Excited State Absorption Spectra Research Articles

Transient excited-state absorption and gain spectroscopy of a two-photon absorbing probe with efficient superfluorescent properties.

The synthesis, linear photophysical properties, two-photon absorption (2PA), excited-state transient absorption, and gain spectroscopy of a new fluorene derivative tert-butyl 4,4'-(4,4' (1E,1'E)-2,2'-(9,9-bis(2- (2-ethoxyethoxy)ethyl)-9H-fluorene-2,7-diyl)bis(ethene-2,1-diyl)bis(4,1 phenylene)]dipiperazine-1-carboxylate (1) are reported. The steady-state linear absorption and fluorescence spectra, along with excitation anisotropy, fluorescence lifetimes, and photochemical stability of 1 were investigated in a number of organic solvents at room temperature. The 2PA spectra of 1 with a maximum cross-section of ~ 300 GM were obtained with a 1 kHz femtosecond laser system using open-aperture Z-scan and two-photon-induced fluorescence methods. The transient excited-state absorption (ESA) and gain kinetics of 1 were investigated by a femtosecond pump-probe methodology. Fast relaxation processes (~1-2 ps) in the gain and ESA spectra of 1 were revealed in ACN solution, attributable to symmetry-breaking effects in the first excited state. Efficient superfluorescence properties of 1 were observed in a nonpolar solvent under femtosecond excitation. One- and two-photon fluorescence microscopy imaging of HCT 116 cells incubated with probe 1 was accomplished, suggesting the potential of this new probe in two-photon fluorescence microscopy bioimaging.

Excited state absorption in glasses activated with rare earth ions: Experiment and modeling

We present semiempirical approach based on the Judd–Ofelt theory and apply it for modeling the spectral properties of fluoride glasses activated with the rare earth (RE) ions. This method provide a powerful tool for simulating both ground state absorption (GSA) and excited state absorption (ESA) spectra of RE ions, e.g. Nd3+, Ho3+, Er3+ and Tm3+ in the ZBLAN glass matrix. The results of theoretical calculations correspond to the experimentally measured data. We also demonstrate that the spectra obtained using the presented approach are applicable in the analysis of up-conversion excitation schemes in these optoelectronically relevant materials.

Optically pump-induced athermal and nonresonant refractive index changes in the reference Cr-doped laser materials: Cr:GSGG and ruby

The refractive index of most ion-doped materials increases with the excited state population. This effect was studied in many laser materials, particularly those doped with Cr3+ and rare earth ions, using several techniques, such as interferometry, wave mixing, and Z-scans. This refractive index variation is athermal (has an electronic origin) and is associated with the difference in the polarizabilites of the Cr3+ ion in its excited and ground states, Δαp. The Cr3+ optical transitions in the visible domain are electric-dipole forbidden, and they have low oscillator strengths. Therefore, the major contribution to Δαp has been assigned to allowed transitions to charge transfer bands (CTBs) in the UV with strengths ∼3 orders of magnitude higher. Although this CTB model qualitatively explains the main observations, it was never quantitatively tested. In order to further investigate the physical origin of Δαp in Cr3+-doped crystals, excited state absorption (ESA) and Z-scan measurements were thus performed in Cr:Al2O3 (ruby) and Cr:GSGG. Cr:GSGG was selected because of the proximity of its E2 and T24 emitting levels, and thus the possibility to explore the role of the spin selection rule in the ESA spectra and the resulting variations in polarizability by comparing low and room temperature data, which were never reported before. On the other hand, Cr:Al2O3 (ruby) was selected because it is the only crystal for which it is possible to obtain CTB absorption data from both ground and excited states, and thus for which it is possible to check the CTB model more accurately. Thanks to these more accurate and more complete data, we came to the first conclusion that the spin selection rule does not play any significant role in the variation of the polarizability with the E2–T24 energy mismatch. We also discovered that using the CTB model in the case of ruby would lead to a negative Δαp value, which is contrary to all refractive index variation (including Z-scan) measurements.

Ground- and excited-state absorption and emission spectroscopy of Nd:GGG

Complete room temperature absorption, excited-state absorption and emission spectra of Nd:GGG have been registered and calibrated in cross section units in the near infrared around the main pump and laser emission transitions of Nd:GGG.

