Hidden electronic excited state of enhanced green fluorescent protein

Precise two-photon absorption spectra of the green fluorescent protein (GFP) and the mutants sapphire-GFP (T203I) and enhanced GFP (S65T/F64L), as well as a model compound for the chromophore, 4'-hydroxybenzylidene-2,3-dimethylimidazolinone (HBDI) were measured by multiplex two-photon absorption spectroscopy. The observed TPA bands of the anionic forms of enhanced GFP and HBDI were significantly shifted to the higher energy compared with the lowest-energy bands in one-photon absorption spectra. This result indicated the existence of a hidden electronic excited state in the vicinity of the lowest excited singlet (S1) state of the anionic form of the GFP chromophore, which is the origin of the blue shift of the two-photon absorption spectra as well as two-photon fluorescence excitation spectra.     

Highly Fluorescent Green Fluorescent Protein Chromophore Analogues Made by Decorating the Imidazolone Ring

The synthesis and photophysical behavior of an unexplored family of green fluorescent protein (GFP)‐like chromophore analogues is reported. The compound (Z)‐4‐(4‐hydroxybenzylidene)‐1‐propyl‐2‐(propylamino)‐1H‐imidazol‐5(4 H)‐one (p‐HBDNI, 2 a ) exhibits significantly enhanced fluorescence properties relative to the parent compound (Z)‐5‐(4‐hydroxybenzylidene)‐2,3‐dimethyl‐3,5‐dihydro‐4H‐imidazol‐4‐one (p‐HBDI, 1 ). p‐HBDNI was considered as a model system and the photophysical properties of other novel 2‐amino‐3,5‐dihydro‐4H‐imidazol‐4‐one derivatives were evaluated. Time‐dependent DFT calculations were carried out to rationalize the results. The analogue AIDNI ( 2 c ), in which the 4‐hydroxybenzyl group of p‐HBDNI was replaced by an azaindole group, showed improved photophysical properties and potential for cell staining. The uptake and intracellular distribution of 2 c in living cells was investigated by confocal microscopy imaging.     

Efficient Photoconversion Distorts the Fluorescence Lifetime of GFP in Confocal Microscopy: A Model Kinetic Study on Mutant Thr203Val

Phototransformations of autofluorescent proteins are applied in high‐resolution microscopy and in studying cellular transport, but they are detrimental when accidentally occurring in blinking or photobleaching (BL). Here, we investigate the kinetics of phototransformations of a photoactivatable green fluorescent protein (GFP) in confocal microscopy. Photoconversion (PC) is achieved by excitation of the barely present anionic chromophore state R_eq^? in the GFP mutant Thr203Val. Besides the shift of the equilibrium between the neutral chromophore state RH and R_eq^?, the photoconverted anionic chromophore R_PC^? exhibits a reduced fluorescence lifetime τ_fl=2.2 ns. In fluorescence lifetime imaging microscopy, τ_fl is found to depend, however, on the excitation conditions and history. The underlying photochemistry is described by the kinetic scheme of consecutive reactions, R_eq^?→R_PC^?→P_dark, in which the anionic chromophore species and the dark protein P_dark are coupled by PC and BL. Time‐correlated single‐photon‐counting detection in a confocal geometry of freely diffusing species is used to compute the quantum yields for PC and BL, Φ_PC and Φ_BL. The assessed values are Φ_PC=5.5×10^?4 and Φ_BL>1×10^?5. Based on these values, PC provokes misinterpretation in fluorescence resonance energy transfer experiments and is responsible for spectroscopic peculiarities in single‐molecule detection.     

The role of the protein matrix in green fluorescent protein fluorescence

In the ground state of the highly conjugated green fluorescent protein (GFP), the chromophore should be planar. However, numerous crystal structures of GFP and GFP-like proteins have been reported with slightly twisted chromophores. We have previously shown that the protein cavity surrounding the chromophore in wild-type GFP is not complementary with a planar chromophore. This study shows that the crystal structure of wild-type GFP is not an anomaly: most of the GFP and GFP-like proteins in the protein databank have a protein matrix that is not complementary with a planar chromophore. When the pi-conjugation across the ethylenic bridge of the chromophore is removed the protein matrix will significantly twist the freely rotating chromophore from the relatively planar structures found in the crystal structures. The possible consequences of this nonplanar deformation on the photophysics of GFP are discussed. A volume analysis of the cis-trans-isomerization of HBDI, a GFP chromophore model compound, reveals that its hula-twist motion is volume conserving. This means that, if the GFP chromophore or GFP chromophore model compounds undergo a cis-trans-isomerization in a volume-constricting medium, such as a protein matrix or viscous liquid, it will probably isomerize by means of a HT-type motion.     

