Two-photon Absorption Spectra Research Articles

Arylphthalimidoporphyrins: New Approaches to Imaging pH and Temperature Simultaneously with Oxygen

We will discuss a family of recently developed tetraarylphalimidoporphyrins (TAPIP) and diarylphalimidoporphyrins (DAPIP), which possess unique photophysical properties and suggest new approaches to the design of superior biological sensors for pH and temperature in addition to their already established ability as sensors for oxygen. The synthesis of TAPIP has already been developed by our group previously. Here we present a synthetic route to asymmetric diarylphthalimidoporphyrins (DAPIP) based on [2+2] condensation of the corresponding dipyrromethanes, followed by oxidative aromatization. We report the properties of free-base (H2), Zn(II), Pt(II) and Pd(II) DAPIP's, including their two-photon absorption (2PA) spectra. Using a combination of phosphorescent Pt complexes of TAPIP and DAPIP we are designing a phosphorescent sensor for pH and oxygen, termed pHOx, which operates by measuring ratios of phosphorescence lifetimes. Unlike all existing optical pH sensors, measurements by pHOx are not affected by optical heterogeneities of the medium and provide unbiased pH readings in vivo simultaneously with pO2. At the same time, Pd complexes of TAPIP have been shown to emit both phosphorescence and thermally activated E-type delayed fluorescence, providing a functional element for optical sensing of temperature. However, to make the temperature readings unobstructed by optical heterogeneities of biological tissue, a PdTAPIP-based probe is supplemented by another probe system, based on H2DAPIP, which emits prompt fluorescence at the same wavelengths and can serve as a correction standard for unbiased temperature measurements.

Nondegenerate two-photon absorption in GaAs/AlGaAs multiple quantum well waveguides

We present femtosecond pump-probe measurements of the nondegenerate (1960$\,$nm excitation and 1176--1326$\,$nm probe) two-photon absorption spectra of 8 $\,$nm GaAs/12$\,$nm Al$_{0.32}$Ga$_{0.68}$As quantum well waveguides. Experiments were performed with light pulses co-polarized normal and tangential to the quantum well plane. The results are compared to perturbative calculations of transition rates between states determined by the $\mathbf{k}\cdot\mathbf{p}$ method with an 8 or 14 band basis. We find excellent agreement between theory and experiment for normal polarization, then use the model to support predictions of orders-of-magnitude enhancement of nondegenerate two-photon absorption as one constituent photon energy nears an intersubband resonance.

Benzenesulfonyl incorporated chalcones: Synthesis, structural and optical properties

Although the optical properties and applicability of chalcones as potential nonlinear optical(NLO) materials is well known, hybrids having both chalcone and sulfonamide portions are relatively scarce. In this sense, we are seeking to show if combining both into a double functionalized compound will still have similar or better NLO responses. For this, we have synthetized three sulfonamide-chalcone analogues by changing the substituent bonded to the sulfonamide ring (I = ethoxy; II = Cl and III = Br). These compounds were characterized by spectroscopic methods (NMR, IR and HRMS) and thermal methods (HSM and DSC/TGA). Their crystal structures were determined by Single Crystal X-ray Diffraction (SCXRD) and their molecular structures were compared. Crystallographic results showed that one compound crystallizes in a triclinic system whereas the others crystallize in a monoclinic crystal system. Moreover, their crystal packing is dominated by C–H⋯O interactions. In addition to this study and to first characterize these compounds, linear and nonlinear optical (NLO) properties were performed on all three compounds dissolved in dimethyl sulfoxide. One-photon and two-photon absorption (2PA) spectra and incoherent second harmonic generation were obtained by employing different spectroscopic techniques. The interesting results observed in the linear and NLO measurements showed that different groups bounded to the same main backbone did not significantly modify their optical properties, although they strongly affect their crystal structures. σ2PA spectra reveal that the first excited state is allowed by both 1PA and 2PA, which indicates asymmetry in the charge distribution along the π-conjugated molecule structure. NLO properties of compounds I-III agree with other chalcones previously studied, which indicates that adding the benzenesulfonyl group does not influence these properties. Considering the similar experimental values for different substituents, the results further motivate investigation on substituent-based optical properties for other sulfonamide-chalcone hybrids.

