ARTICLE pubs.acs.org/JPCA
Toward Panchromatic Organic Functional Molecules: Density Functional Theory Study on the Electronic Absorption Spectra of Substituted Tetraanthracenylporphyrins Dongdong Qi and Jianzhuang Jiang* Department of Chemistry, University of Science and Technology Beijing, Beijing 100083, China
bS Supporting Information ABSTRACT: To achieve full solar spectrum absorption of organic dyes for organic solar cells and organic solar antenna collectors, a series of tetraanthracenylporphyrin derivatives including H2(TAnP), H2(α-F4TAnP), H2(β,β0 -F8TAnP), H2(γ,γ0 -F8TAnP), H2(δ,δ0 F8TAnP), H2[α-(NH2)4TAnP], H2[β,β0 -(NH2)8TAnP], H2[γ,γ0 -(NH2)8TAnP], and H2[δ,δ0 -(NH2)8TAnP] was designed and their electronic absorption spectra were systematically studied on the basis of TDDFT calculations. The nature of the broad and intense electronic absorptions of H2(TAnP) in the range of 5001700 nm is clearly revealed, and different types of π f π* electronic transitions associated with different absorption bands are revealed to correspond to different electron density moving direction between peripherally fused 14-electron-π-conjugated anthracene units and the central 18-electron-π-conjugated porphyrin core. Introduction of electron-donating groups onto the periphery of the H2(TAnP) macrocycle is revealed to be able to lead to novel NIR dyes such as H2[α-(NH2)4TAnP] and H2[δ,δ0 -(NH2)8TAnP] with regulated UVvisNIR absorption bands covering the full solar spectrum in the range of 3002400 nm.
’ INTRODUCTION Near-IR (NIR) dyes have received considerable attention in recent years due to their potential applications in organic solar cells,1 photodynamic therapy for cancer,2 NIR imaging,3 and organic solar antenna collectors.4 Tetrapyrrole derivatives, in particular, porphyrins, phthalocyanines, and naphthalocyanines, are among the most important functional molecular materials with intense NIR absorption due to their high photochemical stability and intense absorption in the NIR region.58 Very lately a new “molecular graphene” type of tetrapyrrole derivatives, tetraanthracenylporphyrins (TAnPs), with significantly intensified NIR absorption at 1417 nm (ε = 1.2 105 M1 cm1) was reported.8 Nevertheless, the very intense absorptions covering an astonishing broad UV, visible, and NIR range from 300 to 1600 nm revealed in the electronic absorption spectrum of these novel TAnPs suggest their great potential applications in different fields including organic solar antenna collectors and organic solar cells.6,9,10 Investigations in inorganic solar antenna collectors have made significant progress.11 Actually a multilayered inorganic solar antenna collector with absorption ability as high as nearly 90% of the full solar energy was reported.12 In addition, a solar cell fabricated from inorganic semiconductors can also reach a high photoelectron transition efficiency of ∼75% due to the intense absorption over the full UVvisNIR region of the solar spectrum.13 However, exploration in molecular functional materials with application in organic solar antenna collectors14 and/or organic solar cells10 appears to be still limited in the UVvis r 2011 American Chemical Society
region of the solar spectrum without extending into the NIR region. Taking account of the quite high ratio of the NIR region emission over the total solar energy,6 43%, easily synthesized, stable organic NIR dyes become highly desired toward highefficiency solar antenna collectors and solar cells.9,10 Recently, an inorganic band anticrossing full solar spectrum absorption material GaNxAs1x was prepared by Lopes and coworkers.1b The main point of this work involves inserting an intermediate energy band between the former valence band (VB) and the conduction band (CB), leading to the very narrow gaps between them and resulting in the obvious absorption in the NIR region.1b In a similar manner, in the case that more extraneous orbitals are inserted into the original frontier orbitals of specific organic dyes with conjugated molecular structure, novel dyes with absorption extending into the NIR region are also expected to improve the efficiency of organic solar antenna collectors and solar cells.1e Significant interaction exists between the central 18-electronπ-conjugated porphyrin core and the 4 peripherally fused 14electron-π-conjugated anthracene units in the TAnP skeleton, leading to densely distributed frontier orbitals and narrower gaps between these orbitals. This in turn results in remarkable redshifted broad Q bands into the NIR region with a complicated electron absorption spectrum of tetraanthracenylporphyrins Received: September 16, 2011 Revised: October 25, 2011 Published: October 27, 2011 13811
