, molecular orbitals, geometric conformations, and optical properties of fluorenone-based molecular materials. Due to the important difference of electronegativity for O, S, Se, and Te atoms in the same group, the ground-state dipole moment of these fluorenone-based molecular materials is gradually decreased in the order FN, FNsCdS, FNsCdSe, and FNsCdTe. At the same time, the ground-state bond length of the CdX (X refers O, S, Se, and Te) is gradually increased in the order of FN, FNsCdS, FNsCdSe, and FNsCdTe. Due to the different nature of the S1 state for FN (ππ* character) and FNsCdS, FNsCdSe, and FNsCdTe (σπ* character), the excitedstate dipole moment of FN in the S1 state is dramatically increased in comparison with that in the ground state; however, the excited-state dipole moments of FNsCdS, FNsCdSe, and FNsCdTe are significantly diminished. In addition, the excited-state bond length of CdX (X refers O, S, Se, and Te) in the S1 state is lengthened in comparison with that in the ground state due to the photoexcitation of the CdX bond FN, FNsCdS, FNsCdSe, and FNsCdTe. On the other hand, the energy level of the HOMO orbital is heightened and that of LUMO orbital is lowered with the introduction of heteroatoms in the order of S, Se, and Te. Consequently, the energy gap between LUMO and HOMO orbtials is gradually decreased in the order of the FN, FNsCdS, FNsCdSe, and FNsCdTe. Consequently, the calculated fluorescence wavelengths are strongly red-shifted from the visible region for FN to the near-infrared (NIR) region for FNsCdS, FNsCdSe, and FNsCdTe. These newly designed fluorenone-based molecules may be potential NIR fluorescent molecular functional materials. 1. Introduction Fluorenone-based materials as a type of important luminescent functional material have wide ranging applications in very diverse fields of current research from uses as photochemical sensitizers, bulk heterojunction solar cells, polymer and organic light-emitting diodes (PLEDs and OLEDs), etc.1-6 Mu¨llen and co-workers have reported the synthesis route, absorption, photoluminescence (PL), electroluminescence (EL) properties, and their applications on the PLED/OLED devices of the poly(fluorenone) materials.7-13 Recently, Demadrille et al. have conceptually designed and synthesized a series of four conjugated molecules consisting of a fluorenone central unit symmetrically coupled to different oligothiophene segments to provide new electroactive materials for application in bulk heterojunction solar cells.14-16 Grisorio et al. has performed an interesting study on the influence of keto groups on the optical, electronic, and electroluminescent properties of random fluorenone-containing poly(fluorenylenevinylene)s.17 Moreover, many wonderful studies on the nonlinear optical properties of a variety of organic molecular materials have been reported by Goodson and Twieg.18-20 At the same time, the photophysics and photochemistry of fluorenone (FN) and its derivatives have been widely investigated in the past decade.21-35 Fluorenone is very sensitive to * To whom correspondence should be addressed, [email protected].

both the intramolecular and intermolecular interactions, such as hydrogen bonding, polarity, steric interaction, etc.36-42 Biczok et al. have extensively reported on the photophysical properties of fluorenone with various substitutes at different sites of fluorenone molecule, and the intramoleuclar and intermolecular hydrogen bonding induced deactivation of photoexcited molecules as well as details of the quenching processes for the excited state.43-51 It should be noted that Zhao and co-workers have done benchmark studies in the important field of excitedstate hydrogen bonding.52-62 In particular, Zhao et al. have demonstrated for the first time that the excited-state hydrogen bonding of coumarin chromophore in protic solvents can be significantly strengthened upon photoexcitation, which has been a milestone for the investigation of the hydrogen bonding structures and dynamics in excited states.53 They also investigated the intramolecular and intermolecular hydrogen bonding in both the singlet and triplet electronic excited states of fluorenone and its derivates as well as some other important organic and biological chromophores in alcoholic solutions and their important roles on the excited-state photophysical processes of these chromophores, such as internal conversion (IC), intersystem crossing (ISC), twisted intramolecular charge transfer (TICT), etc.54-72 Moreover, incorporating heteroatoms into the molecular skeleton is an intriguing target because the introduction of heteroatoms can dramatically change the electronic structures

