Efficient Intersystem Crossing Research Articles

Deciphering the intersystem crossing in near-infrared BODIPY photosensitizers for highly efficient photodynamic therapy.

Deciphering singlet-to-triplet intersystem crossing (ISC) in organic near-infrared photosensitizers (PSs) is of fundamental importance in the designing of high-performance PSs to boost the clinical usage of photodynamic therapy (PDT). However, in-depth investigations of the ISC dynamics in near-infrared PSs have not been performed to date. Here, systematical investigations of the ISC dynamics in organic near-infrared BODIPY derivatives are presented, in which a multi-channel yet remarkably efficient ISC process is revealed by ultrafast femtosecond transient absorption (fs-TA) spectroscopy and theoretical calculation. The fs-TA verifies an exceptionally enhanced ISC efficiency (Φ ISC = 91%) in iodine-substituted BODIPY (2I-BDP) which is further supported by the calculation results. This endows 2I-BDP with an ultrahigh singlet oxygen quantum yield (Φ Δ = 88%), thus enabling a proof-of-concept application of highly efficient PDT in vivo under ultralow near-infrared light power density (10 mW cm-2). The in-depth understanding of ISC dynamics in organic near-infrared materials may provide valuable guidance in the designing of novel organic theranostic materials for clinical cancer treatment.

Directing energy transfer in Pt(bodipy)(mercaptopyrene) dyads.

We report on the photophysical properties of three dyads that combine a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (bodipy, BDP) and a mercaptopyrene (SPyr) dye ligand at a Pt(PEt3)2 fragment. σ-Bonding of the dyes to the Pt ion promotes intersystem crossing (ISC) via the external heavy atom effect. The coupling of efficient ISC with charge-transfer from the electron-rich mercaptopyrene to the electron-accepting BDP ligand (PB-CT) gives rise to a multitude of (potentially) emissive states. This culminates in the presence of four different emissions for the mono- and dinuclear complexes BPtSPyr and BPtSPyrSPtB with an unsubstituted BDP ligand and either a terminal 1-mercaptopyrene or a bridging pyrene-1,6-dithiolate ligand. Thus, in fluid solution, near IR emission at 724 nm from the 3PB-CT state is observed with a quantum yield of up to 15%. Excitation into the BDP-based 1ππ* or the pyrene-based 1ππ* band additionally trigger fluorescence and phosphorescence emissions from the BDP-centred 1ππ* and 3ππ* states. In frozen solution, at 77 K, phosphorescence from the pyrene ligand becomes the prominent emission channel, while PB-CT emission is absent. Alkylation of the BDP ligand in KBPtSPyr funnels all excitation energy into fluorescence and phosphorescence emissions from the KBDP ligand. The assignments of the various excited states and the deactivation cascades were probed by absorption and emission spectroscopy, transient absorption spectroscopy, electrochemical and UV/Vis/NIR spectroelectrochemical measurements, and by quantum chemical calculations. Our conclusions are further corroborated with the aid of suitable reference compounds comprising of just one chromophore. All dyads are triplet sensitizers and are able to generate singlet oxygen.

Mechanism of Ultrafast Intersystem Crossing in 2-Nitronaphthalene.

Nitronaphthalene derivatives efficiently populate their electronically excited triplet states upon photoexcitation through ultrafast intersystem crossing (ISC). Despite having been studied extensively by time‐resolved spectroscopy, the reasons behind their ultrafast ISC remain unknown. Herein, we present the first ab initio nonadiabatic molecular dynamics study of a nitronaphthalene derivative, 2‐nitronaphthalene, including singlet and triplet states. We find that there are two distinct ISC reaction pathways involving different electronic states at distinct nuclear configurations. The high ISC efficiency is explained by the very small electronic and nuclear alterations that the chromophore needs to undergo during the singlet–triplet transition in the dominating ISC pathway after initial dynamics in the singlet manifold. The insights gained in this work are expected to shed new light on the photochemistry of other nitro polycyclic aromatic hydrocarbons that exhibit ultrafast intersystem crossing.

Control of triplet state generation in heavy atom-free BODIPY-anthracene dyads by media polarity and structural factors.