Study of singlet excited state absorption spectrum of lutetium bisphthalocyanine using the femtosecond Z-scan technique

In this study we investigate the singlet excited state absorption of lutetium bisphthalocyanine (LuPc2) over a wide spectral range. It was observed distinct nonlinear absorption behaviors; saturable (SA) and reverse saturable absorption (RSA). The RSA effect was observed below 640 and above 680nm, while SA occurs around the Q-band region, located around 660nm. To describe the main singlet–singlet transitions, we employed the rate equation model considering the simplified three-energy level diagram. Our results reveal a ratio between excited and ground state absorption smaller than 0.05 at the Q-band region, and of approximately 4 for the other regions.

Solvent Effects on Isomerization and Spectral Properties of Photochromic-Switching Diarythene Derivatives in Polar and Apolar Solutions

The photocyclization behavior and dynamic conformational transition of photochromic switches of diarythene derivatives in solutions are investigated by using the density functional theory (DFT) and molecular dynamics (MD) simulations. Three possible conformations, antiparallel (anti), parallel (para), and twist, for the open-ring isomers of 1,2-bis(2-methylbenzothiophene-3-yl)maleic anhydride are located. Both PCM-B3LYP/6-31G* calculations and MD simulations demonstrate that anti and twist open-ring isomers can interconvert freely in n-hexane and acetonitrile solutions at room temperature. The statistical ratio of twist to anti isomers from MD simulations is 2.09 in n-hexane and 1.07 in CH(3)CN, in qualitative agreement with those (1.18 in n-hexane and 1.05 in CH(3)CN) estimated from Arrhenius analysis of DFT activation energies. The solvent polarity has little influence on the isomerization of open-ring isomers in the ground state. Due to the evident charge transfer upon excitations, the solvent effects on the electronic structures and absorption spectra of low-lying excited states (S(1) and S(2)) are more significant. For such charge-transfer excited states, the long-range corrected functional CAM-B3LYP gives better agreement with the experimental spectra than B3LYP. The solvent polarity and polarization of the charge-transfer excited states are crucial for fabricating the novel functionalized photochromic molecular switches.

Photoinduced intramolecular charge-transfer dynamics of a red-emitting dicyanovinyl-based triarylamine dye in solution

Abstract The photophysics of a red-emitting push–pull triarylamine compound (fvin) comprising a triphenylamino electron donor core and a dicyanovinylene electron acceptor group is investigated by steady-state absorption and emission and femtosecond time-resolved absorption spectroscopy in room-temperature n -hexane, toluene, ethanol, and acetonitrile solvents. Fvin is strongly fluorescent in apolar solvents upon excitation of the S 0 → S 1 intramolecular charge transfer (ICT) transition, but hardly emissive in polar solvents. Time-resolved spectra reveal a strong dependence of the excited-state dynamics on both the solvent polarity and viscosity. Unlike n -hexane solutions where the fluorescent ICT excited state remains weakly solvated and keeps a structure close to that of the Franck–Condon level, significant stabilization of the ICT state operates in toluene, leading to a relaxed fluorescent ICT′ form with stronger charge transfer character. This was featured by a notable dynamic Stokes shift of the stimulated emission (SE) band occurring in 15 ps. However the similarly high fluorescence quantum yield and lifetime in n -hexane and toluene excludes any large structural distortion on going from ICT to ICT′. In acetonitrile and ethanol, the SE shift dynamics is enhanced and clearly biphasic. The first step can be ascribed to an ICT → ICT′ conformational relaxation as in toluene. The second step, accompanied by a drastic drop of the SE intensity and a strong evolution of the excited-state absorption spectrum (time-constant 1.5 ps in acetonitrile, 9 ps in ethanol), corresponds to the appearance of a new ICT″ state with enhanced charge localization and separation, most probably associated with a rotation of the phenyldicyanovinylene group. This large-amplitude distortion is followed by fast internal conversion to the ground state (9 ps in acetonitrile, 21 ps in ethanol), accounting for the almost nonemissive character of fvin in polar solvents.

