Article pubs.acs.org/JPCA
Impact of Electronic Coupling, Symmetry, and Planarization on Oneand Two-Photon Properties of Triarylamines with One, Two, or Three Diarylboryl Acceptors Nikolay S. Makarov, Sukrit Mukhopadhyay, Kada Yesudas, Jean-Luc Brédas, and Joseph W. Perry* School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States
Agnieszka Pron, Milan Kivala, and Klaus Müllen* Max Planck Institute for Polymer Research, Ackermannweg 10, Mainz, D-55128, Germany S Supporting Information *
ABSTRACT: We have performed a study of the one- and two-photon absorption properties of a systematically varied series of triarylamino-compounds with one, two, or three attached diarylborane arms arranged in linear dipolar, bent dipolar, and octupolar geometries. Two-photon fluorescence excitation spectra were measured over a wide spectral range with femtosecond laser pulses. We found that on going from the single-arm to the two- and three-arm systems, the peak in two-photon absorption (2PA) cross-section is suppressed by factors of 3−11 for the lowest excitonic level associated with the electronic coupling of the arms, whereas it is enhanced by factors of 4−8 for the higher excitonic level. These results show that the coupling of arms redistributes the 2PA cross-section between the excitonic levels in a manner that strongly favors the higher-energy excitonic level. The experimental data on oneand two-photon cross-sections, ground- and excited-state transition dipole moments, and permanent dipole moment differences between the ground and the lowest excited states were compared to the results obtained from a simple Frenkel exciton model and from highly correlated quantum-chemical calculations. It has been found that planarization of the structure around the triarylamine moiety leads to a sizable increase in peak 2PA cross-section for the lowest excitonic level of the two-arm system, whereas for the three-arm system, the corresponding peak was weakened and shifted to lower energy. Our studies show the importance of the interarm coupling, number of arms, and structural planarity on both the enhancement and the suppression of two-photon cross-sections in multiarm molecules.
1. INTRODUCTION Understanding of the structure−property relationships for twophoton absorption (2PA) cross-sections (σ2) of a broad range of chromophores and multichromophore motifs is essential for the design of highly sensitive molecules for various applications of 2PA including three-dimensional (3D) fluorescence microscopy,1,2 optical memory,3,4 and nanofabrication,5,6 as well as optical power limiting7−9 and photodynamic therapy.10−12 Structure−property relationships for 2PA have been studied intensively over the past decade,13−16 and systems with various numbers of molecular units have been synthesized including those with two,17,18 three,19−22 or more23−26 units in supermolecular compounds. The properties of some of these multichromophoric systems have been modeled based on the properties of their single-arm parent chromophores via computational approaches including essential-state (2−4 states) models,27−29 the Frenkel exciton model,30 and correlated quantum-chemical approaches.31,32 © 2012 American Chemical Society
Despite the numerous studies of 2PA in multiarm systems, there is still a lack of understanding33 as to (i) whether multiarm branching will result in more than a simple additive enhancement,25,34−36 additive behavior,37−39 or even suppression40,41 of 2PA activity and (ii) how the effects depend on the nature of the single-arm units and the strength of the interarm electronic coupling. Therefore, a thorough characterization of multiarm synthetic chromophores with well-defined, prototypical units is desirable, both experimentally and theoretically. Here, we report a comprehensive study of the one-photon absorption (1PA) and 2PA properties, over a broad spectral range (550−950 nm), of a systematically varied series of six well-defined triarylamino-compounds with one, two, or three attached diarylborane arms arranged in linear dipolar, bent dipolar, and octupolar geometries (see Chart 1). Because the Received: December 9, 2011 Revised: February 16, 2012 Published: March 19, 2012 3781
dx.doi.org/10.1021/jp211878u | J. Phys. Chem. A 2012, 116, 3781−3793
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Chart 1. Chemical Structures of the Six Studied Triarylamino-Triarylborane Compounds with One, Two, or Three Arms and Varied Planarity
addition of an excess of Mes2BF (see Scheme 1). The halogenmetal exchange reaction time was reduced to 20 min to
molecules feature simple phenyl linkages and strong donor (triarylamino) and acceptor (diarylboryl) groups, large changes in dipole moment upon excitation are expected. We also examine the effects of planarization by comparing the properties of molecules wherein the phenyls of triarylamine group are free to undergo hindered torsion (referred to as the A series) or are planarized via ring locking of the phenyl groups (the B series). Through combined experimental investigations, modeling based on Frenkel excitons, and highly correlated quantumchemical calculations, we aim to address the following questions: (1) What is the main impact of the coupling of two and three arms in a bent dipolar and octupolar arrangement on the 1PA and 2PA properties of the molecules? (2) What are the main effects of planarization at the triaryl amine on the 1PA and 2PA properties? (3) To what extent can the 1PA and 2PA spectra be explained using the simple Frenkel exciton model? (4) How well do two- and three-state models and sum-over-states quantum-chemical calculations describe the 2PA spectra and cross-sections? We show that a given multiarm molecule can exhibit 2PA bands with the crosssections being significantly enhanced or suppressed as compared to the peak cross-sections of the single-arm analogs. We also show that even though the simple Frenkel exciton model is capable of predicting accurately many linear properties of the molecules, it fails in quantitatively predicting 2PA crosssections. Quantum-chemical calculations, on the other hand, provide results much closer to the experimental 2PA measurements, due to the important role played by interactions with higher excited states in the 2PA response.