A family of heavy atom-free BODIPY-anthracene dyads (BADs) exhibiting triplet excited state formation from charge-transfer states is reported. Four types of BODIPY scaffolds, different in the alkyl substitution pattern, and four anthracene derivatives have been used to access BADs. Fluorescence and intersystem crossing (ISC) in these dyads depend on donor-acceptor couplings and can be accurately controlled by substitution or media polarity. Under conditions that do not allow charge transfer (CT), the dyads exhibit fluorescence with high quantum yields. Formation of charge-transfer states triggers ISC and the formation of long-lived triplet excited states in the dyads. The excited state properties were studied by steady-state techniques and ultrafast pump-probe spectroscopy to determine the parameters of the observed processes. Structural information for various BADs was derived from single crystal X-ray structure determinations alongside DFT molecular geometry optimization, revealing the effects of mutual orientation of subunits on the photophysical properties. The calculations showed that alkyl substituents on the BODIPY destabilize CT states in the dyads, thus controlling the charge transfer between the subunits. The effect of the dyad structure on the ISC efficiency was considered at the M06-2X level of theory, and a correlation between mutual orientation of the subunits and the energy gap between singlet and triplet CT states was studied using a multireference CASSCF method.

Controlling Plasmon-Enhanced Fluorescence via Intersystem Crossing in Photoswitchable Molecules.

By harnessing photoswitchable intersystem crossing (ISC) in spiropyran (SP) molecules, active control of plasmon-enhanced fluorescence in the hybrid systems of SP molecules and plasmonic nanostructures is achieved. Specifically, SP-derived merocyanine (MC) molecules formed by photochemical ring-opening reaction display efficient ISC due to their zwitterionic character. In contrast, ISC in quinoidal MC molecules formed by thermal ring-opening reaction is negligible. The high ISC rate can improve fluorescence quantum yield of the plasmon-modified spontaneous emission, only when the plasmonic electromagnetic field enhancement is sufficiently high. Along this line, extensive photomodulation of fluorescence is demonstrated by switching the ISC in MC molecules at Au nanoparticle aggregates, where strongly enhanced plasmonic hot spots exist. The ISC-mediated plasmon-enhanced fluorescence represents a new approach toward controlling the spontaneous emission of fluorophores near plasmonic nanostructures, which expands the applications of active molecular plasmonics in information processing, biosensing, and bioimaging.

Protonation‐State‐Driven Photophysics in Phenothiazinium Dyes: Intersystem Crossing and Singlet‐Oxygen Production

AbstractThe impact of altering the solvent pH value on the photodynamic activity of thionine has been studied computationally by means of density functional theory and multi‐reference interaction methods. To this end, we have investigated the electronic structure of the ground and excited states of diprotonated (TH22+) and neutral imine (T) forms of thionine (TH+). It is well known experimentally that the T1 state of TH+ undergoes acid–base equilibrium reactions resulting in a pronounced pH effect for the efficiency of singlet‐oxygen (1O2) production. Our results show that the energy‐transfer reactions from the T1 state of TH22+ and T to 3O2 correspond to reversible equilibrium processes, whereas in TH+ this process is very exothermic in a vacuum (−0.66 eV) and in aqueous solution (−0.49 eV). These facts explain the experimental observation of a much smaller efficiency of 1O2 production for TH22+ than for TH+. Moreover, we found that the pH value significantly effected the intersystem crossing (ISC) kinetics impacting the concentration of triplet‐state species available for energy transfer. In very acidic aqueous solution (pH<2) where TH22+ is the prevailing species, the ISC proceeds with a rate constant of ≈108 s−1. In a basic medium where T is the dominant species, ISC decay occurs by means of a thermally activated channel (≈108 s−1) which competes with fluorescence (5.32×107 s−1). According to these results, maximum ISC efficiency is expected for intermediate acidic pH values (TH+, ≈109 s−1).

Protonation-Induced Room-Temperature Phosphorescence in Fluorescent Polyurethane.

Room-temperature phosphorescence (RTP) from purely organic systems is of practical importance in biological imaging, oxygen sensing and displaying technologies. The key step to obtaining RTP from organic molecules is efficient intersystem crossing (ISC), which is usually low compared to inorganic materials. Here we show that protonation of a dye molecule, a thioflavin derivative, in strongly polar polyurethane can be used to effectively harness RTP. Prior to protonation, the predominant transition is π-π* for the polymer, which has nearly undetectable RTP due to the large singlet-triplet energy splitting (0.87 eV); when Brønsted acids are gradually added to the system, increasingly strong RTP is observed due to the presence of a new intramolecular charge-transfer state (ICT). The ICT state serves to lower the singlet-triplet energy gap (0.46 eV). The smaller gap results in more efficient ISC and thus strong RTP under deoxygenated conditions. The thioflavin-polyurethane system can be tuned via proton concentration and counterions and opens new doors for RTP-based polymeric sensors and stimuli-responsive materials.