Nonlinear Optical Study in a Set of Dibenzylideneacetone Derivatives with Potential for Optical Frequency Conversion

The search for advanced optical materials, in particular, materials with nonlinear optical responses, has, in the last years, experienced substantial growth due to their vast applications in the photonics field. One of those applications is ultra-fast optical frequency conversion, in the optics communications field. Organic compounds have emerged as promising candidates for raw materials to develop nonlinear optical devices, such as optical converters, due to their intrinsic ultra-fast electronic responses. Also, the easy tailoring of organic molecular structures makes organic materials much more appealing than the inorganic ones. In this work, we have performed a linear and nonlinear optical characterization of a set of dibenzylideneacetone derivatives. The nonlinear optical responses investigated correspond to second- and third-order nonlinear processes, namely, first electronic molecular hyperpolarizability and two-photon absorption cross-section, respectively. The value of the first electronic molecular hyperpolarizability, up to 52 cm4·statvolt−1, could be considered a robust value when compared to the short-sized π-electron backbone length of the studied compounds. Such results suggest that these compounds exhibit the potential to be used as optical frequency converters. Quantum chemical calculations were used to predict the theoretical value of the first molecular hyperpolarizability, as well as to simulate the one- and two-photon absorption spectra for all compounds.

On the spectral line width broadening for simulation of the two-photon absorption cross-section of para-Nitroaniline in liquid environment

The calculated two-photon absorption (TPA) cross-section amplitude depends on the spectral line width (Γ) of the transition and there is not a consensus on the most appropriate way to obtain the Γ value. Usually it is arbitrarily defined or ambiguously estimated based on experimental data or using a transition energy variation determined from a quantum mechanics/molecular mechanics approach. In this study we compare different procedures to estimate Γ values and different methods of spectral convolution to simulate TPA cross-section for para-Nitroaniline (pNA) in liquid environment. The effects of the liquid environment on the nonlinear absorption were approximated using Monte Carlo and Born-Oppenheimer Molecular Dynamics as well as the commonly used Polarizable Continuum Model. The different procedures adopted here provided large difference in the calculated TPA amplitudes, mainly because of the underestimate/overestimate of the homogeneous and/or inhomogeneous contributions to the spectral line width. Based on these difficulties, we suggest a new procedure in which the value assigned to Γ is the one used to induce the total line width of the simulated TPA spectrum to reproduce the experimental one-photon absorption line width. Within this procedure the Γ value, which becomes a fitting parameter (Γfit), represents part of the homogeneous contribution to the spectral line width. Finally, although time dependent density functional theory calculations reproduced only reasonably well the energy of the lowest-energy π → π* transition of pNA, the fitting procedure here proposed provided the calculated TPA cross-section in good agreement with the available experimental data.

Characterizing the Two-photon Absorption Properties of Fluorescent Molecules in the 680-1300 nm Spectral Range.

Two-photon laser scanning microscopy (2PLSM) is a state-of-the-art technique used for non-invasive imaging deep inside the tissue, with high 3D resolution, minimal out-of-focus photodamage, and minimal autofluorescence background. For optimal application of fluorescent probes in 2PLSM, their two-photon absorption (2PA) spectra, expressed in absolute cross sections must be characterized. Excitation at optimum wavelength will make it possible to reduce the laser power and therefore minimize photodamage. Obtaining 2PA spectra and cross sections requires correcting the two-photon excited fluorescence signals for a combination of laser properties, including the beam spatial profile, pulse duration, and absolute power, at each wavelength of the tuning range. To avoid such tedious day-to-day laser characterization required in the absolute measurement method, a relative method based on independently characterized 2PA reference standards is often used. By carefully analyzing the available literature data, we selected the most reliable standards for both the 2PA spectral shape and cross section measurements. Here we describe a protocol for measuring the 2PA spectral shapes and cross sections of fluorescent proteins and other fluorophores with the relative fluorescence method using these reference standards. Our protocol first describes how to build an optical system and then how to perform the measurements. In our protocol, we use Coumarin 540A in dimethyl sulfoxide and LDS 798 in chloroform for the spectral shape measurements to cover the range from 680 to 1300 nm, and Rhodamine 590 in methanol and Fluorescein in alkaline water (pH 11) for the absolute two-photon cross section measurements.

Photoinduced charge transfer in quasi-one-dimensional polymers in two-photon absorption.