Excited state absorption: a key phenomenon for the improvement of biphotonic based optical limiting at telecommunication wavelengths

Spectroscopic properties, two-photon absorption (TPA) and excited state absorption (ESA), of two organic cyanine dyes and of a ruthenium based organometallic cyanine are compared in order to rationalize their similar ns-optical power limiting (OPL) efficiency in the telecommunication wavelength range. The TPA contribution to the ns-OPL behavior is higher for both organic cyanines, while the main process is a TPA-induced ESA in the case of the organometallic system, in which the ruthenium induces a broadening of the NIR-ESA band and resulting in a strong spectral overlap between TPA and ESA spectra.

Quantum Chemical Simulations of Excited-State Absorption Spectra of Photosynthetic Bacterial Reaction Center and Antenna Complexes

The semiempirical ZINDO/S CIS configuration interaction method has been used to study the ground- and excited-state absorption spectra of wild type and heterodimer M202HL reaction centers from purple bacterium Rhodobacter sphaeroides as well as of peripheral LH2 and LH3 light harvesting complexes from purple bacterium Rhodopseudomonas acidophila. The calculations well reproduce the experimentally observed excited-state absorption spectra between 1000 and 17,000 cm(-1), despite the necessarily limited number of chromophores and protein subunits involved in the calculations. The electron density analysis reveals that the charge transfer between adjacent chromophores dominates the excited-state absorption spectra. Clear spectroscopic differences observed between the wild type and heterodimer reaction centers as well as between the LH2 and LH3 antenna complexes arise from differences in the energy level manifolds of the complexes, particularly those of the charge transfer states. The calculations also imply that the lowest excited state of the bacterial reaction centers has charge transfer character that is related to charge transfer within the special pair and between the special pair and the accessory bacteriochlorophyll of the photosynthetically active electron transfer branch.

Femtosecond Stimulated Raman Spectroscopy of Flavin after Optical Excitation

In blue-light photoreceptors using flavin (BLUF), the signaling state is formed already within several 100 ps after illumination, with only small changes of the absorption spectrum. The accompanying structural evolution can, in principle, be monitored by femtosecond stimulated Raman spectroscopy (FSRS). The method is used here to characterize the excited-state properties of riboflavin and flavin adenine dinucleotide in polar solvents. Raman modes are observed in the range 90-1800 cm(-1) for the electronic ground state S(0) and upon excitation to the S(1) state, and modes >1000 cm(-1) of both states are assigned with the help of quantum-chemical calculations. Line shapes are shown to depend sensitively on resonance conditions. They are affected by wavepacket motion in any of the participating electronic states, resulting in complex amplitude modulation of the stimulated Raman spectra. Wavepackets in S(1) can be marked, and thus isolated, by stimulated-emission pumping with the picosecond Raman pulses. Excited-state absorption spectra are obtained from a quantitative comparison of broadband transient fluorescence and absorption. In this way, the resonance conditions for FSRS are determined. Early differences of the emission spectrum depend on excess vibrational energy, and solvation is seen as dynamic Stokes shift of the emission band. The nπ* state is evidenced only through changes of emission oscillator strength during solvation. S(1) quenching by adenine is seen with all methods in terms of dynamics, not by spectral intermediates.

Ultrafast electrocyclic ring opening of 7-dehydrocholesterol in solution: The influence of solvent on excited state dynamics

Broadband UV-visible femtosecond transient absorption spectroscopy and steady-state integrated fluorescence were used to study the excited state dynamics of 7-dehydrocholesterol (provitamin D(3), DHC) in solution following excitation at 266 nm. The major results from these experiments are: (1) The excited state absorption spectrum is broad and structureless spanning the visible from 400 to 800 nm. (2) The state responsible for the excited state absorption is the initially excited state. Fluorescence from this state has a quantum yield of ∼2.5 × 10(-4) in room temperature solution. (3) The decay of the excited state absorption is biexponential, with a fast component of ∼0.4-0.65 ps and a slow component 1.0-1.8 ps depending on the solvent. The spectral profiles of the two components are similar, with the fast component redshifted with respect to the slow component. The relative amplitudes of the fast and slow components are influenced by the solvent. These data are discussed in the context of sequential and parallel models for the excited state internal conversion from the optically excited 1(1)B state. Although both models are possible, the more likely explanation is fast bifurcation between two excited state geometries leading to parallel decay channels. The relative yield of each conformation is dependent on details of the potential energy surface. Models for the temperature dependence of the excited state decay yield an intrinsic activation barrier of ∼2 kJ/mol for internal conversion and ring opening. This model for the excited state behavior of DHC suggests new experiments to further understand the photochemistry and perhaps control the excited state pathways with optical pulse shaping.