Excited‐State Intramolecular Proton Transfer in a Blue Fluorescence Chromophore Induces Dual Emission

Compared with green fluorescence protein (GFP) chromophores, the recently synthesized blue fluorescence protein (BFP) chromophore variant presents intriguing photochemical properties, for example, dual fluorescence emission, enhanced fluorescence quantum yield, and ultra‐slow excited‐state intramolecular proton transfer (ESIPT; J. Phys. Chem. Lett., 2014 , 5, 92); however, its photochemical mechanism is still elusive. Herein we have employed the CASSCF and CASPT2 methods to study the mechanistic photochemistry of a truncated BFP chromophore variant in the S₀ and S₁ states. Based on the optimized minima, conical intersections, and minimum‐energy paths (ESIPT, photoisomerization, and deactivation), we have found that the system has two competitive S₁ relaxation pathways from the Franck–Condon point of the BFP chromophore variant. One is the ESIPT path to generate an S₁ tautomer that exhibits a large Stokes shift in experiments. The generated S₁ tautomer can further evolve toward the nearby S₁/S₀ conical intersection and then jumps down to the S₀ state. The other is the photoisomerization path along the rotation of the central double bond. Along this path, the S₁ system runs into an S₁/S₀ conical intersection region and eventually hops to the S₀ state. The two energetically allowed S₁ excited‐state deactivation pathways are responsible for the in‐part loss of fluorescence quantum yield. The considerable S₁ ESIPT barrier and the sizable barriers that separate the S₁ tautomers from the S₁/S₀ conical intersections make these two tautomers establish a kinetic equilibrium in the S₁ state, which thus results in dual fluorescence emission.     

Synthesis of new tripodal receptors—a ‘PET’ based ‘off-on’ recognition of Ag

A new set of tripodal receptors based upon an aromatic platform have been synthesized in high yields. The compounds have been characterized by spectroscopic techniques and by single crystal X-ray crystallography. These receptors are found to have good extraction ability and high transport rate for Ag(I). The receptor with imine linkages exhibits weak fluorescence emission bands at λ_max=413 and 540 nm, upon excitation at λ_max=365 nm. The fluorescence spectrum of the receptor shows enhancement in the intensity of the signal at 413 nm on binding with the Ag⁺ cation. No such significant changes are observed with other metal ions. An absorption at ∼365 nm is typical of an intraligand (π-π^∗) transition involving the imine chromophore, which produces a weak emission band at 413 nm due to quenching caused by PET from a neighboring -OH group. Participation of OH group in coordination to the metal ion reduces PET and an enhancement of fluorescence intensity is observed, signaling recognition of the metal ion.     

What is Best Strategy for Water Soluble Fluorescence Dyes?—A Case Study Using Long Fluorescence Lifetime DAOTA Dyes**

The lipophilic nature of organic dyes complicates their effectiveness in aqueous solutions. In this work we investigate three different strategies for achieving water-solubility of the diazaoxatriangulenium (DAOTA⁺) chromophore: hydrophilic counter ions, aromatic sulfonation of the chromophore, and attachment of charged side chains. The long fluorescence lifetime (FLT, τ_f=20 ns) of DAOTA⁺ makes it a sensitive probe to analyze solvation and aggregation effects. Direct sulfonation of the chromophore was found to increase solubility drastically, but at the cost of greatly reduced quantum yields (QYs) due to enhanced non-radiative deactivation processes. The introduction of either cationic (4) or zwitterionic side chains (5), however, brings the FLT (τ_f=18 ns) and QY (ϕ_f=0.56) of the dye to the same level as the parent chromophore in acetonitrile. Time-resolved fluorescence spectroscopy also reveals a high resistance to aggregation and non-specific binding in a high loading of bovine serum albumin (BSA). The results clearly show that addition of charged flexible side chains is preferable to direct sulfonation of the chromophore core.     

