Franck-Condon State Research Articles

Determination of 0–0-Transition Frequencies from Diffuse Vibronic Spectra

The equality of the probabilities of direct and inverse vibronic transitions between the initial and final elementary quantum Franck–Condon states in the adiabatic approximation is taken as the equality of the probabilities of one electronic transition in which only the initial and final elementary vibrational ensembles exchange places. The transition frequencies will have mirror symmetry with respect to the 0–0-transition frequency ν0. Accounting for this and selecting hot molecules in the anti-Stokes regions with the initial state in thermal equilibrium leads to a restoration of the known relation for the transition cross sections σ(ν) in the form [σ(ν)/ν] exp (±hν/2kT) = 𝜑[(ν – ν0)2], where ″+″ denotes emission; ″–″, absorption. Examples of using the relation to determine ν0 from absorption, fluorescence, and phosphorescence spectra are given and compared with ν0 obtained by jet-cooling.

Synthesis of 5-Acyl-2-Amino-3-Cyanothiophenes: Chemistry and Fluorescent Properties.

Independent of the substrate structure and reaction conditions, 3-amino-2-cyanothioacrylamides, which contain two active electrophilic centers, were shown to interact with various active halo methylene compounds under mild conditions to afford 5-acyl-2-amino-3-cyanothiophenes as the only products. A series of new polyfunctional thiophene derivatives with a rare combination of functionalities were synthesized, and their photophysical properties were experimentally and computationally investigated. The calculated electronic characteristics of the ground and excited states were compared to the experimental results, which provided a good understanding of the relationship between the optoelectronic properties and the molecular structures. After absorption of light quanta, the systems populate an intramolecular charge-transfer (ICT) Franck-Condon state, and emission occurs from a twisted ICT minimum.

Structural symmetry breaking in octupolar tetrastyrylpyrazines and their dipole moments in equilibrium ground and Franck-Condon excited state

From electrooptical absorption measurements (EOAM) follows that the dipole moment of octupolar tetrastyrylpyrazines in the equilibrium ground state is large and that the change of dipole moments upon transition to the excited Franck–Condon state is significant. Obtained results unambiguously testify to structural symmetry breaking in the studied octupolar tetrastyrylpyrazines. Molecular mechanics and semi-empirical calculations evidence that tetrastyrylpyrazines have non-planar configurations. The non-planar geometry of the molecules causes large μg values and significant change of dipole moment Δaμ after excitation.Due to large μg and Δaμ values the maxima of the first absorption band and fluorescence band of the studied tetrastyrylpyrazines in various solvents shift to the long-wavelength region with an increasing solvent polarity. Both absorption and fluorescence spectra of the dyes do not change the shape during bathochromic shift in the solvents. This phenomenon evidences about the defining role of universal van der Waals interactions of dyes with the environment, namely dispersive, orientational, and inductive nature.

Solvent effects on the photophysical properties of distyrylnaphthalene-based conjugated oligoelectrolytes

In this study we have characterized electronic structure and photophysical properties of the new-synthetized distyrylnaphthalene DSNN derivatives in solvents of different polarity and viscosity with the use of both experimental (absorption and fluorescence spectroscopy) and theoretical (TD DFT(B3LYP)/PCM) methods. The obtained results were interpreted in terms of the reasonable scenario for the change in both geometrical conformation and charge distribution upon photoexcitation. Direct excitation of DSNN conformers with non-planar geometries concerning the conjugated backbone leads to the locally excited Franck-Condon excited states LE FC, from which the ICT state is formed. Most probably, the formation of this ICT state involves planarization and quinoidization of the conjugated core. The emissive non-relaxed LE and relaxed ICT states differ in geometry, charge distribution, polarity as well as in radiative and nonradiative decay rates. Environmental effects have a profound effect on the rate of ICT formation and also on the relative stability of these states. Thus, controlling the environment can be a very effective way of tuning the linear and nonlinear responses of the compounds studied.

Ultrafast Dynamics in Light-Driven Molecular Rotary Motors Probed by Femtosecond Stimulated Raman Spectroscopy.