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Scheme 1. Molecular Structures of H2(TAnP) and Its Derivatives
(TAnPs) relative to almost all of the monomeric tetrapyrrole derivatives such as porphyrins (Pors),15,16 tetraazaporphyrins (TAPs),16 phthalocyanines (Pcs),16,17 naphthalocyanines (Ncs),16,18 anthracocyanine (Acs),19 azulenocyanines (Azcs),6 and even many core-modified porphyrins.20 In addition, the absorption spectrum of some sandwich-type multi(tetarpyrrole) metal multiple-decker complexes can also cover such a broad near-IR region. However, the absorption intensity in this region is much weaker than TAnPs.21 Osuka and co-workers synthesized a series of fully conjugated porphyrin tapes and porphyrin arrays also with a broad and an intense NIR electronic absorption in their spectrum, which intensely increase the utilization of solar energy.1a,22 However, difficulty in their large scale of preparation appears to preclude their wide range of applications in comparison with TAnPs as a simple molecule, revealing the great application potential of TAnPs as a new skeleton of organic NIR dyes. In the present work, the electronic structures and electronic absorption spectra of a series of H2(TAnP) have been theoretically investigated on the basis of density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. Introduction of electron-withdrawing or electron-donating groups onto the periphery of the TAnP skeleton was revealed to further tune the HOMOLUMO gap, inducing an obvious red/blue shift of the NIR electronic absorption bands of H2[α-(NH2)4TAnP]/H2[δ,δ0 -(NH2)4TAnP] into the range of 10002400 nm.
’ COMPUTATIONAL DETAILS The hybrid-generalized gradient approximation (hGGA) method B3LYP is proved suitable for geometry optimization of porphyrins, phthalocyanines, as well as their various analogues.23
However, B3LYP always overestimates the transition energies for this type of large conjugated system, while the generalized gradient approximation (GGA) method significantly improves the agreement between theoretical and experimental results.6,24 In addition, Pople basis sets proved excellent for calculating the structure and properties of porphyrins and phthalocyanines as well as their analogues,25 leading to many applications in this field.26 DFT and TDDFT calculations were carried out using the Gaussian 03 program27 on an IBM P690 system housed at the Shandong Province High Performance Computing Center. Geometry Optimization and Electronic Structure Calculation. The density functional theory (DFT) method of hybrid B3LYP with the Becke three-parameter exchange functional28 and LeeYangParr correlation functional29 were used to simulate the molecular structures of the series of TAnP derivatives including H2(TAnP), H2(α-F4TAnP), H2(β,β0 -F8TAnP), H2(γ,γ0 -F8TAnP), H2(δ,δ0 -F8TAnP), H2[α-(NH2)4TAnP], H2[β,β0 -(NH2)8TAnP], H2[γ,γ0 -(NH2)8TAnP], and H2[δ,δ0 (NH2)8TAnP], Scheme 1. Comparative molecular structural calculations on the B3LYP/6-311++G(d,p) and B3LYP/6-31G(d) levels were carried out for H2(TAnP). The largest difference between the bond lengths of the two levels is less than 0.002 Å, indicating that the 6-31G(d) basis set used in this work is accurate enough. For the reason of time efficiency, molecular structural calculations on other compounds are all performed at the B3LYP/6-31G(d) level. The Berny algorithm using redundant internal coordinates30 was employed in energy minimization, and the default cutoffs were used throughout. Electronic Absorption Spectrum. The time-dependent density functional theory (TDDFT) method of BP86 with the Becke88 exchange functional31 and Perdew86 correlation functional32 was used xto simulate the electronic absorption spectra. The UVvisNIR spectra were calculated using the 13812
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Scheme 2. Distortion Angle α of TAnPs