10.1021/jp909594e  2010 American Chemical Society Published on Web 01/15/2010

Fluorenone-Based Molecular Materials and photochemical properties of these heteroaromatic fluorenones. Zhao et al. have performed density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods to theoretically design and investigate the ground-state and excited-state electronic structures as well as the photochemical properties of unsubstituent perylene and a variety of heterocyclic annulated perylene (HAP) materials.62 As a result, the differences in electronic structures and photochemistry for these heterocyclic annulated perylenes have been theoretically predicted and explained.62 Furthermore, the theoretical prediction will be useful and helpful for the design and synthesis of new novel molecular materials. Therefore, we are motivated to theoretically investigate the ground-state and excited-state electronic structures and optical properties by incorporating heteroatoms into the fluorenone skeleton. As we have known, the carbonyl group plays a very important role in the photophysical and photochemical properties of fluorenone (FN) and its derivatives.43-51 Hence, the electronic structures and optical properties of FN and its derivatives can be tuned by incorporating heteroatoms into the carbonyl group of the fluorenone skeleton. In general, people would choose the elements of the same group to substitute the original atoms. Consequently, we are motivated to use the elements of group VI to substitute the O atom of the carbonyl group. Furthermore, the ground-state and excited-state electronic structures and optical properties of these newly designed fluorenone-based functional materials are also theoretically investigated using the DFT/TDDFT method.

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Figure 1. Optimized ground-state geometric conformations of fluorenone (FN) and various substituent fluorenones. Some important atoms are labeled.

TABLE 1: Computed Bond Lengths L (in Å) and Bond Angles A (in deg) as Well as Dipole Moments µ (in D) for the Ground-State and Excited-State Geometric Conformations of Fluorenone (FN) and Various Substituent Fluorenonesa FNsCdS

FNsCdSe

FNsCdTe

S0

FN S1

S0

S1

S0

S1

S0

S1

1.212 1.499 1.406 1.483 1.406 1.499 2.378 104.9 109.0 108.6 108.6 109.0 3.520

1.250 1.462 1.461 1.407 1.461 1.462 2.304 104.0 110.1 107.9 107.9 110.1 5.982

1.646 1.477 1.412 1.475 1.412 1.477 2.350 105.4 109.2 108.1 108.1 109.2 3.245

1.719 1.441 1.428 1.460 1.428 1.441 2.331 108.0 108.3 107.8 107.8 108.3 0.974

1.789 1.471 1.414 1.473 1.414 1.471 2.346 105.7 109.2 108.0 108.0 109.2 3.129

1.864 1.438 1.429 1.460 1.429 1.438 2.328 108.1 108.3 107.7 107.7 108.3 0.783

1.999 1.466 1.417 1.472 1.417 1.466 2.339 105.8 109.3 107.8 107.8 109.3 2.955

2.068 1.437 1.430 1.461 1.430 1.437 2.323 107.8 108.5 107.6 107.6 108.5 0.537

2. Theoretical Methods The ground-state geometry optimizations are performed using density functional theory (DFT) with Becke’s three-parameter hybrid exchange function with Lee-Yang-Parr gradientcorrected correlation functional (B3-LYP functional).73,74 The triple-ζ valence quality with one set of polarization functions (def-TZVP) is chosen as the basis sets throughout.75 The timedependent density functional theory (TD-DFT) method with B3LYP hybrid functional and the def-TZVP basis set is used to investigate the excited-state electronic structures. All the electronic excited-state geometric conformations are fully optimized using the TDDFT method. Fine quadrature grids, 4, were also employed.76,77 Both the convergence thresholds for the ground-state and excited-state optimization were set to be 10-7. All the quantum chemistry calculations are carried out using the Turbomole program suite.73-78 3. Results and Discussion Optimized ground-state geometric conformations of the fluorenone (FN) and substituent FN by S (FNsCdS), Se (FNsCdSe), and Te (FNsCdTe) are shown in Figure 1. In addition, some important atoms are labeled in Figure 1. It is distinct that all these fluorenone-based molecular materials are of planar conformations. However, their geometric conformations of the planar skeleton can be tuned by the incorporating heteroatoms. For example, the carbonyl group is clearly lengthened for FNsCdS, FNsCdSe, and FNsCdTe in comparison with that of FN. Moreover, geometric conformations of all these fluorenones and their derivatives in their first singlet excited (S1) states are fully optimized using the TDDFT method. These fluorenone-based molecular materials in the S1 state are also of planar conformations. Some bond lengths and bond angles as well as dipole moments for the fully optimized geometric conformations of FN and FNsCdS, FNsCdSe, and FNsCdTe in both the S0