Unravelling the early photochemical behavior of (8-substituted-7-hydroxyquinolinyl)methyl acetates through electronic structure theory and ultrafast transient absorption spectroscopy.

The photophysical processes and photochemical reactions in the ultrafast time region of (8-bromo-7-hydroxyquinolin-2-yl)methyl acetate (BHQ-OAc) in acetonitrile and neutral aqueous solutions were investigated using quantum chemical calculations and femtosecond transient absorption spectroscopy. After initial excitation into the π,π* excited state, BHQ-OAc undergoes an ultrafast intersystem crossing (ISC) into a π,π* excited triplet state on a timescale of 16 ps. The n,π* and π,π* excited singlet and triplet states involved in the photochemistry were identified by means of their characteristic excited state absorption (ESA) bands and from second order coupled-cluster (CC2) calculations. The high ISC rate of BHQ-OAc and related compounds is traced back to involvement of almost energetically degenerate n,π* excited states that enable efficient ISC that obeys El-Sayed's rules.

Efficient Intersystem Crossing in Heavy-Atom-Free Perylenebisimide Derivatives

Efficient intersystem crossing (ISC) in heavy-atom-free organic chromophores remains rare because of the lack of strong spin–orbit coupling effects in such compounds. Finding organic chromophores with ISC ability is important for applications in several areas, e.g., photocatalysis and photodynamic therapy. Herein, we report new perylenebisimide (PBI) chromophores with tetraphenylethynyl substituents at the 2,5,8,11-positions of the PBI core (ortho-positions, not the usually reported bay-positions of PBI), which show efficient ISC without the presence of any heavy atoms. Steady-state and picosecond–nanosecond transient absorption spectroscopies as well as time-dependent density functional theory computations were used to reveal the photophysical properties. For one of the PBI derivatives, excitation wavelength-dependent ISC was observed. The efficient ISC was attributed to the S1/S2 → Tn (n > 1) processes. Photochemical reduction of the PBI derivatives in the presence of a sacrificial electron donor (triet...

Accessing the Triplet State in Heavy‐Atom‐Free Perylene Diimides

Previous studies of perylenediimides (PDIs) mostly utilized the lowest singlet excited state S1 . Generation of a triplet excited state (T1 ) in PDIs is important for applications ranging from photodynamic therapy to photovoltaics; however, it remains a formidable task. Herein, we developed a heavy-atom-free strategy to prompt the T1 ←S1 intersystem crossing (ISC) by introducing electron-donating aryl (Ar) groups at the head positions of an electron-deficient perylenediimide (PDI) core. We found that the ISC efficiency increases from 8 to 54 % and then to 86 % by increasing the electron-donating ability of head-substituted aryl groups from phenyl (p-PDI) to methoxyphenyl (MeO-PDI) and then to methylthioxyphenyl (MeS-PDI). By enhancing the intramolecular charge-transfer (ICT) interaction from p-PDI to MeO-PDI, and then to MeS-PDI, singlet oxygen generation via energy-transfer reactions from T1 of PDIs to (3)O2 was demonstrated with the highest yield of up to 80 %. These results provide guidelines for developing new triplet-generating PDIs and related rylene diimides for optoelectronic applications.

Excited state relaxation pathways of 4-dimethylamino-β-nitrostyrene: Effect of solvent polarity and donor–acceptor conjugation

Abstract Solvent polarity dependent relaxation pathways of the excited state of 4-dimethylamino-β-nitrostyrene (DANS) have been investigated using steady state and transient absorption spectroscopic techniques in femtosecond and nanosecond timescale. The molecule undergoes ultrafast twisted intramolecular charge transfer (TICT) relaxation involving the rotation of dimethylaniline group in polar solvents. In the medium and low polarity solvents, TICT relaxation rate is retarded due to larger barrier of twisting along TICT coordinate. In nonpolar solvents, the S1 state population undergo efficient intersystem crossing (ISC) to triplet surface without any conformational change. However, trans → cis isomerisation in nonpolar solvents takes place from the excited triplet manifold, as monitored by steady state photo-irradiation experiments. The ISC efficiency decreases with the increase in solvent polarity, which is explained by invoking solvent polarity dependent stabilization of charge transfer S1 state relative to a receiver triplet state. Comparison of the excited state dynamics of DANS with less conjugated para-dialkylamino nitrobenzene derivatives, reported previously, suggests that additional π-conjugation between the donor and acceptor groups stabilizes planar ICT state resulting to slower TICT relaxation.