Vinyl monomers bearing chromophore moieties and their polymers. V. Synthesis and fluorescence behavior of a vinyloxy monomer bearing electron-accepting chromophore moiety,N-(2-(vinyloxy)ethyl)-1,8-naphthalimide,and its polymer

A vinyloxy monomer bearing electron-accepting chromophore, N-(2-(vinyloxy)ethyl)-1,8-naphthalimide (VOENI), was synthesized by reaction of potassium 1,8-naphthalimide with 2-chloroethyl vinyl ether. VOENI can be homopolymerized by cationic initiation and copolymerized with maleic anhydride (MAn) under radical initiation. The fluorescence behaviors of VOENI and its polymers were investigated. It has been found that the fluorescence intensity of the VOENI monomer is much lower than that of its polymers at the same chromophore concentration. This means that a “structural self-quenching effect” (SSQE) has been also observed in the vinyloxy monomer consisting of an electron-accepting chromophore, which has opposite electronic structure in comparison with acrylates bearing electron-donating chromophores as we have reported previously. The SSQE is attributed to the charge-transfer interaction between the electron-accepting chromophore and the electron-donating double bond in the same molecule. The fluorescence quenching of 1,8-naphthalic anhydride and P(VOENI-co-MAn) by ethyl vinyl ether (EVE), dihydrofuran, triethylamine (TEA), etc. evidences that the electron-rich vinyloxy group does act as an important role in the SSQE of VOENI. C₆₀ can also quench the fluorescence of the polymers, and an upward deviation from the linearity of the Stern–Volmer plot was observed showing that C₆₀ acted as a powerful electron donor to quench the fluorescence of the copolymer. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1111–1116, 1998     

Synthesis and Nonlinear Optical Properties of a New A-π-A Two-Photon Compound Utilizing Dibenzothiophene as Center

A new two‐photon material, 3E,6E‐bis(2‐pyrid‐4′‐ylvinyl)dibenzothiophene (BPVDBT), has been firstly synthesized by an efficient Pd‐catalyzed Heck coupling route. The single‐ and two‐photon fluorescence, quantum yields, lifetimes, solvent effects of the chromophore were studied in detail and the compound exhibited solvent‐sensitivity. The fluorescence intensity (I_out) and input excitation intensity (I_in) can fit in well with the quadratic parabolas, which indicates that the up‐converted fluorescence was induced by the two‐photon absorption (TPA). TPA cross‐section of BPVDBT has been measured using the two‐photon‐induced fluorescence method, whose value is 14.24×10^?50 cm⁴·s·photon^?1·molecule^?1 at 750 nm. The experimental results confirm that BPVDBT is a good two‐photon absorbing chromophore with an A‐π‐A type.     

Resonance Energy Transfer in a Genetically Engineered Polypeptide Results in Unanticipated Fluorescence Intensity

The fluorescence intensity of a C-terminal acceptor chromophore, N-(7-dimethylamino-4-methyl coumarin (DACM), increased proportionally with 280 nm irradiation of an increasing number of donor tryptophan residues located on a β-sheet forming polypeptide. The fluorescence intensity of the acceptor chromophore increased even as the length of the β-sheet edge approached 256 Å, well beyond the Förster radius for the tryptophan–acceptor chromophore pair. The folding of the peptides under investigation was verified by circular dichroism (CD) and deep UV resonance Raman experiments. Control experiments showed that the enhancement of DACM fluorescence occurred concomitantly with peptide folding. In other control experiments, the DACM fluorescence intensity of the solutions of tryptophan and DACM did not show any enhancement of DACM fluorescence with increasing tryptophan concentrations. Formation of fibrillar aggregates of the substrate peptides prepared for the fluorescence studies was undetectable by thioflavin T (ThT) fluorescence.     

Synthesis,crystal structures,and photophysical properties of dibromo-2-(2′-pyridyl)imidazole and its corresponding boron–fluorine complex

2-(2′-Pyridyl)imidazole L1 and its corresponding boron–fluorine complex, 1, were synthesized and their crystal structures correlated with their photophysical properties. L1 forms a rigid supramolecular network through hydrogen bonds and halogen bond in the single crystal, which induces amplified spontaneous emission in crystals; it emits rather poor fluorescence in solution and powder states. Its boron chelate 1 emits intense fluorescence in solution since boron chelate is an excellent chromophore, and it exhibits large Stokes shift (136?nm in acetonitrile), due to the charge-transfer transition from the electron-donating π system to the electron-accepting boron moiety. Interestingly, 1 is also highly fluorescent in amorphous powder and crystal states; C–C rotation between pyridyl and imidazole groups is inhibited by the formation of a five-member ring containing BF₂, and the formation of intermolecular non-covalent bonds is the key factor. Solid emission with large Stokes shift makes it a valuable chromophore for synthesis of functional materials.     