Scheme 1. Main Steps of the Synthesis of the Planar Compounds B1−B3
2. EXPERIMENTAL SECTION 2.1. Synthesis. Compounds A1, A2, and A3 were prepared in one step from bromo-, dibromo-, and tribromo-triphenylamine, respectively, according to the previously published procedure.42 The chemical structures of all six compounds are shown in Chart 1. Further details on the synthesis are summarized in the Supporting Information. The synthesis of compounds B1, B2, and B3 involved treatment of bromo-, dibromo-, and tribromo-triangulenes43,44 with 1, 2, and 3 equiv of n-BuLi, respectively, followed by an
minimize the formation of side products. After purification by size exclusion chromatography, 2-(dimesitylboryl)4,4,8,8,12,12-hexamethyl-4H,8H,12H-benzo[1,9]quinolizino[3,4,5,6,7-defg]acridine (B1), 2,6-bis(dimesitylboryl)4,4,8,8,12,12-hexamethyl-4H,8H,12H-benzo[1,9]quinolizino[3,4,5,6,7-defg]acridine (B2), and 2,6,10-tris(dimesitylboryl)4,4,8,8,12,12-hexamethyl-4H,8H,12H-benzo[1,9]quinolizino[3,4,5,6,7-defg]acridine (B3) were obtained as yellow solids in 30, 40, and 50% yield, respectively. Because of the presence of two mesityl groups on each boron center, synthesized 3782
dx.doi.org/10.1021/jp211878u | J. Phys. Chem. A 2012, 116, 3781−3793
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where ℏωi and ℏωf are the excitation energies for the intermediate state |i⟩ and final state |f⟩. For molecules in gas phase or in solution, the 2PA cross-section is given by the orientational average of the transition matrix (S):55,56
compounds are stable at the ambient conditions. Their solubility increases with an increased number of B-moieties. Thus, B1 is sparingly soluble in hexane or acetone, whereas B3 is well soluble in all common organic solvents. 2.2. Commercial Materials. THF (EMD Chemical), methanol (Sigma-Aldrich), chloroform (EMD Chemical), toluene (Sigma-Aldrich), ethanol (Sigma-Aldrich), hexane (VWR International), ethyl acetate (Sigma-Aldrich), and dichloromethane (EMD Chemical) were used as received without further purification. All solvents were of spectral grade quality. 2PA and excited-state absorption (ESA) spectra were measured in THF. 2PA spectra and cross-sections were measured relative to coumarin 485 (Exciton) dissolved in methanol. 2.3. Experimental Techniques. The 1PA spectra of the molecules were measured using Shimadzu UV-3101PC and Perkin-Elmer Lambda 15 scanning spectrometers. Emission spectra were recorded using a J&M TIDAS spectrofluorometer. Fluorescence anisotropy was measured using a Jobin Yvon SPEX Fluorolog-3 spectrofluorimeter. Fluorescence lifetimes were measured using the photon-counting system described earlier.45 ESA was measured using an Ultrafast Systems Helios nonlinear spectrometer as was described earlier.8 The excited state extinction coefficients were estimated based on the comparison of the ground-state bleach absorbance change to the 1PA extinction coefficient.46 2PA was measured using a two-arm, two-photon excited fluorescence spectrometer as described elsewhere.47 2.4. Quantum-Chemical Calculations. The quantumchemical calculations were performed with the Gaussian 09 program.48 The geometry optimizations were carried out at the DFT-B3LYP/6-31G** level with a C2/D3 symmetry constraint and followed by vibrational-frequency calculations to confirm that real minima were obtained. On the basis of the DFToptimized geometries, semiempirical intermediate neglect of differential overlap (INDO) Hamiltonian49,50 were performed and coupled to a multireference determinant single configuration interaction (MRD-CIS)51 scheme; the Mataga− Nishimoto potential was used to express the Coulomb repulsion term.52,53 In the MRD-CIS calculations, Rumer CI diagrams are generated from the reference determinants with an active space of 20 occupied and 20 unoccupied orbitals. The reference determinants include the following configurations: (i) the HF ground state, (ii) a single excitation from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO), (iii) a single excitation from the HOMO to LUMO+1, (iv) a single excitation from HOMO-1 to LUMO, and (v) a double excitation from HOMO to LUMO. Because low-lying singly and doubly excited configurations are taken as reference determinants, single particle-hole excitations over these determinants result in doubly and triply excited configurations, which leads to the incorporation of substantial correlation effects in both the ground state and the low-lying excited states. The results provide the transition energies as well as the state- and transition-dipole moments, which are then plugged into sum-over-states (SOS) calculations to obtain the 2PA cross-sections. The 2PA transition matrix elements (Sαβ) were calculated using the following expression:54 Sαβ =