In this work, we theoretically investigate the structure and the transition characteristics of one- (OPA) and two-photon absorption (TPA) spectra of different length neutral and charged thiophene polymers. The effects and regulation of different charges on photoinduced charge transfer are discovered and their physical mechanisms are explained. We find that both the OPA and TPA spectra undergo a sizeable redshift after the charge is injected into the polymer, and the redshift after the positive charge injection is excellent. The alternating charge transfer that occurs in a two-photon transition of a charged system is derived from the alternating distribution of charge (dipole moment) in the dynamics of the system. To study the gradual behavior of infinite polymers, we also theoretically calculated the optical properties and electronic structures of infinitely long thiophene polymers under different electrical charge injections by a one-dimensional periodic model. The redshift of the OPA and TPA spectra is found to be due to orbital energy level movement.

Two-dimensional two-photon absorptions and third-order nonlinear optical properties of Ih fullerenes and fullerene onions.

The third order static and dynamic nonlinear optical (NLO) responses of Ih symmetry fullerenes (C60, C240, and C540) and fullerene onions (C60@C240 and C60@C240@C540) are predicted using the ZINDO method and the sum-over-states model. The static second hyperpolarizability of Ih symmetry fullerenes increases exponentially with fullerene size [from 10.00 × 10-34 esu in C60 to 3266.74 × 10-34 esu ≈ γ0(C60) × 92.63 in C540]. The external fields of strong third order NLO responses of Ih symmetry fullerenes change from ultra-violet (C60) to the visible region (C540) as the fullerene size increases. The outer layer fullerene in the fullerene onions has dominant contributions to the third order NLO properties of the fullerene onions, and the inter-shell charge-transfer excitations have conspicuous contributions to the third order NLO properties. The two-dimensional two-photon absorption spectra of C60 and C240 show that those fullerenes have strong two-photon absorptions in the visible region with short wavelength and in the ultra-violet region.

The importance of relativistic effects on two-photon absorption spectra in metal halide perovskites

Despite intense research into the optoelectronic properties of metal halide perovskites (MHPs), sub-bandgap absorption in MHPs remains largely unexplored. Here we recorded two-photon absorption spectra of MHPs using the time-resolved microwave conductivity technique. A two-step upward trend is observed in the two-photon absorption spectrum for methylammonium lead iodide, and some analogues, which implies that the commonly used scaling law is not applicable to MHPs. This aspect is further confirmed by temperature-dependent conductivity measurements. Using an empirical multiband tight binding model, spectra for methylammonium lead iodide were calculated by integration over the entire Brillouin zone, showing compelling similarity with experimental results. We conclude that the second upward trend in the two-photon absorption spectrum originates from additional optical transitions to the heavy and light electron bands formed by the strong spin-orbit coupling. Hence, valuable insight can be obtained in the opto-electronic properties of MHPs by sub-bandgap spectroscopy, complemented by modelling.

Chalcone-based molecules: Experimental and theoretical studies on the two-photon absorption and molecular first hyperpolarizability.

Five chalcone-based molecules denominated by C-3 ((E)-1-(4-methoxyphenyl)-3-phenylprop-2-en-1-one), C-4 ((E)-1,3-bis(4-methoxyphenyl)prop-2-en-1-one), C-5 ((E)-1-(benzo[d][1,3]dioxol-5-yl)-3-(4-methoxyphenyl)prop-2-en-1-one), C-6 ((E)-3-(naphthalen-1-yl)-1-phenylprop-2-en-1-one) and C-7 ((E)-1-(4-methoxyphenyl)-3-(naphthalen-1-yl)prop-2-en-1-one) were synthesized by Claisen-Schmidt reaction in solution of NaOH in water/ethanol 2:1. The aldehydes used were benzaldehyde, anisaldehyde, and β-naphthaldehyde, while the used ketones were acetophenone, p-methoxyacetophenone, and 3,4-methylenedioxyacetophenone. Z-scan and hyper-Rayleigh scattering techniques were used to study the nonlinear optical properties of these compounds in dichloromethane medium. By using Z-scan technique with femtosecond pulses, two-photon absorption cross-sections (σTPA) were determined, while the first molecular electronic hyperpolarizabilities (βHRS) were evaluated by the hyper-Rayleigh scattering technique, with picosecond pulses. From the recorded two-photon absorption spectra, it was identified that compound C-7 presented the highest σTPA, regarding the HOMO-LUMO transition, with a value of 40 GM, while C-6 achieved the lowest value for the same transition with 13 GM. Concerning the values of the first molecular hyperpolarizability, compound C-4 presented the highest value, 38 × 10-30 cm4 statvolt-1, while C-3 presented the lowest βHRS value of about 16 × 10-30 cm4 statvolt-1. Time-dependent density functional theory calculations were used to simulate the one- and two-photon absorption spectra, as well to predict the theoretical value of βHRS in dichloromethane and vacuum medium.