Quantification of Photosensitized Singlet Oxygen Production by a Fluorescent Protein

Fluorescent proteins are increasingly becoming actuators in a range of cell biology techniques. One of those techniques is chromophore‐assisted laser inactivation (CALI), which is employed to specifically inactivate the function of target proteins or organelles by producing photochemical damage. CALI is achieved by the irradiation of dyes that are able to produce reactive oxygen species (ROS). The combination of CALI and the labelling specificity that fluorescent proteins provide is useful to avoid uncontrolled photodamage, although the inactivation mechanisms by ROS are dependent on the fluorescent protein and are not fully understood. Herein, we present a quantitative study of the ability of the red fluorescent protein TagRFP to produce ROS, in particular singlet oxygen (¹O₂). TagRFP is able to photosensitize ¹O₂ with an estimated quantum yield of 0.004. This is the first estimation of a quantum yield of ¹O₂ production value for a GFP‐like protein. We also find that TagRFP has a short triplet lifetime compared to EGFP, which reflects relatively high oxygen accessibility to the chromophore. The insight into the structural and photophysical properties of TagRFP has implications in improving fluorescent proteins for fluorescence microscopy and CALI.     

Native Biomolecules in the Gas Phase? The Case of Green Fluorescent Protein

Green fluorescent protein (GFP) was ionized by native electrospray ionization and trapped for many seconds in high vacuum, allowing fluorescence emission to be measured as a probe of its biological function, to answer the question whether GFP exists in the native form in the gas phase or not. Although a narrow charge‐state distribution, a collision cross‐section very close to that expected for correctly folded GFP, and a large stability against dissociation all support a near‐native gas‐phase structure, no fluorescence emission was observed. The loss of the native form is attributed to the absence of residual water in the gas phase, which normally stabilizes the para‐hydroxybenzylidene imidazolone chromophore of GFP.     

Developing Bright Green Fluorescent Protein (GFP)-like Fluorogens for Live-Cell Imaging with Nonpolar Protein−Chromophore Interactions

Fluorescence-activating proteins (FAPs) that bind a chromophore and activate its fluorescence have gained popularity in bioimaging. The fluorescence-activating and absorption-shifting tag (FAST) is a light-weight FAP that enables fast reversible fluorogen binding, thus advancing multiplex and super-resolution imaging. However, the rational design of FAST-specific fluorogens with large fluorescence enhancement (FE) remains challenging. Herein, a new fluorogen directly engineered from green fluorescent protein (GFP) chromophore by a unique double-donor-one-acceptor strategy, which exhibits an over 550-fold FE upon FAST binding and a high extinction coefficient of approximately 100,000 M⁻¹ cm⁻¹, is reported. Correlation analysis of the excited state nonradiative decay rates and environmental factors reveal that the large FE is caused by nonpolar protein−fluorogen interactions. Our deep insights into structure-function relationships could guide the rational design of bright fluorogens for live-cell imaging with extended spectral properties such as redder emissions.     

Synthesis and Electronic Properties of 1,2‐Hemisquarimines and Their Encapsulation in a Cucurbit[7]uril Host

The synthesis of a new class of robust squaraine dyes, colloquially named 1,2‐hemisquarimines (1,2‐HSQiMs), through the microwave‐assisted condensation of aniline derivatives with the 1,2‐squaraine core is reported. In CH₃CN, 1,2‐HSQiMs show a broad absorption band with a high extinction coefficient and a maximum at around λ=530 nm, as well as an emission band centered at about λ=574 nm, that are pH dependent. Protonation of the imine nitrogen causes a redshift of both absorption and emission maxima, with a concomitant increase in the lifetime of the emitting excited state. Encapsulation of the chromophore into a cucurbit[7]uril host revealed fluorescence enhancement and increased photostability in water. The redox characteristics of 1,2‐HSQiMs indicate that charge injection into TiO₂ is possible; this opens up promising perspectives for their use as photosensitizers for solar energy conversion.     