Gaussian distribution model with SWizard 4.6.33 The simulated continuous UVvisNIR spectrum is calculated by ε(ω) = ∑I (fI)/(Δ21/2,I)1/2 exp[B(ω ωI)2/(Δ21/2,I)1/2], where ε(ω) is the simulated relative extinction coefficient, Δ1/2,I the half-bandwidth defined as 500 cm1, fI the calculated oscillator strength, ωI the electron transition energy, and B defined as 2.773 here.33 The total integrated intensity under the absorption profile is proportional to the sum of the oscillator strengths, R ε(ω)dω µ ∑IfI. Photon-Induced Electron Density Movement. The total electron density difference between the ground and the excited states (∑ fmfn) is calculated by the molecular orbital electron density difference fmfn = [(cmfn2)/(∑ cmfn2)](Fn Fm), where Fn and Fm are the electron density of the two molecular orbitals relative to the electron transition model of MO(m) f MO(n), cmfn is the orthogonal coefficient in the TD-DFT equation, and (cmfn2)/(∑ cmfn2) can be considered as the contribution of this electron transition model to this absorption peak.6 The electron density difference between the ground and the excited states is the linear combination of various electron transition models. Also, for the reason of time efficiency, only the electron transition models with the configuration larger than 5.0% are taken into account. The electron density difference map is plotted using the isovalue of 4.0 104 au. Noncovalent Interaction. The noncovalent interaction is defined by the reduced density gradient (RDG, SRDG) at the level of B3LYP/6-31++G(d,p), which is calculated by SRDG = (1/(2(3π2)1/3))((|3F|)/F4/3), where F is the electron density of the whole system. The interaction type is identified as Ω = Sign(λ2)F, where Sign(λ2) is the sign of the second largest eigenvalue of the electron density Hessian matrix. According to Johnson and Yang0 s theory, Ω < 0.01, Ω > 0.01, and 0.01 < Ω < 0.01 indicate an attractive interaction (such as dipoledipole or hydrogen bonding), nonbonding interaction, and van der Waals interaction, respectively.34 The scheme of SRDGΩ is plotted using MultiWFN 2.01.35 The reduced density gradient map is plotted using VMD 1.8.736 with an isovalue of 0.60 au.
Localized Orbital Locator and Magnetically Induced Ring Current Density. The localized orbital locator (LOL) is a
function for locating high-localization regions, which is defined 5/3 as LOL(r B) )/((1/2)B) =2 (t(r B))/(1 + t(r B)), t(r B) = CF(F(r ∑iηi|3ji(r B)| ), and ji(r B) = ∑lCl,iχl(r B), where the coefficient CF = (3(9π4)1/3)/10 is the Fermi constant, ηi the occupation number of orbital i, Cl,i the element of coefficient matrix, and χl(r B) the corresponding basis function to Cl,i.35,37 The magnetically induced ring current density is calculated at the level of B3LYP/6311+g(2df,p) using the Gauge-independent atomic orbital (GIAO) method.27,38
’ RESULTS AND DISCUSSION The structure of metal-free tetraanthracenylporphyrin [H2(TAnP)] is shown in Scheme 1, which can be considered as the fusing combination of porphyrin (central core) and anthracene (peripheral substituents). As can be found, there are four types of substituent sites α, β, γ, and δ. NH2 and F are chosen as representative of electron-donating and -withdrawing groups, respectively,6,39,40 leading to the tetraanthracenylporphyrin derivatives, including H2(TAnP), H2(α-F4TAnP), H2(β,β0 -F8TAnP), H2(γ,γ0 -F8TAnP), H2(δ,δ0 -F8TAnP), H2[α(NH2)4TAnP], H2[β,β0 -(NH2)8TAnP], H2[γ,γ0 -(NH2)8TAnP], and H2[δ,δ0 -(NH2)8TAnP]. In line with the previous experimental and thoeratical studies,21b,23d defining the mean plane formed by the four inner nitrogen atoms named as Plane N4, the distortion degree is then measured by the angle θ between Plane N4 and the average plane of one of the four peripheral conjugated anthracene units, Scheme 2. Molecular Structure and Electronic Structure. Figure 1a and 1b shows the optimized molecular structure of metal-free tetraanthracenylporphyrin [H2(TAnP)] together with a scheme indicating that fusion between the four 14-electron-π-conjugated anthracene units (πAn14 units) and a central 18-electron-πconjugated porphyrin core (πPor18 core) forms the tetraanthracenylporphyrin. As shown in Figure 1b, the magnetically induced 13813
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Figure 2. Reduced density gradient (RDG) model.
