Controlling phosphorescence color and quantum yields in cationic iridium complexes: a combined experimental and theoretical study

We report a combined experimental and theoretical study on cationic Ir(III) complexes for OLED applications and describe a strategy to tune the phosphorescence wavelength and to enhance the emission quantum yields for this class of compounds. This is achieved by modulating the electronic structure and the excited states of the complexes by selective ligand functionalization. In particular, we report the synthesis, electrochemical characterization, and photophysical properties of a new cationic Ir(III) complex, [Ir(2,4-difluorophenylpyridine)2(4,4'-dimethylamino-2,2'-bipyridine)](PF(6)) (N969), and compare the results with those reported for the analogous [Ir(2-phenylpyridine)2(4,4'-dimethylamino-2,2'-bipyridine)](PF(6)) (N926) and for the prototype [Ir(2-phenylpyridine)2(4,4'-tert-butyl-2,2'-bipyridine)](PF(6)) complex, hereafter labeled N925. The three complexes allow us to explore the (C/\N) and (N/\N) ligand functionalization: considering N925 as a reference, we investigate in N926 the effect of electron-releasing substituents on the bipyridine ligand, while in N969, we investigate the combined effect of electron-releasing substituents on the bipyridine ligand and the effect of electron-withdrawing substituents on the phenylpyridine ligands. For N969 we obtain blue-green emission at 463 nm with unprecedented high quantum yield of 85% in acetonitrile solution at room temperature. To gain insight into the factors responsible for the emission color change and the different quantum yields, we perform DFT and TDDFT calculations on the ground and excited states of the three complexes, characterizing the excited-state geometries and including solvation effects on the calculation of the excited states. This computational procedure allows us to provide a detailed assignment of the excited states involved in the absorption and emission processes and to rationalize the factors determining the efficiency of radiative and nonradiative deactivation pathways in the investigated complexes. This work represents an example of electronic structure-driven tuning of the excited-state properties, thus opening the way to a combined theoretical and experimental strategy for the design of new iridium(III) phosphors with specific target characteristics.     

Color tuning in rhodopsins: the mechanism for the spectral shift between bacteriorhodopsin and sensory rhodopsin II

The mechanism of color tuning in the rhodopsin family of proteins has been studied by comparing the optical properties of the light-driven proton pump bacteriorhodopsin (bR) and the light detector sensory rhodopsin II (sRII). Despite a high structural similarity, the maximal absorption is blue-shifted from 568 nm in bR to 497 nm in sRII. The molecular mechanism of this shift is still a matter of debate, and its clarification sheds light onto the general mechanisms of color tuning in retinal proteins. The calculations employ a combined quantum mechanical/molecular mechanical (QM/MM) technique, using a DFT-based method for ground state properties and the semiempirical OM2/MRCI method and ab initio SORCI method for excited state calculations. The high efficiency of the methodology has allowed us to study a wide variety of aspects including dynamical effects. The absorption shift as well as various mutation experiments and vibrational properties have been successfully reproduced. Our results indicate that several sources contribute to the spectral shift between bR and sRII. The main factors are the counterion region at the extracellular side of retinal and the amino acid composition of the binding pocket. Our analysis allows a distinction and identification of the different effects in detail and leads to a clear picture of the mechanism of color tuning, which is in good agreement with available experimental data.     

Excited‐State Proton Transfer Can Tune the Color of Protein Fluorescent Markers

We show by quantum mechanical/molecular mechanical (QM/MM) simulations that phenylbenzothiazoles undergoing an excited‐state proton transfer (ESPT) can be used to probe protein binding sites. For 2‐(2′‐hydroxy‐4′‐aminophenyl)benzothiazole (HABT) bound to a tyrosine kinase, the absolute and relative intensities of the fluorescence bands arising from the enol and keto forms are found to be strongly dependent on the active‐site conformation. The emission properties are tuned by hydrogen‐bonding interactions of HABT with the neighboring amino acid T766 and with active‐site water. The use of ESPT tuners opens the possibility of creating two‐color fluorescent markers for protein binding sites, with potential applications in the detection of mutations in cancer cell lines.     

Ab initio prediction of the emission color in phosphorescent iridium(III) complexes for OLEDs

We performed fully first principles quantum mechanical calculations of the ground and excited state geometries and harmonic vibrational frequencies of two prototype cationic Ir(III) complexes showing high emission quantum efficiencies. Thanks to recent theoretical advances, we have been able for the first time to simulate their vibrationally resolved emission spectra. Our results, in good agreement with the experiment, allow us to calculate the CIE coordinates and therefore the emission color of this important class of emitters for OLEDs and LECs.     