Controlling Light-Induced Proton Transfer from the GFP Chromophore

Green Fluorescent Protein (GFP) is known to undergo excited-state proton transfer (ESPT). Formation of a short H-bond favors ultrafast ESPT in GFP-like proteins, such as the GFP S65T/H148D mutant, but the detailed mechanism and its quantum nature remain to be resolved. Here we study in vacuo, light-induced proton transfer from the GFP chromophore in hydrogen-bonded complexes with two anionic proton acceptors, I⁻ and deprotonated trichloroacetic acid (TCA⁻). We address the role of the strong H-bond and the quantum mechanical proton-density distribution in the excited state, which determines the proton-transfer probability. Our study shows that chemical modifications to the molecular network drastically change the proton-transfer probability and it can become strongly wavelength dependent. The proton-transfer branching ratio is found to be 60 % for the TCA complex and 10 % for the iodide complex, being highly dependent on the photon energy in the latter case. Using high-level ab initio calculations, we show that light-induced proton transfer takes place in S₁, revealing intrinsic photoacid properties of the isolated GFP chromophore in strongly bound H-bonded complexes. ESPT is found to be very sensitive to the topography of the highly anharmonic potential in S₁, depending on the quantum-density distribution upon vibrational excitation. We also show that the S₁ potential-energy surface, and hence excited-state proton transfer, can be controlled by altering the chromophore microenvironment.     

Vinyl monomers bearing chromophore moieties and their polymers. VIII. synthesis and fluorescence behavior of a vinyloxy monomer having an electron‐accepting chromophore moiety,p‐((vinyloxy)methyl)benzonitrile,and its polymers

A vinyloxy monomer having an electron‐accepting chromophore moiety, p‐((vinyloxy)methyl)benzonitrile (VOMBN), was synthesized by reaction of p‐(hydroxymethyl)benzonitrile with ethyl vinyl ether (EVE) in the presence of mercuric acetate. VOMBN can easily be cationically homopolymerized and copolymerized with EVE by using Lewis acids such as boron trifluoride etherate (BF₃ · OEt₂) as a catalyst and radically copolymerized with maleic anhydride (MAn) using AIBN as an initiator. The fluorescence behaviors of VOMBN, its copolymer P(VOMBN‐co‐MAn), and its saturated model compound p‐(ethoxymethyl)benzonitrile (EOMBN) were investigated in acetonitrile. It has been found that the fluorescence intensity of VOMBN is much lower than its copolymer and EOMBN at the same chromophore concentration. A fluorescence “structural self‐quenching effect” (SSQE) is also observed for VOMBN as we have reported previously [Li, F. M.; Chen, S. J.; Li, Z. C.; Qiu, J. J Polym Sci Polym Chem 1996, 34, 1881]. This phenomenon has been attributed to the inter‐ and intramolecular charge transfer interaction between the electron‐accepting cyanophenyl chromophore and the electron‐donating vinyloxy group in the same molecule. The dependence of the fluorescence intensity of VOMBN on solvents of different viscosities is evidence that the SSQE of VOMBN mainly occurs intramolecularly. The fluorescence of EOMBN and P(VOMBN‐co‐MAn) was quenched by a series of electron‐rich vinyloxy compounds which do not have chromophore moieties, such as dihydrofuran (2H‐furan), dihydropyran (2H‐pyran), furan, and EVE. It is observed that the higher the electron‐donating ability of the quenchers, the greater the quenching efficiency. P(VOMBN) and the random copolymers of VOMBN with EVE show broader fluorescence spectra as compared to the alternating copolymer P(VOMBN‐co‐MAn). This indicates that there is a mutual interaction between the adjacent cyanophenyl groups in P(VOMBN) and P(VOMBN‐co‐EVE), whereas such an interaction does not exist for P(VOMBN‐co‐MAn) in which the cyanophenyl groups are isolated by the rigid succinic anhydride rings. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 179–187, 1999     

The electronic effect in supramolecular self-assembly of <Emphasis Type="Italic">para-</Emphasis>substituted tetraphenylporphyrinatozinc(II) with imidazolyl-linked porphyrinatomanganese(III) by coordinative bonding