⎡ ⟨g |μ |i⟩⟨i|μ |f ⟩ α β
∑⎢ i
⎢⎣ ℏ(ωi − ωf /2)
+
⟨g |μβ |i⟩⟨i|μα |f ⟩ ⎤ ⎥ ℏ(ωi − ωf /2) ⎥⎦
σ2 =
2 π ⎛ ℏωf ⎞ ⎜⎛ 6.4386 ⎟⎞ δavgg (ωf ) ⎜ ⎟ 15 ⎝ 2 ⎠ ⎝ 600 ⎠
(2)
where δavg = ∑ij(SiiSjj + 2SijSji) and g(ωf) is the normalized line shape function of the final state ∫ ωg(ω) = 1), determined from either 1PA or 2PA experimental spectra. The factor of 1/15 arises from the orientational averaging, and the prefactor is a conversion factor between MKS and GM units. In our calculations, we assumed a Gaussian line shape function: g (ω) =
exp −2[(ω − ωf )/w]2
{
}
w π /2
where w is twice the standard deviation.
3. FRENKEL EXCITON MODEL FOR 1PA AND 2PA IN TWO- AND THREE-ARM SYSTEMS Molecules A2, B2, A3, and B3 consist of two and three building blocks, derived from A1 and B1. To a first approximation, the photophysical properties of the two- and three-arm compounds can be considered to be due to electronic coupling of the excited states of the “linear” molecules. The Frenkel exciton model57,58 describes the excited-state wave functions of the multiarm molecules as a simple superposition of the wave functions of the single arms. The Frenkel exciton model has been used to treat the state and transition dipole moments and the nonlinear optical properties, such as 2PA cross-sections, of bent or cruciform type two-arm and planar three-arm molecules.30,41 Using the Frenkel exciton model, the peak 2PA cross-sections of the two lowest energy bands (11A → 11E and 11A → 21A) of triple-arm molecules can be expressed as:30 1
σ2(1 E) = A
1 σ2(2 A)
( )2 ( )2 3 μ01* Δμ01* 2 (E − V )2 Γ
=A 3
(*)2 (*)2 μ01 Δμ01
(E − 4V )2 Γ
= σ2(*)
=
σ2(*)
E2 (E − V )2
2E2 (E − 4V )2
(3)
where V denotes the electronic coupling constant, Γ is the damping factor related to the finite line width of the transition, μ(01*) is the transition dipole moment from the ground state to the first excited state of the single-arm molecule, Δμ(01*) is the permanent dipole moment difference between the ground state and the first excited state of the single-arm molecule, σ(2*) stands for the peak 2PA cross-section of a single-arm molecule, ⟨ ⟩ denotes isotropic orientational averaging of the crosssection, A is a coefficient combining physical constants, and E is the transition energy for the single-arm molecule from the ground state to the first excited state.
4. RESULTS AND DISCUSSION To investigate the effects of the number and symmetry of coupled arms and of planarization at the nitrogen position on the optical and electronic properties of the molecules, we performed spectroscopic studies of a systematically varied set of arylamine-arylborane-based compounds with one, two, and three donor−acceptor arms (series A in Chart 1) and an
(1) 3783
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Figure 1. One-photon (black curves, top and right axis), two-photon (black squares, bottom and left axis), and excited state (red dashed lines, top and right axis) absorption spectra of compounds A1−B3 in tetrahydrofuran solution. The spectra are shown with the 2PA pump laser wavelength on the lower abscissa and with the 1PA excitation wavelength on the upper abscissa.
discussed below, and the numbering of states is based on the energy ordering of states with discernible intensities in 1PA or 2PA spectra). This band is followed by a peak at shorter wavelength, ∼296 nm (4.19 eV) (11A → 31A) and a shoulder around ∼318 nm (3.90 eV) (11A → 11B) for A1; the analogous band for B1 resolves into a series of three discernible bands, the middle of which is at ∼293 nm (4.23 eV). This sharpening and increased resolution of electronic absorption bands are a consequence of the planarization of the triarylamine group in B1, which is expected to greatly reduce torsional disordering about the phenyl-N bonds and the corresponding modulation of the electronic transition energy and vibronic structure. There is also an increase of 1PA toward shorter wavelengths,