Photochemical isomerization in sterically crowded chiral alkenes is the driving force for molecular rotary motors in nanoscale machines. Here the excited-state dynamics and structural evolution of the prototypical light-driven rotary motor are followed on the ultrafast time scale by femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption (TA). TA reveals a sub-100-fs blue shift and decay of the Franck-Condon bright state arising from relaxation along the reactive potential energy surface. The decay is accompanied by coherently excited vibrational dynamics which survive the excited-state structural evolution. The ultrafast Franck-Condon bright state relaxes to a dark excited state, which FSRS reveals to have a rich spectrum compared to the electronic ground state, with the most intense Raman-active modes shifted to significantly lower wavenumber. This is discussed in terms of a reduced bond order of the central bridging bond and overall weakening of bonds in the dark state, which is supported by electronic structure calculations. The observed evolution in the FSRS spectrum is assigned to vibrational cooling accompanied by partitioning of the dark state between the product isomer and the original ground state. Formation of the product isomer is observed in real time by FSRS. It is formed vibrationally hot and cools over several picoseconds, completing the characterization of the light-driven half of the photocycle.

Fluorescence and picosecond induced absorption from the lowest singlet excited states of quercetin in solutions and polymer films

The spectroscopic and photophysical properties of the biologically important plant antioxidant quercetin in organic solvents, polymer films of polyvinyl alcohol, and a buffer solution at pH 7.0 are studied by stationary luminescence and femtosecond laser spectroscopy at room temperature and 77 K. The large magnitude of the dipole moment of the quercetin molecule in the excited Franck–Condon state μ e FC = 52.8 C m indicates the dipolar nature of quercetin in this excited state. The transient induced absorption spectra S 1→S n in all solvents are characterized by a short-wave band at λ abs max = 460 nm with exponential decay times in the range of 10.0–20.0 ps. In the entire spectral range at times of >100 ps, no residual induced absorption was observed that could be attributed to the triplet–triplet transitions Т 1 → Т k in quercetin. In polar solvents, two-band fluorescence was also recorded at room temperature, which is due to the luminescence of the initial enol form of quercetin (~415 nm) and its keto form with a transferred proton (550 nm). The short-wave band is absent in nonpolar 2-methyltetrahydrofuran (2-MTHF). The spectra of fluorescence and fluorescence excitation exhibit a low dependence on the wavelength of excitation and detection, which may be related to the solvation and conformational changes in the quercetin molecule. Decreasing the temperature of a glassy-like freezing quercetin solution in ethanol and 2-MTHF to 77 K leads to a strong increase in the intensity (by a factor of ~100) of both bands. The energy circuits for the proton transfer process are proposed depending on the polarity of the medium. The main channel for the exchange of electronic excitation energy in the quercetin molecule at room temperature is the internal conversion S 1 ⇝ S 0, induced by the state with a proton transfer.

Effect of Bridge Alteration on Ground- and Excited-State Properties of Ruthenium(II) Complexes with Electron-Donor-Substituted Dipyrido[3,2-a:2′,3′-c]phenazine Ligands

A series of Ru(II) 2,2'-bipyridine (bpy) complexes with an electron-accepting dipyrido[3,2-a:2',3'-c]phenazine (dppz) ligand coupled to an electron-donating triarylamine (TAA) group have been investigated. Systematic alteration of a bridging unit between the dppz and TAA allowed exploration into how communication between the donor and acceptor is perturbed by distance, as well as by steric and electronic effects. The effect of the bridging group on the electronic properties of the systems was characterized using a variety of spectroscopic methods, including Fourier transform-Raman (FT-Raman) spectroscopy, resonance Raman spectroscopy, and transient resonance Raman (TR2) spectroscopy. These methods were used in conjunction with ground- and excited-state absorption spectroscopy, electrochemical studies, and DFT calculations. The ground-state electronic absorption spectra show distinct variation with the bridging group, with the wavelength observed for the lowest energy electronic transition ranging from 449 nm to 522 nm, accompanied by large changes in the molar absorptivity. The lowest-energy Franck-Condon state was determined to be intra-ligand charge transfer (ILCT) in nature for most compounds. The presence of higher-energy metal-to-ligand charge transfer (MLCT) Ru(II) → bpy and Ru(II) → dppz transitions was also confirmed via resonance Raman spectroscopy. The TR2 spectra showed characteristic dppz• - and TAA• + vibrations, indicating that the THEXI state formed was also ILCT in nature. Excited-state lifetime measurements reveal that the rate of decay is in accordance with the energy gap law and is not otherwise affected by the nature of the bridging unit.

Electrooptical Absorption Measurements (EOAM) Testify Existence of two Conformers of Prodan and Laurdan with Different Dipole Moments in Equilibrium Ground and Franck-Condon Excited State.