Figure 1. Uniform TAnP conjugated skeleton. (a) Conjugated model of TAnP skeleton. (b) Magnetically induced ring current density (isovalue = 1.0 104 au). (c) Localized orbital locator (LOL) analysis for all occupied π orbitals of H2(TAnP).
Gauge-independent atomic orbital (GIAO) current density reveals the uniform noncoplanar conjugated system πTAnP82 including all 80 C/N atoms and 82 π electrons, which therefore can be considered as a “molecular graphene”.8 Figure 1c shows the localized orbital locator (LOL) of all the π orbitals of the TAnP skeleton. In the LOL scheme, the π electron density is significantly localized between the anthracene and the porphyrin units, confirming that fusion of the central πPor18 core and four πAn14 units leads to the uniform noncoplanar conjugated system πTAnP82 in the TAnP skeleton. In comparison with the anthracene 14-electron conjugated system and porphyrin 18-electron conjugated system, this type of tetraanthracenylporphyrin with the 82-electron conjugated system provides much more densely distributed frontier orbitals, which in turn results in many narrow gaps between frontier molecular orbitals. This actually just resembles the inorganic full solar spectrum absorption material GaNxAs1x reported recently as mentioned above.1b Briefly summarizing the above, bond interaction analysis for TAnP shows that fusion of the central πPor18 core and four πAn14 units forms the conjugated system in TAnP and results in the densely distributed TAnP frontier orbitals. The nonbond interaction analysis of TAnP is shown in Figure 2. As can be found, due to the steric hindrance of the peripheral anthracene units as well as the δ,δ0 -H atoms, H2(TAnP) employs a saddle structure instead of the planar structure as for metal-free porphyrin [H2(Por)] and anthracene (An), therefore leading to a distorted πTAnP82 system. It is worth pointing out that the balance between the dominant repulsive force (between the δ,δ0 -H atoms of the peripheral anthracene units) and the πTAnP82 planar tendency leads to the saddle H2(TAnP) structure. As the substituted TAnPs are concerned, the peripheral substituents have an influence on the distortion
angle. As can be found in Scheme 2, the distortion angle θ increases from 14.6° for H2(α-F4TAnP), 14.7° for H2(β,β0 F8TAnP), 14.9° for H2(γ,γ0 -F8TAnP), to 15.2° for H2(δ,δ0 F8TAnP), and similarly from 14.0° for H2[α-(NH2)4TAnP], 14.1° for H2[β,β0 -(NH2)8TAnP], 14.5° for H2[γ,γ0 -(NH2)8TAnP], to 15.9° for H2[δ,δ0 -(NH2)8TAnP], indicating the most intense distortion degree of H2[δ,δ0 -(NH2)8TAnP]/H2(δ,δ0 F8TAnP) since the volume of NH2/F is bigger than H. These results reveals that the steric hindrance of H2[δ,δ0 -(NH2)8TAnP]/H2(δ,δ0 -F8TAnP) is higher than other analogues. Nature of the Broad Electronic Absorption of H2(TAnP). Figure 3 shows the simulated electronic absorption spectrum of H2(TAnP), which can be divided into three regions due to the different electron transition models, Table S1 (Supporting Information). In region NIR-1, an astonishingly red-shifted NIR absorption band from 1300 to 1600 nm was observed, which corresponds well with the experimental band.8 Absorptions in this region are mainly due to the electron transitions of HOMO f LUMO/ LUMO+1, which are assigned to the Q bands of H2(TAnP).39 According to TDDFT, the Q bands are assigned as L þ 1ðb3 Þ
Φ1512nm ≈0:672ΨHðb2 Þ 1B 1
L þ 2ðaÞ
Lðb Þ
1 þ 0:204ΨH-2ðaÞ
Hðb Þ
þ 0:154ΨH-1ðb1 Þ 0:146ΨL þ21ðb3 Þ Lðb Þ
Hðb Þ
L þ 1ðb3 Þ
Φ1436nm ≈0:695ΨHðb12 Þ 0:183ΨLðb12Þ 0:149ΨH-2ðaÞ 1B 3
L þ 2ðaÞ
þ 0:129ΨH-4ðb3 Þ
It is noteworthy that the excited states with |cmfn| < 0.1 are ignored in the TD-DFT transition equations since the electron transition model of MO(m) f MO(n) does not have an obvious effect on the TD-DFT result when (c mfn 2 )/(∑ c mfn 2 ) < 0.02. As shown in Figure 3, the LUMO/LUMO+1 energy is 3.44/3.42 eV for H2(TAnP), while the HOMO energy is 13814
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Figure 3. Simulated UVvisNIR spectrum and main electron transition models for H2(TAnP) (red, region NIR-1; blue, region NIR-2; green, region vis).