Theoretical study of the visible photodissociation spectrum of Ar3

The six A′ potential energy surfaces were computed by a DIM-like method involving a valence-bond quasidiabatic basis. Transition dipole moments were also determined using a similar method. The 4D dynamics of this system (restricted to a molecular plane fixed in space) was obtained with the HWD method (hemiquantal dynamics with the whole DIM basis) and the visible photoabsorption spectrum was determined with the help of a 1D full quantum mechanical program applied to each normal mode. The photoabsorption spectrum of Ar₃⁺ was calculated in the range 440–710 nm. It corresponds to photodissociation since the excited Ar₃⁺ ions almost all dissociate into the Ar⁺ +Ar+Ar channel by a rapid explosion of the cluster, and only very few into Ar₂⁺ +Ar. It is dominated by a transition to the second excited state along with a symmetric stretching motion. We found a prominent 80 nm wide peak centered at 530 nm, with a maximum cross section of 4.2 × 10⁻¹⁶cm². The Ar₂⁺ formation results from a transition to the first excited state under low-energy laser excitation and is controlled by non-adiabatic transitions.     

Devising Efficient Red‐Shifting Strategies for Bioimaging: A Generalizable Donor‐Acceptor Fluorophore Prototype

Long emission wavelengths, high fluorescence quantum yields (FQYs), and large Stokes shifts are highly desirable features for fluorescent probes in biological imaging. However, the current development of many fluorescent probes remains largely trial‐and‐error and lacks efficiency. Moreover, to achieve far‐red/near‐infrared emission, a significant extension in the ‐conjugation is usually adopted but accompanied by other drawbacks such as fluorescence loss. In this review, we discuss an effective red‐shifting strategy built upon the green fluorescent protein chromophore, which enables a synergistic tuning of both the electronic ground and excited states. This approach could shorten the path toward redder emission in comparison to the conventional intramolecular charge transfer (ICT) strategy. We envision that this spectroscopy and computation‐aided strategy may advance the noncanonical fluorescent protein design and be generalized to various fluorophore scaffolds for redder emission while preserving other superior properties such as high FQYs.     

Linear-scaling quantum mechanical methods for excited states

The poor scaling of many existing quantum mechanical methods with respect to the system size hinders their applications to large systems. In this tutorial review, we focus on latest research on linear-scaling or O(N) quantum mechanical methods for excited states. Based on the locality of quantum mechanical systems, O(N) quantum mechanical methods for excited states are comprised of two categories, the time-domain and frequency-domain methods. The former solves the dynamics of the electronic systems in real time while the latter involves direct evaluation of electronic response in the frequency-domain. The localized density matrix (LDM) method is the first and most mature linear-scaling quantum mechanical method for excited states. It has been implemented in time- and frequency-domains. The O(N) time-domain methods also include the approach that solves the time-dependent Kohn-Sham (TDKS) equation using the non-orthogonal localized molecular orbitals (NOLMOs). Besides the frequency-domain LDM method, other O(N) frequency-domain methods have been proposed and implemented at the first-principles level. Except one-dimensional or quasi-one-dimensional systems, the O(N) frequency-domain methods are often not applicable to resonant responses because of the convergence problem. For linear response, the most efficient O(N) first-principles method is found to be the LDM method with Chebyshev expansion for time integration. For off-resonant response (including nonlinear properties) at a specific frequency, the frequency-domain methods with iterative solvers are quite efficient and thus practical. For nonlinear response, both on-resonance and off-resonance, the time-domain methods can be used, however, as the time-domain first-principles methods are quite expensive, time-domain O(N) semi-empirical methods are often the practical choice. Compared to the O(N) frequency-domain methods, the O(N) time-domain methods for excited states are much more mature and numerically stable, and have been applied widely to investigate the dynamics of complex molecular systems.     

Excitation energies and photoabsorption oscillator strengths of the Rydberg series in CF3Cl. A linear response and quantum defect study

Vertical excitation energies of the CF(3)Cl molecule have been obtained from a response function approach with a CC reference function to determine absolute photoabsorption oscillator strengths in the molecular-adapted quantum defect orbital formalism (MQDO). The present work covers more highly excited Rydberg states than have been experimentally reported. Assessing of the reliability of the present calculations is provided through a comparative analysis between the results of the molecule and the Cl atom. This can be used to allow for predictions of the same type of properties in other analogous systems.     