The results from the electrooptical absorption measurements (EOAM) on the equilibrium ground and excited Franck-Condon state dipole moments of Prodan and Laurdan in 1,4-dioxane are presented. As follows from experiments Prodan and Laurdan in the equilibrium ground and excited Franck-Condon state have two conformers with considerably different dipole moments. The electrical dipole moments and the transition dipole moment, obtained from the short-wavelength region of the absorption spectrum are parallel. The electrical dipole moments measured at the long-wavelength spectral region are parallel to each other but not parallel to the transition dipole moment m a. The angle θ between the transition dipole moment m a and the dipole moment in the equilibrium ground state μ g of the long-wavelength conformer is about 30(0) for both probes. Obtained results evidence that donor-acceptor pairs of the short-wavelength and long-wavelength conformers are not located on the same axis. Two low-energy conformers of Prodan have been found by density functional theory (DFT) calculations, differing in the orientation of the carbonyl group towards the naphthalene system.

Potential‐Energy Surfaces, the Born–Oppenheimer Approximations, and the Franck–Condon Principle: Back to the Roots

The concept of a potential-energy surface (PES) is central to our understanding of spectroscopy, photochemistry, and chemical kinetics. However, the terminology used in connection with the basic approximations is variously, and somewhat confusingly, represented with such phrases as "adiabatic", "Born-Oppenheimer", or "Born-Oppenheimer adiabatic" approximation. Concerning the closely relevant and important Franck-Condon principle (FCP), the IUPAC definition differentiates between a classical and quantum mechanical formulation. Consequently, in many publications we find terms such as "Franck-Condon (excited) state", or a vertical transition to the "Franck-Condon point" with the "Franck-Condon geometry" that relaxes to the excited-state equilibrium geometry. The Born-Oppenheimer approximation and the "classical" model of the Franck-Condon principle are typical examples of misused terms and lax interpretations of the original theories. In this essay, we revisit the original publications of pioneers of the PES concept and the FCP to help stimulate a lively discussion and clearer thinking around these important concepts.

(Invited) Photoinduced Charge Transfer Reactions and Excited State Properties in Triphenylamine C60 Donor-Acceptor Conjugates

The molecular heterojunction triphenylamine/C60 is a potential material to construct high-efficient dye-sensitized solar cells. We have carried out a combined molecular dynamics and time-dependent density functional theory study to analyze charge separation and exciton delocalization in excited states of two constitutional isomers of this interface.[1] Our results reveal the presence of exciton states characterized by partial charge-transfer character (hybrid states); which are suggested to promote the direct charge separation process by excitation. These states may have a significant impact on the efficiency of the light-harvesting ensembles. That is to say, a greater amount of such hybrid states is found at short distances between the triphenylamine fragment and the C60 cage; as a result, charge transfer reactions can occur faster. This trend resembles the experimental evidence since rate constants for photoinduced charge transfer reactions kct are bigger for the constitutional isomer with the shortest distance between the triphenylamine fragment and the C60 cage.[2] We have also evaluated the performance of U-CAM-B3LYP/6-31G* and TD-CAM-B3LYP/6-31G* level of theories to compute the different parameters affecting kct ; that is the reorganization energy λ, the electronic coupling Vij , and free energies ΔG.[3] Our study is done under the theory of nonadiabatic electron transfer. By using the standard Marcus formulation, our results show that the outer-sphere contribution to λ and ΔG for the recombination reaction are the parameters that more uncertainty introduces to the estimation of kct . We implemented a low-cost methodology based on U-DFT for the evaluation of the outer-sphere part of λ and ΔG; wherein Franck-Condon and relaxed charge-transfer states are conveniently modeled, including properly the solvent reorganization energy, by U-CAM-B3LYP/6-31G* coupled to continuum solvation models. Finally, it is worth to mention that the replacement of the C60 cage by Ih -Sc3N@C80 generates longer-lived charge-transfer states; which is attributed to the lower ΔG of the charge recombination reaction.[2] Our group is currently analyzing this trend for the whole family of endohedral metallofullerenes; for which dynamics is expected to be in the normal region of the Marcus parabola. [1] J. P. Martínez, S. Osuna, M. Solà, A. Voityuk, Theor. Chem. Acc. 2015, 134, 12. [2] J. R. Pinzón, D. C. Gasca, S. G. Sankaranarayanan, G. Bottari, T. Torres, D. M. Guldi, L. Echegoyen, J. Am. Chem. Soc. 2009, 131, 7727–7734. [3] J. P. Martínez, M. Solà, A. Voityuk, 2015, in preparation. Figure 1