Dual Fluorescence and Ultrafast Intramolecular Charge Transfer with 6-N,N-Dialkylaminopurines. A Two-State Model

6-N,N-Dimethyl-9-methyladenine (DMPURM) and 6-N,N-dimethyladenine (DMPURH) show dual fluorescence from a locally excited (LE) and an intramolecular charge transfer (ICT) state in solvents of different polarity over extended temperature ranges. The fluorescence quantum yields are very small, in particular those of LE. For DMPURM in acetonitrile (MeCN) at 25 °C, for example, Φ'(ICT) = 3.2 × 10(-3) and Φ(LE) = 1.6 × 10(-4). The large value of Φ'(ICT)/Φ(LE) indicates that the forward LE → ICT reaction is much faster than the back reaction. The data obtained for the intersystem crossing yield Φ(ISC) show that internal conversion (IC) is the dominant deactivation channel from LE directly to the ground state S(0). For DMPURM in MeCN with Φ(ISC) = 0.22, Φ(IC) = 1 - Φ(ISC) - Φ'(ICT) - Φ(LE) = 0.78, whereas in cyclohexane an even larger Φ(IC) of 0.97 is found. The dipole moment gradually increases upon excitation, from 2.5 D (S(0)), via 6 D (LE) to 9 D (ICT) for DMPURM and from 2.3 D (S(0)), via 7 D (LE) to 8 D (ICT) for DMPURH. From the temperature dependence of Φ'(ICT)/Φ(LE), a reaction enthalpy -ΔH of 11 kJ/mol is obtained for DMPURM in n-hexane (ε(25) = 1.88), increasing to 17 kJ/mol in the more polar solvent di-n-butyl ether (ε(25) = 3.05). With DMPURM in diethyl ether, an activation energy of 8.3 kJ/mol is determined for the LE → ICT reaction (k(a)). The femtosecond excited state absorption spectra at 22 °C undergo an ultrafast decay: 1.0 ps in CHX and 0.63 ps in MeCN for DMPURM, still shorter (0.46 ps) for DMPURH in MeCN. With DMPURM in n-hexane, the LE fluorescence decay time τ(2) increases upon cooling from 2.6 ps at -45 °C to 6.9 ps at -95 °C. The decay involves ICT and IC as the two main pathways: 1/τ(2) ≅ k(a) + k(IC). As a model compound (no ICT) is not available, its lifetime τ(0)(LE) ∼ 1/k(IC) is not known, which prevents a separate determination of k(a). The excited state reactions of DMPURM and DMPURH are treated with a two-state model: S(0) → LE ⇄ ICT. With 6-N-methyl-9-methyladenine (MPURM) and 9-methyladenine (PURM), the fluorescence quantum yield is very low (<5 × 10(-5)) and dominated by impurities, due to enhanced IC from LE to S(0).

Photophysical and quantum chemical study on a J-aggregate forming perylene bisimide monomer

Perylene bisimides (PBIs) are excellent dyes and versatile building blocks for supramolecular structures. Only recently have PBIs been shown to depict absorption characteristics of J-aggregates. We apply electronic structure calculations and femtosecond pump-probe spectroscopy to the monomeric, bay-substituted building-block of a PBI aggregate in dichloromethane to investigate its electronically excited states in order to provide the ingredients for the description of excitons in the aggregates and their annihilation processes. The PBI S(1)←S(0) absorption spectrum and the S(1)→S(0) emission spectrum have been assigned based on time-dependent Density Functional Theory calculations for the geometry-optimized electronic ground state and excited state structures in the gas phase. The monomeric absorption spectrum contains a strong transition at 580 nm and a broad shoulder between 575-500 nm, both features are attributed to a vibrational progression with an effective vibrational mode of 1415 cm(-1) whose major contributing vibrational normal modes are breathing modes of the perylene body. The effective vibrational mode of the emission spectrum is characterized by a frequency of 1369 cm(-1), whose major contributing vibrational normal modes are characterized by perylene and phenol (bay-substituent) CH bendings. The S(n)←S(1) excited state absorption spectrum is assigned based on Multi-Reference Configuration Interaction methodology. Here, we identify three transitions which give rise to two broad experimental features, one being located between 500 and 600 nm and the other one ranging from 650 to 750 nm.