Molecule in soft-crystal at ground and excited states: Theoretical approach

This account discusses first two computational methods which can be applied to electronic structure calculations of soft-crystals; one is a method composed of the periodic-density functional theory (DFT) for an infinite crystal and the post-Hartree-Fock method for a cluster model, named here cluster-model/periodic-model combined method (abbreviated as CM/PM-Combined method). The other is a quantum mechanics/periodic-molecular mechanics (named QM/Periodic-MM) method, in which a target molecule is calculated by the DFT or the post-Hartree-Fock method and the other moiety is calculated by the MM method under the periodic boundary condition. Then, the performance of these two methods is discussed. The CM/PM-Combined method exhibited good performance for investigating the gas adsorption into MOF and the QM/Periodic-MM succeeded in reproducing geometry of single crystal of platinum(II) complexes. The QM/periodic-MM method has been applied to theoretical studies of the excited state and the emission spectrum in soft-crystals: In a theoretical study of a gold(I) phenyl phenylisocyanide complex, the geometries of a triplet ligand-to-ligand charger transfer (³LLCT) and a triplet metal-metal to ligand charge-transfer (³MMLCT) excited states were optimized in the crystal and the dependences of absorption and emission energies on crystal phase were discussed. In a theoretical study of a platinum(II) dicyano bipyridine complex, the geometries of several delocalized ³MMLCT excited states, emission spectra, and their temperature dependences were investigated in the crystal. In both gold(I) and platinum(II) complexes, the characteristic features of the excited state and the emission spectra were elucidated by the theoretical calculations. Although the CM/PM-Combined method has not been applied to photochemistry issue, brief discussion is presented for its possibility for the application.     

Photophysics of fluorinated benzene. III. Hexafluorobenzene

A theoretical study of the photoabsorption spectroscopy of hexafluorobenzene (HFBz) is presented in this paper. The chemical effect due to fluorine atom substitution on the electronic structure of benzene (Bz) saturates in HFBz. State- of-the-art quantum chemistry calculations are carried out to establish potential energy surfaces and coupling surfaces of five energetically low-lying electronic (two of them are orbitally degenerate) states of HFBz. Coupling of these electronic states caused by the Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) type of interactions are examined. The impact of these couplings on the nuclear dynamics of the participating electronic states is thoroughly investigated by quantum mechanical methods and the results are compared with those observed in the experiments. The complex structure of the S(1) ← S(0) absorption band is found to originate from a very strong nonadiabatic coupling of the S(2) (of πσ* origin) and S(1) (of ππ* origin) state. While S(2) state is orbitally degenerate and JT active, the S(1) state is nondegenerate. These states form energetically low-lying conical intersections (CIs) in HFBz. These CIs are found to be the mechanistic bottleneck of the observed low quantum yield of fluorescence emission, non overlapping absorption, and emission bands of HFBz and contribute to the spectral width. Justification is also provided for the observed two peaks in the second absorption (the unassigned "c band") band of HFBz. The peaks observed in the third, fourth, and fifth absorption bands are also identified and assigned.     

Linear-scaling computation of excited states in time-domain

The applicability of quantum mechanical methods is severely limited by their poor scaling.To circumvent the problem,linearscaling methods for quantum mechanical calculations had been developed.The physical basis of linear-scaling methods is the locality in quantum mechanics where the properties or observables of a system are weakly influenced by factors spatially far apart.Besides the substantial efforts spent on devising linear-scaling methods for ground state,there is also a growing interest in the development of linear-scaling methods for excited states.This review gives an overview of linear-scaling approaches for excited states solved in real time-domain.     

Substituent effects of iridium complexes for highly efficient red OLEDs

This study reports substituent effects of iridium complexes with 1-phenylisoquinoline ligands. The emission spectra and phosphorescence quantum yields of the complexes differ from that of tris(1-phenylisoquinolinato-C2,N)iridium(iii)(Irpiq) depending on the substituents. The maximum emission peak, quantum yield and lifetime of those complexes ranged from 598-635 nm, 0.17-0.32 and 1.07-2.34 micros, respectively. This indicates the nature of the substituents has a significant influence on the kinetics of the excited-state decay. The substituents attached to phenyl ring have an influence on a stability of the HOMO. Furthermore, those substituents have effect on the contribution to a mixing between 3pi-pi* and (3)MLCT for the lowest excited states. Some of the complexes display the larger quantum yield than Irpiq, which has the quantum yield of 0.22. The organic light emitting diode (OLED) device based on tris [1-(4-fluoro-5-methylphenyl)isoquinolinato-C2,N]iridium(iii)(Ir4F5Mpiq) yielded high external quantum efficiency of 15.5% and a power efficiency of 12.4 lm W(-1) at a luminance of 218 cd m(-2). An emission color of the device was close to an NTSC specification with CIE chromaticity characteristics of (0.66, 0.34).     