Optical properties of S2 and S3 excited states of protonated schiff-base retinal chromophores in TPA, ECD and ROA.

The electronic transitions of the protonated Schiff base of 1l-cis-retinal (PSB11) and protonated Schiff bases of all-trans retinal (PSBT) for the second or higher electronic excited states are hard to be observed experimentally, due to weak intensities of electronic state excitations. In this paper, we propose visualizations method to investigate these weak electronic state transitions of PSB11 and PSBT, using two-photon absorption (TPA), electronic circular dichroism (ECD) and Raman optical activity (ROA) spectra. Because of the resonance excitations of PSB11 and PSB11 in TPA, the transition intensity of the third electronic state is significantly enhanced, which are much larger than that of S1 and S2 electronic transitions. The charge transfer and electron-hole coherence of these electronic transitions in each step in TPA are visualized with charge difference density and transition density matrix. Also, the strong absorptions of S1 and S2 electronic excited states are observed with ECD spectra, and the physical mechanism of electric and magnetic interactions for these electronic transitions are revealed by visualization method. The large intensity of ROA at S3 excited state results from transition electric and magnetic dipole interactions, not from transition electric dipole and transition electric quadrupole interactions. Our results provide a new visualization method to study the optical properties of biological system using TPA and ECD spectra.

Signatures of Through‐Space Charge Transfer in Two‐Photon Absorption of Paracyclophane Derivatives

Third order polarizability, (γ) taken from the collective electronic oscillator (CEO) method was used to calculate the two‐photon absorption (TPA) of tetrastyryl‐[2,2]paracyclophane derivatives with different through‐space charge transfer configurations considering various donor and acceptor combinations at the terminal styryl groups. For the virtually same linear absorption, different TPA spectra were obtained. For controlling and fine‐tuning frequency and cross‐sections of TPA the through‐space charge transfer interactions can be used. The results are explained by the electronic density matrices corresponding to governing oscillators in one‐ and two‐photon absorption and the ground state. It is indicated that for the studied systems mainly the lowest four oscillators are responsible for the TPA cross‐sections rather than a simple effective three‐state model.

Illuminating the origins of two‐photon absorption properties in fluorescent protein chromophores

AbstractWe provide here a structural impact on two‐photon absorption cross‐section (σTPA) for 22 distinct fluorescent protein (FP) chromophores. By employing time‐dependent density functional theory, we gain insight into two‐photon absorption (TPA) process by investigating relationship between σTPA and one‐photon electronic transition dipole moment and permanent dipole moment change (Δμ) upon transition. Our results reveal that for the S1 excited state, σTPA is proportional to (Δμ)2 in agreement with two‐state model of TPA process. On the contrary, the TPA spectroscopy of higher excited states (S n, n > 1) is much more complex. We do not find a main driving force of large σTPA that would be common for investigated chromophores. Instead, it seems that channel interference between one‐photon transition dipole moment vectors is responsible for enhancement or diminishment of σTPA. Our in vacuo results may serve as a benchmark to investigate a role of chromophore‐protein interaction in shaping TPA spectra of FPs.

Resonance enhanced two-photon cavity ring-down spectroscopy of vibrational overtone bands: A proposal.