A superfluorescent fluorenyl probe with efficient two-photon absorption

The linear photophysical, excited state absorption (ESA), superfluorescence, and two-photon absorption (2PA) properties of 4,4'-(1E,1'E)-2,2'-(7,7'(1E,1'E)2,2'(4,4'-sulfonylbis(4,1-phenylene))bis(ethane-2,1-diyl)bis(9,9-didecy-9H-fluorene7,2-diyl))bis(ethane-2,1-diyl)bis(N,N-diphenylaniline) (1) were investigated in organic and aqueous media with respect to its potential application in biological imaging. The analysis of linear photophysical properties revealed a rather complex nature of the main one-photon absorption band, strong solvatochromic effects in the steady-state fluorescence spectra, single-exponential fluorescence decay, and high fluorescence quantum yields in organic solvents (≈1.0). The ESA spectra of 1 suggested potential for light amplification in nonpolar media while efficient superfluorescence in cyclohexane was demonstrated. The degenerate 2PA spectra of 1 were obtained over a broad spectral range (640-900 nm), using a standard two-photon induced fluorescence method under 1 kHz femtosecond excitation. Two well defined 2PA bands with maximum 2PA cross sections up to 1700 GM in the higher energy, short wavelength band and ≈1200 GM in the lower energy, long wavelength band of 1 were shown. The potential use of 1 in bioimaging was demonstrated via one- and two-photon in vitro fluorescence imaging of HCT 116 cells.

Theoretical study of solvent influence on the electronic absorption and emission spectra of kynurenine

Neutral/zwitterionic form equilibrium, excited state wave functions, absorption and emission spectra of kynurenine (KN) in various solvents (water, methanol, ethanol, and dimethylsulfoxide) have been studied theoretically. The ground electronic state geometries have been optimized by density functional theory methods; the geometries of the first two singlets excited electronic states have been optimized using the CASSCF technique. The influence of the solvent was taken into account by the calculation of the solvation free energies using the Polarizable Continuum Model (PCM). The spectra of electronic absorption and fluorescence emission have been calculated by the CS-INDO S-CI and SDT-CI methods [Momicchioli, Baraldi, and Bruni, Chem Phys, 1983, 82, 229]. The calculated data reproduce the experimental positions of maxima and the solvent-induced shifts of the absorption and emission bands well. The energy gap between the two lowest excited states of KN increases from aprotic to protic solvents. This fact suggests that the “proximity effect” cannot be responsible for the ultrafast decay of KN fluorescence in protic solvents. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Pentacyano-N,N-Dimethylaniline in the Excited State. Only Locally Excited State Emission, in Spite of the Large Electron Affinity of the Pentacyanobenzene Subgroup