Photophysics and Nonlinear Absorption of Gold(I) and Platinum(II) Donor-Acceptor-Donor Chromophores.

A series of Au(I) and Pt(II) acetylide complexes of a π-conjugated donor-acceptor-donor (D-A-D) chromophore were studied to develop quantitative structure-property relationships for their photophysical and nonlinear optical properties. The D-A-D chromophore consists of a "TBT" unit, where T = 3-hexyl-2,5-thienylene and BTD = 2,1,3-benzothiadiazole, capped with ethynylene groups. The D-A-D chromophore is functionalized with Au(I)PR3 (R = -Me and -Ph) and trans-Pt(II)(PR3)2-CCPh (R = -Me and -Bu) "auxochromes". All of the metal complexes were characterized by ground-state absorption, photoluminescence, nanosecond transient absorption, and two-photon absorption (2PA) spectroscopy. The experiments provided quantitative values of the photophysical parameters, including rates for radiative decay and intersystem crossing (ISC), triplet yields, and two-photon absorption cross sections. Pronounced solvatochromism in the fluorescence spectra suggests an enhanced dipole moment in the excited state of the complexes compared to the unmetalated TBT chromophore. The gold complexes feature larger fluorescence quantum yields and longer emission lifetimes compared to platinum. The Pt(II) complexes exhibit enhanced triplet-triplet absorption, reduced triplet-state lifetimes, and larger singlet oxygen quantum yields, consistent with more efficient ISC compared to the Au(I) complexes. When excited by 100 fs pulses, all of the D-A-D chromophores exhibit moderate two-photon absorption in the near-infrared between 700 and 900 nm. The 2PA cross section for the Au(I) complexes is almost the same as the unmetalated D-A-D chromophore (∼100 GM). The Pt(II) complexes exhibit significantly enhanced 2PA compared to the other chromophores, reaching 1000 GM at 750 nm. Taken together, the results indicate that the Pt(II) center is considerably more effective in inducing singlet-triplet ISC and in enhancing the 2PA cross section. This result reveals the greater promise for Pt(II) acetylides in chromophores for temporal and frequency agile nonlinear absorption.

Impact of non-symmetric 2,9,10-aryl substitution on charge transport and optical properties of anthracene derivatives

A new methodology for the synthesis of non-symmetric 2,9,10-arylanthracenes is proposed. The anthracenes were intentionally decorated with non-symmetric aryls to improve charge transport properties. The impact of non-symmetric aryl substituents on optical and electrical properties was thoroughly studied. Non-symmetric 9,10-aryl substituents ensured blue emission in the range of 422–436 nm and fluorescence quantum efficiency of ca. 0.45, while the type of substituents had a minor impact on excited state deactivation due to their twisted configuration. Optical properties of the compounds obtained in dilute solution and in a rigid polymer matrix revealed the dominating non-radiative decay mechanism to be due to the intersystem crossing (ISC), which was mainly governed by 2-phenyl substitution. Favourable alignment of the singlet and triplet energy levels for efficient ISC was confirmed by density functional theory calculations. Some of the non-symmetrically substituted anthracene derivatives showed very high hole drift mobilities reaching 10−2 cm2/Vs in solution-processed neat films.