Solvent effect on the singlet excited-state lifetimes of nucleic acid bases: A computational study of 5-fluorouracil and uracil in acetonitrile and water

The first comprehensive quantum mechanical study of solvent effects on the behavior of the two lowest energy excited states of uracil derivatives is presented. The absorption and emission spectra of uracil and 5-fluorouracil in acetonitrile and aqueous solution have been computed at the time-dependent density-functional theory level, using the polarizable continuum model (PCM) to take into account bulk solvent effects. The computed spectra and the solvent shifts provided by our method are close to their experimental counterpart. The S0/S1 conical intersection, located in the presence of hydrogen-bonded solvent molecules by CASSCF (8/8) calculations, indicates that the mechanism of ground-state recovery, involving out-of-plane motion of the 5 substituent, does not depend on the nature of the solvent. Extensive explorations of the excited-state surfaces in the Franck-Condon (FC) region show that solvent can modulate the accessibility of an additional decay channel, involving a dark n/pi* excited state. This finding provides the first unifying explanation for the experimental trend of 5-fluorouracil excited-state lifetime in different solvents. The microscopic mechanisms underlying solvent effects on the excited-state behavior of nucleobases are discussed.     

Hydration‐Facilitated Fine‐Tuning of the AIE Amphiphile Color and Application as Erasable Materials with Hot/Cold Dual Writing Modes

Hydration water greatly impacts the color of inorganic crystals, but it is still unknown whether hydration water can be utilized to systematically manipulate the emission color of organic luminescent groups. Now, metal ions with different hydration ability allow fine‐tuning the emission color of a fluorescent group displaying aggregation induced emission (AIE). Because the hydration water can be removed easily by gentle heating or mechanical grinding and re‐gained by solvent fuming, rewritable materials can be fabricated both in the hot‐writing and cold‐writing modes. This hydration‐facilitated strategy will open up a new vista in fine‐tuning the emission color of AIE molecules based on one synthesis and in the design of smart luminescent devices.     

S1 and S2 excited states of gas-phase Schiff-base retinal chromophores: a time-dependent density functional theoretical investigation

In concert with the recent photoabsorption experiments of gas-phase Schiff-base retinal chromophores (Nielsen et al. Phys. Rev. Lett. 2006, 96, 018304), quantum chemical calculations using time-dependent density functional theory coupled with different functionals and under the Tamm-Dancoff approximation were made on the first two excited states (S1 and S2) of two retinal chromophores: 11-cis and all-trans protonated Schiff bases. The calculated vertical excitation energies (Tv) and oscillator strengths (f) are consistent with the experimental absorption bands. The experimentally observed phenomenon that the transition dipole moment (mu) of S2 is much smaller that of S1 was interpreted by 3D representation of transition densities. The different optical behaviors (linear and nonlinear optical responds) of the excited states were investigated by considering different strengths of external electric fields.     

CHEMIEXCITATION IN THE ARACHIDONIC ACID CASCADE

As investigated in neutrophils, the very weak luminescence accompanying the arachidonic acid cascade is associated with the lipoxygenase pathway. The emission is dramatically enhanced by energy transfer to chlorophyll a. The number of chlorophyll molecules excited to the fluorescent state per oxygen consumed, (the S1/O2 ratio), equal to the product of the quantum yields of chemiexcitation and of energy transfer, is 5.4 x 10(-6). The quantum yield of chemiexcitation is inferred to be higher than 1 x 10(-3). The two most likely chemiexcitation routes point to triplet conjugated carbonyls as the most likely candidates for the excited species that transfer to chlorophyll. As such the emission intensity may reflect the level of hydroperoxyeicosatetraenoic acid. This is the first case where addition of a biotic substrate to a cellular system results in substantial generation of electronic excited states without any drastic loss of cell viability. Whether the formation of excited states in the arachidonic acid cascade in neutrophils is accidental or has a biological role is an open question.     

Strategic emission color tuning of highly fluorescent imidazole-based excited-state intramolecular proton transfer molecules

Highly fluorescent molecules harnessing the excited state intramolecular proton transfer (ESIPT) process are promising for a new generation of displays and light sources because they can offer very unique and novel optoelectronic properties which are different from those of conventional fluorescent dyes. To realize innovative ESIPT devices comprising full emission colors over the whole visible region, a molecular design strategy for predictable emission color tuning should be established. Here, we have developed a general strategy for a wide-range spectral tuning of imidazole-based ESIPT materials based on three different strategies--introduction of a nodal plane model, extension of effective conjugation length, and modification of heterocyclic rings. A series of nine ESIPT molecules were designed, synthesized and comprehensively investigated for their characteristic emission properties. All these molecules commonly showed no clear and transparent visible range absorption with no absorption color, but showed different colors of intense photoluminescence over broad visible regions from 450 nm (HPI) to 630 nm (HPNO) depending on their molecular structure. With the aid of density functional theory and time-dependent DFT calculations using M06, wB97XD, and B3LYP parameters with the 6-31G(d,p) basis set, these tuned emission bands of nine emitters were assigned from the stabilized excited state conformations that were derived from modified molecular structures.