Pentacyano-N,N-dimethylaniline (PCDMA) does not undergo an intramolecular charge transfer (ICT) reaction, even in the strongly polar solvent acetonitrile (MeCN), in clear contrast to 4-(dimethylamino)benzonitrile (DMABN). Within the twisted ICT (TICT) model, this is unexpected, as the electron affinity of the pentacyanobenzene moiety of PCDMA is much larger than that of the benzonitrile subgroup in DMABN. According to the TICT model, the energy of the ICT state of PCDMA would be 2.05 eV (∼16550 cm(-1)) lower than that of DMABN, on the basis of the reduction potentials E(A(-)/A) of pentacyanobenzene (-0.29 V vs saturated calomel electrode (SCE)) and benzonitrile (-2.36 V vs SCE), more than enough to compensate for the decrease in energy of the locally excited (LE) state of PCDMA (E(S(1)) = 19990 cm(-1)) relative to that of DMABN (E(S(1)) = 29990 cm(-1)). This absence of a LE → ICT reaction shows that the TICT hypothesis does not hold for PCDMA in the singlet excited state, similar to what was found for DMABN, N-phenylpyrrole, and their derivatives. In this connection, the six dicyano-substituted dimethylanilines are also discussed. The energy gap ΔE(S(1),S(2)) between the two lowest singlet excited states is, at 7170 cm(-1) for PCDMA in MeCN, considerably larger than that for DMABN (2700 cm(-1) in n-hexane, smaller in MeCN). The absence of ICT is therefore in accord with the planar ICT (PICT) model, which considers a sufficiently small ΔE(S(1),S(2)) to be an important condition determining whether an ICT reaction will take place. The fluorescence quantum yield of PCDMA is very small: Φ(LE) = 0.0006 in MeCN at 25 °C, predominantly due to LE → S(0) internal conversion (IC), as the intersystem crossing yield Φ(ISC) is practically zero (<0.01). From the LE fluorescence decay time of 27 ps for PCDMA in MeCN at 25 °C, a radiative rate constant k(f)(LE) = 2 × 10(7) s(-1) results, comparable to the k(f)(LE) of DMABN (6.5 × 10(7) s(-1)) and 2,4,6-tricyano-N,N-dimethylaniline (TCDMA) (1.2 × 10(7) s(-1)) in this solvent, but clearly larger than the k'(f)(ICT) = 0.79 × 10(7) s(-1) of DMABN in MeCN. The IC reaction with PCDMA in MeCN at room temperature, with a rate constant k(IC) of 3.6 × 10(10) s(-1), is much faster than with TCDMA (25 × 10(7) s(-1)) and DMABN (1.3 × 10(7) s(-1), in n-hexane). This is connected with the nonzero (37°) amino twist angle of PCDMA, which leads to a decrease of the effective LE-S(0) energy gap. The femtosecond excited state absorption (ESA) spectra of PCDMA in MeCN at 22 °C are similar to the LE ESA spectra of TCDMA and DMABN and are therefore attributed to the LE state, confirming that an ICT reaction does not occur. The decay of the LE ESA spectra of PCDMA is single exponential, with a decay time of 22 ps, in reasonable agreement with the LE fluorescence decay time of 27 ps at 25 °C. The spectra decay to zero, showing that there is no triplet or other intermediate.

Study of absorption spectrum and dynamics evaluation of the indocyanine‐green first singlet excited state

AbstractExcited state absorption and excited state dynamics of indocyanine‐green (ICG) dissolved in dymethyl sulfoxide were measured using white‐light continuum Z‐scan (WLCZScan) and white‐light continuum pump–probe (WLCPP) techniques. The excited state absorption spectrum, obtained through Z‐scan measurements, revealed saturable absorption (SA) for wavelengths longer than 630 nm, while reverse saturable absorption (RSA) appeared, as indicated by a band at approximately 570 nm. Both processes were modeled by a three‐energy‐level diagram, from which the excited state cross‐section values were determined. SA and RSA were also observed in pump–probe experiments, with a recovery time in the hundreds of picoseconds time scale due to the long lifetime of the first excited state of ICG. Such results contribute to the understanding of ICG optical properties, allowing application in photonics and medicine. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Ultrafast Intramolecular Charge Transfer with N-(4-Cyanophenyl)carbazole. Evidence for a LE Precursor and Dual LE + ICT Fluorescence