LCdX LC-C1 LC1-C2 LC2-C3 LC3-C4 LC4-C LC1-C4 AC1CC4 ACC1C2 AC1C2C3 AC2C3C4 AC3C4C µ

a X refers to different heteroatoms in various substituent fluorenones.

and S1 states are listed in Table 1. From the listed bond lengths and bond angles, one can find that the bond length of C1-C2 is equal to that of C3-C4 in both the ground and excited states of all these fluorenone-based molecular materials. At the same time, the angles formed by CC1C2 and C1C2C3 are equal to those of C3C4C and C2C3C4, respectively. Thus, the geometric conformations of all these fluorenone-based molecular materials are of good symmetry. It is also noted that the original conformation of the FN skeleton can be changed by the substitutes of O atom in the carbonyl group with S, Se, and Te. The bond length of the carbonyl group is markedly increased via the substitutes by S, Se, and Te. Furthermore, the bond length of the carbonyl group is gradually increased in the order of FN, FNsCdS, FNsCdSe, and FNsCdTe due to the electronegativity reduction for the heavy elements in the same group. On the contrary, the bond length of C2-C3 and the distances between C1 and C4 are correspondingly decreased. In addition, the angles formed by C1CC4, CC1C2, and C3C4C are slightly enlarged by the substitutes of O atom of the carbonyl group with S, Se, and Te.

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Song and Ma

TABLE 2: The Lowest Unoccupied Molecular Orbital (LUMO) and Highest Occupied Molecular Orbital (HOMO) of all the Fluorenone-Based Molecular Materials

At the same time, the angles formed by C1C2C3 and C2C3C4 of all these fluorenone-based molecular materials are decreased in comparison with those of unsubstituent fluorenone. Upon photoexcitation to the S1 state, some of the excitedstate geometric conformations for these fluorenone-based molecular materials are changed in comparison with those in the ground state. One can note that the bond lengths of CdX (X refers S, Se, and Te) in the S1 state are remarkably lengthened, which resembles the case of the fluorenone. Therefore, it can be expected that the CdX bond of these molecules should be photoexcited. Moreover, distances of the C-C1, C-C4, C1-C4, and C2-C3 are significantly shortened upon excitation to the S1 state. While the distances of the C1-C2 and C3-C4 are lengthened in the S1 state. At the same time, the angles formed by C1CC4 of FNsCdS, FNsCdSe, and FNsCdTe are enlarged in the S1 state. The excited-state angles formed by CC1C2, C3C4C, C1C2C3, and C2C3C4 are slightly decreased in comparison with those in ground state of the substituent fluorenone-based molecules. It should be noted that the change tendency of the geometric conformations for the substituent fluorenone-based molecules is somewhat different from that for fluorenone. It indicates that the nature of the S1 state for FNsCdS, FNsCdSe, FNsCdTe may be different from the S1 state of FN. This can also be confirmed by the different change of the dipole moments between the S1 and S0 states. One can note that the ground-state dipole moments of all these fluorenone-based molecular materials are relatively small and gradually decreased in the order of FN, FNsCdS, FNsCdSe, and FNsCdTe due to the electronegativity reduction of the elements in group VI. Furthermore, it is distinct that the dipole moment of FN in the S1 state is dramatically increased to 5.982 from 3.520 D in the ground state. However, the excited-state dipole moments of FNsCdS, FNsCdSe, and FNsCdTe are significantly diminished and very close to zero. Table 2 shows the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of all these fluorenone-based molecular materials in order to delineate the nature of the S1 state which is dominantly related to the HOMO-LUMO orbital transition from our TDDFT results. One can note that all the LUMO orbitals of FNsCdS, FNsCdSe, and FNsCdTe are nearly the same as the LUMO orbital of FN. However, the HOMO orbitals of FNsCdS, FNsCdSe, and FNsCdTe are very different from that of FN. The most important difference is the populated electron densities on heteroatoms. The electron density of the HOMO orbital of FN is localized on the fluorene moiety. While the electron densities of HOMO orbitals of FNsCdS, FNsCdSe, and FNsCdTe are dominantly localized on the substituent heteroatoms. As a result, it is suggested that the S1 state of FN is