as high as 3.95 eV, leading to the very narrow gap between HOMO and LUMO/LUMO+1 of 0.51/0.53 eV. The HOMO LUMO/LUMO+1 gaps for H2(TAnP) are much lower in comparison with other tetrapyrrole counterparts such as 2.93 and 2.91 eV for porphyrin, 2.73 and 2.63 eV for tetraazaporphyrin, 2.19 and 2.15 eV for phthalocyanine, 1.92 and 1.78 eV for napthalocyanine, 0.86 and 0.96 eV for anthracocyanine, and 0.84 and 0.91 eV for azulenocyanine.6,7a,19a This clearly explains the significantly red-shifted nature of these bands in comparison with those for other tetrapyrrole compounds. An intense NIR absorption band from 700 to 1000 nm was observed, which also corresponds well with the experimental result.8 This band is mainly attributed to the electronic transitions of HOMO-2/HOMO-3/HOMO-4 f LUMO/LUMO+1 and assigned to the Soret bands of H2(TAnP).39 According to TDDFT, the Soret band is assigned as Lðb1 Þ ≈0:519ΨH-3ðb Φ902nm 1B 2Þ 3
L þ 1ðb Þ 0:467ΨH-2ðaÞ 3
Lðb Þ
L þ 1ðb Þ
L þ 1ðb3 Þ
Lðb Þ
L þ 1ðb Þ
L þ 1ðb1Þ
1 Φ861nm ≈0:575ΨH-2ðaÞ þ 0:371ΨH-3ðb2 Þ3 0:155ΨHðb2 Þ 1B 1
3 Φ839nm ≈0:500ΨH-2ðaÞ þ 0:458ΨH-3ðb2 Þ1 0:166ΨHðb2 Þ 1B 3
L þ 1ðb Þ
L þ 2ðaÞ
Lðb Þ
1 Φ795nm ≈0:448ΨH-3ðb2 Þ3 0:419ΨH-1ðb1 Þ 0:242ΨH-2ðaÞ 1B 1
L þ 1ðb3 Þ
þ 0:217ΨHðb2 Þ
Lðb Þ
Hðb Þ
1 þ 0:146ΨH-6ðaÞ 0:107ΨL þ21ðb3 Þ
L þ 2ðaÞ
Φ793nm ≈ 0:655ΨH-4ðb3 Þ 1B 3
Lðb Þ
L þ 1ðb3 Þ
1 þ 0:168ΨH-5ðb 0:132ΨH-6ðaÞ 2Þ
Lðb Þ
0:129ΨHðb12 Þ
Lðb Þ
1 þ 0:104ΨH-3ðb 2Þ
Lðb Þ
1 Φ755nm ≈ 0:460ΨH-4ðb 1B 3Þ 2
L þ 1ðb Þ
L þ 2ðaÞ
L þ 2ðaÞ
Hðb Þ
L þ 2ðaÞ
þ 0:338ΨH-1ðb1 Þ3 0:330ΨH-3ðb2 Þ 0:221ΨHðb2 Þ þ 0:131ΨH-5ðb2 Þ þ 0:101ΨL þ22ðaÞ
As shown in Figure 3, the gaps between HOMO-2/HOMO-3/ HOMO-4 and LUMO/LUMO+1 are still only in the range of 1.11.5 eV for H2(TAnP), leading to the significantly red-shifted Soret band in the range of 7001000 nm for H2(TAnP). The result indicates the intense influence of the peripheral πAn14 units on the electronic structure of the central πPor18 core. The bands in region vis (from 500 to 700 nm) are mainly attributed to the electron transitions of HOMO-6 f LUMO/ LUMO+1. As displayed in Figure 3, HOMO-6 mainly locates on the peripheral πAn14 units, indicating the origin of the new absorption band associated with introduction of the peripherally fused conjugated system. Moreover, the absorption region from 500 to 700 nm in the electronic absorption spectrum is occupied by this new type of transition from the peripherally fused conjugated πAn14 systems to the central πPor18 core. According 13815
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Table 1. Electron Density Difference Plots of Electron Transitions of H2(TAnP) (isovalue 2.0 104 e 3 au3)a
a Electron densities move from the green area to the blue area. Excited states with less than 17 000 cm1 and configurations which contribute more than 5% are shown (assignment: H = HOMO, L = LUMO, L+1 = LUMO+1, H-1 = HOMO-1, etc.).