The photophysics of N-(4-cyanophenyl)carbazole (NP4CN) was investigated by using absorption and fluorescence spectra, picosecond fluorescence decays, and femtosecond transient absorption. In the nonpolar n-hexane as well as in the polar solvent acetonitrile (MeCN), a locally excited (LE) state is detected, as a precursor for the intramolecular charge transfer (ICT) state. A LE → ICT reaction time τ(2) at 22 °C of 0.95 ps in ethyl cyanide (EtCN) and 0.32 ps in MeCN is determined from the decay of the LE excited state absorption (ESA) maximum around 620 nm. In the ESA spectrum of NP4CN in n-hexane at a pump-probe delay time of 100 ps, an important contribution of the LE band remains alongside the ICT band, in contrast to what is observed in EtCN and MeCN. This shows that a LE ⇄ ICT equilibrium is established in this solvent and the ICT reaction time of 0.5 ps is equal to the reciprocal of the sum of the forward and backward ICT rate constants 1/(k(a) + k(d)). In the photostationary S(0) → S(n) absorption spectrum of NP4CN in n-hexane and MeCN, an additional CT absorption band appears, absent in the sum of the spectra of its electron donor (D) and acceptor (A) subgroups carbazole and benzonitrile. This CT band is located at an energy of ∼4000 cm(-1) lower than for N-phenylcarbazole (NPC), due to the larger electron affinity of the benzonitrile moiety of NP4CN than the phenyl subunit of NPC. The fluorescence spectrum of NP4CN in n-hexane at 25 °C mainly consists of a structured LE emission, with a small ICT admixture, indicating that a LE → ICT reaction just starts to occur under these conditions. In di-n-pentyl ether (DPeE) and di-n-butyl ether (DBE), a LE emission is found upon cooling at the high-energy edge of the ICT fluorescence band, caused by the onset of dielectric solvent relaxation. This is not the case in more polar solvents, such as diethyl ether (DEE) and MeCN, in which a structureless ICT emission band fully overlaps the strongly quenched LE fluorescence. For the series of D/A molecules NPC, N-(4-fluorophenyl)carbazole (NP4F), N-[4-(trifluoromethyl)phenyl]carbazole (NP4CF), and NP4CN, with increasing electron affinity of their phenyl subgroup, an ICT emission in n-hexane 25 °C only is present for NP4CN, whereas in MeCN an ICT fluorescence is observed with NP4CF and NP4CN. The ICT fluorescence appears when for the energies E(ICT) of the ICT state and E(S(1)) of the lowest excited singlet state the condition E(ICT) ≤ E(S(1)) holds. E(ICT) is calculated from the difference E(D/D(+)) - E(A(-)/A) of the redox potentials of the D and A subgroups of the N-phenylcarbazoles. From solvatochromic measurements with NP4CN an ICT dipole moment μ(e)(ICT) = 19 D is obtained, somewhat larger than the literature values of 10-16 D, because of a different Onsager radius ρ. The carbazole/phenyl twist angle θ = 45° of NP4CN in the S(0) ground state, determined from X-ray crystal analysis, has become smaller for its ICT state, in analogy with similar conclusions for related N-phenylcarbazoles and other D/A molecules in the literature.

On the absorption properties of the excited states of DMABN

There is some recent controversy on whether the excited state absorption spectrum features of DMABN can be explained only with the two excited states responsible for the dual fluorescence in this compound. We have performed CASPT2 calculations on the optimized ππ∗ excited state in gas phase and in water. The results reproduce the main experimental features of the spectra, leading to the conclusion that the participation of another excited state of different nature is not needed to explain the observed absorption bands.

Analysis of the excited-state absorption spectral bandshape of oligofluorenes

We present ultrafast transient absorption spectra of two oligofluorene derivatives in dilute solution. These spectra display a photoinduced absorption band with clear vibronic structure, which we analyze rigorously using a time-dependent formalism of absorption to extract the principal excited-state vibrational normal-mode frequencies that couple to the electronic transition, the configurational displacement of the higher-lying excited state, and the reorganization energies. We can model the excited-state absorption spectrum using two totally symmetric vibrational modes with frequencies 450 (dimer) or 400 cm(-1) (trimer), and 1666 cm(-1). The reorganization energy of the ground-state absorption is rather insensitive to the oligomer length at 230 meV. However, that of the excited-state absorption evolves from 58 to 166 meV between the oligofluorene dimer and trimer. Based on previous theoretical work [A. Shukla et al., Phys. Rev. B 67, 245203 (2003)], we assign the absorption spectra to a transition from the 1B(u) excited state to a higher-lying mA(g) state, and find that the energy of the excited-state transition with respect to the ground-state transition energy is in excellent agreement with the theoretical predictions for both oligomers studied here. These results and analysis permit profound understanding of the nature of excited-state absorption in pi-conjugated polymers, which are the subject of general interest as organic semiconductors in the solid state.