TABLE 3: Calculated Electronic Excitation Energies (in nm) and Corresponding Oscillator Strengths of FN, FNsCdS, FNsCdSe, and FNsCdTe (H and L Refer to HOMO and LUMO) FNa 392(0.004) H f L 98.5% λFlu: 521 S2 390(0.000) S3 305(0.016) S4 276(0.032) S5 255(0.000) S6 249(0.869) S7 242(0.043) S8 233(0.005) S9 226(0.003) S10 214(0.000) S1

a

FNsCdS

FNsCdSe

FNsCdTe

672(0.000) H f L 99.4% 781 496(0.000) 349(0.074) 338(0.193) 303(0.000) 280(0.075) 257(0.615) 253(0.013) 248(0.000) 237(0.002)

820(0.000) H f L 99.6% 957 529(0.000) 370(0.251) 362(0.081) 322(0.000) 293(0.051) 264(0.035) 261(0.000) 259(0.523) 253(0.003)

1041(0.000) H f L 99.8% 1198 574(0.000) 426(0.291) 379(0.087) 342(0.000) 309(0.022) 281(0.050) 279(0.005) 274(0.000) 262(0.388)

Taken from ref 53.

of ππ* character, while the S1 state of FNsCdS, FNsCdSe, and FNsCdTe is of σπ* nature. The different nature of the excited states of these fluorenone-based molecular materials can significantly influence their photochemical properties. In addition, the LUMO orbitals of all these molecules at the site of CdX bonds are antibonding orbitals. Thus, the CdX bonds in the S1 state are lengthened upon photoexcitation. Moreover, the LUMO orbitals at the sites of the C-C1, C-C4, and C2-C3 bonds are bonding orbitals and for C1-C2 and C3-C4 bonds are antibonding orbitals. So bond lengths of C-C1, C-C4, and C2-C3 are significantly shortened and the C1-C2 and C3-C4 bonds are lengthened in the S1 state. These are well consistent with the calculated geometric results. The calculated electronic excitation energies and the corresponding oscillator strengths of FN, FNsCdS, FNsCdSe, and FNsCdTe are shown in Table 3. In addition, the calculated emission wavelengths from the S1 state of all these molecules are also listed here. To distinctly describe the spectral characters of these fluorenone-based molecular materials, the absorption and fluorescence spectra of FN, FNsCdS, FNsCdSe, and FNsCdTe are calculated and presented together in Figure 2. One can note that all the absorption spectra have a strong absorption band at the range from 200 to 300 nm. At the same time, their key characters are very similar to each other. Thus, this absorption band should originate from the fluorene moiety of these fluorenone-based molecular materials. Furthermore, the absorption wavelength at this band is red-shifted from the 249 nm of FN to 262 nm of FNsCdTe. It should be noted that the absorption band of lower excitation energy for FN is very weak. However, a new strong absorption peak at this band for

Fluorenone-Based Molecular Materials

J. Phys. Chem. A, Vol. 114, No. 5, 2010 2233 with the marked red-shift of the absorption and fluorescence spectra for these fluorenone-based molecular materials. It is evident that the introduction of heteroatoms in group VI into the carbonyl group of fluorenone can induce the absorption and fluorescence spectral shift to the near-infrared (NIR) region. Therefore, photochemists may be inspired from this to design and synthesize NIR fluorescent molecular functional materials by the introduction of heteroatoms in group VI into the original molecular skeletons. 4. Conclusions

Figure 2. Calculated absorption and fluorescence spectra of all fluorenone-based molecular materials.

Figure 3. Calculated energy levels of LUMO and HOMO orbitals for all fluorenone-based molecular materials.