Triplet Exciton Dynamics in Fluorene−Amine Copolymer Films

Effect of aromaticity on triplet exciton dynamics was studied by transient absorption spectroscopy for two fluorene-based random copolymers with different aromatic amine, poly(9,9′-di-n-octylfluorene-ran-N,N′-bis(4-n-butylphenyl)-N,N′-diphenyl-1,4-benzenediamine) (F8-PDA) and poly(9,9′-di-n-octylfluorene-ran-N,N′-bis(4-t-butylphenyl)-N,N′-diphenyl-9,10-anthracenediamine) (F8-ADA). On a time scale of nanoseconds, triplet exciton was efficiently formed in F8-PDA through the intersystem crossing (ISC) from singlet exciton with a rate constant of 2.0 × 108 s–1. On the other hand, the ISC was not efficient in F8-ADA, resulting in efficient fluorescence emission. On a time scale of micro- to milliseconds, F8-PDA exhibited bimolecular triplet exciton decay due to triplet–triplet annihilation (TTA), but the TTA was negligible in F8-ADA, indicating that triplet excitons can diffuse freely in F8-PDA while they are trapped at ADA units in F8-ADA. The difference in the ISC efficiency and triplet exciton diffusion is ...

ChemInform Abstract: Triplet Photosensitizers: From Molecular Design to Applications

AbstractReview: 248 refs.

Donor–acceptor–donor thienyl/bithienyl-benzothiadiazole/quinoxaline model oligomers: experimental and theoretical studies

A comprehensive spectral and photophysical investigation of four donor-acceptor-donor (DAD) oligomers consisting of electron-deficient 2,1,3-benzothiadiazole or quinoxaline moieties linked to electron-rich thienyl or bithienyl units has been undertaken. Additionally, a bis(dithienyl) substituted naphthalene was also investigated. The D-A-D nature of these oligomers resulted in the presence of an intramolecular charge transfer (ICT) state, which was further substantiated by solvatochromism studies (analysis with the Lippert-Mataga formalism). Hereby, significant differences have been obtained for the fluorescence quantum yields of the oligomers in the non-polar solvent methylcyclohexane vs. the polar ethanol. The study was further complemented with the determination of the optimized ground-state molecular geometries for the oligomers together with the prediction of the lowest vertical one-electron excitation energy and the relevant molecular orbital contours using DFT calculations. The electronic transitions show a clear HOMO to LUMO charge-transfer character. In contrast to the thiophene oligomers (the oligothiophenes with n = 1-7), where the intersystem crossing (ISC) yield decreases with n, the studied DAD oligomers were found to show an increase in the ISC efficiency with the number of (donor) thienyl units.

Triplet photosensitizers: from molecular design to applications

Triplet photosensitizers (PSs) are compounds that can be efficiently excited to the triplet excited state which subsequently act as catalysts in photochemical reactions. The name is originally derived from compounds that were used to transfer the triplet energy to other compounds that have only a small intrinsic triplet state yield. Triplet PSs are not only used for triplet energy transfer, but also for photocatalytic organic reactions, photodynamic therapy (PDT), photoinduced hydrogen production from water and triplet-triplet annihilation (TTA) upconversion. A good PS should exhibit strong absorption of the excitation light, a high yield of intersystem crossing (ISC) for efficient production of the triplet state, and a long triplet lifetime to allow for the reaction with a reactant molecule. Most transition metal complexes show efficient ISC, but small molar absorption coefficients in the visible spectral region and short-lived triplet excited states, which make them unsuitable as triplet PSs. One obstacle to the development of new triplet PSs is the difficulty in predicting the ISC of chromophores, especially of organic compounds without any heavy atoms. This review article summarizes some molecular design rationales for triplet PSs, based on the molecular structural factors that facilitate ISC. The design of transition metal complexes with large molar absorption coefficients in the visible spectral region and long-lived triplet excited states is presented. A new method of using a spin converter to construct heavy atom-free organic triplet PSs is discussed, with which ISC becomes predictable, C60 being an example. To enhance the performance of triplet PSs, energy funneling based triplet PSs are proposed, which show broadband absorption in the visible region. Applications of triplet PSs in photocatalytic organic reactions, hydrogen production, triplet-triplet annihilation upconversion and luminescent oxygen sensing are briefly introduced.

Femtosecond Intersystem Crossing in the DNA Nucleobase Cytosine.

Ab initio molecular dynamics including nonadiabatic and spin-orbit couplings on equal footing is used to unravel the deactivation of cytosine after UV light absorption. Intersystem crossing (ISC) is found to compete directly with internal conversion in tens of femtoseconds, thus making cytosine the organic compound with the fastest triplet population calculated so far. It is found that close degeneracy between singlet and triplet states can more than compensate for very small spin-orbit couplings, leading to efficient ISC. The femtosecond nature of the ISC process highlights its importance in photochemistry and challenges the conventional view that large singlet-triplet couplings are required for an efficient population flow into triplet states. These findings are important to understand DNA photostability and the photochemistry and dynamics of organic molecules in general.