Instantaneous Intervalley Transition Just at the Franck–Condon State in the Conduction Band of GaAs

We propose two possible quantum microscopic mechanisms for intervalley transitions just at the Franck-Condon state in GaAs. The first is the simultaneously combined coulombic elastic transitions of two electrons,from the originally photo-excited $\Gamma$ valley to the mutually opposite L and -L valleys in the Brillouin zone. The second is the "elastic" scattering of an electron from the $\Gamma$ valley to the L one by the frozen phonon, which is not newly created in the excited state, but already exists, being inherited from the starting ground state, according to the Franck-Condon principle of the photo-excitation. Surprisingly, we can theoretically show that such the elastic electron-phonon scattering gives extremely fast time constant of the order of a few tens femtoseconds, as well as the above elastic electron-electron scattering does.

Exploration of Quenching Pathways of Multiluminescent Acenes Using the GRRM Method with the SF-TDDFT Method.

The quenching pathways were investigated for three types of multiluminescent acene derivatives, which show environment-dependent fluorescence. Spin-flip time dependent density functional theory (SF-TDDFT) combined with the Global Reaction Route mapping (GRRM) strategy is employed to locate minimum-energy conical intersections (MECIs). The energies and geometries of the MECIs relative to the Franck-Condon (FC) state control the difference in fluorescence behavior among the three derivatives. For the molecule with a phenyamide moiety, a MECI with energy lower than the FC state with large geometrical change from V-type to flat structure provides an efficient internal conversion (quenching) pathway in solution. For the same molecule, in a solid, this large geometrical change is inhibited, and the second MECI, with an energy lower than FC but higher than the first MECI requiring only a small geometry change of CH out-of-plane bending, contributes to the quenching. The molecule with the napthaleneimide moiety has only one low-energy MECI that requires large geometrical change from the V-type to flat structure. Although this MECI provides the quenching pathway in solution, in the solid, this large motion is inhibited, and the molecule will stay in the excited state and emit. The molecule with an anthraceneimide moiety has no conical intersection lower than the FC state, and no quenching pathway is available in solution or solid. In addition, in this molecule, at the local minimum of the excited state, the dipole transition to the ground state is allowed, and this molecule prefers emission rather than internal conversion.

Excited State Features and Dynamics in a Distyrylbenzene-Based Mixed Stack Donor-Acceptor Cocrystal with Luminescent Charge Transfer Characteristics.

Combined structural, photophysical, and quantum-chemical studies at the quantum mechanics/molecular mechanics (QM/MM) level precisely reveal the structure-property relationships in a mixed-stack donor-acceptor cocrystal, which displays vibronically structured fluorescence, strongly red-shifted against the spectra of the parent donor and acceptor, with high quantum yield despite the pronounced CT character of the emitting state. The study elucidates the reasons for this unusual combination, quantifies the ordering and nature of the collective excited singlet and triplet state manifold, and details the deactivation pathways of the initially created Franck-Condon state.

Kinetics of Thermal Conversion of Conjugated Polymers Investigated from Their Optical Absorption Spectra.

We report on a phenomenological, theoretical model to calculate the rate constants and activation energies for the thermal conversion reactions from poly(xylylidene tetrahydrothiophenium chloride) (PTHT) into poly(p-phenylenevinylene) (PPV) using the optical absorption spectra of spin-coated films. The probabilities of electron transitions were calculated considering Franck-Condon states with a Gaussian distribution of conjugated segments and molecular excitons. The dependence on the conjugation degree (n) for the energy gap, transition dipole moment, and electron-phonon coupling were obtained semiempirically using published data for PPV. Fitting was performed for the C-C stretching of the aromatic ring 1550 cm(-1), for it is considered the most optically active. The isotherms for consumption and formation of PPV segments were fitted using a first-order and consecutive reactions, respectively. With this modeling we could identify the most probable reactions, where the formation of longer PPV segments n ≥ 3 occurs only via reactions from smaller conjugated segments (i.e., n = 1 and 2). The activation energies tend to decrease with longer conjugation lengths. Significantly, the modeling allows us to predict the conversion temperature and chemical composition yielding a predefined distribution of conjugated segments, which can be applied to any polymer undergoing thermal conversion, decomposition, or photo-oxidation.