FNsCdS, FNsCdSe, and FNsCdTe appears. These absorption peaks should be contributed by the substituent carbonyl groups. It is consistent with the molecular orbital analysis above. Moreover, the peak value is also red-shifted in the order of FNsCdS, FNsCdSe, and FNsCdTe. The absorption spectral red-shift of these fluorenone-based molecules suggests that the substitution of the O atom of the carbonyl group by S, Se, and Te can significantly decrease their energy gap between HOMO and LUMO orbitals. As a result, the calculated fluorescence peaks are remarkably red-shifted from the visible region (521 nm for FN) to the near-IR region (781 nm for FNsCdS, 957 nm for FNsCdSe, and 1198 nm for FNsCdTe). Figure 3 presents the calculated energy levels of LUMO and HOMO orbitals for all fluorenone-based molecular materials. At the same time, the energy gap between LUMO and HOMO orbitals is also shown. It is distinct that the energy levels of HOMO and LUMO orbitals as well as their energy gap can be remarkably tuned by these heteroatoms. One can note that the calculated energy level of the HOMO orbital for FN, FNsCdS, FNsCdSe, and FNsCdTe is -6.48, -6.14, -5.86, and -5.59 eV, respectively. Hence, the HOMO energy level is heightened with the introduction of heteroatoms in the order of S, Se, and Te. However, the energy level of LUMO orbital for FN, FNsCdS, FNsCdSe, and FNsCdTe is calculated to be -2.51, -3.14, -3.28, and -3.45 eV, respectively, which is lowered in the order of O, S, Se, and Te. Consequently, the energy gap between LUMO and HOMO orbtials for FNsCdS, FNsCdSe, and FNsCdTe will be significantly decreased in comparison with that of FN. The calculated energy gap between LUMO and HOMO orbtials of FN is 3.97 eV, while the energy gaps of FNsCdS, FNsCdSe, and FNsCdTe decreased to 3.00, 2.58, and 2.14 eV, respectively. This is very consistent

In this work, the ground-state and excited-state electronic structures as well as the tunable optical properties of a variety of fluorenone-based molecular materials have been theoretically investigated using the density functional theory (DFT) and timedependent density functional theory (TDDFT) methods. The ground-state and excited-state geometric conformations for all these fluorenone-based molecular materials are fully optimized using the B3LYP/def-TZVP and the TD-B3LYP/def-TZVP methods, respectively. Both the ground-state and excited-state geometric conformations of fluorenone can be significantly changed by the substitutes of the O atom in the carbonyl group with S, Se, and Te. The ground-state bond length of the carbonyl group is gradually increased in the order of FN, FNsCdS, FNsCdSe, and FNsCdTe due to the electronegativity reduction for the heavy elements in the same group. In addition, the excited-state bond length of CdX (X refers O, S, Se, and Te) in the S1 state is remarkably lengthened in comparison with that in the ground state due to the photoexcitation of the CdX bond FN, FNsCdS, FNsCdSe, and FNsCdTe. On the other hand, the ground-state dipole moments of all these fluorenonebased molecular materials are relatively small and gradually decreased in the order of FN, FNsCdS, FNsCdSe, and FNsCdTe due to the electronegativity reduction of the elements from O to Te. Furthermore, it is distinct that the dipole moment of FN in the S1 state is dramatically increased in comparison with that in the ground state. However, the excitedstate dipole moments of FNsCdS, FNsCdSe, and FNsCdTe are significantly diminished. This should be ascribed to the different nature of the S1 state between FN and the substituent fluorenones. From the molecular orbital analysis, it is indicated that the S1 state of FN is of ππ* character, while the S1 state of FNsCdS, FNsCdSe, and FNsCdTe is of σπ* nature. Furthermore, the LUMO orbitals of all these fluorenone-based molecular materials at the site of CdX (X refers O, S, Se, and Te) bonds are antibonding orbitals, which is attributed to the lengthened CdX bonds in the S1 state upon photoexcitation compared with that in the ground state. At the same time, the energy level of the HOMO orbital is heightened and the energy level of the LUMO orbital is lowered with the introduction of heteroatoms in the order of S, Se, and Te. Consequently, the energy gap between LUMO and HOMO orbtials for FNsCdS, FNsCdSe, and FNsCdTe will be significantly decreased in comparison with that of FN. Both the different nature of the excited states and the remarkable decrease of the energy gap between LUMO and HOMO orbtials for these fluorenone-based molecular materials can drastically influence their photochemical properties. The newly appeared strong absorption peak of lower excitation energy for FNsCdS, FNsCdSe, and FNsCdTe can be contributed by the substituent carbonyl groups. Moreover, the fluorescence peaks are strongly red-shifted from the visible region for FN to the near-infrared (NIR) region for FNsCdS, FNsCdSe, and FNsCdTe because of the remarkable decrease of the energy gap between LUMO and HOMO orbtials.

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