This paper presents an analysis of near-resonant, rovibrational two-photon spectroscopy and the use of cavity ring-down spectroscopy for its detection. Expressions are derived for the photon absorption rate of a three-level system, correct to all orders and the simpler expressions that result from various approximations. The analysis includes the angular momentum projection degeneracies and linear or circular polarization of the exciting field. Expressions are derived for the rate of two-photon power loss for light inside a resonant cavity. Explicit calculations are made for excitation of the ν3 mode of 12C16O2 for which the two-photon excitation spectrum is dominated by a single v3 = 0 → 2, Q(16) line at ν̃=2335.826 cm-1. This transition has an intermediate v3 = 0 → 1, P(16) one-photon transition that is off resonance by 0.093 cm-1 (2.8 GHz). At 1 Torr total pressure, the Q(16) two-photon transition is calculated to have a cross section of 2.99 × 10-38 cm4 s per CO2 molecule in the J = 16 state or 2.24 × 10-39 cm4 s per CO2 molecule at 300 K. The analysis of the sensitivity limits for 2-photon cavity ring-down spectroscopy predicts a theoretical detection limit of 32 ppq (10-15) Hz-1/2 for 12C16O2, higher sensitivity than has been realized using one-photon absorption. The analysis predicts that most polyatomic molecules will have sparse, Doppler-free two-photon absorption spectra, which will dramatically increase the selectivity of trace gas detection of samples with multiple components with overlapping absorption bands. This is demonstrated by the predicted mid-IR two-photon absorption spectrum of butadiene using theoretical spectroscopic constants.

Two-photon absorption property of Cl-substituted diphenylacetylenes by optical-probing photoacoustic spectroscopy.

Two-photon absorption spectra and two-photon absorption cross sections of Cl-substituted diphenylacetylenes (ClDPAs) were investigated by optical-probing photoacoustic spectroscopy and quantum chemical calculations for the first time. The two-photon absorption spectra of ClDPAs exhibited intense two-photon absorption bands at around 480 nm, which are forbidden by one-photon absorption. The two-photon absorption cross sections σ(2) of o-, m-, and p-ClDPAs at 476 nm were determined to be 22 ± 1, 23 ± 1, and 38 ± 2 GM, respectively. Compared with diphenylacetylene (DPA) (27 GM at 472 nm), the σ(2) values of o- and m-ClDPAs were lower, while that of p-ClDPA was higher. Simulated two-photon absorption spectra using the TD-B3LYP/6-311+G(d,p) level of calculations within the Tamm-Dancoff approximation, based on the three-state model, well agreed with the experimental results. The difference in the σ(2) values of DPA and ClDPAs was responsible for those in the transition dipole moments between the intermediate and the final states.

Multistate Multiconfiguration Quantum Chemical Computation of the Two-Photon Absorption Spectra of Bovine Rhodopsin.

Recently, progress in IR sources has led to the discovery that humans can detect infrared (IR) light. This is hypothesized to be due to the two-photon absorption (TPA) events promoting the retina dim-light rod photoreceptor rhodopsin to the same excited state populated via one-photon absorption (OPA). Here, we combine quantum mechanics/molecular mechanics and extended multiconfiguration quasi-degenerate perturbation theory calculations to simulate the TPA spectrum of bovine rhodopsin (Rh) as a model for the human photoreceptor. The results show that the TPA spectrum of Rh has an intense S0 → S1 band but shows also S0 → S2 and S0 → S3 transitions whose intensities, relative to the S0 → S1 band, are significantly increased when compared to the corresponding bands of the OPA spectrum. In conclusion, we show that IR light in the 950 nm region can be perceived by rod photoreceptors, thus supporting the two-photon origin of the IR perception. We also found that the same photoreceptor can perceive red (i.e., close to 680 nm) light provided that TPA induces population of S2.

Electronic Structure of Liquid Methanol and Ethanol from Polarization-Dependent Two-Photon Absorption Spectroscopy.

Two-photon absorption (2PA) spectra of liquid methanol and ethanol are reported for the energy range 7-10 eV from the first electronic excitation to close to the liquid-phase ionization potential. The spectra give detailed information on the electronic structures of these alcohols in the bulk liquid. The focus of this Article is to examine the electronic structure change compared with water on substitution of a hydrogen by an alkyl group. Continuous 2PA spectra are recorded in the broadband pump-probe fashion, with a fixed pump pulse in the UV region and a white-light continuum as a probe. Pump pulses of two different energies, 4.6 and 6.2 eV, are used to cover the spectral range up to 10 eV. In addition, theoretical 2PA cross sections for both molecules isolated in the gas phase are computed by the equation-of-motion coupled-cluster method with single and double substitutions (EOM-CCSD). These computational results are used to assign both the experimental 2PA and literature one-photon linear absorption spectra. The most intense spectral features are due to transitions to the Rydberg states, and the 2PA spectra are dominated by the totally symmetric 3pz ← 2pz transition in both alcohols. The experimental 2PA spectra are compared with the simulated 2PA spectra based on ab initio calculations that reveal a general blue shift of the excited transitions upon solvation. The effective 2PA thresholds in methanol and ethanol decrease to 6.9 eV compared with 7.8 eV for water. The analysis of the 2PA polarization ratio leads us to conclude that the excited states of ethanol deviate more markedly from water in the lower energy region compared with methanol. The polarization dependence of the 2PA spectra reveal the symmetries of the excited states within the measured energy range. Natural transition orbital calculations are performed to visualize the nature of the transitions and the orbitals participating during electronic excitation.