The Interactions with Proton-Donor Solvents and Stability of some Borondipyrrinates

The constants of acid-base equilibrium were estimated with spectroscopic method at the ground (S0) and at the fluorescent (S1fl) states. The constants of acid-base equilibrium for Franck-Condon excited states (S1F-C) were estimated from spectral shifts for basic and acid forms. This results are compared with quantum yields of photostability of complexes in neutral solvents. The mechanisms of complexes protonation are discussed dependence from the structure of ligands.

The Photophysics of Three Naphthylmethylene Malononitriles

The solvent dependence of the photophysical properties of three naphthylmethylene malononitriles, 1-(1-naphthalenylmethylene)-propanedinitrile (1-MN), 2-(2-naphthalenylmethylene)-propanedinitrile (2-MN), and 2-(3,4-dihydro-1(2H)-phenanthrenylidene)-propanedinitile (r2-MN), was studied in order to determine their potential utility as fluidity probes and to make comparisons to the better studied benzylidene malononitriles. Density functional calculations were used to understand the possible conformational states related to rotation about the vinyl-aromatic bond ("τ"). Absorption and emission frequencies, extinction coefficients, fluorescence quantum yields, and fluorescence lifetimes were measured in 11 representative solvents. Both the computational and experimental results indicate that the S0 → S1 transitions of these molecules have substantial charge-transfer character and produce highly polar excited states. Emission appears to result from relaxed S1 states which do not differ qualitatively from the Franck-Condon states reached by absorption. In 2-MN, time-resolved emission reveals the presence of two ground-state conformers ("a" and "b" differing by ∼180° rotation about τ) coexisting in low-polarity solvents. In contrast, 1-MN appears to exist primarily as a single dominant ground-state conformer. Fluorescence lifetimes vary from ∼1 ps in 1-MN to ∼200 ps in 2-MN(a) at room temperature. With the exception of 2-MN(a), the lifetimes vary systematically with solvent in a manner similar to what is observed in the benzylidene malononitriles. Both solvent polarity and fluidity appear to be important determinants of lifetime. The primary mechanism of fluorescence decay in naphthylmethylene malononitriles is likely to be the same as that of the benzylidene malononitriles-twisting about the double bond in S1, which leads to rapid internal conversion via a conical intersection with S0.

Diverse photoinduced dynamics in an organic charge-transfer complex having strong electron-phonon interactions.

CONSPECTUS: Phenomena that occur in nonequilibrium states created by photoexcitation differ qualitatively from those that occur at thermal equilibrium, and various physical theories developed for thermal equilibrium states can hardly be applied to such phenomena. Recently it has been realized that understanding phenomena in nonequilibrium states in solids is important for photoenergy usage and ultrafast computing. Consequently, much effort has been devoted to revealing such phenomena by developing various ultrafast observation techniques and theories applicable to nonequilibrium states. This Account describes our recent studies of diverse photoinduced dynamics in a strongly correlated organic solid using various ultrafast techniques. Solids in which the electronic behavior is affected by Coulomb interactions between electrons are designated as strongly correlated materials and are known to exhibit unique physical properties even at thermal equilibrium. Among them, many organic charge-transfer (CT) complexes have low dimensionality and flexibility in addition to strong correlations; thus, their physical properties change sensitively in response to changes in pressure or electric field. Photoexcitation is also expected to drastically change their physical properties and would be useful for ultrafast photoswitching devices. However, in nonequilibrium states, the complicated dynamics due to these characteristics prevents us from understanding and using these materials for photonic devices. The CT complex (EDO-TTF)2PF6 (EDO-TTF = 4,5-ethylenedioxytetrathiafulvalene) exhibits unique photoinduced dynamics due to strong electron-electron and electron-phonon interactions. We have performed detailed studies of the dynamics of this complex using transient electronic spectroscopy at the 10 and 100 fs time scales. These studies include transient vibrational spectroscopy, which is sensitive to the charges and structures of constituent molecules, and transient electron diffraction, which provides direct information on the crystal structure. Photoexcitation of the charge-ordered low-temperature phase of (EDO-TTF)2PF6 creates a new photoinduced phase over 40 fs via the Franck-Condon state, in which electrons and vibrations are coherently and strongly coupled. This new photoinduced phase is assigned to an insulator-like state in which the charge order differs from that of the initial state. In the photoinduced phase, translations of component molecules proceed before the rearrangements of intramolecular conformations. Subsequently, the charge order and structure gradually approach those of the high-temperature phase over 100 ps. This unusual two-step photoinduced phase transition presumably originates from steric effects due to the bent EDO-TTF as well as strong electron-lattice interactions.