Elucidating the Early Steps in Photoinitiated Radical Polymerization via Femtosecond Pump–Probe Experiments and DFT Calculations

The excited states and dynamics of the three triplet radical photoinitiators benzoin (2-hydroxy-1,2-diphenylethanone, Bz), 2,4,6-trimethylbenzoin (2-hydroxy-1-mesityl-2-phenylethanone, TMB), and mesitil (1,2-bis(2,4,6-trimethylphenyl)-1,2-ethanedione, Me)—employed in our previous studies for quantifying net initiation efficiencies in pulsed laser polymerization with methacrylate monomers [Voll, D.; Junkers, T.; Barner-Kowollik, C. Macromolecules2011, 44, 2542–2551]—are investigated via both femtosecond transient absorption (TA) spectroscopy and density functional theory (DFT) methods to elucidate the underlying mechanisms causing different initiation efficiencies when excited at 351 nm. Bz and TMB are found to have very similar properties in the calculated excited states as well as in the experimentally observed dynamics. After excitation into the first excited singlet state (S1) Bz and TMB undergo rapid intersystem crossing (ISC). The ISC can compete with ultrafast internal conversion (IC) processes because an excited triplet state (Tn) of nearly the same energy is present in both cases. ISC is therefore the dominating depopulation channel of S1, and subsequent α-cleavage to produce radicals takes place on the picosecond time scale. In contrast, Me is excited into the second excited singlet state (S2). In this case no isoenergetic triplet state is available, which inhibits ISC from competing with ultrafast deactivation processes. ISC is therefore assigned to be a minor deactivation channel in Me. Employing these findings, quantitative photoinitiation efficiency relations of Bz, TMB, and Me obtained by pulsed laser polymerization can be directly correlated with the relative TA intensities found in the femtosecond experiments. The ISC efficiency is thus a critical parameter for evaluating the overall photoinitiation efficiency and demonstrates that the employment of the herein presented method represents a powerful tool for attaining a quantitative picture on the suitability of a photoinitiator.

The photophysics of flavins: What makes the difference between gas phase and aqueous solution?

The ground and low-lying excited electronic states of isoalloxazine, 10-methylisoallox-azine and lumiflavin, three flavin-related compounds, were investigated by means of quantum chemical methods. Minimum structures were determined employing (time-dependent) Kohn–Sham density functional theory. Spectral properties were computed utilizing a combined density functional and multi-reference configuration interaction (DFT/MRCI) method. Solvent effects were mimicked by a conductor like screening model and micro-hydration with four explicit water molecules. At selected points along a linearly interpolated path connecting the Franck–Condon region and the S 1minimum, spin–orbit interaction was computed employing a nonempirical mean-field Hamiltonian. For isoalloxazine, intersystem crossing (ISC) rate constants were computed, taking both direct and vibronic spin–orbit coupling into account. On the basis of these calculations we suggest the following photo relaxation model. In the vacuum, efficient ISC ( k ISC ≈ 1 0 9 s − 1 ) takes place between the primarily excited ( π → π ∗ ) 1 state (S 1) and the lowest ( n → π ∗ ) 3 state (T 2). The energetic proximity of the ( n → π ∗ ) 1 state (S 2) enhances the nonradiative relaxation of S 1 by internal conversion (IC). In aqueous solution these ISC and IC channels are energetically not accessible due to the blue shift of the ( n → π ∗ ) states. The high triplet quantum yield observed in experiment [J.T.M. Kennis, S. Crosson, M. Gauden, I.H.M. van Stokkum, K. Moffat, R. van Grondelle, Biochemistry 42 (2003) 3385–3392] is explained by the intersection between the ( π → π ∗ ) 1 state (S 1) potential energy hypersurface (PEH) and the second ( π → π ∗ ) 3 (T 2) PEH along the relaxation pathway and the strong enhancement of their spin–orbit coupling by vibronic interactions. The calculated ISC rate for this channel ( k ISC ≈ 1 0 8 s − 1 ) is in good agreement with experimental results. According to our model, lack of an efficient IC channel leads to an increased fluorescence quantum yield in aqueous solution.