The supramolecular self-assembly of para-substituted tetraphenylporphyrin complexes of zinc(II), Zn(p-X)TPP, with imidazolyl-linked porphyrinatomanganese(III), Mn(p-lmBPTPP)Cl, driven by coordinative bonding has been investigated by fluorescence spectra, electrospray mass spectrometry, ¹H-n.m.r. and u.v.–vis. spectra. The association constants of the supramolecular complexes, K_c, were calculated using fluorescence spectroscopic titration data at suitable dilute concentration ranges in which the fluorescent quenching of Zn(p-X)TPP by Mn(p-ImBPTPP)Cl is a static process. The electronic effect in the supramolecular self-assembly of para-substituted . porphyrinatozinc(II) with imidazolyl-linked porphyrinatomanganese(III) is discussed. The non-linear dependence of log K_c on the Hammett constants was found, which suggested that the electronic effect in para-substituents of tetraphenylporphyrin complexes of zinc(II) is an important, but not a sole factor effecting the association constants of the Zn(p-X)TPP–Mn(p-ImBPTPP)Cl supramolecular complexes. The results indicate that the closed conformation of the Zn(p-X)TPP–Mn(p-ImBPTPP)Cl supramolecular complex is another important factor effecting the association constants of the Zn(p-X)TPP–Mn(p-ImBPTPP)Cl supramolecular complexes.     

The Contrasting Recognition Behavior of β‐Cyclodextrin and Its Sulfobutylether Derivative towards 4′,6‐Diamidino‐2‐phenylindole

The noncovalent interactions between 4′, 6‐diamidino‐2‐phenylindole (DAPI) and sulfobutylether β‐cyclodextrin (SBE₇β‐CD) are evaluated by using photochemical measurements and compared with that of native β‐CD. Contrasting recognition behavior and intriguing modulations in the photochemical behavior of DAPI were observed. In particular, a large enhancement in the fluorescence emission and excited‐state lifetime were seen upon binding to SBE₇β‐CD, with the SBE₇β‐CD inclusion complex being approximately 1000 times stronger than that of β‐CD. The ensuing fluorescence “turn on” was demonstrated to be responsive to chemical stimuli, such as metal ions and adamantylanmine (AD). Upon addition of Ca²⁺/AD, nearly quantitative dissociation of the complex was established to regenerate the free dye and result in fluorescence “turn off”. The SO₃^? groups are believed to be critical for the strong and selective binding of the chromophore and the stimuli‐responsive tuning. This is as an important design criterion for the optimization of host–guest properties through supramolecular association, which is relevant for drug‐delivery applications.     

Multi‐Pyrene Assemblies Supported on Stannoxane Frameworks: Synthesis,Structure and Photophysical Studies

Organostannoxanes have been used as scaffolds for the preparation of multi‐chromophore assemblies. A single‐step synthesis procedure allows the preparation of compounds in which the number of chromophore units can be varied from two to six. Thus, the reactions of pyrene sulfonic acid (PySO₃H) or C₁₆H₉CHNC₆H₃(COOH)₂ (LH₂) with various organotin precursors gave pyrene‐containing organostannoxanes, that is, [Ph₃SnPySO₃]₆ ( 1 ), [{(Me₂Sn)₂(μ₃‐O)(μ‐OH)PySO₃}₂{(Me₂Sn)₂(μ₃‐O)(μ‐OH)H₂O}₂ ? 2 PySO₃] ( 2 ), [{tBu₂Sn(OH)PySO₃}₂] ( 3 ), [{(nBuSn)₁₂(O)₁₄(OH)₆{PySO₃}₂] ( 4 ), and [{(nBu₂Sn)L}₃]₂ ? C₆H₅CH₃ ( 5 ). Compounds 1 – 5 were characterized by using X‐ray crystallography. Compounds 1 and 5 are 24‐membered macrocycles. Macrocycle 1 possesses intramolecular π–π stacking interactions. An unusual co‐crystal of two tetrameric ladders in 2 was observed in which one of the components of the co‐crystal is neutral whereas the other is dicationic and two pyrenesulfonate counterions are present to balance the overall charge. In the solid state these compounds reveal rich supramolecular structures. Photophysical studies on 1 – 5 reveal that interactions in the solid state lead to considerable broadening of the emission bands.