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Figure 4. Simulated UVvisNIR spectra for H2(TAnP) and its derivatives with peripheral electron-donating and electron-withdrawing groups.
to TD-DFT theory, this new band is assigned as Lðb1 Þ Φ616nm ≈0:670ΨH-6ðaÞ 1B 1
L þ 2ðaÞ 0:120ΨH-7ðb1 Þ
L þ 1ðb3 Þ
Φ599nm ≈0:637ΨH-6ðaÞ 1B 3
L þ 2ðaÞ
L þ 2ðaÞ 0:103ΨH-1ðb1 Þ
Lðb Þ
1 0:202ΨH-5ðb2Þ
Lðb Þ
þ 0:161ΨH-4ðb3 Þ 0:110ΨHðb12 Þ The maps of the electron difference between the ground state and the exited states are shown in Table 1. Obviously, the electronic absorption bands in region NIR-1, region NIR-2, and region vis are due to the electron density movements between the x axis and the y axis, from the peripheral πAn14 units to the central πPor18 core, and from the central πPor18 core to the peripheral πAn14 units, respectively. As a total result, the UVvis absorption bands of H2(TAnP) actually originate from various types of π f π* electron transitions among the frontier molecular orbitals. Influence of Electron-Donating and Electron-Withdrawing Groups. To reveal the influence of the electron-donating
groups attached at the periphery of the H2(TAnP) macrocycle skeleton on the NIR electronic absorption spectra of tetraanthracenylporphyrin derivatives, H2[α-(NH2)4TAnP], H2[β,β0 (NH2)8TAnP], H2[γ,γ0 -(NH2)8TAnP], and H2[δ,δ0 -(NH2)8TAnP] were calculated at the same BP86/6-31G(d)//B3LYP/631G(d) level.40 As exhibited in Figures 4 and 5, the NIR absorption bands of H2[α-(NH2)4TAnP] are significantly red shifted due to introduction of four strongly electron-donating NH2 groups onto the periphery of TAnP macrocycle at the α positions while significantly blue shifted due to introduction of the 8 strongly electron-donating NH2 groups onto the periphery of TAnP macrocycle at the δ positions. However, the Q bands of H2[β,β0 -(NH2)8TAnP] and H2[γ,γ0 -(NH2)8TAnP] are similar to H2(TAnP), indicating the slight substituent effect at the β/γ positions. According to our calculation results, the frontier orbital energies are also significantly affected by incorporating peripheral NH2 groups. The HOMOLUMO gap decreases to 0.35 eV for H2[α-(NH2)4TAnP], while it increases to 0.69 eV for 13817
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half-bandwidth of 500 cm1 is concerned in order to simulate the real absorption, the NIR absorption band for H2[α-(NH2)4TAnP] and H2[δ-(NH2)4TAnP] will cover the region of 15002500 and 10001500 nm, respectively, which are obviously different from the H2(TAnP) NIR absorption band in the range of 13001700 nm. This reveals that introduction of electrondonating groups onto different positions of the periphery of TAnP ring is an effective way to generate novel near-IR (NIR) dyes with potential applications in different fields such as organic solar antenna collectors and solar cells.9,10,14
Figure 5. Frontier molecular orbitals for H2(TAnP) and its derivatives with peripheral electron-donating and electron-withdrawing groups.