Femtosecond dynamics of primary processes in visual pigment rhodopsin

The dynamics of the coherent photoisomerization of the 11-cis-retinal in bovine rhodopsin is studied by femtosecond time-resolved laser absorption spectroscopy with a resolution of 30 fs. Rhodopsin is excited with 500-, 535-, and 560-nm femtosecond pulses to produce different initial Franck-Condon states with different vibrational energies of the molecule in its electronically excited state. The time evolution of the photoinduced differential absorption spectra of rhodopsin is measured upon excitation with a femtosecond pulse in a spectral range from 400 to 720 nm. Oscillations in the time-resolved absorption of the rhodopsin photoproducts, such as photorhodopsin with a vibrationally excited all-trans-retinal and in its initial-state rhodopsin with a vibrationally excited 11-cis-retinal, are examined. It is demonstrated that these oscillations reflect the dynamics of coherent vibrational wavepackets. The Fourier transform of these oscillatory components yields frequencies, amplitudes, and initial phases of various vibrational modes involved in the motion the wavepackets in both photoproducts. The main vibrational modes manifest themselves at frequencies of 62 and 160 cm−1 for photorhodopsin and 44 and 142 cm−1 for initial-state rhodopsin. It is shown that these vibrational modes are directly involved in the coherent reaction under the study, with their amplitudes in the power spectrum produced by the Fourier transform of the kinetic curves being dependent on the wavelength of rhodopsin excitation.

Chemically optimizing operational efficiency of molecular rotary motors.

Unidirectional molecular rotary motors that harness photoinduced cis-trans (E-Z) isomerization are promising tools for the conversion of light energy to mechanical motion in nanoscale molecular machines. Considerable progress has been made in optimizing the frequency of ground-state rotation, but less attention has been focused on excited-state processes. Here the excited-state dynamics of a molecular motor with electron donor and acceptor substituents located to modify the excited-state reaction coordinate, without altering its stereochemistry, are studied. The substituents are shown to modify the photochemical yield of the isomerization without altering the motor frequency. By combining 50 fs resolution time-resolved fluorescence with ultrafast transient absorption spectroscopy the underlying excited-state dynamics are characterized. The Franck-Condon excited state relaxes in a few hundred femtoseconds to populate a lower energy dark state by a pathway that utilizes a volume conserving structural change. This is assigned to pyramidalization at a carbon atom of the isomerizing bridging double bond. The structure and energy of the dark state thus reached are a function of the substituent, with electron-withdrawing groups yielding a lower energy longer lived dark state. The dark state is coupled to the Franck-Condon state and decays on a picosecond time scale via a coordinate that is sensitive to solvent friction, such as rotation about the bridging bond. Neither subpicosecond nor picosecond dynamics are sensitive to solvent polarity, suggesting that intramolecular charge transfer and solvation are not key driving forces for the rate of the reaction. Instead steric factors and medium friction determine the reaction pathway, with the sterically remote substitution primarily influencing the energetics. Thus, these data indicate a chemical method of optimizing the efficiency of operation of these molecular motors without modifying their overall rotational frequency.

Cooperative Conformational Change and Excitation Migration of Biphenyl-PMO Amorphous Film, As Revealed by Femtosecond Time-Resolved Spectroscopy

Excited state dynamics of biphenyl-bridged mesoporous organosilica (Bp-PMO) film was investigated by femtosecond transient absorption and dichroism measurements at various excitation intensities. Under the excitation condition with low intensity (ca. 0.1 μJ/pulse), the relaxation from the excited Franck–Condon state with skewed structure of the two phenyl rings to the preplanar state occurred with a time constant of 550 fs, followed by the excimer formation with two time constants of 9.0 and 140 ps. Under higher excitation condition with 1.0 μJ/pulse, very rapid excimer formation within 500 fs was observed. From the analysis of the transient absorption spectra, it was revealed that the cooperative geometrical relaxation from skewed to planar structures, in addition to the energy migration, led to the rapid excimer formation under the high excitation condition. By integrating these results with the fluorescence dynamics, the photoprimary processes in Bp-PMO, such as energy migration, annihilation, and excimer formation, were discussed.