Width Dependent Two-Photon Absorption in Monolayer Black Phosphorus Nanoribbons

Black phosphorus nanoribbons (BPNs) might offer alternatives to narrow-gap compound semiconductors for tunable optoelectronics in infrared region. In this work we present a quantum perturbation theory on two-photon absorption (TPA) in monolayer armchair-edged black phosphorus nanoribbons (acBPNs) employing the reduced two-band model within the long-wavelength BP Hamiltonian. The matrix elements for one-photon transition have been derived and the TPA spectrum associate with intra conduction band transition and inter band transition have been drawn. The calculations predict that the TPA coefficient in acBPNs is in the magnitude of 10−6 m/W in visible region, which is 4 orders higher than the conventional semiconductor quantum dots. And in infrared region, there is a giant TPA coefficient, which is mainly contributed from intra band transitions and can reach up to10−1 m/W. The TPA peaks can be tuned both by the width of BPNs and the electron relaxation energy.

Theoretical modeling of two-photon absorption of benzoxazoles exhibiting excited state proton transfer

Two-photon absorption spectra of 2-(2'-hydroxyphenyl)benzoxazole, 2,5-bis(2-benzoxazolyl)phenol and 2,5-bis(2-benzoxazolyl)hydroquinone are modeled by time-dependent density functional theory calculations. In contrast to the linear absorption, where the transitions to the two lowest excited states are leading, the two-photon absorption cross sections are significant for the transitions to higher states which are weak in one-photon absorption spectra. The 2,5-bis(2-benzoxazolyl)phenol and 2,5-bis(2-benzoxazolyl)hydroquinone demonstrate the maximum cross section of about ten times larger than that of 2-(2'-hydroxyphenyl)benzoxazole. The largest value of the cross section is calculated for 2,5-bis(2-benzoxazolyl)hydroquinone to be about 1500 units of Goeppert-Mayer for excitation at 600 nm.

Assessment of Functionals for TDDFT Calculations of One- and Two-Photon Absorption Properties of Neutral and Anionic Fluorescent Proteins Chromophores

Performance of DFT functionals with different percentages of exact Hartree-Fock exchange energy (EX) is assessed for recovery of the CC2 reference one- (OPA) and two-photon absorption (TPA) spectra of fluorescent proteins chromophores in vacuo. The investigated DFT functionals, together with their EX contributions are BLYP (0%), B3LYP (20%), B1LYP (25%), BHandHLYP (50%), and CAM-B3LYP (19% at short range and 65% at long range). Our test set consists of anionic and neutral chromophores as naturally occurring in the fluorescent proteins. For the first time, we compare TDDFT and CC2 methods for higher excited states than the S1 state, exhibiting relatively large TPA intensity. Our TDDFT results for neutral chromophores reveal an increase in excitation energies as well as TPA and OPA intensities errors, compared to CC2-derived results, as the DFT functional contains less exact exchange. The long-range-corrected CAM-B3LYP functional performs the best, closely followed by BHandHLYP, while BLYP usually significantly underestimates all investigated spectral properties, hence being the worst in reproducing the reference CC2 results. The hybrid B3LYP and B1LYP functionals can be roughly placed in between. We propose that TDDFT may underestimate the TPA intensities for neutral chromophores of fluorescent proteins due to underestimated oscillator strengths between some excited states. In the case of anionic chromophores, we find that B3LYP and B1LYP functionals overcome others in terms of reproducing CC2 excitation energies. On the other hand, however, TPA intensity is usually significantly underestimated, and in this respect, CAM-B3LYP functional seems to be again superior. In contrast to the case of neutral chromophores, it seems that a large magnitude of excited-state dipole moments or changes in dipole moments upon excitation may be the driving force behind high TPA transition moments.