H2[δ,δ0 -(NH2)8TAnP], leading to the red-shifted NIR absorption bands at 1846 and 1717 nm for H2[α-(NH2)4TAnP] and blue-shifted NIR absorption bands at 1120 and 1177 nm for H2[δ,δ0 -(NH2)8TAnP]. In addition, the HOMOLUMO gap remains almost unchanged at 0.50 eV for H2[β,β0 -(NH2)8TAnP] and H2[γ,γ0 -(NH2)8TAnP], similar to that for H2(TAnP) of 0.51 eV, leading to the Q bands in the similar region for H2(TAnP), H2[β,β0 -(NH2)8TAnP], and H2[γ,γ0 -(NH2)8TAnP]. This is also true for H2(α-F4TAnP), H2(β,β0 -F8TAnP), H2(γ,γ0 -F8TAnP), and H2(δ,δ0 -F8TAnP) incorporating the peripheral electron-withdrawing F atoms.40 As shown in Figures 4 and 5, the NIR absorption bands of H2(α-F4TAnP) are red shifted due to introduction of four intense electron-withdrawing F atoms onto the periphery of the TAnP ring at the α positions due to the decreased HOMOLUMO gap of 0.47 eV while being blue shifted due to introduction of eight intense electronwithdrawing F atoms onto the periphery of the TAnP ring at the δ positions due to the increased HOMOLUMO gap of 0.62 eV. In addition, the HOMOLUMO gap of H 2 (β,β 0 -F 8 TAnP) and H2(γ,γ0 -F8TAnP) is quite similar to that of H2(TAnP), resulting in the Q bands for the three derivatives locating also in a similar region. This result confirms the slight substituent effect at the β/γ positions on the HOMOLUMO gap and NIR bands. Nevertheless, the substituent effect of electron-withdrawing groups is revealed to be much slighter than that of electrondonating groups, Figure 4. In summary, the HOMOLUMO gap changes significantly along with introduction of electron-withdrawing and in particular electron-donating substituents at the α/δ positions, leading to regulation of NIR absorption by the positions of peripheral substituents. In the case that the Gaussian band model with the
’ CONCLUSION The nature of the broad and intense NIR absorptions of H2(TAnP) derivatives is revealed on the basis of DFT and TDDFT calculations, and the electron density moving direction between peripherally fused 14-electron-π-conjugated systems and the 18-electron-π-conjugated core due to different types of electron transitions (actually associated with different absorption bands) has also been clarified. Theoretical calculation results indicate that the astonishing narrow HOMOLUMO gap is responsible for the significant red shift of the Q bands of TAnPs. A new absorption band appearing in region vis of TAnPs is considered as the result of electron density transfer from the peripherally fused anthracene rings to the 18-electron-πconjugated porphyrin core. Introduction of electron-donating groups onto the periphery of anthracene ring, in particular, the α/δ positions, is able to lead to novel NIR dyes with regulated red-shifted NIR absorption bands covering the full solar spectrum. ’ ASSOCIATED CONTENT
bS
Supporting Information. Map of frontier orbitals of H2(TAnP), H2[α-(NH2)4TAnP], and H2[δ,δ0 -(NH2)4TAnP]; frontier orbital coupling of H2(TAnP); reduced density gradient (RDG) model and ΩSRDG scheme for H2(TAnP); calculated transition energies, oscillator strength (f), and configurations for all the molecules. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT Financial support from the Natural Science Foundation of China, Fundamental Research Funds for the Central Universities of Ministry of Education of China, Beijing Municipal Commission of Education, Fundamental Research Funds for the Central Universities, and University of Science and Technology Beijing is gratefully acknowledged. We are also grateful to the Shandong Province High Performance Computing Center for a grant of computer time. We are also grateful to Prof. Dr. Feiwu Chen and Dr. Yuexing Zhang at the University of Science and Technology Beijing for kind discussion and help. ’ REFERENCES (1) (a) Tsuda, A.; Osuka, A. Science 2001, 293, 79–82. (b) Lopez, N.; Reichertz, L. A.; Yu, K. M.; Campman, K.; Walukiewicz, W. Phys. Rev. 13818
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